uk rockall prospectivity: re-awakening exploration in a ... · the ukcs. this is against an overall...

Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. The manuscript will undergo copyediting,

typesetting and correction before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Although reasonable efforts have been made to obtain all necessary permissions from third parties to include their

copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record once published for full citation and

copyright details, as permissions may be required.

Accepted Manuscript

Petroleum Geoscience

UK Rockall Prospectivity: Re-awakening exploration in a

frontier basin

Lena Broadley, Nick Schofield, David Jolley, John Howell & John R. Underhill

DOI: https://doi.org/10.1144/petgeo2019-098

This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Received 8 July 2019

Revised 10 February 2020

Accepted 14 February 2020

© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative

Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London for GSL and EAGE. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

To cite this article, please follow the guidance at https://www.geolsoc.org.uk/~/media/Files/GSL/shared/pdfs/Publications/AuthorInfo_Text.pdf?la=en

by guest on June 24, 2020http://pg.lyellcollection.org/Downloaded from

https://doi.org/10.1144/petgeo2019-098

https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

http://creativecommons.org/licenses/by/4.0/

http://www.geolsoc.org.uk/pub_ethics

https://www.geolsoc.org.uk/~/media/Files/GSL/shared/pdfs/Publications/AuthorInfo_Text.pdf?la=en

http://pg.lyellcollection.org/

UK Rockall Prospectivity: Re-awakening exploration in a frontier basin

Lena Broadley1, Nick Schofield1, David Jolley1, John Howell1, John R. Underhill2

1Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Meston Building, Aberdeen, AB24 3UE, UK 2Shell Centre for Exploration Geoscience, Applied Geoscience Unit, Institute of GeoEnergy Engineering (IGE),

Heriot-Watt University, Edinburgh EH14 4AS, UK

Corresponding author: [email protected]

Abstract

The UK Rockall, located to the west of Scotland and the Hebrides, is a frontier petroleum

bearing basin. Exploratory drilling in the basin took place over a quarter of a century (1980-

2006), during which time a total of 12 wells were drilled, leading to the discovery of a single,

sub-commercial gas accumulation. We argue that the basin, which has seen no drilling

activity for more than a decade, has not been sufficiently tested by the existing well stock.

We examine the reasons for the absence of key Jurassic source rocks in the UK Rockall

wells, that are widely distributed elsewhere on the UK continental shelf (UKCS), and argue

that their absence in the wells does not preclude their existence in the basin at large. An

evaluation of the Permian to Early Eocene successions based upon the seismic interpretation

of new 2D seismic data, has been integrated with legacy data and regional evidence, to

establish the potential for source, reservoir and sealing elements within each interval.

Finally, we look at the future for exploration in the UK Rockall and suggest a way forward in

the drilling of a new joint governmental-industry test well that may help to unlock the

exploration potential of this under-explored yet prospective basin.

ACCEPTED MANUSCRIPT



Introduction

The UK continues to undergo rapid change in its domestic Oil and Gas supply chain, with

supermajors (e.g. Conoco-Philips and Chevron) now exiting exploration and production on

the UKCS. This is against an overall decrease in production across the Southern, Central

and Northern North Sea Basins (SNS, CNS and NNS). Even with the increased focus within

the UK on energy transition to renewables, at current best estimates, the global demand for

Oil and Gas will still remain high beyond 2040 (e.g. BP, 2019a; BP, 2019b). This coupled with

socio-political global uncertainty, it remains important for UK energy security to fully

evaluate areas of the UKCS which are under-explored with a view to feeding the funnel of

new projects that could yield production and decrease the country‟s reliance on oil and gas

imports.

The UK Rockall Basin represents a highly underexplored area within the UKCS. To

date, only 12 exploration wells (with 1 discovery) have been drilled in the UK Rockall Basin

(Figure 1), an area comparable in size to the entire developed area of the UK North Sea.

However, our knowledge of the UK Rockall stratigraphy is severely lacking, as only 4 of the

12 wells drilled, 132/15-1, 154/03-1, 164/25-1Z and 164/25-2, penetrated rocks pre-dating

the Cretaceous, and not a single exploration well has found strata of Jurassic age. The

results of these 4 wells have contributed to an informal, but widely held, perception that

Jurassic sequences are largely absent from the Rockall Basin. The perceived absence of key

stratigraphic intervals has been central to a generally negative industry view of the basin

because the Jurassic hosts the main source rock intervals in the more mature NW

European basins, including the prolific North Sea and Faroe-Shetland Basin (FSB). However,

the 154/01-1 Benbecula thermal gas discovery not only proved the existence of a source

rock but also underlined the fact that a working petroleum system exists in the basin, at

least locally. Associated geochemical analyses indicated that the gas could potentially have

ACCEPTED MANUSCRIPT



been derived from an Upper Jurassic source (Kleingeld. 2005). This is further corroborated

by analysis of oils from the 164/28-1A well, in which biomarker data from oil shows in the

Paleocene age Vaila Formation indicated multiple sourcing of hydrocarbons, with one

component corresponding to a siliciclastic marine source type, suggested to be similar to

the Upper Jurassic of the North Sea (Ferriday, 2000).

The lack of source rock penetration in the basin continues to supress confidence,

particularly in the context of the low levels of hydrocarbon industry activity that have

persisted in the UKCS for the past 5 years. To compound this, exploration within the UK

Rockall remains costly due to the deep water and risky due to poor seismic data quality;

biostratigraphy from existing wells is inconsistent, complicating correlation; and the impact

of intrusive and extrusive volcanics is manifold, with potential impact not only on seismic

imaging, but also on migration, trap formation and breaching, maturation or cracking of

source rocks and reservoir diagenesis through fracturing, local heating and enhanced fluid

flow (Schofield et al. 2017a; Karvelas et al., 2016).

In this paper, we discuss aspects of the results of a two-year study of the Rockall

Basin conducted at the University of Aberdeen as part of the UK Oil and Gas Authority

(OGA) Frontier Basins project. The stimulus for the study was the release of new regional

2D seismic data in the Rockall Basin by the OGA in 2016 to reinvigorate exploration in this

frontier basin. We present an overview of the geological background and the exploration

history and narrative of the basin. This is followed by a series of well correlations, based on

new biostratigraphy carried out in order to place the wells within the widely used T-

sequence framework (Figure 4), established by BP stratigraphers in the 1990s (e.g. Jones &

Milton, 1994; Ebdon et al., 1995) for the Paleogene sequences of the North Sea and the

Faroe-Shetland Basin. Paleogeographies at key intervals are presented, followed by a

ACCEPTED MANUSCRIPT



summary of play concepts. Finally, we consider the main impediments to exploration

progress and discuss potential ways forward.

Importantly, we argue, that as it stands, even taking into account regional

information from outside of the Basin (e.g. Inner Hebrides, Solan Basin and Irish Rockall),

insufficient direct subsurface data exists within the UK Rockall, particularly in terms of wells,

to make firm conclusions about UK Rockall prospectivity, or the lack thereof. We suggest

that this impasse can only be broken by the provision of hard data provided by a new

stratigraphic test well, similar to the first well drilled within the basin in 1980, which was

drilled by a consortium of government and oil companies (to share cost), only then may

enough data be potentially available to finally decide on the prospectivity of Rockall, whether

positive or negative. Without undertaking such an admittedly bold move, we would argue

that questions of the UK Rockall prospectivity are likely to remain for decades to come and

exploration interest is likely to remain supressed.

Geological Background and Stratigraphy

The detailed geological evolution of the Rockall Basin is still largely unknown, due to the

lack of deep well penetrations in the basin, and limited coverage of good quality seismic

data, however its development is thought to be comparable with that of the adjacent Faroe-

Shetland Basin (Archer et al., 2005), whilst the Inner Hebrides may provide an insight into

the Jurassic development of the basin. A petroleum systems events chart is presented in

Figure 3, whilst the lithostratigraphic scheme for the Permian to the Eocene of these areas is

shown in Figures 4-7.

The basement is likely comprised of two major Caledonian basement terranes,

sutured across the Anton Dohrn Lineament (Figure 1; Doré et al. 1997a; Doré et al., 1999;

ACCEPTED MANUSCRIPT



Hitchen et al. 2013). The presence of Middle Proterozoic through to Earliest Devonian

sequences is currently largely unknown in the Rockall area, however the existence of such

sequences within the Faroe-Shetland Basin (e.g. Rona Ridge) and, possibly, in the Inner

Hebrides (e.g. Minch-1 well; Peacock, 1990) infers that they may be present within the UK

Rockall. The North East-South West structural grain dominating the rift orientation along

the Atlantic Margin is an inherited trend, first developed during compression associated with

the Caledonian Orogeny from Silurian to Early Devonian times (Hitchen et al. 2013).

During the subsequent Variscan orogeny, later Devonian to Carboniferous sediments were

distributed across large parts of mainland Britain and Ireland. Offshore, Devonian and/or

Carboniferous fluvio-deltaic sediments are proven in both the Faroe-Shetland Basin to the

north and the Irish Rockall to the south. Whilst their occurrence remains unproven in the

UK Rockall, it has been postulated that these Upper Palaeozoic sequences may underlie

many of the Mesozoic basins along the North East Atlantic margin, including the UK Rockall

(Hitchen et al., 2013).

Although now bathymetrically separated by the Wyville-Thomson Ridge, a Cenozoic

inversion structure, the NE Rockall Basin and the Faroe-Shetland Basin to the north are

assumed to have undergone a similar rift history (Archer et al., 2005). A complex, multi-

phase rift history has evolved following the collapse of the Caledonides. A Triassic to Early

Cretaceous rift phase and Late Cretaceous to Paleogene post-rift phase appear to

characterise much of the stratigraphy of the Rockall Basin (Musgrove and Mitchener, 1996;

Archer et al., 2005). Jurassic rifting is thought to have resulted in a series of restricted

North-South orientated basins that were later overprinted by more widespread Early

Cretaceous rifting (Scotchman et al., 2018). The Rockall may have become hyperextended

(Roberts et al., 2018) during this subsequent rifting, potentially leading to highly segmented

ACCEPTED MANUSCRIPT



and discontinuous Jurassic and older sequences towards the centre of the basin (Lundin and

Doré, 2011).

In the Faroe-Shetland basin, rifting continued into the Middle and Late Cretaceous,

evolving into an initial phase of post-rift subsidence by the latest Cretaceous (e.g. Dean et

al., 1999; Doré et al., 1999). Evidence for Late Cretaceous rifting in the UK Rockall is

unclear, based on currently available data, and it is argued by Musgrove and Mitchener

(1996) that thermal subsidence dominated from the Cenomanian age onwards. The

Cenozoic was a period of overall post-rift thermal sag, with accelerated subsidence from the

Late Eocene leading to the present-day expression of the Rockall Basin as a single, deep

water basin (Hitchen et al., 2013). Against this backdrop of net subsidence, episodes of

inversion during the Thanetian, late Ypresian, Late Eocene, Early Oligocene and Early –

Middle Miocene (e.g. Stoker et al., 2005; Ritchie et al., 2008; Tuitt, 2009; Tuitt et al., 2010;

Hitchen et al., 2013) were widespread along the NE Atlantic margin and were important

events for the formation of large compressional structures. such as the Wyville-Thomson

Ridge (Johnson et al., 2005; Ritchie et al., 2008; Hitchen et al., 2013) and the Ormen Lange

Dome in Norway (Lundin & Doré, 2002; Tuitt et al., 2010).

Exploration History and Narrative

To date, only 12 exploration wells have been drilled in the UK Rockall (Figures 1 and 2),

comprising 11 dry holes, one with oil shows, and a single gas discovery. This in an area of

some 180,000 km2, comparable in size to that of the developed areas of the UK North Sea

hydrocarbon province. Nine of the wells are confined to the Northeast Rockall Basin, with a

single well in the adjacent West Lewis Basin, and two in the South Rockall Basin (Figure 1).

For a full review of UK Rockall exploration, the reader is referred to Schofield et al. (2017a;

2019), who detail the objectives and results of each well in context. Schofield et al. (2019)

ACCEPTED MANUSCRIPT



summarise the stratigraphy encountered in each well and present a geological interpretation

for each well. They conclude that 5 of the 12 Rockall wells were drilled on invalid tests and

based upon inadequate geological understanding and/or poor seismic data quality. This

analysis increases the ratio of exploration successes versus failures to a respectable 1:6

wells, rather than 1:11, similar to early technical success rates quoted for the Faroe-Shetland

Basin, of 1 well in every 7 wells drilled in the Faroe-Shetland Basin (Loizou, 2003). In this

section, we present a brief overview of the existing wells and consider the factors that

contributed to the negative industry perception of the basin and whether they can be

viewed in a different light. In particular, the four wells that penetrated pre-Cretaceous

strata are discussed in more detail, with an emphasis on possible reasons for the absence of

Jurassic strata.

Basin Opening Well – 163/06-1A joint industry and governmental stratigraphic test

well

Exploration in the UK Rockall was kicked off in 1980 with the 163/06-1A well. This was a

stratigraphic test well, with the objective of testing the stratigraphy of the Rockall basin and

investigating the potential of a set of clinoforms prognosed to be a Mesozoic sedimentary

foreset succession. However, the well encountered significant losses in the volcanic section

and was first sidetracked and eventually abandoned after problematic drilling through almost

a kilometre of basalt. Although the occurrence of lavas had been predicted, the thickness of

the volcanics was grossly underestimated, and the well was abandoned without having

penetrated the base of the volcanics (Morton et al., 1988). With hindsight and knowledge

gleaned from the study of volcanic stratigraphy (e.g. Nelson et al., 2009; Davison et al., 2010;

Ellefsen et al., 2010; Wright et al., 2012; Rawcliffe, 2016), it is considered likely that the

ACCEPTED MANUSCRIPT



original well target was, in fact, a hyaloclastite delta rather than a sedimentary delta

(Schofield et al., 2017a).

