Petroleum Systems of the Central Atlantic Margins, from Outcrop and Subsurface Data

Wach, GrantDalhousie University1355 Oxford StreetHalifax, Nova Scotia, Canada, B3H 4R2e-mail: Grant.Wach@dal.ca

Pimentel, NunoCentro de Geologia, Faculdade de Ciências da

Universidade LisboaCampo Grande C-61749-016 Lisboa, Portugale-mail: Pimentel@fc.ul.pt

Pena dos Reis, RuiCentro de Geociências, Faculdade de Ciências e

Tecnologia da Universidade de CoimbraLg Marquês de Pombal3000-272 Coimbra, Portugale-mail: penareis@dct.uc.pt

Abstract

Coastal exposures of Mesozoic sediments in theWessex basin and Channel subbasin (southern UK),and the Lusitanian basin (Portugal) provide keys to thepetroleum systems being exploited for oil and gas off-shore Atlantic Canada. These coastal areas havestriking similarities to the Canadian offshore regionand provide insight to controls and characteristics ofthe reservoirs. Outcrops demonstrate a range of deposi-

tional environments from terrigenous and non-marine,shallow siliciclastic and carbonate sediments, throughto deep marine sediments, and clarify key stratigraphicsurfaces representing conformable and non-conform-able surfaces. Validation of these analog sections andsurfaces can help predict downdip, updip, and lateralpotential of the petroleum systems, especially sourcerock and reservoir.

Introduction

Outcrops from analogous outcrop sections alongthe UK and European margins may provide new playopportunities when the petroleum systems of the Cen-

The Wessex and Channel basins lie on Paleozoicbasement deformed during the Variscan Orogeny,which culminated during the late Carboniferous. Inht 2

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Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems 1

tral Atlantic margin are explored and developed(Fig. 1). These outlier basins typically are marked bysignificant unconformities that can mark key intervalsfor reservoir generation and reservoir distribution, andhiatal surfaces that may provide key data on condensedand sediment starved intervals. These basins have rela-tively complex geological histories, multiple sources ofsediment input, source rock analogs, and variable dep-ositional settings. The Wessex-Channel basins ofsouthern England and the Lusitanian basin of Portugalprovide excellent outcrops to examine these intervalsand develop new concepts that can be applied to theWestern Margin where there are wells and significantproduction, but no outcrops of producing intervals(Fig. 1).

early Mesozoic, north-south extension created a seriesof east-west faults possibly related to reactivation ofVariscan thrusts. Extensional activity lasted until theend of Barremian times and possibly well into theAptian, while thermal relaxation continued until theend of the Cretaceous.

The Lusitanian basin is an epeiric basin on thecoastal areas and offshore western Portugal. It isbounded to the east and west by emergent Paleozoichighlands that provide the source of siliciclastic sedi-ments to the basin. The basin provides good outcropanalogs for a range of depositional environments rang-ing from fluvial to estuarine mixed sediments andcoastal platform carbonates.

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Background and Previous Work

Srivastava and Verhoef (1992) establishedboundaries for the basins of the Central Atlantic thatremain valid today with minor modifications. Hiscott etal. (1978) helped to establish the tectono-stratigraphicevents between the Moroccan margin and easternNorth America and determined that rates of sedimenta-tion were relatively constant along the basin marginsbut there was some increase in rifting along the New-foundland margin (Hiscott et al., 1990). Sinclair et al.(1994) and Shannon et al. (1995) examined wells fromthe Jeanne d’Arc basin offshore Newfoundland withthose of the Porcupine and Moray Firth basins with the

objective of determining similarities in basin fill andthe tectonic controls on reservoir architecture. Wu's(2007, 2013) work on crustal structure of the Scotianmargin correlated the refraction seismic data from theScotian and Moroccan margins, noting the similaritiesand key differences between the margins. The Channeland Wessex basin outcrops have been investigated inseveral studies (e.g. Channel basin - Ruffell and Wach,1991; Wessex Basin - Hesselbo et al. 1990). Lusitanianbasin studies include those of Cunha and Pena dos Reis(1995), and Dinis et al., (2008 and references therein).

Petroleum Systems

An understanding of basin evolution is crucial todeciphering the petroleum systems and controls on sed-imentation and the depositional history of the Eastern

margin basins. Regional tectonic events coupled witheustatic variations had direct impact on the petroleumsystems.

