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72nd

EAGE Conference & Exhibition incorporating SPE EUROPEC 2010

Barcelona, Spain, 14 - 17 June 2010

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72nd



Introduction

Despite being a major reservoir in large parts of NW Europe (e.g. in Poland and Germany),

carbonates belonging to the Zechstein Supergroup have never played a significant role in contiguous

parts of the North Sea Basin. Until recently, production from Zechstein Supergroup carbonates was

limited to the Hewett Field in the Southern North Sea, the Auk and Argyll fields of the Central North

Sea and Ettrick in the Moray Firth and its broader exploration potential has been largely overlooked

and remains uncertain. Reawakening of and the successful production from a gas discovery, now

called Wissey but originally christened “Scram”, when found over forty years ago, has highlighted

that the Zechstein Supergroup may still hold potential. The aim of this paper is to document the

geological factors that control gas production from this poorly understood naturally fractured

carbonate reservoir and provide new insights into its prospective potential elsewhere in the basin.

Location and Field History

The Wissey gas field is located on the southern margin of the UK sector of the Southern North Sea

(SNS) in the region of the Southern Permian Basin (SPB) known as the Sole Pit Basin (see Figure 1).

Initially termed “Scram” upon discovery by Signal Oil in 1967, the field changed hands numerous

times and was subjected to a series of appraisals before finally coming on-stream in 2008 becoming

the first and currently only UK gas field to produce solely from Zechstein Supergroup carbonates.

Production continues from the

fractured Plattendolomit

Formation reservoir under

operatorship from Tullow Oil,

with joint venture partnership from

First Oil and Faroe Petroleum.

Stratigraphy

The stratigraphy of the Sole Pit

Basin is generally well

documented and understood. For a

succinct review of the major

stratigraphic features of the local

area the reader is referred to

Underhill et al. (2009). A

stratigraphic column for the

immediate vicinity of Wissey is

presented in Figure 2.

Carboniferous sediments contain

the prolific Westphalian Coal

Measures Group, which sources all

gas fields in the vicinity of the

Sole Pit Basin. Unconformably

overlying the Carboniferous are Lower Permian aeolian, continental sandy sabkha and fluvial

sediments of the Rotliegend Group clastic play fairway – which remains the main reservoir rock for

the majority of Sole Pit Basin gas fields.

Marine incursion during the Upper Permian and subsequent recharge-evaporation led to the

deposition of cyclic (Z1-Z6) carbonate-evaporite sediments ascribed to the Zechstein Supergroup

(Tucker, 1991). Regionally across the SPB there exist a maximum of six flooding-evaporation cycles,

however locally across the Sole Bit Basin the upper two-three are missing due to non-deposition or

erosion in the latest Permian times. Broadly, anhydrite and halite dominate in basinal settings;

Figure 1. Location map. Wissey is in the boxed area in block

53/4 (Modified after Underhill et al., 2009)

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whereas proportions of carbonate greatly increase towards the

basin margins where a rim of carbonate deposition is observed

(Tucker, 1991).

Triassic sedimentation is initiated by Brockelschiefer Formation

deposition and then clastic red bed deposition of the Bacton and

Haisborough Groups. Locally, Jurassic sediments have been

completely eroded, with Triassic deposits overlain unconformably

by the Cretaceous Cromer Knoll Group at the Base Cretaceous

Unconformity. Overlying the Cromer Knoll Group are marine

deposits of the Cretaceous Chalk Group, in turn overlain by

Cenozoic deposits.

Database and Interpretation Methods

Regional-scale and field-scale structural and stratigraphic

understanding have been gained upon interpretation of a well-

calibrated 3D seismic volume acquired over a total of 19 SNS

license blocks surrounding the Wissey gasfield (block 53/4).

Composite well logs and time-depth information were utilised in

order to produce an accurate structural and stratigraphic

interpretation across the volume. Additionally, depth conversion

and attribute analysis allowed seismic information to be combined

with well log and core data in the construction of a stochastic

reservoir model for the field.