Although the 163/06-1A well failed to penetrate stratigraphy deeper than the

volcanics, it is an interesting case study in UKCS collaboration between the public and

private sectors. It was drilled in unlicensed acreage by a consortium of 19 companies and

the then Department of Energy (DOE) (Morton et al., 1988; Hitchen et al., 2013), with the

DOE and national oil and gas companies (British National Oil Corporation, BG

Corporation) splitting 51% of the costs, and 49% apportioned between the remaining

private sector companies. This unusual operational model permitted the (many)

stakeholders to reduce their cost exposure and financial risk, increasing the ratio of value of

information to exploration cost.

Late 1980s to early 1990s: Pre-Rift Play

Following the disappointing 163/06-1A well, there was a hiatus in drilling activity in the UK

Rockall. A second planned stratigraphic test well was never drilled and activity didn‟t

resume until 1988, which marked the start of the first proper phase of exploration in the

basin. This period saw a (relative) burst of activity, with 164/25-1Z drilled in 1988, 164/25-2

in 1990 and 132/15-1 and 154/03-1 in 1991. These wells predominantly targeted the pre-rift

play and it was during this phase of exploration that the idea that there was no widely

distributed source rock in the basin was conceived. The first well to hint at the absence of a

Jurassic source rock, was 164/25-1Z, which was a test of the West Lewis sub-basin. The

well objective was Middle Jurassic deltaic sands, but instead the well found Late Albian

(latest Early Cretaceous) sandstones and mudstones lying unconformably upon Permo-

ACCEPTED MANUSCRIPT



Triassic red beds, with the remainder of the Early Cretaceous and the Jurassic entirely

absent.

Two years later, in 1990, 164/25-2 tested the West Lewis Ridge, prognosed to be a

pre-rift fault block capped by a Devonian-Jurassic sedimentary succession. Unfortunately,

the well found Lewisian Basement directly underlying the Paleocene lavas, with the pre-rift

sedimentary succession being markedly absent. Similarly, 154/03-1, was drilled the following

year, targeting Paleocene deltaic sands with a secondary objective of Jurassic and Early

Cretaceous sands in the pre-rift succession of the West Lewis Ridge. However, the

foresets identified on the seismic as the primary deltaic objective proved to be volcanic in

origin and beneath the thick volcanic succession, a very thin (< 2 m) Cenomanian interval

capped a thick breccia of unknown age which, in turn, rested on Palaeozoic-aged Basement.

Well 132/15-1, also drilled in 1991, targeted a pre-rift fault block on the eastern

margin of the South Rockall Basin, with the expectation that the fault block would be

capped by a Permo-Triassic to Jurassic succession. Once again, the pre-rift sedimentary

succession was found to be missing. Wells 164/25-2 and 154/03-1 were both drilled into

the West Lewis Ridge, a fault-bound, NNE-SSW trending Palaeozoic basement high that

separates the Northeast Rockall Basin from the West Lewis Basin, in order to test the pre-

rift succession. However, whilst the wells tagged crystalline basement without encountering

pre-Cretaceous sediments, the results do not truly test the age of the basin fill, due to their

position on a structural high. Rather, they demonstrate that the West Lewis Ridge was

either emergent and a site of non-deposition or that erosion had stripped the sedimentary

cover by Late Cretaceous times. 164/25-1Z and 132/15-1 require more discussion and are

revisited later in this paper in order to discuss the possible reasons for the absence of the

Jurassic.

ACCEPTED MANUSCRIPT



Late 1990s to early 2000s: 4-way Dip Closure & Forced folds

Following the 154/03-1 well, there was another gap in exploration activity (for 6 years), with

no further drilling in the UK Rockall until 1997, at which time the final phase of UK Rockall

exploration was launched, with drilling operations commencing on licences awarded during

the 17th UK Licensing Round.

The first well to be drilled during this final phase of exploration within the basin was

the 164/07-1 well, which targeted a four-way dip closure in the Northeast Rockall Basin,

interpreted to be a rollover anticline comprising a sequence of alternating sandstones and

shales of Triassic to Cretaceous age. The well was, however, dry and found a thick volcanic

sequence underlain by a heavily intruded and mud-prone Late Cretaceous section. Post-

well analyses concluded that the structure was, in fact, a result of doming due to the

inflation of an underlying laccolith, rather than being linked to extension (Archer et al., 2005;

Schofield et al., 2017a; Schofield et al., 2019), with significant implications for the timing of

structuration.

After the drilling of 164/07-1, the primary focus of exploration in the UK Rockall

shifted to the Paleocene plays, probably stimulated by the release of data from, and

development sanction of, early to mid-1990s Paleocene discoveries in the Faroe-Shetland

Basin, such as Foinaven, Schiehallion and Loyal (Leach et al., 1999; Cooper et al., 1999;

Loizou, 2014; Austin et al., 2014) in addition to the recognition that Paleocene Vaila

formation sandstones of reservoir quality had been penetrated by the 164/25-1Z well.

Unfortunately, only limited success was found during this phase of exploration, with the

discovery of gas by the 154/01-1 Benbecula South well and the only evidence for oil in any

of the UK Rockall wells found in the form of oil shows in the Vaila Formation of 164/28-1A.

ACCEPTED MANUSCRIPT



However, the wells drilled during this phase of exploration firmly established the presence

of thermogenically derived oil and gas and the occurrence of reservoir quality Vaila

Formation sandstones in the NE Rockall Basin with wells 164/27-1 and 154/01-2, drilled in

2002 and 2006, respectively, also encountering the interval.

A common theme with wells drilled during this phase is that they targeted four-way

dip closures. It was shown by Schofield et al. (2017a) that a number of these structures

could be linked to underlying sill complexes and were potentially generated as forced folding

(e.g. Møller Hansen & Cartwright, 2006; Magee et al., 2014) in response to Paleocene

intrusion. This interpretation is significant as it has implications for the relative timing of

petroleum system events. A solid understanding of the relationship between networks of

sills and overlying forced folds may, furthermore, hold the key to understanding why the

154/01-1 Benbecula well found hydrocarbons whilst other forced folds, presumably

generated at a similar time, were found to be dry (Schofield et al., 2017a).

Reassessment of early exploration wells

164/25-1z – Questions raised about Source Rock Presence

In a stratigraphic review of 164/25-1Z, Ebdon & Payne (1990) note that the lack of Jurassic

rocks, in conjunction with the absence of shows in the well raised questions on „both the

presence and effectiveness of a source in the immediate area‟. However, Ebdon and Payne

(1990) concede that the evidence from this well, which is in a marginal location immediately

adjacent to the West Lewis Ridge, cannot preclude the existence of such rocks in the

Rockall basin proper nor even in deeper parts of the West Lewis Basin. In fact, the BGS

borehole 88/01, drilled on the eastern margin of the West Lewis Basin at around the same

time that 164/25-1Z was approaching TD, proved Middle Jurassic mudstones in the basin;

ACCEPTED MANUSCRIPT



whilst in 1990, shallow boreholes 90/02 and 90/05, also in the West Lewis Basin,

respectively found Middle Jurassic sandstones and mudstones, and Ryazanian (earliest

Cretaceous) black shales. Subsequent analyses from these boreholes (Hitchen & Stoker,

1993; Isaksen et al., 2000) have established that black shales from 90/05 were equivalent to

the Kimmeridge Clay Formation (KCF) of the North Sea and that both the Middle Jurassic

mudstones and the KCF-equivalent mudstones had good to excellent potential for

generating oil or transitional gas to oil, with all samples yielding high Total Organic Carbon

and Potential Yield and intermediate to high Hydrogen Index. The results from these

shallow boreholes are in contrast with those from 164/25-1Z. It is possible that either

Jurassic rocks were once present across the West Lewis basin and stripped off by erosion

prior to the Late Albian or that they were never deposited. Certainly, there is evidence in

164/25-1Z for erosion of Late Jurassic and, to a lesser degree, Middle Jurassic rocks based

on reworked fossil assemblages (Ebdon & Payne, 1990, Schofield et al., 2019) and mudstones

(Figure 8). However, the observed reworking occurs within the Late Paleocene Vaila

Formation, and although there is also some reworking within this interval of Albian and Late

Cretaceous fossil assemblages, no reworked Jurassic assemblages are found within the

Cretaceous sequence of 164/25-1Z, suggesting that the Jurassic assemblages are derived

from elsewhere in the basin, whilst in the vicinity of the well, the period is represented by a

hiatus. This evaluation is supported by the interpretation of seismic data carried out as part

of this study (Figure 9a), which suggests that the West Lewis Basin is divided into two

Permo-Triassic half grabens. The more easterly of these saw a subsequent period of rifting

activity, resulting in a syn-rift succession of Middle Jurassic to Earliest Cretaceous, age, as

proven by the 88/01, 90/02 and 90/05 boreholes. In contrast, the western half graben,

encompassing the 164/25-1Z well, was either inactive until at least the Late Albian or

Jurassic sediments were deposited, thickening towards the eastern bounding fault, and

ACCEPTED MANUSCRIPT



subsequently stripped away from the western part of the sub-basin as the fault block tilted

during further extension. This interpretation reconciles the results from both the

exploration well and the shallow boreholes. It should be noted, though, that due to poor

data quality, the interpretation is not of high confidence, as it cannot be readily mapped on

all lines in the basin. Additional confidence is supplied, however, by depth-to-basement

mapping from potential field data (Frogtech Geoscience, 2016), which shows a N-S-striking

basement high coincident with the fault separating the two sub-basins (Figure 9b), suggesting

that the West Lewis Basin is indeed composed of two separate sub-basins.

132/15-1 – Missed crest of pre-rift fault block

The primary objective of well 132/15-1, located in the South Rockall Basin, was to test the

hydrocarbon potential of the pre-rift succession in a large tilted fault block. However, this

well also found Early Cretaceous sediments, predominantly mudstones, directly above

Lewisian basement. This could be construed to corroborate the results of 164/25-1Z and

confirm that the Jurassic was a period of non-deposition in the UK Rockall. However, an

alternative interpretation is provided in Figure 10, which shows an interpreted seismic line

through the well, from the OGA 2015 survey. The tilted fault block penetrated by the well

is clearly evident on the interpretation, and it can be seen that the well penetrated the

bounding fault itself, failing to penetrate what is interpreted as a pre-rift sedimentary

succession capping the fault block by approximately 1.4 km to the SW. The observation in

the post-well reporting that the basement was highly fragmented and fractured (Ebdon et al.,

1992; Schofield et al., 2019) is consistent with our interpretation that this is a fault zone.

Inspection of the pre-drill interpretation based on the seismic data available at the time

(Figure 10c) illustrates clearly that the target was missed because the structural

ACCEPTED MANUSCRIPT



interpretation differed significantly from that presented here (Figure 10b). In the pre-drill

interpretation faults apparently dip to the SE instead of the NW (Figure 10b), and the

surface that we interpret as a major fault (1, Figure 9b) was interpreted in the prognosis as

the top of the pre-rift succession (1, Figure 10c). The nature of the interpreted pre-rift

succession is, of course, unknown, other than that it must pre-date the Early Cretaceous

syn-rift package that was penetrated by the well. Based on borehole and outcrop data from

the Irish Rockall, the Sea of Hebrides and the Isle of Mull (Broadley et al., 2019), it could be

any combination of Carboniferous, Permo-Triassic or Early-Late Jurassic. The absence of

shows in the 132/15-1 well could indicate that the pre-rift succession in the vicinity of the

well does not contain mature source rocks or, alternatively, it may be a symptom of the

mudstone-dominated well stratigraphy, with any generated hydrocarbons bypassing the well

location via more permeable lithologies.

Basin modelling, hydrocarbon generation and occurrences in the UK Rockall

The interpretation of geochemical data from the Faroe-Shetland Basin to the north of the

UK Rockall suggests that the primary source rock within the former is the Upper Jurassic to

Lowermost Cretaceous KCF (Doré et al., 1997b; Gardiner et al., 2019), whilst in the

Hebridean basins immediately to the east, Middle Jurassic source rocks are widely

documented (e.g. Thrasher, 1992; Butterworth et al., 1999). There is evidence to suggest

that similar rocks may occur at depth within the Rockall Basin. This evidence is discussed

below with occurrences shown in Figures 12, 14 and 16-18.

Whilst source rocks may be present, however, there is a great deal of uncertainty regarding

their maturity and, in particular, the timing of maturity relative to trap formation. Vis (2017)

identified a large slick cluster along much of the eastern margin of the Rockall Basin,

ACCEPTED MANUSCRIPT



between the Rosemary Bank and Anton Dohrn igneous centres (Figure 1) and recently

released satellite seepage slick data (OGA CGG UKCS EEZ Seeps, 2019) show a correlation

between slick centre points and faults mapped as part of this study (Figure 11). These data

may be an indication of active oil seepage. Published petroleum system and basin modelling

for the UK Rockall (e.g. Isaksen et al., 2001; Karvelas et al., 2016) suggests that charge from

a buried Jurassic marine source rock is likely to have commenced post-deposition of

Paleocene reservoirs and after trap formation.

However, it is important to note that historically, models of hydrocarbon generation along

the NE Atlantic Margin have been shown to be troublesome and often incorrect. In the

analogous FSB, which possesses a relatively large quantity of subsurface data and direct

information to constrain source rock intervals and sedimentary thicknesses, standard basin

models predict early hydrocarbon generation, occurring during the Late Cretaceous, prior

to reservoir deposition and trap formation in major fields and discoveries in the FSB (e.g.

Lamers & Carmichael, 1999; Iliffe et al., 1999). In this respect, if basin models had been

adhered to in screening the FSB for prospectivity, the giant Paleocene Schiehallion and

Foinaven fields, respectively estimated to hold > 2 (Freeman et al., 2008) and 1.07 (Carruth,

2003) billion barrels in-place, would be predicted to never have existed.

The quandary of timing was resolved initially by invoking the transient trapping of

hydrocarbons in deep Cretaceous reservoirs which were subsequently breached by

hydrofracturing resulting from rapid burial and overpressure development (e.g. Lamers and

Carmichael, 1999; Iliffe et al., 1999) or by Cenozoic compression (e.g. Doré et al., 1997b).

Carr and Scotchman (2003) showed that the onset of hydrocarbon generation could be

delayed by sufficient overpressure development, suggesting that the KCF could still be

mature for oil generation today, despite being buried to a depth of 8 km. More recent

studies have highlighted the thickening effects of Paleocene intrusives on the Cretaceous of

ACCEPTED MANUSCRIPT



the FSB (Schofield et al., 2017b; Mark et al., 2019), a critical period for petroleum system

development due to the deep burial of the Jurassic source rock thought to have occurred at

this time (e.g. Doré et al., 1997b). Mark et al. (2019) estimated that up to 2000 m

cumulative thickness of igneous material may be hosted within the Cretaceous section of

the FSB. Subsequently, Gardiner et al. (2019) showed that within the FSB, the thickening

effects and the long term change of Cretaceous thermal properties resulting from intrusion,

combined with more accurate representation of basement composition, could delay the

onset of petroleum expulsion by up to 40 Myr.

Similar overthickening of the Cretaceous sequence by intrusives is observed in the UK

Rockall. For example, in the 164/07-1 well, approximately 70% of the thickness of the

Cretaceous was actually comprised of igneous material, emplaced during the Paleocene

epoch (Schofield et al., 2019). Conversely, isotopic analyses of the Benbecula (154/01-1) gas

suggest that the gas is comprised of oil-associated gas plus additional gas that may have been

derived by oil-to-gas cracking (Goodwin, 2000; Schofield et al., 2017a), an effect that may

have been a result of heating by igneous intrusion. The implications of this are two-fold.

Firstly, that the accumulation was originally oil and secondly, that cracking may originally

have taken place in situ, or it may support earlier models of transient accumulations, with

both cracking and remigration of hydrocarbons related to later intrusion.

Karvelas et al. (2016) made steps towards incorporating intrusives into their 2D Rockall

basin model, which was based on the OGA 2015 seismic dataset, but they only considered

the local heating effects of intrusions. In order to build upon these results, proper 3D

seismic imaging of the Rockall sill complex and constraints on source rock type and

presence are required.

ACCEPTED MANUSCRIPT



The preceding discussion illustrates how much variability can be produced in a basin model,

based on the input data. We suggest, therefore, that in basins along the Atlantic Margin, like

Rockall, where a dearth of subsurface data exists, great care must be exercised when

making even a 1D basin model, as this could be made to fit virtually any subsurface scenario

of generation, based on current lack of hard data.

Play elements

In this section we consider the elements required for a successful petroleum system by

discussing a series of paleogeography and present-day distribution maps for key intervals in

the UK Rockall. The maps were constructed based on review and analysis of well, borehole

and outcrop data; regional data from the literature; and isochrons derived from the regional

seismic interpretation carried out as part of this study (Broadley et al., 2019). For each

interval we consider the potential for reservoir, seal and source rocks. It should be noted

that the emphasis is on evaluating the potential character of the Mesozoic of the Rockall

Basin, by looking at evidence from the seismic and from regional borehole and outcrop data,

and therefore these intervals are discussed extensively.

The Cenozoic section, which hosts the Benbecula discovery in Paleocene Vaila

Formation reservoir sandstones, is considered to contain important reservoir intervals in

the UK Rockall. However, it is relatively well represented by exploration wells, and a more

extensive discussion of the Paleocene Vaila formation play and the Early Eocene Colsay

Member play is presented by Schofield et al. (2017a). Instead, we present a brief overview,

along with well correlations based on our new biostratigraphic re-evaluation.

ACCEPTED MANUSCRIPT



Permo-Triassic

The Permo-Triassic was a time of widespread continental deposition, interrupted by

episodes of marine ingress, across much of Northwest Europe (Doré, 1992; Coward, 1995;

Benton et al., 2002) (Figure 12). In Britain, alluvial to aeolian deposits of the Early to Middle

Permian gave way to marine evaporites and carbonates in the Late Permian (Benton et al.,

2002). This Late Permian transgression was followed by a return to predominantly fluvial-

alluvial continental conditions during the Early Triassic. The Mid to Late Triassic is an

overall transgressive succession with deposition in low-lying playa and sabkha environments

punctuated by minor marine incursions and eventually conceding to marginal marine

conditions in the Latest Triassic (Rhaetian), a transgression that continued into the Liassic

with the establishment of fully marine conditions (Coward, 1995; Benton et al., 2002).

It is unclear how significant these events were in the UK Rockall, due to limited

dating resulting from poor fossil recovery within the succession and to the limited

stratigraphy sampled by shallow boreholes, which provide much of the evidence for the

distribution of the Permo-Triassic succession on the Hebrides Shelf and the UK Rockall.

However, the periods of relative low-sea level and of transgression are widely represented

in other areas of the UKCS, such as the North Sea, controlling the deposition of the

regionally important reservoir-seal pairs of the Early Permian to Late Permian Rotliegendes

and Zechstein Groups and the Latest Permian to Triassic Sherwood Sandstone and Mercia

Mudstone Groups (e.g. Meadows and Beach, 1993; Benton et al., 2012).

The Rotliegendes-Zechstein reservoir-seal pair is significant in the SNS, in particular,

with over 80% of production to end 2012 coming from this interval (Gray, 2013).

Rotliegendes-equivalent sandstones are proven in the Irish Rockall, with Early Permian

sandstones forming the deepest of the reservoirs in the 12/2-1Z Dooish discovery (Tyrrell

ACCEPTED MANUSCRIPT



et al., 2010). In the discovery well, however, Zechstein-equivalent strata are absent and the

Early Permian reservoir is unconformably overlain by Middle Jurassic reservoir sands.

The Triassic-age Sherwood Sandstone, meanwhile, is a key reservoir interval in

western UK and Irish offshore basins. Equivalent to the Early Papa and Heron Groups

(Figure 6), it forms the principal reservoir interval in sizeable fields, such as the North and

South Morecambe fields (combined GIIP 6.7 TCF) in the East Irish Sea Basin (Meadows and

Beach, 1993; Bastin et al., 2003; Cowan & Boycott-Brown, 2003) and, in Irish waters, the

Corrib field (> 1 TCF gas in place) of the Slyne Basin (Schmid et al., 2004; Corcoran &

Mecklenburgh, 2005; Tyrrell et al., 2010). Strathmore, in the East Solan basin located in the

FSB is an oil discovery in Triassic-aged sandstones equivalent to the Sherwood Sandstone

Group with an estimated 200 MMBBL oil in place (Herries et al., 1999; Morton et al., 2007).

In the Donegal Basin, undifferentiated Permo-Triassic sandstones with log porosities in the

range 10-20% were found to be overlying an interval tentatively dated as Zechstein-

equivalent (Odell & Walker, 1979) and therefore might potentially be assigned to the

Sherwood Sandstone Group equivalent. To the north, the 202/18-1 and 202/19-1 wells in

the Papa basin also proved a Permo-Triassic succession at least 2.9 km in thickness,

consisting of a thick sandstone succession overlying anhydrites that may be equivalent in age

to the Zechstein. Core and log porosities in 202/19-1 are noted to be up to 20%. In the

Sula Sgeir Basin, the 202-12-1 well also found presumed Triassic fluvial sandstones, although

they were found to have little reservoir potential due to diagenesis (Macchi, 1995).

Palaeomagnetic dating of the Stornoway Formation exposed on the Isle of Lewis is

consistent with a Late Permian to Triassic age and exposures are dominated by sandstone

and conglomerates (Storevedt & Steel, 1977; Hitchen et al., 1995). In addition to these

regional data, 164/25-1Z in the West Lewis Basin also penetrated a thick presumed Permo-

Triassic succession, albeit with poor reservoir potential at the well location.

ACCEPTED MANUSCRIPT



The observations listed above support the regional occurrence of Triassic and, to a

lesser degree, Permian sandstone reservoirs to the West of Britain. Whilst there is only a

single deep well penetration proving Permo-Triassic rocks in the UK Rockall (164/25-1Z), it

is considered likely that both Permian and Triassic reservoirs and seals are present at depth

in the Rockall Basin.

Our interpretation of the Permo-Triassic paleogeography of the UK Rockall (Figure

12) portrays the region as one of continental rifting in a series of NE-SW trending basins.

This is consistent with older reconstructions, with evidence from the NW European

continental shelf including the North Sea (e.g. Faerseth, 1996; Tomasso et al., 2008) and

with the evidence for transgressive-regressive cycles within the succession prior to the

development of open marine conditions by the Early Jurassic (Coward, 1995; Benton et al.,

2002).

Evidence of Permo-Triassic rifting can be found in the Sula Sgeir Basin, where the

Permo-Triassic of the 202/12-1 well can be observed to thicken into the fault that bounds

this half graben basin (Figure 13a). Similar into-the-fault thickening of the Permo-Triassic

package found in the 202/19-1 well in the Papa Basin (Figure 13b) and, less convincingly, that

found in the Minch-1 well in the Minches, is also observed on the reprocessed legacy seismic

included as part of the OGA 2016 Rockall data release. The Permo-Triassic succession

penetrated by 164/25-1Z was found to consist of a fining upward sequence with fluvial

deposits giving way to shallow ephemeral lakes, possibly indicative of subsidence associated

with rifting.

Preservation of the Permian and Triassic of the Rockall Basin is difficult to address

because of the poor seismic imaging beneath the volcanics. It remains speculative as to

whether the entire succession is preserved or was, indeed, deposited in any given location.

ACCEPTED MANUSCRIPT



This is borne out by contrasting the stratigraphies of the 12/2-1Z and 12/2-2 wells that both

targeted pre-rift fault blocks in the Irish Rockall; the former encountered only Permian

strata, whilst the latter also found a thin veneer of Late Triassic rocks.

One area where it has been possible to pick out likely areas where the succession is

thin or absent is that just to the east and southeast of the Anton Dohrn Centre (Figure 12),

where the Basement to Base Cretaceous isochron from this study has been interpreted to

represent a series of tilted fault blocks (Broadley et al., 2019). The Pre-Rift succession,

including the Permo-Triassic and Jurassic sequences, is interpreted to be thin or absent in

the footwalls of faults active during Cretaceous extension due to fault cut out and crestal

erosion (Figure 12). It is likely that other areas exist where the succession is absent or

highly thinned due to Cretaceous extension, but that these are not readily identified on

currently available data.

Early Jurassic

The Late Triassic was a period of transgression (Hitchen et al., 1995; Stoker et al., 2017),

leading to open marine conditions that prevailed during the Early Jurassic (Morton, 1989;

Hesselbo et al., 1998; Hesselbo & Coe, 2000). A paleogeography map has not been

constructed for the Early Jurassic of the Rockall Basin, because to date, Early Jurassic strata

have not been penetrated by boreholes to the west of the Hebrides, so no constraining data

exist and the sequence cannot be identified within the Pre-Rift succession on seismic with

any confidence.

The Early Jurassic is well represented by shallow boreholes and exploration wells in

the Minch Basins and the North Lewis Basin as well as in outcrop at Ardnamurchan,

Movern, Skye, Scalpay, Raasay and Mull (e.g. Turner, 1966; Morton, 1989; Hesselbo et al.,

1998). Together, these occurrences prove an overall transgressive Early Jurassic succession

ACCEPTED MANUSCRIPT



of Hettangian to Early Toarcian age in the Inner Hebrides (e.g. Morton, 1989; Hesselbo et

al., 1998), with a maximum proven thickness of at least 1.3 km in the Sea of Hebrides-1 well.

Of particular note is the occurrence of organic rich shales of moderate to excellent

source rock potential within the Toarcian and the Sinemurian-Pliensbachian in boreholes,

exploration wells and outcrop on Skye and in the Minch Basin. Source rock evaluation of

the Toarcian Portree Shale has yielded TOC values in the region of 3-7%, whilst values in

the range 0.5 – 3.1% are noted in the Sinemurian-Pliensbachian Pabay Shale (Abernethy,

1989; Thrasher, 1992; Butterworth et al., 1999; Scotchman, 2001; Schofield et al., 2016;

Broadley et al., 2019).

Based on outcrops on Skye and Raasay, reservoir potential within the Early Jurassic

may lie within the Pliensbachian Scalpa Sandstones and the Hettangian-Sinemurian Upper

Broadford Beds, which consists of sandstones and limestones (Morton, 1989; Hesselbo et

al., 1998; Hesselbo & Coe, 2000). Respectively, these were deposited in a shelf

environment and a nearshore marine setting (Morton, 1989). In the Solan Basin, Early

Jurassic sandstones and shales are found in wells 202/04-1 and 202/03a-3, in which a

Pliensbachian – Hettangian aged sequence of shallow marine origin is proven. It should be

noted, however, that the subsurface equivalents encountered thus far are not encouraging ,

either comprising unfavourable fine-grained facies, e.g. the Scalpa Sandstone in the Sea of

Hebrides-1 and Upperglen-1 wells, or suffering from porosity occlusion due to diagenetic

cements (e.g. the sandstone-dominated Broadford Beds in the Minch-1 well).

On the basis of the above observations, it is considered very likely that an Early

Jurassic sequence is present in the UK Rockall and that it contains source rocks and

reservoirs, although reservoir quality may be a risk.

ACCEPTED MANUSCRIPT



Middle Jurassic

The Middle Jurassic is widely distributed in the Inner Hebrides, both onshore and offshore,

with the outcrops on the east coast of Skye and Raasay forming one of the most complete

Middle Jurassic sections in Great Britain (Barron et al., 2012; Archer et al., 2019) (Figure

14). Middle Jurassic source rocks of Bathonian and Aalenian age cropping out on Skye are

shown to have good to excellent potential for the generation of oil and gas (Thrasher, 1992;

Vincent & Tyson, 1999). Similarly good to excellent source rocks are found in the Aalenian

– Bathonian section of the Upperglen-1 well (Pentex Oil Ltd., 1989; Scotchman, 2001;

Schofield et al., 2016). At both outcrop and in the subsurface, the greatest potential is

found within the Bathonian Cullaidh Shale and the Aalenian Dun Caan Shale.

Critically, the occurrence of Middle Jurassic source rocks is noted in shallow

boreholes to the west of the Hebrides, proving that the Middle Jurassic fairway was not

confined to the Inner Hebrides basins. The 90/02 and 88/01 boreholes, located NNE of

Lewis on the eastern margin of the West Lewis Basin (Figure 14), found Bathonian organic-

rich mudstones, interpreted to have been deposited in a lagoonal to lacustrine environment.