Field Structure

A near-N-S striking representative seismic line through the Wissey

field is shown in Figure 3. The Wissey field exists as a structurally

high horst block bounded by northern- and southern-bounding

faults. A series of inverted normal faults are observed south of the

Wissey field, however further analysis of the structures shows that little structural inversion has taken

place on the field-bounding faults themselves.

Figure 4 shows a depth converted top reservoir (Plattendolomit Formation) structure map for Wissey

and the local area of the SNS. At the reservoir level, two fault trends can be identified: a general

NNW-SSE strike determined in this study to be initially active as normal faults during the Lower

Permian; and a general WNW-ESE strike, the ‘Wissey trend’, initially active during the Lower

Triassic, directly following Upper Permian Zechstein Supergroup deposition. The two field bounding

faults can be seen to be of the Wissey trend, and bound a WNW-ESE striking elongated structural

high approximately 1km wide - this is the Wissey field. Shown on Figure 4 are the locations of all

wells drilled in the vicinity including four on-structure wells: 53/4-1, 53/4-3, 53/4a-9 and the current

producer 53/4d-11, along with one off-structure well: 53/4-2 to the SE of the field.

As well as the series of Wissey trend faults, structural inversion is also present on a number of the

NNW-SSE trending Lower Permian faults in the region and appears to have taken place in two

separate inversion events: one in the Late-Cretaceous and a second period in the Mid-Tertiary (as

originally identified from 2D interpretations undertaken and reported by Badley et al., 1989). Thus,

the structural geometries present in this region appear to result from the subsequent structural

inversion of two pre-existing extensional fault trends and not for example related to strike-slip

movements as interpreted by some. Also evident from Figure 3 and Figure 4 is the nature of the

resulting trap geometry for the field whereby the tilted Plattendolomit Formation reservoir is sealed

against the sealing Bunter Shale Formation to the south and exhibits dip closure to the north. It can be

Figure 2. Stratigraphic

column for Wissey and

local area (After Underhill

et al., 2009)

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determined from the structure map that dip closure is present to the west and a structural saddle exists

to the east of the field.

Reservoir Description

On analysis of log and core information

for the Wissey field it is clear that the

Plattendolomit Formation reservoir at

Wissey documents a regressional

sequence of events, directly overlying the

laminated mudstones of the Grauer

Salzton Member: the base of the

sequence. Within the Plattendolomit

Formation itself three broad units can be

defined and correlated across the field: a

lower organic rich argillaceous limey

mudstone; middle massive and bedded

dolomitised wackestones, packstones and

boundstones with regions of high

brecciation throughout; and upper thinly

bedded fine-grained dolomitised oolitic

grainstones. Overlying the reservoir itself

is an anhydritic mudstone, previously

referred to as the ‘Plattenmudstone’,

which may however be more

appropriately ascribed to the Scolthead

Formation documented to directly overly the

Plattendolomit Formation to the south (Johnson

et al., 1994). The facies association thus appears

consistent with the NNE directed basinward

progradation of the basin rimming carbonate

margin, with regional thickness assessments

suggesting that Wissey in fact lies at the

approximate shelf break of the system. Seal to the

reservoir is provided by the overlying anhydritic

mudstone which has a thin anhydrite between it

and the Brockelschiefer Formation above

marking the onset of Triassic deposition.

Depositional environment and post-depositional

diagenesis have long been recognised as factors

determining reservoir quality in carbonates

(Clark, 1986). Primary porosity at Wissey

appears to increase from the distal mudstones at

the base of the formation through the brecciated

middle unit to the oolitic grainstones of the upper

unit, as would be expected for the shoaling

upward sequence present. Diagenesis also clearly plays a part in the quality of the Plattendolomit

Formation reservoir which has been heavily dolomitised throughout, both enhancing and inhibiting

porosity, with occasional vuggy porosity existing as evidence for early leaching upon burial. From

analysis of the 53/4a-9 core, fracturing appears widespread throughout the reservoir, with increased

fracturing in the two upper most units. Both primary and secondary porosity can be observed to be

Figure 3. 2D N-S dip-seismic section through well

53/4d-11 of the Wissey field. Upper figure un-

interpreted; Lower figure is interpreted with key

horizons and major faults. See Figure 4 for the

location of this line.