On the basis of these borehole observations and of seismic interpretation carried out as

part of this study, it can be inferred that Middle Jurassic rocks may exist in the Rockall Basin

(Figure 14, Figure 15).

Further north, in the Solan Basin and Rona Ridge area, up to 1.5 km of Middle

Jurassic and older rocks were stripped off by peneplanation during the late Middle to early

Late Jurassic (Booth et al., 1993). Observations from exploration wells constrain the size of

this area on the present day distribution map (Figure 14). Where preserved, however, the

Middle Jurassic sampled in Solan Basin wells is generally dominated by sandstones.

ACCEPTED MANUSCRIPT



Middle Jurassic reservoirs remain unproven west of the Hebrides in UK waters,

more likely as a function of sparse sampling rather than an actual absence of potential

reservoirs. However, the upper reservoir interval in the Dooish discovery in the Irish

Rockall comprises Middle Jurassic fluvial-aeolian to estuarine sandstones (Tyrrell et al.,

2010), thus establishing that there is potential for Middle Jurassic reservoirs in the Rockall

Basin at large. Evidence from outcrop and well data from the Inner Hebrides suggests that

there is reservoir potential at multiple levels through the Middle Jurassic, with a number of

sandstone formations occurring within both the Aalenian to Bajocian Bearreraig Sandstone

Group and the Bathonian Great Estuarine Group (Pentex Oil Ltd., 1989; Hesselbo & Coe,

2000; Schofield et al., 2016). Field observations from the Isle of Skye indicate large lateral

variation in facies and thickness of the Bearreraig Sandstone, whilst the Great Estuarine

Group has greater lateral uniformity of both facies and thickness (Morton, 1989; Archer et

al., 2019). The Bearreraig Sandstone of the Hebrides contains the thickest Jurassic

sandstone succession of any onshore area of the UK and is considered to be an analogue for

the Brent Group of the North Sea (Archer et al., 2019). If present in the UK Rockall, it may

be an important reservoir interval.

In conclusion, it is likely that any Middle Jurassic section preserved within the Pre-

Rift of the Rockall Basin has good potential for both reservoirs and source rocks.

Late Jurassic

In this study, the Late Jurassic is taken to span the interval from the Oxfordian to the

Earliest Berriasian. This period is largely equivalent to the deposition of the Kimmeridge

Clay Formation (KCF) of the North Sea, where deposition extends into the Earliest

Berriasian (Fraser et al., 2003).

ACCEPTED MANUSCRIPT



Marine conditions are thought to have resumed to the West of Britain with the

advent of the Late Jurassic epoch. This is supported by offshore wells and boreholes and

field studies in the Hebrides (Morton, 1989). The paleogeography map for this interval

depicts the UK Rockall as a series of open marine basins with the paleo Outer Hebrides

landmass separating an Inner Hebridean seaway from an outer Rockall-Faroe-Shetland trend

(Figure 16).

Deposition in a restricted marine environment is documented in the West Lewis

(WLB) and West Flannan Basins (WFB) by BGS boreholes 90/05 and 90/09, respectively,

which found organic-rich shales of Earliest Berriasian-age, considered equivalent to the KCF

and with excellent potential for hydrocarbon generation (Hitchen & Stoker, 1993; Isaksen,

2000). These rocks are immature at the borehole locations, but their presence west of the

Hebrides has significance for the UK Rockall where, if present, it is likely that the rocks have

been buried to sufficient depth to reach maturity.

Similar organic-rich, oil prone shales of Earliest Berriasian age are found in a large

number of wells in the West of Shetlands, including 202/08-1, 202/02-1 and the Lancaster

discovery well, 205/21-1A. The Lancaster Field, with an estimated 1 billion barrels oil in

place (Belaidi et al., 2018), is located on the Rona Ridge and is considered to have a KCF

source, although it is most likely sourced by mature equivalents of the Earliest Berriasian

buried to sufficient depth in the basins either side of the ridge (Trice, 2014) than by the

source rocks sampled by the well, which are very thin and likely immature by virtue of their

location on the Rona Ridge.

In the Solan Basin and on the Rona Ridge, reservoir potential is found in the shallow

marine Rona Sandstone member or the deeper water turbiditic Solan Sandstone, which

forms the main reservoir interval in the Solan Field (78 MMBBL oil in place), charged by oils

ACCEPTED MANUSCRIPT



from the KCF and with average porosity ≤27.5% and permeability ≤185 mD (Herries et al.,

1999). The Late Jurassic geology of the Solan Basin is echoed in the WFB, where BGS

borehole 90/08 found shallow marine sandstones assigned a Kimmeridgian to Oxfordian age

(Hitchen & Stoker, 1993; Isaksen, 2000). The sandstones are roughly age-equivalent to the

Solan Sandstone and may represent a shallow marine lateral equivalent. Their occurrence in

the WFB is a very important data point, as it represents the most westerly occurrence of

Jurassic rocks to the west of Scotland. Younger sandstones are also proven in the WFB by

the 90/09 borehole, in which interbedded Earliest Berriasian sandstones and mudstones

were found to overlie the aforementioned organic-rich shales.

The two control points in the WFB suggest at least two episodes of coarse clastic

input to basins west of the Hebrides during the Late Jurassic and Earliest Cretaceous. It is

envisaged that during this period, a sand-dominated marine shelf may have existed along

much of the eastern margin of the Rockall Basin, fed by detritus shed from the Hebridean

and Scottish landmasses (Figure 16). Preservation of the Late Jurassic sequence in the UK

Rockall, similar to that of the Permo-Triassic and Middle Jurassic, is uncertain and we

propose a similar distribution to that of the Permo-Triassic, as the Late Jurassic largely post-

dated the peneplanation event in the Solan Basin-Rona Ridge area (Booth et al., 1993).

Although Late Jurassic rocks remain unproven in the Rockall Basin, the borehole

results can be correlated into the basin from the WLB and the WFB on seismic, albeit with

varying degrees of confidence. If present, it is highly likely that the sequence contains both

rich source rocks and potential reservoir sands.

Early Cretaceous

The Early Cretaceous paleogeography map (Figure 17) represents the main phase of rifting

in the Rockall Basin and its development into a largely deepwater marine basin. The

ACCEPTED MANUSCRIPT



interpretation that the western half of the basin is shallower is based on the Cretaceous

isochron, as are the outlines of the deep water clastics and shelf sands facies bands.

Early Cretaceous sediments are penetrated by just 2 of the 12 UK Rockall wells. In

the 164/25-1Z well, the top ~177 m of the interval originally assigned entirely to the Permo-

Triassic was subsequently found to be Late Albian in age (Plumb et al., 1989; Ebdon & Payne,

1990). Ebdon & Payne (1990) interpret a mid-shelf depositional setting for this section,

which comprises red-brown calcareous mudstones, siltstones and sandstones that resemble

Albian sediments described from exploration wells in the West of Shetlands area, such as

202/12-1, 204/30-1 and 205/26-1.

Well 132/15-1, in the South Rockall Basin, found a thick Albian - Barremian

mudstone-dominated sequence, with fossil assemblages indicating a deep water marine

setting (Ebdon et al, 1992). The mudstone-dominated facies penetrated to date in the Early

Cretaceous of the UK Rockall suggest that there is good sealing potential within this

interval.

There is regional evidence for sandstones and limestones within the Early

Cretaceous, indicating that there may be some reservoir potential in this interval. The

88/01 borehole in the West Lewis Basin found Barremian shallow marine sandstones

(Hitchen & Stoker, 1993). The presence of both marine and terrestrial palynomorphs and of

primary carbonate and glauconite mud matrix with the sandstones (Hitchen & Stoker, 1993)

is indicative of a clastic-dominated continental shelf depositional environment in the West

Lewis Basin at this time. Conversely, wells in the Solan and Sula Sgeir Basins and in the

Rona Ridge area provide evidence of a mid-to-outer carbonate shelf setting in the Late

Berriasian and Barremian, becoming clastic-dominated by the Aptian and Albian. The Albian

section in 204/30-1 contained an 18 m thick sandstone unit. In 202/12-1, bioclastic

ACCEPTED MANUSCRIPT



limestones of Late Berriasian to Earliest Barremian age are found, although they are noted

to lack visible porosity. IRL 12/13-1A in the Erris Basin found a predominantly clastic Lower

Cretaceous sequence containing numerous thin sandstones. Based on the evidence

presented, it is considered likely that there is some reservoir potential in Early Cretaceous

of the UK Rockall, particularly in the NE Rockall Basin, where marine shelf conditions may

have prevailed.

It is anticipated that the Early Cretaceous interval is generally preserved throughout

the Rockall Basin, although it may be condensed over the crests of fault blocks. The

succession is considered to be absent in the Inner Hebrides, where the Early Cretaceous

was a time of non-deposition or erosion due to basin inversion (Morton, 1987); this is

evident by the lack of Early Cretaceous sediments in boreholes and in outcrop in this area.

Late Cretaceous

The Late Cretaceous sequence is equivalent to the Shetland Group and is taken to extend

from the Cenomanian to the Danian. In the FSB, the Late Cretaceous is interpreted to

represent a continuation and intensification of Early Cretaceous rifting followed by an initial

phase of post-rift subsidence by the latest Cretaceous (Dean et al., 1999; Doré et al., 1999;

Larsen et al, 2011). Evidence from the Irish Porcupine Basin suggests post-rift subsidence

from the Cenomanian onwards (e.g. Shannon et al., 1993; Johnston et al., 2001). In the UK

Rockall, however, the timing of Cretaceous rifting and subsidence remains uncertain due to

the poor seismic imaging and lack of well control (Doré et al., 1999; Hitchen et al., 2013),

although Musgrove and Mitchener (1996) propose that post-rift subsidence occurred from

the Cenomanian onwards.

In the NE Rockall and South Rockall Basins, the interval is interpreted to represent a

bathyal setting (Figure 18), with no shelfal area proven adjacent to the West Lewis Ridge,

ACCEPTED MANUSCRIPT



nor the eastern margin of the South Rockall Basin (132/15-1). However, the NE Rockall

Basin wells that sample the Shetland Group (164/07-1, 164/27-1, 164/28-1A, 154/01-1 and

154/01-2) generally TD within the Maastrichtian or Late Campanian, so it is possible that a

shelf environment may have prevailed on the margins of the basin during the early stages of

post-rift subsidence but is not penetrated by the existing well stock. Further to the

northwest, wells from the West Lewis Basin through to the Sula Sgeir and Solan Basins

consistently confirm a mid- to outer shelf setting.

Relatively speaking, the Late Cretaceous succession is well represented in UK

Rockall wells, and is commonly entirely composed of mudstone with subordinate limestones

and is commonly heavily intruded by igneous sills. These observations are consistent with

observations from the West of Shetlands, where the Late Cretaceous is also well

represented in the subsurface record. The abundant mudstones represent good sealing

potential within the succession, but are thought to have little generative potential as source

rocks, based on the overall low TOC of Cretaceous rocks penetrated thus far in the

Atlantic margin basins of the UK, Irish and Faroese continental shelves (Scotchman et al.,

2018).

164/25-1Z and 132/15-1 are the only UK Rockall wells to have penetrated the base

of the Late Cretaceous. In both wells, a relatively complete Cenomanian to Danian section

is present (Ebdon & Payne, 1990; Ebdon et al., 1992) and, though mudstone-dominated, both

show evidence of either coarse clastic input or carbonate deposition that could signal the

presence of potential reservoirs within this succession. 164/25-1Z, found ~62 m of

interbedded Danian sandstone and shale, indicating coarse clastic input at least into the

West Lewis Basin at this time. Meanwhile, the Late Cretaceous mudstones of 132/15-1 are

interspersed with occasional thin sandstone and limestone interbeds that are generally of

insignificant thickness. However, at the base of the section, Late Cenomanian limestones

ACCEPTED MANUSCRIPT



reach a thickness of ~38 m, and whilst they are noted to have no visible porosity in cuttings,

similar limestones within the Cenomanian may present viable reservoirs if fractured or if

secondary porosity has been developed. It is possible that sandstones and limestones may

be present in marginal areas of the UK Rockall, representing low stands within the overall

pattern of Late Cretaceous transgression. Outcrop exposures of Cenomanian to

Campanian shallow marine sandstones and limestones on the Isles of Skye, Raasay, Eigg and

Mull and on the Movern Peninsula of mainland Scotland (Braley, 1990; Broadley et al., 2019),

may be analogous to sandstone and limestone members that may be developed within the

Late Cretaceous succession on the margins of at least the South Rockall Basin and may shed

light on their reservoir potential.

Paleocene to Early Eocene

This is, arguably, with present knowledge the most important period for reservoir

development in the Faroe-Shetland Basin. Numerous sizeable oil and gas discoveries are

hosted in sandstones of the Selandian (T22-T35) Vaila Formation, the Ypresian (T40-T45)

Flett Formation and, to a lesser degree, the Thanetian (T36-T38) Lamba Formation (e.g.

Loizou et al., 2014). The UK Rockall wells, with the exception of 153/05-1 and 163/06-1A,

all penetrate the base of the succession and prove the presence of rocks ranging in age from

T32-T50 (Figure 4). The presence of reservoir rocks within this interval is therefore less

speculative than that at earlier stratigraphic levels. The emphasis, therefore, of the work

done on the Paleocene to Early Eocene was to improve upon the understanding of the

interval by placing the wells within the T-sequence framework (Ebdon et al., 1995)

employed in the Faroe-Shetland basin in order to better define which intervals had been

penetrated and proven by each well. For full details of the biostratigraphic analyses

conducted and the resulting well correlations the reader is referred to Broadley et al.

(2019).

ACCEPTED MANUSCRIPT



The Vaila Formation is the only proven hydrocarbon play in the UK Rockall to date.

It forms the reservoir interval in the 154/01-1 Benbecula gas discovery in the Northeast

Rockall Basin, in which gas, that may have originated from thermally altered oil (Goodwin,

2000; Schofield et al., 2017a), is trapped in a four-way closure most likely formed by the

jacking up of strata above an igneous intrusion. However, although 154/01-1, 164/28-1A,

164/27-1 and 154/01-2 all targeted and found Vaila formation sandstones of reservoir

quality, belonging to sequences T34 and T35 and in similar four-way closures, 154/01-1

remains the only success case. It has been argued that sills underlying forced folds may act

as baffles or barriers to hydrocarbon migration, diverting hydrocarbons away from some

structures, whilst channelling them into others (Rateau et al., 2013; Schofield et al., 2016;

Schofield et al., 2017). In order to build upon previous basin modelling work for the UK

Rockall (Isaksen et al., 2001; Karvelas et al., 2016) and begin to predict migration patterns,

however, 3D seismic data is required, with acquisition parameters and processing designed

to improve sub-basalt imaging. This approach has been successful in the Faroe-Shetland

Basin, where Mark et al., (2018) were able to carry out detailed mapping of intrusions of the

Faroe Shetland Sill Complex on reprocessed 3D data optimised for sub-basalt imaging

(Schofield et al., 2017b).

Based on the new biostratigraphic analyses carried out as part of this study, the T36-

T38 Lamba Formation is recognised in wells 132/06-1, 132/15-1, 153/05-1, 154/01-1,

154/01-2, 164/27-1 and 164/28-1A (Broadley et al., 2019). In the Faroe Shetland Basin, the

Lamba Formation forms the reservoir interval in a number of discoveries (Louizou, 2014).

However, in the UK Rockall wells, the formation documents a period of sequence T36 rift

flank volcanism, with a series of hyaloclastite deltas prograding westward from the eastern

margin of both the NE and South Rockall Basins. As a consequence, the sequence

comprises volcanic and shale-dominated lithologies, and reservoir potential within the

ACCEPTED MANUSCRIPT



Lamba Formation remains unproven. It does, however, provide an important intra-

Paleocene seal, as demonstrated by the 154/01-1 Benbecula discovery, in which the

hyaloclastites and shales that provide the seal are shown to be of Lamba Formation age

(Broadley et al., 2019).

The 164/25-2 well penetrated a 150 m thick sandstone package, sandwiched

between two lava units and assigned to the Lamba Formation at the time of dril ling.

However, Schofield et al. (2017a) present evidence that the interval should be assigned to

the Colsay Member of the T40 Flett Formation, which forms the reservoir interval in the

Rosebank discovery in the Faroe-Shetland Basin. Whilst the Colsay sands in 164/25-2 were

water-bearing, they were found to possess excellent reservoir properties, with porosities of

up to 34% and multi-Darcy permeabilities. Based on seismic data, Schofield et al. (2017a)

correlated the interval with a volcaniclastic package immediately overlying the Cretaceous

package in 164/07-1. This interpretation is substantiated by re-interpretation of the 164/07-

1 biostratigraphy conducted as part of this study, which indicates that a thin clastic interval

between the volcanics and the Late Cretaceous Shetland Group can be assigned to the T40

Colsay Member (Broadley et al., 2019). Whilst the lithology encountered in 164/07-1 is

non-reservoir, this result is important because it confirms that the top of the Intra-Volcanic

package can be recognised on the seismic data.

Discussion

UK Rockall Potential Play Elements: a summary

Based on the evidence presented thus far, it is clear that the source, reservoir and seal

elements required for a successful petroleum system are likely to exist at multiple levels in

the UK Rockall (Figure 19) and the challenge remains to detect where they occur,

ACCEPTED MANUSCRIPT



particularly where source rocks are mature and how Paleogene sills may have affected

hydrocarbon generation and migration. Reservoirs and seals are proven within the

Paleocene to Early Eocene megasequence, and are shown to be equivalent to those found in

a number of fields in the Faroe-Shetland Basin. Sandstone reservoirs are also likely to exist

within the Permo-Triassic, Early, Middle and Late Jurassic, Barremian, Albian and Danian,

whilst carbonate reservoirs may be present within the Cenomanian of the South Rockall

Basin. Sealing potential is not seen as a risk, as all proven successions contain intra-

formational shales of significant thickness, with hyaloclastites and lavas adding to the sealing

potential within the Paleocene to Early Eocene megasequence. Potential source rocks are

most likely to be of Early, Middle and Late Jurassic age. These elements are summarised in

Figures 20 and 21. A variety of trapping geometries may be present, for example, in the

form of tilted fault blocks, stratigraphic pinchouts, fault juxtapositions, truncation, forced

folding and Cenozoic compressional folding.

However, what is equally clear is that much of the evidence for play elements comes

from regional observations and inferences. In fact, the only proven elements are reservoirs

and seals within the Paleocene and Eocene. Even the Late Cretaceous is only penetrated in

its entirety by two wells, from which we cannot extrapolate to the entire basin. Older

elements, particularly the pre-rift succession (Permo-Triassic to Late Jurassic) remain highly

speculative in the absence of additional well data and an accurate evaluation of source rock

maturity is, we would argue, presently near impossible to conduct successfully.

It is important to note, that although this study has focussed on the potential

occurrence of Permian and younger rocks, regional evidence from the Irish Rockall (IRL

12/02-1Z and 12/02-2) and from the Faroe-Shetland Basin (e.g. Clair Field) shows that

Carboniferous rocks also occur to the West of Britain and Ireland and there is potential for

Carboniferous source rocks and reservoirs to be present in the UK Rockall.

ACCEPTED MANUSCRIPT



Comparison with the UK North Sea Exploration – How many wells are needed to know if a basin is

prospective?

The question of whether 12 exploration wells within the UK Rockall is enough to establish if

the basin is prospective or non-prospective is open to debate. However, a comparison that

can be drawn when evaluating the prospectivity of any part of the UKCS is to the early

history and exploration narrative which underlies the world class North Sea basins, which

took considerable investment and drilling before the first commercial oil discovery was

made.

The notion that the North Sea might host hydrocarbon resources was born after the

discovery of the giant Groningen gas field in the South Permian Basin, onshore Netherlands

in 1959 (Stäuble and Milius, 1970; Kemp, 2012). Groningen comprises a Lower Permian

Rotliegendes reservoir, sealed by Zechstein evaporates and sourced by Pennsylvanian

(Carboniferous) coals (Cook, 1964; Stäuble and Milius, 1970). By the time of its discovery,

onshore evidence from the UK and continental Europe (Kent, 1949; Visser and Sung, 1958;

Bentz, 1958) already indicated that the Permian geology of these regions was similar, and

there would have been reasonable confidence that this geology was replicated in the

Southern North Sea. This was confirmed by early gravity and seismic refraction studies in

the North Sea and, despite the shallow water and the presence of salt, it was possible to

map the top and base Permian on early seismic reflection data (Cook, 1964), so already,

arguably prior to drilling, North Sea explorers were able to identify key play elements and

understand some of the broad basin stratigraphy far more effectively than was possible in

the UK Rockall prior to the first wells drilled in the basin.

ACCEPTED MANUSCRIPT



In the first year of North Sea exploration, 1965, 10 wells were spudded, in water

depths averaging ~33 m, all within the Southern North Sea. These initial wells resulted in

four gas discoveries, three of which went on to become producing gas fields, opening up the

Southern North Sea Rotliegendes play (National Data Repository, 2019; Gage, 1980; Hillier

& Williams, 1991; Winter & King, 1991). The drilling rate accelerated over the next few

years with continuing high success rates and exploration was expanded northwards into the

Central North Sea and the Moray Firth, but it wasn‟t until 1969, when the 158th well to be

spudded across the SNS and CNS found the first commercial oil accumulation, now known

as the Arbroath Field, in CNS Block 22/18 (Kemp, 2012). A huge number of wells were

drilled before the first commercial oil discovery was made, despite the better quality seismic

data and the ever-increasing well data sets. Whilst it could reasonably be assumed that

seismic data quality today is of far better quality than that on which the early North Sea

wells were drilled, we argue that much of current data quality in the Rockall Basin is

comparable to this early North Sea data due to the pervasive volcanics present within the

subsurface.

In contrast, when the number of UK North Sea wells drilled to find commercial oil is

compared with the exploration history of the UK Rockall (which is the same size as the

SNS, CNS, and NNS combined) it can be said with some degree of certainty that the latter

has not been adequately tested by the 12 wells drilled to date, particularly as only 7 of these

are considered to be valid tests of the basin (Schofield et al., 2019), and of these, one well

reached TD in the late Paleocene, and only one well, 164/25-1Z, penetrated below the late

Campanian.

ACCEPTED MANUSCRIPT



Where next for the UK Rockall?

In conclusion, we have argued that a decision cannot yet be made regarding the

prospectivity, either positive or negative, of the UK Rockall, as the wells drilled thus far have

failed to establish the basin stratigraphy and sub-basalt seismic imaging is generally too poor

to constrain the deep structure of the basin. For these reasons, the basin‟s rifting history

remains unknown, with rifting episodes prior to the Early Cretaceous remaining

unconstrained by well control, despite regional evidence that suggests that Permo-Triassic

and Jurassic rifting is likely to have occurred. Furthermore, the fact remains that the UK

Rockall remains a high risk, but potentially high reward, basin to enter – in addition to the

geological risk: operating conditions are harsh; water depths and distance to shore can be

excessive; and there is no existing infrastructure. All of these factors combine to dictate

that any discovery must be of a substantial size, and likely be oil, in order to be deemed

commercial. For seismic acquisition contractors, the impetus to conduct a speculative 3D

survey is driven by the perceived appetite (or lack thereof) for exploration, and thus a

vicious circle of lack of exploration confidence due to a lack of data continues. So how to

break the deadlock?

In order to increase industry confidence, it is imperative to properly constrain the

full basin stratigraphy and, by extension, it‟s structural evolution. This can only be achieved

by exploratory drilling. Although seismic data quality in the UK Rockall is still poor, we

suggest that it is now of sufficient quality and coverage to identify a number of suitable well

locations to test the basin‟s full stratigraphy. Moreover, we propose that the time is now

ripe for a government-industry collaboration in order to drill one or more stratigraphic test

wells, similar to the model employed in drilling the 163/06-1A well in 1980 in the North

Rockall. This modus operandi permitted the individual stakeholders to spread the financial

risk of the well, so that it was possible, in theory, to conduct an extensive data acquisition

ACCEPTED MANUSCRIPT



programme at very low individual stakeholder cost, whilst retaining full access to all the

valuable subsurface data.

Three locations (Figures 20 and 21) have been identified as potential sites for a

stratigraphic test well, each with their advantages and disadvantages. Site 1 (Figure 20) is

located in the northernmost part of the NE Rockall Basin, close to the Wyville-Thomson

Ridge. Our interpretation below the top Cretaceous in this area delineates a number of

pre-rift fault blocks capped by a pre-rift succession and overlain by wedge-shaped syn-rift

packages. The eastern end of the seismic line shows the pre-rift succession of the West

Lewis Basin, where rich source rocks of Middle and Upper Jurassic age are proven, but

immature. Seismic data quality is reasonable in this area and there is a good chance that the

pre-rift geology observed in the West Lewis Basin is repeated at depth within the pre-rift

fault blocks. The pre-rift is also at sufficient depth to increase the likelihood of any source

rocks being mature and not post-mature. The top of fault block A can be mapped as a

structural closure, even on the widely spaced 2D data currently available. However, in our

view, targeting Fault Block A in a down dip position, rather than testing the closure, in the

first instance, would have the greatest benefit from the perspective of constraining the pre-

Late Cretaceous stratigraphy, as this is where the pre- and syn-rift successions are observed

to be thickest, where there is a lower chance of erosion of the pre-rift succession and

therefore greater chance of preservation of the complete succession. Another advantage of

this site is that it gives the opportunity to evaluate the T40 (Late Paleocene – Early Eocene)

Colsay Member, which is correlated on the seismic from 164/25-2 and is a fairly clear pick

(Figure 15). Penetrating this unit to determine the nature of the Colsay Member and

whether it is clastic or volcanic would aid in future delineation of the Colsay play fairway.

The disadvantages of this site are that the Late Paleocene succession is most likely thin or

absent, based on the seismic data, and the results of 164/07-1 and 163/06-1A. The Late

ACCEPTED MANUSCRIPT



Cretaceous, based on our interpretation, is also likely to be incomplete. However, wells

further to the south have already proven the Late Paleocene succession and more limited

information on the Late Cretaceous is also available from the 164/25-1Z and 132/15-1 wells.

Site 2 (Figure 20) is located close to the Benbecula discovery well. This is a low risk

option, as a working petroleum has already been proven here and operational learnings

from the 154/01-1 well could be applied to streamline the planning process. The purpose of

the well would be to establish the Pre-Paleocene stratigraphy and identify the source rock.

Issues with this well location, however, are that it is in currently licensed acreage and that it

has less potential to yield new information, as it is already known that Benbecula

hydrocarbons most likely originated from a marine source rock (Kleingeld, 2005). A

cheaper alternative might be to re-drill the Benbecula well itself, if this could be done safely

and without compromising the objective of testing the deep, pre-rift stratigraphy.

The final location, Site 3 (Figure 21), is in the South Rockall Basin, where a complex

series of pre-rift fault blocks, similar in form to the Irish Dooish discovery (well IRL 12/02-

1Z), can be mapped on the seismic. Targeting one of these structures could establish the

extension of the proven geology of the Irish Rockall into UK waters. Water depths here

are substantial, however 132/06-1, close to the proposed drilling location, was drilled to a

TD of 4,370 m in water depths close to 1,900 m, with drilling activities taking 27 days and

completion a further 10 days. The top of the pre-rift at the proposed location is slightly

shallower than TD of 132/06-1 and at a similar depth in TWT to the Dooish discovery. As

with Site 1, a slightly down-dip position might ensure penetration of a more complete pre-

rift succession. A risk at this location is that it can be argued that any well drilled here may

only constrain the pre-rift geology very locally. Comparison of the results from IRL 12/02-

1Z (Dooish) and IRL 12/02-2 (Dooish West), located just 7 km apart, demonstrates that the

pre-rift stratigraphy of these wells varies significantly, most likely due to complex and highly

ACCEPTED MANUSCRIPT



localised pre-Cretaceous rifting, with the Permian succession penetrated in 12/02-1Z absent

in 12/02-2, whilst the Upper Jurassic found in 12/02-2 is not represented in 12/02-1Z.