Figure 4. Top Plattendolomit structure map

over parts of blocks 53/4 and 53/5

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highly occluded by anhydrite cement, likely sourced locally, however many open fractures are present

in the core samples available.

Remaining prospective potential of the Plattendolomit Formation

Whilst it is evident that the majority of Plattendolomit Formation penetrations in the region exhibit

poor reservoir quality, Wissey has shown that in the correct circumstances, gas can accumulate in

economic quantities in a producible reservoir with adequate permeability within Zechstein

Supergroup carbonates in the UK sector. It seems highly likely, therefore, that an approximate WNW-

ESE trending ‘belt of prospectivity’ exists – reliant upon the exact positioning of the basin margin

axis – running through the Wissey field, where depositional circumstances provide adequate storage

opportunity. Secondly, the presence of natural fractures throughout the reservoir at Wissey are,

although poorly constrained by well positioning and data, an extremely important factor in explaining

the success of Wissey’s reservoir. These tectonically induced fractures have been related to two

periods of positive tectonic inversion on surrounding extensional precursor faults, and as such provide

an additional consideration factor in prospective exploration and development targets in the adjacent

region of the SNS.

Conclusions

The Wissey gas field, situated in the UK SNS, produces from a naturally fractured Plattendolomit

Formation (Zechstein Supergroup) reservoir. The structure of the field and surrounding area is

resultant upon tectonic inversion of two sets of extensional precursor fault trends. Reservoir quality is

reliant upon deposition near the shelf break of a regressive system, dolomitisation and especially

natural fracturing, likely induced during one or both of two periods of tectonic inversion documented

in the stratigraphy of the surrounding area. These controls on prospectivity can usefully be tested

elsewhere in the basin in areas of prospective exploration targets.

Acknowledgements

The results documented in this paper are based upon initial investigation contributing to the

undergraduate thesis of one of us (CD) – submitted to the University of Edinburgh in 2008 – and

further investigation conducted during a placement coordinated by Tullow Oil in the summer of 2009.

Rupert Hoare, WesternGeco is thanked for providing access to and subsequent permission to publish

images from the 3D seismic volume, interpretation of which formed the basis of this investigation.

Interpretation was principally conducted using Petrel software kindly provided by Schlumberger, in

the University of Edinburgh’s seismic interpretation laboratory. Tullow Oil, First Oil and Faroe

Petroleum are all kindly thanked for their permission to publish confidential data. Joel Corcoran,

Tullow’s Exploration Manager for Europe is especially thanked for help and support of the project.

References

Badley, M.E., Price, J.D. & Backshall, L.C. 1989. Inversion, reactivated faults and related structures: seismic examples from

the southern North Sea. In: Cooper, M.A. & Williams, G.D. (Eds.): Inversion Tectonics. Geological Society Special

Publication, 44, 201-219.

Clark, D.N. 1986. The Distribution of Porosity in Zechstein Carbonates. In: Brooks, J., Goff, J.C. & Van Hoorn, B. (Eds):

Habitat of Palaeozoic Gas in N.W. Europe. Geological Society Special Publication, 23, 121-149.

Johnson, H., Warrington, G & Stoker, S.J. 1994, Permian and Triassic of the southern North Sea, v. 6. In: Knox, R.W.O’B.&

Cordey, W. G. (Eds.): Lithostratigraphic nomenclature of the UK North Sea: Nottingham, British Geological Survey, 141 p.

Tucker, M.E. 1991. Sequence Stratigraphy of carbonate-evaporite basins: models and application to the Upper Permian

(Zechstein) of northeast England and adjoining North Sea. Journal of the Geological Society, 148, 1019-1036.

Underhill, J.R., Lykakis, N. & Shafique, S. 2009. Turning Exploration Risk into a Carbon Storage Opportunity in the UK

Southern North Sea. Petroleum Geoscience, 15, 291-304.

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