Our preferred location is Site 1, as we believe that this location offers the highest

value of information and could open up an area that has greater potential for commercial

success by virtue of its proximity to shore. However, all three sites offer potential to

improve our understanding of the pre-Cretaceous evolution of the UK Rockall. It should

be noted that a limited amount of speculative 3D seismic data is available in the basin and

that the three suggested stratigraphic test well locations each fall within the area of one of

these surveys. Whilst the data vintage is around 20-25 years old, a logical first step towards

any well planning might be to license and apply modern reprocessing techniques to one or

more of these surveys, similar to those applied by PGS on the Faroe-Shetland Basin

MegaSurvey Plus. This delivered a substantial improvement in the imaging of the Base

Cretaceous unconformity and of the Paleogene sills (Schofield et al., 2017b; Mark et al.,

2018), both of which are important uncertainties that need to be captured for both well

planning and basin modelling purposes. The interpretation of these data is seen as a step in

the decision process on any proposed well location.

It is recognised by the authors that drilling a new stratigraphic test well in deep

water and in the challenging conditions of the NE Atlantic Margin will always be a costly and

bold endeavour and that full technical and commercial justification would be required for

any such enterprise. In reality, it is likely that at least part of this justification must come

from work done in the more mature, analogous FSB, where 3D seismic and good well

coverage (spatially and stratigraphically) are available. Ongoing research in the FSB is

underway to model the impact, or lack thereof, of igneous intrusions upon hydrocarbon

migration. If this work can be used to reproduce known accumulations within the basin

model, and then be carried forward to successful exploration, this would provide a strong

ACCEPTED MANUSCRIPT



justification for the acquisition of additional data in the UK Rockall, whether this be in the

form of 3D seismic or a stratigraphic test well, in order to identify and de-risk prospects

with a strong likelihood of hydrocarbon charge.

As the UK transitions to a green future, hydrocarbons, in particular gas, will still

need to be an essential part of the energy mix, in particular for electricity generation to

enable fuelling of transition to electrified infrastructure. Relying on gas imports, degrade the

UK‟s ability to enable its energy security. In relation to Rockall, a clear forward strategy

needs to be developed by government and industry to decide if the area will (or will) not

play a part in the future of the UK‟s energy and hydrocarbon demands.

Conclusions

On the basis of the work presented in this paper, we conclude that:

1. The information available from the existing database of 12 wells in the UK Rockall is

insufficient to determine whether or not the basin is prospective.

2. Reservoirs are proven within the Paleocene to Early Eocene of the UK Rockall.

Whilst the Early and Late Cretaceous successions have been penetrated by UK

Rockall wells, their characteristics are not well constrained, as the well evidence is

patchy and incomplete. The presence of any rocks older than the Early Cretaceous

remains speculative, however we draw on regional evidence and our new seismic

interpretation to demonstrate that there is a strong likelihood that Permo-Triassic

and Jurassic rifting occurred in the UK Rockall. It is probable that the associated

rock record contains both reservoir, seal and, in the case of the Jurassic, restricted

marine source rocks, as evidenced by neighbouring basins, such as the Faroe-

Shetland and the Inner Hebrides Basins.

ACCEPTED MANUSCRIPT



3. We propose that in order to reboot exploration in the UK Rockall, a bold and

different approach must be applied in order to establish the basin‟s

tectonostratigraphic evolution. We suggest that a government-industry

collaboration to drill one or more stratigraphic test wells might succeed in obtaining

the requisite information whilst reducing overall risk for the individual stakeholders.

Acknowledgments

We acknowledge a governmental research grant from the Oil and Gas Authority (OGA)

Frontier Basins Research Program and British Government for funding a 2-year independent

academic research project “Evaluating the Prospectivity of the Rockall Trough – Towards a

Complete View of the Petroleum System West of Britain”. We would like to acknowledge

PGS and GeoPartners for the donation of seismic datasets in Rockall, and for continued

support of research. Spectrum Geophysical are also thanked for donation of data for

research in the Rockall. Well data used in the paper has been accessed through the OGA‟s

National Data Repository (NDR). Frogtech Geoscience are thanked for access to Rockall

SEEBASE products. Interpretation was carried out using IHS Kingdom software and

Schlumberger Petrel software. WGM 2012 gravity data courtesy of International

Gravimetric Bureau. This manuscript contains information provided by the Oil and Gas

Authority and/or other third parties. Michael Lentini and an anonymous reviewer are

thanked for their detailed and constructive reviews.

ACCEPTED MANUSCRIPT



References

1. Abernethy, I., 1989, Well Upperglen-1 petroleum geochemical database for the interval

5180 ft – 8510 ft, Paleo Services Project no. G2210.

2. Archer, S., Bergman, S., Illiffe, J., Murphy, C. & Thornton, M., 2005, Palaeogene igneous

rocks reveal new insights into the geodynamic evolution and petroleum potential of the

Rockall Trough, NE Atlantic Margin, Basin Research, 17, pp. 171-201.

3. Archer, S., Steel, R., Mellere, D., Blackwood, S. & Cullen, B., 2019, Response of the

Middle Jurassic shallow-marine environments to syn-depositional block tilting: Isles of

Skye and Raasay, NW Scotland, Scottish Journal of Geology, 55, pp. 35-68.

4. Austin, J., Cannon, S. & Ellis, D., 2014, Hydrocarbon exploration and exploitation West

of Shetlands. In: Cannon, S. & Ellis, D. (eds), Hydrocarbon Exploration to Exploitation

West of Shetlands, Geological Society, London, Special Publications, 397, pp. 1-10.

5. Barron, A., Lott, G., Riding, J., 2012, Stratigraphical framework for the Middle Jurassic

strata of great Britain and the adjoining continental shelf, BGS Research Report,

RR/11/06, British Geological Survey, Keyworth, Nottingham.

6. Bastin, J., Boycott-Brown, T., Sims, A. & Woodhouse, R., 2003, The South Morecambe

gas field, Blocks 110/2a, 110/3a, 110/7a and 110/8a, East Irish Sea. In Gluyas, J. &

Hichens, H. (eds), United Kingdom Oil and Gas Fields, Commemorative Millennium

Volume, Geological Society, London, Memoir, 20, pp. 107-118.

7. Benton, M., Cook, E. & Turner, P., 2002, Permian and Triassic Red Beds and the Penarth

Group of Great Britain, Geological Conservation Review Series, 24, Joint Nature

Conservation Committee, Peterborough.

8. Booth, J., Swiecicki, T. & Wilcockson, P., 1993, The tectono-stratigraphy of the Solan

Basin, west of Shetland. In: Parker, J. (ed.), Petroleum Geology of Northwest Europe:

Proceedings of the 4th Conference, Geological Society, London, 987-998.

9. BP Plc, 2019a, Insights from the Evolving transition scenario- Global. In: BP Energy

Outlook 2019.

10. BP Plc, 2019b, Insights from the Rapid transition scenario- Global. In: BP Energy

Outlook 2019.

11. Braley, S., 1990, The Sedimentology, Palaeoecology and Stratigraphy of Cretaceous

Rocks in NW Scotland, PhD Thesis, Polytechnic Southwest (University of Plymouth),

Plymouth.

12. Broadley, L., Schofield, N., Jolley, D. & Howell, J., 2019, Rockall Regional Study – Final

Report, University of Aberdeen. https://data-

ogauthority.opendata.arcgis.com/datasets/university-of-aberdeen-–-oga-frontier-basins-

rockall-project-a-new-view-of-the-uk-rockall-prospectivity-1

13. Butterworth, P., Holba, A., Hertig, S., Hughes, W. & Atkinson, C., 1999, Jurassic non-

marine source rocks and oils of the Porcupine Basin and other North Atlantic margin

basins. In: Fleet, A. & Boldy, S. (eds), Petroleum Geology of Northwest Europe:

Proceedings of the 5th Conference, pp. 471-486.

14. Carr, A. & Scotchman, I., 2003, Thermal history modelling in the southern Faroe-

Shetland Basin, Petroleum Geoscience, 9, pp. 333 – 345.

ACCEPTED MANUSCRIPT


https://data-ogauthority.opendata.arcgis.com/datasets/university-of-aberdeen-–-oga-frontier-basins-rockall-project-a-new-view-of-the-uk-rockall-prospectivity-1




15. Carruth, A., 2003, The Foinaven Field, Blocks 204/19 and 204/24a, UK North Sea. In

Gluyas, J. & Hichens, H. (eds), United Kingdom Oil and Gas Fields, Commemorative

Millennium Volume, Geological Society, London, Memoir, 20, pp. 121-130.

16. Cooper, M., Evans, A., Lynch, D., Neville, G. & Newley, T., 1999, The Foinaven Field:

managing reservoir development uncertainty prior to start-up. In: Fleet, A. & Boldy, S.

(eds), Petroleum Geology of Northwest Europe: Proceedings of the 5 th Conference, pp.

675-682.

17. Corcoran, D. & Mecklenburgh, R., 2005, Exhumation of the Corrib Gas Field, Slyne

Basin, offshore Ireland, Petroleum Geoscience, 11, pp. 239-256.

18. Cowan, G. & Boycott-Brown, T., 2003, The North Morecambe Field, Block 110/2a, East

Irish Sea. In Gluyas, J. & Hichens, H. (eds), United Kingdom Oil and Gas Fields,

Commemorative Millennium Volume, Geological Society, London, Memoir, 20, pp. 97-

105.

19. Coward, M., 1995, Structural and tectonic setting of the Permo-Triassic basins of

northwest Europe. In: Boldy, S. (ed), Permian and Triassic Rifting in Northwest Europe,

Geological Society, London, Special Publication, 91, pp. 7-39.

20. Davison, I., Stasiuk, S., Nuttall, P. & Keane, P., 2010, Sub-basalt hydrocarbon

prospectivity in the Rockall, Faroe-Shetland and Møre basins, NE Atlantic. In: Vining, B.

& Pickering, S. (eds), Petroleum Geology: From Mature Basins to New Frontiers –

Proceedings of the 7th Petroleum Geology Conference, pp. 1025-1023.

21. Dean, K., McLachlan, K. & Chambers, A., 1999, Rifting and the development of the

Faroe-Shetland Basin. In: Fleet, A. & Boldy, S. (eds), Petroleum Geology of Northwest

Europe: Proceedings of the 5th Conference, pp. 533-544.

22. Doré, A. 1992, Synoptic palaeogeography of the Northeast Atlantic Seaway: late Permian

to Cretaceous. In: Parnell, J. (ed), Basins on the Atlantic Seaboard: Petroleum Geology,

Sedimentology and Basin Evolution. Geological Society London Special Publication, 62,

pp. 421-446.

23. Doré, A., Lundin, E., Fichler, C. & Olesen, O., 1997a, Patterns of basement structure and

reactivation along the NE Atlantic margin, Journal of the Geological Society, London,

154, pp. 85-92.

24. Doré, A., Lundin, E., Birkeland, Ø., Eliassen, P. & Jensen, L., 1997b, The NE Atlantic

Margin: implications of late Mesozoic and Cenozoic events for hydrocarbon

prospectivity, Petroleum Geoscience, 3, pp. 117-131.

25. Doré, A., Lundin, E., Jensen, L., Birkeland, Ø., Eliassen, P. & Fichler, C., 1999, Principal

tectonic events in the evolution of the northwest European Atlantic margin. In: In: Fleet,

A. & Boldy, S. (eds), Petroleum Geology of Northwest Europe: Proceedings of the 5 th

Conference, pp. 41-61.

26. Ebdon, C. & Payne, S., 1990, Stratigraphy of the Wells 164/25-1 and 164/25-1Z, West

Lewis Basin, Rockall. BP Internal Report No. 164/25-1/W22/W23.

27. Ebdon, C., Partington, M. & Wonders, A., 1992, Stratigraphy of the well 132/15-1, South

Rockall. BP Internal Report No. 132/15-1/W22/W23.

28. Ebdon, C., Granger, P., Johnson, H. & Evans, A., 1995, Early Tertiary evolution and

sequence stratigraphy of the Faroe-Shetland Basin: implications for hydrocarbon

prospectivity. In: Scrutton, R., Stoker, M., Shimmield, G. & Tudhope, A. (Eds),The

Tectonics, Sedimentation and Palaeoceanography of the North Atlantic Region,

Geological Society Special Publication, 90, 51-69

ACCEPTED MANUSCRIPT



29. Ellefsen, M., Boldreel, O. & Larsen, M., 2010, Intra-basalt units and base of the volcanic

succession east of the Faroe Islands exemplified by interpretation of offshore 3D seismic

data. In: Vining, B. & Pickering, S. (Eds), Petroleum Geology: From Mature Basins to

New Frontiers – Proceedings of the 7th Petroleum Geology Conference, pp. 1033-1042.

30. Færseth, R. B., 1996, Interaction of Permo-Triassic and Jurassic extensional fault blocks

during the development of the northern North Sea, Journal of the Geological Society,

London, 153, 931-944

31. Ferriday, I., 2000, Geochemical Interpretation Report, Well UKCS 164/28-1A. Geolab

Nor AS, Trondheim, Norway.

32. Fyfe, L-J., Schofield, N., Howell, J., Hartley, A. & Muirhead, D., 2017, An introduction to

hydrocarbon prospectivity of Scotland‟s inner West Coast basins, Poster presentation at

AAPG Annual Convention and Exhibition, Houston, 2-5 April.

33. Gage, M., 1980, A Review of the Viking Gas Field. In: Halbouty, M., Giant Oil and Gas

Fields of the Decade 1968-1978, AAPG Memoirs, Vol. 30, AAPG, Tulsa, Oklahoma,

USA.

34. Freeman, P., Kelly, S., Macdonald, C., Millington, J. & Tothill, M., 2008, The Schiehallion

Field: lessons learned modelling a complex deepwater turbidite. In: Robinson, A.,

Griffiths, P., Price, S., Hegre, J. & Muggeridge, A. (eds), The Future of Geological

Modelling in Hydrocarbon Development, Geological Society, London, Special

Publication, 309, pp. 205-219.

35. Gardiner, D., Schofield, N., Finlay, A., Mark, N., Holt, L., Grove, C., Forster, C. &

Moore, J., 2019, Modelling petroleum expulsion in sedimentary basins: The importance

of igneous intrusion timing and basement composition, Geology, 47, pp. 904-908.

36. Gray, J., 2013, Petroleum prospectivity of the principal sedimentary basins on the United

Kingdom Continental Shelf, Promote UK 2014 Report, UK Department of Energy and

Climate Change.

37. Herries, R., Poddubiuk, R. & Wilcockson, P., 1999, Solan, Strathmore and the back basin

play, West of Shetland. In: Fleet, A. & Boldy, S. (Eds), Petroleum Geology of Northwest

Europe: Proceedings of the 5th Conference, 693-712.

38. Hesselbo, S. & Coe, A., 2000, Jurassic sequences of the Hebrides Basin, Isle of Skye,

Scotland. In: Graham, J. & Ryan, A. (eds.), Field Trip Guidebook, International

Association of Sedimentologists Meeting, Dublin, 2000, 41-58.

39. Hesselbo, S., Oates, M, & Jenkyns, H., 1998, The lower Lias Group of the Hebrides

Basin, Scottish Journal of Geology, 34 (1), pp. 23-60.

40. Hitchen, K. & Stoker, M., 1993, Mesozoic rocks from the Hebrides Shelf and

implications for hydrocarbon prospectivity in the northern Rockall Trough, Marine and

Petroleum Geology, 10, 246-254.

41. Hitchen, K., Stoker, M., Evans, D. & Beddoe-Stephens, B., 1995, Permo-Triassic

sedimentary and volcanic rocks in basins to the north and west of Scotland. In: Boldy, S.

(Ed), Permian and Triassic rifting in Northwest Europe, Geological Society, London,

Special Publication, 91, 87-102.

42. Hitchen, K., Johnson, H. and Gatliff, R., 2013, Geology of the Rockall Basin and adjacent

areas, BGS Research Report RR/12/03, British Geological Survey, Keyworth,

Nottingham.

ACCEPTED MANUSCRIPT



43. Hillier, A. & Williams, B., 1991, The Leman Field, Blocks 49/26, 49/27. 49/28, 53/1, 53/2,

UK North Sea. In: Abbots. I. (Ed), United Kingdom Oil and Gas Fields, 25 Years

Commemorative Volume, Geological Society Memoir 14, 451-458.

44. Iliffe, J., Robertson, A., Ward, G., Wynn, C., Pead, S. & Cameron, N., 1999, The

importance of fluid pressures and migration to the hydrocarbon prospectivity of the

Faroe-Shetland White Zone. In: Fleet, A. & Boldy, S. (eds), Petroleum Geology of

Northwest Europe: Proceedings of the 5th Conference, pp. 601-611.

45. Isaksen, G., Wall, G., Thomsen, M., Tapscott, C., Wilkinson, D. and McLachlan, K., 2001,

Application of petroleum seep technology in mitigating the risk of source-rock adequacy

and yield-timing in a frontier basin: the Rockall Trough, UK, presentation at Earth

System Processes- Global Meeting, Edinburgh, June 24-28, 2001.

46. Johnson, H., Ritchie, J., Hitchen, K., McInroy, D. & Kimbell, G., 2005, Aspects of the

Cenozoic deformational history of the Northeast Faroe-Shetland Basin, Wyville-

Thomson Ridge and Hatton Bank areas. In Doré, A. & Vining, B. (eds), Petroleum

Geology: Northwest Europe and Global Perspectives – Proceedings of the 6th Petroleum

Geology Conference, 993-1007.

47. Johnston, S., Doré, A. & Spencer, A., 2001, The Mesozoic evolution of the southern

North Atlantic region and its relationship to basin development in the south Porcupine

Basin, offshore Ireland. In: Shannon, P., Haughton, P. & Corcoran, D. (eds), The

Petroleum Exploration of Ireland‟s Offshore Basins, Geological Society, London, Special

Publication, 188, pp. 237-263.

48. Jones, R. & Milton, N., 1994, Sequence development during uplift: Paleogene stratigraphy

and relative sea-level history of the Outer Moray Firth, UK North Sea, Marine and

Petroleum Geology, 11(2), pp. 157-165.

49. Karvelas, A., Schenk, O. & Shtukert, O., 2016, The UK‟s Frontier Basin: The Rockall

Basin, PETEX Conference Abstract, London, November 15-17, 2016.

50. Kleingeld, J., 2005, Geochemical investigation of a gas from a sample from Benbecula,

154/1-1, UK, Internal Report EP 2005-6017, Shell International Exploration and

Production B.V., Rijswijk, Netherlands.

51. Lamers, E. & Carmichael, S., 1999, The Paleocene deepwater sandstone play West of

Shetland. In: Fleet, A. & Boldy, S. (eds), Petroleum Geology of Northwest Europe:

Proceedings of the 5th Conference, pp. 645-659.Larsen, M., Rasmussen, T. & Hjelm, L.,

2011, Cretaceous revisited: exploring the syn-rift play of the Faroe-Shetland Basin. In:

Vining, B. & Pickering, S. (eds), Petroleum Geology: From Mature Basins to New

Frontiers – Proceedings of the 7th Petroleum Geology Conference, pp. 953-962.

52. Leach, H., Herbert, N., Los, A. & Smith, R., 1999, The Schiehallion development. In:

Fleet, A. & Boldy, S. (eds), Petroleum Geology of Northwest Europe: Proceedings of the

5th Conference, pp. 683-692.

53. Goodwin, 2000, Gas Analysis of samples from well 154/1-1, LGC Geochemistry report

for Enterprise Oil.

54. Louizou, N., 2003, A post-well analysis of recent years exploration drilling along the UK

Atlantic Margin, First Break, 21(4), pp. 45-49

55. Loizou, N., 2014, Success in exploring for reliable, robust Paleocene traps west of

Shetland. In: Cannon, S. & Ellis, D. (eds), Hydrocarbon Exploration to Exploitation West

of Shetlands, Geological Society, London, Special Publications, 397.

ACCEPTED MANUSCRIPT



56. Lundin, E. & Doré, A., 2002, Mid-Cenozoic post-breakup deformation in the „passive‟

margins bordering the Norwegian-Greenland Sea, Marine and Petroleum Geology, 19,

pp. 79-93.

57. Lundin, E. & Doré, A., 2011, Hyperextension, serpentinization and weakening: A new

paradigm for rifted margin compressional deformation, Geology, 39, pp. 347-350.

58. Macchi, L., 1995, Summary report on the reservoir geology of Texaco well 202/12-1

West of Shetland, Reservoir Associates International Report, Cheshire, UK.

59. Magee, C., Jackson, C. & Schofield, N., 2014, Diachronous sub-volcanic intrusion along

deep-water margins: insights from the Irish Rockall Basin, Basin Research, 26, pp. 85-105.

60. Mark, N., Schofield, N., Pugliese, S., Watson, D., Holford, S., Muirhead, D., Brown, R. &

Healy, D., 2018, Igneous intrusions in the Faroe-Shetland basin and their implications for

hydrocarbon exploration; new insights from well and seismic data, Marine and

Petroleum Geology, 92, pp. 733-753.

61. Mark, N., Schofield, N., Gardiner, D., Holt, L., Grove, C., Watson, D., Alexander, A. &

Poore, H., 2019, Overthickening of sedimentary sequences by igneous intrusions, Journal

of the Geological Society, 176(1), pp. 46-60.

62. McInroy, D. * Hitchen, K., 2008, Geological evolution and hydrocarbon potential of the

Hatton Basin (UK sector), northeast Atlantic Ocean. In: Brown, D. (ed), Central

Atlantic Conjugate Margins Conference, Halifax, Nova Scotia, Canada, 13-15 August

2008. 132-143.

63. Meadows, N. & Beach, A., 1993, Controls on reservoir quality in the Triassic Sherwood

Sandstone of the Irish Sea. In: Parker, J. (ed), Petroleum Geology of Northwest Europe:

Proceedings of the 4th Conference, Geological Society, London, pp. 823-833.

64. Morton, A., Dixon, J., Fitton, J., MacIntyre, R., Smythe, D. & Taylor, P., 1988, Early

Tertiary volcanic rocks in Well 163/6-1A, Rockall Trough. In: Morton, A. & Parson, L.

(eds), 1988, Early Tertiary Volcanism and the Opening of the NE Atlantic, Geological

Society Special Publication, 39, pp. 293-308.

65. Morton, A., Herries, R. & Fanning, M., 2007, Correlation of Triassic sandstones in the

Strathmore Field, West of Shetland, using heavy mineral provenance signatures,

Developments in Sedimentology, 58, pp. 1037-1072.

66. Morton, N., 1987, Jurassic subsidence history in the Hebrides, NW Scotland, Marine and


67. Morton, N., 1989, Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland,

Marine and Petroleum Geology, 6, 243-260.

68. Musgrove, F. and Mitchener, B., 1996, Analysis of the pre-Tertiary rifting history of the

Rockall trough, Petroleum Geoscience, 2, pp. 353-360.

69. Møller Hansen, D. & Cartwright, J., 2006, The three-dimensional geometry and growth

of forced folds above saucer-shaped igneous sills, Journal of Structural Geology, 28,

pp.1520-1535.

70. Nelson, C., Jerram, D. & Hobbs, R., 2009, Flood basalt facies from borehole data:

implications for prospectivity and volcanology in volcanic rifted margins, Petroleum

Geoscience, 15, pp. 313-324.

71. Odell, R. & Walker, D., 1979, Rockall Basin 12/13-1 and 12/13-1A Completion Report,

Amoco Ireland Exploration Company Internal Report.

72. Peacock, S., 1990, 156/17-1 Well Evaluation Report, BP Internal Report No. L156/17-

1/W27, BP Exploration, Glasgow.

ACCEPTED MANUSCRIPT



73. Pentex Oil Ltd., 1989, Final Well Report Upperglen 1, Pentex Oil Internal Report.

74. Plumb, H., Fuchter, E. & Denis, J., 1989, Geological Well Evaluation Report 164/25-1,

1ST, BP Petroleum Development Ltd., Glasgow.

75. Rawcliffe, H., 2016, Lava-water-sediment interaction: processes, products and petroleum

systems, University of Glasgow PhD thesis.

76. Ritchie, J., Johnson, H., Quinn, M. & Gatliff, R., 2008, The effects of Cenozoic

compression with the Faroe-Shetland Basin and adjacent areas. In: Johnson, H., Doré,

A., Gatliff, R., Holdsworth, R., Lundin, E. & Ritchie, J. (eds), The Nature and Origin of

Compression in Passive Margins, Geological Society, London, Special Publications, 306,

pp. 121-136.

77. Roberts, A., Alvey, A. & Kuznir, N., 2018, Crustal structure and heat-flow history in the

UK Rockall Basin, derived from backstripping and gravity-inversion analysis, Petroleum

Geoscience. https://doi.org/10.1144/petgeo2017-063

78. Schmid, S., Worden, R. & Fisher, Q., 2004, Diagenesis and reservoir quality of the

Sherwood Sandstone (Triassic), Corrib Field, Slyne Basin, west of Ireland, Marine and


79. Schofield, N., Jerram, D., Holford, S., Archer, S., Mark, N., Hartley, A., Howell, J.,

Muirhead, D., Green, P., Hutton, D. & Stevenson, C., 2016, Sills in Sedimentary Basins

and Petroleum Systems. In Breitkreuz, C., & Rocchi, S. (Eds.), Physical Geology of

Shallow Magmatic Systems, Advances in Volcanology, pp. 273-294.

80. Schofield, N., Jolley, D., Holford, S., Archer, S., Watson, D., Hartley, A., Howell, J.,

Muirhead, D., Underhill, J. & Green, P., 2017a, Challenges of future exploration within

the UK Rockall Basin, in: Bowman, M. & Levell, B. (eds), Petroleum Geology of NW

Europe, 50 Years of Learning – Proceedings of the 8th Petroleum Geology Conference,

Geological Society, London.

81. Schofield, N., Holford, S., Millett, J., Brown, D., Jolley, D., Passey, S., Muirhead, D.,

Grove, C., Magee, C., Murray, J., Hole, M., Jackson, C. & Stevenson, C., 2017b, Regional

magma plumbing and emplacement mechanisms of the Faroe-Shetland Sill Complex:

implications for magma transport and petroleum systems within sedimentary basins,

Basin Research, 29, pp. 41-63.

82. Schofield, N., Broadley, L., Jolley, D., 2019, Audit of Petroleum Exploration Wells in the

UK Rockall Basin 1980-2006. Final Version, University of Aberdeen. https://data-

ogauthority.opendata.arcgis.com/datasets/university-of-aberdeen-–-oga-frontier-basins-

rockall-project-a-new-view-of-the-uk-rockall-prospectivity-1

83. Scotchman, I., 2001, Petroleum geochemistry of the Lower and Middle Jurassic in

Atlantic margin basins of Ireland and the UK. In: Shannon, P., Haughton, P. & Corcoran,

D. (eds), The Petroleum Exploration of Ireland‟s Offshore Basins, Geological Society,

London, Special Publication, 188, pp. 31-60.

84. Scotchman, I., Doré, A. and Spencer, A., 2018, Petroleum systems and results of

exploration on the Atlantic margins of the UK, Faroes and Ireland: what have we learnt?

In: Bowman, M. & Levell, B. (eds), Petroleum Geology of NW Europe, 50 Years of

Learning – Proceedings of the 8th Petroleum Geology Conference, Geological Society,

London.

85. Shannon, P., Moore, J., Jacob, W. & Makris, J., 1993, Cretaceous and Tertiary basin

development west of Ireland. In: Parker, J. (ed), Petroleum Geology of Northwest

Europe: Proceedings of the 4th Conference, Geological Society London, pp. 1057-1066.

ACCEPTED MANUSCRIPT


https://doi.org/10.1144/petgeo2017-063





86. Steel, R. & Wilson, A., 1975, Sedimentation and tectonism (?Permo-Triassic) on the

margin of the North Minch Basin, Lewis, Journal of the Geological Society London, 131,

pp.183 – 202.

87. Stoker, M., Praeg, D., Shannon, P., Hjelstuen, B., Laberg, J., Nielsen, T., Van Weering, T.,

Sejrup, H. & Evans, D., 2005, Neogene evolution of the Atlantic continental margin of

NW Europe (Lofoten Islands to SW Ireland): anything but passive. In Doré, A. & Vining,

B. (eds), Petroleum Geology: Northwest Europe and Global Perspectives – Proceedings

of the 6th Petroleum Geology Conference, 1057-1076.

88. Stoker, M., Stewart, M., Shannon, P., Bjerager, M., Nielsen, T., Blischke, A., Huelstuen, B.,

Gaina, C., McDermott, K. & Ólavsdóttir, J., 2017, An overview of the Upper Palaeozoic-

Mesozoic stratigraphy of the NE Atlantic region. In: Péron-Pinvidic, G., Hopper, J.,

Stoker, M., Gaina, C., Doornenbal, J., Funck, T. & Árting, U. (Eds), The Northeast

Atlantic region: a reappraisal of crustal structure, tectonostratigraphy and magmatic

evolution, Geological Society, London, Special Publication, 447, 11-68.

89. Storevedt, K. & Steel, R., 1977, Palaeomagnetic evidence for the age of the Stornoway

Formation, Scottish Journal of Geology, 13, 263-269.

90. Thrasher, J., 1992, Thermal effect of the Tertiary Cuillins Intrusive Complex in the

Jurassic of the Hebrides: an organic geochemical study. In: Parnell, J. (ed), Basins on the

Atlantic Seaboard: Petroleum Geology, Sedimentology and Evolution. Geological Society,

London, Special Publication, 62, 35-49.

91. Tomasso, M., Underhill, J.R., Hodgkinson, R.A. & Young, M.J. 2008. Structural styles and

depositional architecture in the Triassic of the Ninian and Alwyn North fields:

Implications for basin development and prospectivity in the Northern North Sea.

Marine & Petroleum Geology, 25, 588-605

92. Trice, R., 2014, Basement exploration, West of Shetlands: progress in opening a new

play on the UKCS. In: Cannon, S. & Ellis, D. (eds), Hydrocarbon Exploration to

Exploitation West of Shetlands, Geological Society, London, Special Publication, 397, pp.

81-105.

93. Tuitt, A., 2009, Timing and controls of structural inversion in the NE Atlantic Margin,

University of Edinburgh PhD Thesis.

94. Tuitt, A., Underhill, J.R., Ritchie, J.D., Johnson, H. & Hitchen, K. 2010. Timing, controls

and consequences of structural inversion in the Rockall-Faroe area of the NE Atlantic

Margin. In: Vining, B.A. & Pickering, S. C. (Eds.) Petroleum Geology: From Mature Basins

to New Frontiers; Proceedings of the 7th Petroleum Geology Conference. 7, 963-977.

95. Turner, J., 1966, The Oxford Clay of Skye, Scalpay and Eigg, Scottish Journal of Geology,

2, 243-252.

96. Tyrrell, S., Souders, A., Haughton, P., Daly, J. & Shannon, P., 2010, Sedimentology,

sandstone provenance and palaeodrainage on the eastern Rockall Basin margin: evidence

from the Pb isotopic composition of detrital K-feldspar. In: Vining, B. & Pickering, S.

(eds), Petroleum Geology: From Mature Basins to New Frontiers – Proceedings of the

7th Petroleum Geology Conference, pp. 937-952.

97. Vincent, A. & Tyson, R., 1999, Organic facies of the Middle Jurassic of the Inner

Hebrides, Scotland, Petroleum Geoscience, 5, pp. 83-92.

98. Waters, C., Gillespie, M., Smith, K., Auton, C., Floyd, J., Leslie, A., Millward, D., Mitchell,

W., McMillan, A., Stone, P., Barron, A., Dean, M., Hopson, P., Krabbendam, M., Browne,

ACCEPTED MANUSCRIPT



M., Stephenson, D., Akhurst, M., and Barnes, R., 2007, Stratigraphical Chart of the

United Kingdom: Northern Britain, British Geological Survey, 1 poster.

99. Winter, D. & King, B., 1991, The West Sole Field, Block 48.6, UK North Sea. In: Abbots

(ed), United Kingdom Oil and Gas Fields, 25 Years Commemorative Volume, Geological

Society Memoir No. 14, pp. 517-523.

100. Wright, K., Davies, R., Jerram, D., Morris, J. & Fletcher, R., 2012, Application of

seismic and sequence stratigraphic concepts to a lava-fed delta system in the Faroe-

Shetland Basin, UK and Faroes, Basin Research, 24, pp. 91-106.

ACCEPTED MANUSCRIPT



List of Figures

Figure 1 Structure map of UK Rockall, showing main sub-basins, intra-basinal highs, faults

and igneous centres. Red boundary marks main area of interest for this study. UK Rockall

exploration wells and shallow boreholes and selected Faroe-Shetland Basin exploration

wells are shown. Boreholes mentioned in the text are labelled. WLR = West Lewis Ridge,

SSH = Sula Sgeir High, SBH = Solan Bank High, RR = Rona Ridge.

Figure 2 Graph showing exploration wells drilled in the UK Rockall between 1980 and 2006.

Figure 3 Petroleum systems events chart for the UK Rockall, showing the timing of play

elements. Data compiled from: BGS UK Rockall shallow borehole records; 164/25-1Z,

164/25-2, 153/05-1, 154/01-1, 154/01-2, 164/28-1A, 164/27-1, 132/15-1, 132/06-1, Minch-1,

Upper Glen-1, Sea of Hebrides-1, 202/08-1, IRL12/2-1Z and IRL12/2-2 well documents;

Hitchen & Stoker, 1993; Vincent & Tyson, 1999; Hesselbo & Coe, 2000; Tuitt, 2009;

McInroy & Hitchen, 2008; Hitchen et al., 2013; Scotchman, 2018; Fyfe et al., 2017. Charge

timing estimates are from Karvelas et al., 2016 (Palaeocene) and Isaksen, 2001 (Miocene) are

included for illustrative purposes.

Figure 4 Paleocene and Eocene lithostratigraphy for West of Britain and Central North Sea,

modified from Schofield et al. (2019) and Waters et al. (2007). BP T-sequence framework

(Ebdon et al., 1995) shown over the Paleocene interval.

Figure 5 Cretaceous lithostratigraphy for West of Britain and Central North Sea, modified

from Schofield et al. (2019) and Waters et al. (2007).

Figure 6 Permian to Jurassic lithostratigraphy for the Inner Hebrides, West of Britain and

Central North Sea, modified from Schofield et al. (2019), Waters et al. (2007) and Morton

(1989). Permo-Triassic of Inner Hebrides based on Steel & Wilson (1975) and Storevedt &

Steel (1977). Note that the Heron Group and the Papa Group are approximately time-

equivalent to the Sherwood Sandstone and Mercia Mudstone discussed in the text.

Figure 7 Detailed lithostratigraphy for the Middle Jurassic of Skye and Raasay, compiled from

Morton (1989), Barron et al. (2012) and Archer et al. (2019).

Figure 8 Section of Core 3 (2646.2-2663.3) from Well 164/25-1Z, showing large mud clasts

in the Selandian Vaila Formation sandstones, interpreted to be reworked Late Jurassic

mudstones by Ebdon & Payne (1990). By virtue of their size, subangularity and preservation

(primary sedimentary structures appear to be preserved in some clasts) these mudstones

are unlikely to have been transported over great distances.

Figure 9 (a) Composite seismic line (from left, OGA 2015 Lines 73 and Line 1) through the

West Lewis Basin, passing through 164/25-1Z and BGS borehole 90/05. The basin can be

divided into two Permo-Triassic half-grabens, a more restricted basin containing the 164/25-

1Z well, which was inactive between the Permo-Triassic and Late Albian, and a more

extensive basin which underwent an additional period of activity from the Middle Jurassic to

Earliest Cretaceous (b) Frogtech Geoscience depth to basement map, (bold line shows

location of (a). Jm = Middle Jurassic, Ju = Late Jurassic, Ber = Early Berriasian.

Figure 10 Seismic line OGA 2015 L78 through 132/15-1 (a) Uninterpreted and (b)

interpreted, showing that the well penetrated the bounding fault of a pre-rift fault block,

ACCEPTED MANUSCRIPT



potentially missing a sedimentary pre-rift package capping the fault block (c) pre-drill

interpretation (modified from Ebdon et al., 1992) of similarly oriented seismic line through

the planned 132/15-1 well location, illustrating the poor data quality available at the time,

and the resulting significantly different interpretation, exact line location unknown, (d) index

map showing location of (a) and (b). “A” indicated on both Figures 10(b) and 10(c) shows

the same surface, interpreted in (b) as a fault and in (c) as the top pre-rift.

Figure 11 Map showing the locations of seepage slick centre points superimposed upon

faults interpreted as part of this study. It can be seen that many of the slicks are aligned

along mapped faults or cluster about fault tips. The large area to the West of Rosemary

Bank in which seeps appear to be unrelated to structure is misleading, as the area is largely

devoid of seismic data and it is possible that even here, there is some correspondence.

Seep data © 2020 Oil and Gas Authority, UKCS CGG NPA Satellite Mapping.

Figure 12 Permo-Triassic (a) paleogeography and (b) present day distribution maps for the

UK Rockall, inset box outlines areas where Permo-Triassic is interpreted to have been

thinned by extension.

Figure 13 Regional examples showing into-the-fault thickening of the Permo-Triassic

package, supporting the interpretation of continental rifting. (a) Seismic line MP86WB-5

across the Sula Sgeir Basin, 202/12-1 well projected onto line, SSH = Sula Sgeir High. (b)

Seismic line WB93-107 across the Papa Basin, 202/19-1 well projected onto line. Ju = Late

Jurassic, Kl = Early Cretaceous, Ku = Late Cretaceous.

Figure 14 Middle Jurassic (a) paleogeography and (b) present day distribution maps for the

UK Rockall.

Figure 15 OGA 2015 Line 16, (a) Uninterpreted, and (b) interpreted, extending from the

northernmost West Lewis Basin to the Northeast Rockall Basin, showing the Upper Jurassic

– Earliest Berriasian, Middle Jurassic and Permo-Triassic picks in the West Lewis Basin,

correlated from the 88/01, 90/01, 90/02 and 90/05 BGS boreholes. Approximate location of

boreholes and seismic line shown in inset map in (a). Interpreted fault blocks in the

Northeast Rockall basin are observed to be capped by a pre-rift succession of unknown

origin. It is possible that this succession is comparable to that observed in the West Lewis

Basin. Note that Top Colsay pick can be correlated from 164/25-1Z.

Figure 16 Late Jurassic (a) paleogeography and (b) present day distribution maps for the UK

Rockall.

Figure 17 Early Cretaceous (a) paleogeography and (b) present day distribution maps for the

UK Rockall. WLR = West Lewis Ridge, RR = Rona Ridge.

Figure 18 Late Cretaceous (a) paleogeography and (b) present day distribution maps for the

UK Rockall. WLB = West Lewis Basin, WLR = West Lewis Ridge, RR = Rona Ridge.

Figure 19 Play concept diagram, modified from a geoseismic line across the South Rockall

Basin to conceptually illustrate potential source rock and reservoir intervals, shown in

Figures 12.14 and 16-18, that are likely to be present in the UK Rockall. Black circles =

potential source rocks, orange circles = potential reservoirs.

ACCEPTED MANUSCRIPT



Source Rocks: (1) Carboniferous coals (proven in Irish Rockall), (2) Sinemurian-Toarcian

organic-rich marine shales (proven onshore Inner Hebrides), (3) Bathonian organic-rich

lagoonal shales (proven in West Lewis Basin), (4) Kimmeridgian-Earliest Berriasian

Kimmeridge Clay Formation equivalent organic-rich marine shales (proven in West Lewis

and West Flannan Basins and Irish Rockall).

Reservoirs: (1) Permo-Triassic continental sandstones (reservoir quality Permo-Triassic

sandstones proven in Minch-1 & IRL 12/2-1Z (Dooish)), (2) Early Jurassic marine sandstones

(Scalpa sandstone and Broadford beds proven at outcrop in the Inner Hebrides, but

subsurface equivalents penetrated in Minch-1, Upper Glen-1 and Sea of Hebrides-1 found to

be cemented or mudstone-dominated), (3) Middle Jurassic marine to estuarine sandstones

(proven in Upper Glen-1, onshore Skye), (4) Late Jurassic – Earliest Berriasian shallow

marine sandstones (proven in West Flannan Basin), (5) Late Cretaceous shallow marine

sandstones (potentially of Barremian or Albian age, see Section 5.3), (6) Late Cretaceous

deepwater sandstones may occur down-dip of shallow marine sands- depends on abundant

clastic supply or cannibalisation of shelf sands, (7) Cenomanian carbonates (e.g. 132/15-1)

may form a viable reservoir if fractured by subsequent intrusive or inversion events, (8)

Danian marine sandstones (e.g. 164/25-1Z), (9) Late Selandian Vaila Formation deepwater

marine sandstones (widely proven in NE Rockall Basin and South Rockall Basin, but likely to

be non-reservoir in South Rockall due to voluminous volcaniclastic input, see Section 5.8),

(10) Ypresian (T40) Colsay Formation continental sandstones (e.g. 164/25-2, see section

5.10), (11) Eocene basin floor fans (e.g. 153/05-1).

Figure 20 Potential sites for a stratigraphic test well in the NE Rockall Basin. (a) Location

map, showing locations of (b)-(d) superimposed on Base Cretaceous Unconformity horizon

and faults, both from this study, blue/purple/pink indicates greater depth, (b) OGA 2015

Line 16, passing through Site 1, (c) TWT closure at Base Cretaceous Unconformity level

close to Site 1, contour interval 50 ms, (d) OGA 2015 Line 32 passing through Site 2.

Figure 21 Potential site for stratigraphic test well in South Rockall Basin (UK). (a) Map on

Base Cretaceous Unconformity showing mapped faults (both this study) and location of (b)

and of Site 3, blue/purple/pink indicates greater depth, (b) OGA 2014 Line 22, passing

through Site 3.

ACCEPTED MANUSCRIPT



ACCEPTED MANUSCRIPT



uk rockall prospectivity: re-awakening exploration in a ... · the ukcs. this is against an overall...

Documents