Wessex and Channel Basins

Three major fault zones divided the Wessexbasin into five subbasins, including the southernmostChannel basin (Fig. 2). Lower sea level in the latestJurassic to Early Cretaceous created two depocentersseparated by the London Brabant massif but withslower sedimentation rates in the northern basins.Within the basin there were minor unconformities andnon-sequences due to eustatic changes and variable

rates of local tectonic and regional tectonism. Thesewere superseded by a major unconformity cutting theMesozoic section in southern England associated withlater Cimmerian tectonism; the unconformity formedin a late extensional setting. The deposition of theAptian-Albian Lower Greensand in southern Englandmarked the end of the late Cimmerian event.

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Bray et al. (1998) recognized four major heatingevents in the Wessex basin complex (Fig. 2): mid-Tri-assic to early Jurassic, early Cretaceous, mid-Tertiary,and late Tertiary. Due to basin tectonics (periods ofuplift and burial), maturity levels vary within differentsubbasins of the Wessex. There are two potential(depending upon maturity) Jurassic source rocks; thelower Oxfordian Clay and Kimmeridge Clay, and aproven source rock in the Lower Jurassic Lias Group.The Kimmeridge Clay consists of the Type II and IIIkerogen whereas the Oxfordian and Lower Lias con-sists of Type II, III, and IV (Ebukanson and Kinghorn,1985). The Kimmeridge and Oxford Clay have hightotal organic carbon (TOC) content (up to 20%) how-ever these rocks are not sufficiently buried to becomemature (Farrimond et al., 1984 cited in Underhill andStoneley, 1998). TOC values in the black shales of the

Lower Lias have values recorded up to 8% in a Type IIkerogen (Ebukanson and Kinghorn, 1985).

The Lias Group shows large variations in matu-ration along the basin due to regional tectonics andsubsequent compartmentalization of the hydrocarbonsystems. This compartmentalization is apparent in theKimmeridge-5 well which experienced source rockgeneration in the early Cretaceous to mid-Tertiary(Bray et al., 1998). It is during this interval within theearly Cretaceous that these hydrocarbons entered theoil window; i.e., the critical moment (Fig. 2). Underhilland Stoneley (1998) suggest that peak generation isaround middle to late Cretaceous. Other areas of thebasin, for example the Wytch Farm Block, does notreach maturity at any time during the Jurassic and earlyCretaceous due to shallow burial.

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Reservoir rock

Within the Wessex basin, siliciclastic units havethe highest reservoir rock potential (Underhill andStoneley, 1998) having high primary porosities, highnet:gross values, and sufficient lateral extent to holdcommercial oil accumulations. Examples of potentialand producing reservoirs are the lower Triassic Sher-wood Sandstone with variable porosity that are faciesdependent: fluvial channel sands (6-18%), sheet flooddeposits (14-22%) and eolian sandstones (14-27%).The thick, fine-grained early Jurassic Bridport Sandshave porosities up to 15% based on outcrop exposure

and up to 32% in the Wytch Farm field based on coredata.

There are other units having higher reservoir riskdue to the reduction in permeability, limited lateralextent (e.g., Permian eolian sandstone), or a high con-tent of fine-grained material (e.g., ThorncombeSandstone), or the fractured mid-Jurassic Frome Clay.Some carbonate units also act as good reservoirs whensecondary porosities are created by fracturing and/ordissolution of cement. These include the Middle Juras-sic Inferior Oolite and Portland Limestone.

Trap

The traps in the Wessex basin were primarilystructural rather than stratigraphic and can be dividedinto two major tectonics events: extensional in theMesozoic and compressional (basin inversion) duringthe Cenozoic. Extensional faulting began during thePaleozoic Variscan fold and thrust belt, progressing tothe Late Cretaceous (Underhill and Stoneley, 1998).The extensional tectonics subdivided the basin intoseveral fault blocks, tilting the strata, and thus creating

potential traps, similar to fields on the Grand Banks.Hydrocarbon exploration shifted to these buried andtilted extensional blocks which are related to the struc-tural plays in the Wytch Farm and Wareham oil fields,whereas structures related to the Cenozoic-based inver-sion are periclinal traps developed from the lateCretaceous to early Cenozoic (Underhill and Stoneley1998).

Seal

Most of the reservoir rocks are sealed by thepresence of thick shale intervals or by thin, imperme-able layers. Some examples of these are the TriassicAylesbeare Mudstone Group overlying Permian eoliansandstone, Mercia Mudstone overlying the SherwoodSandstone, and potentially the Kimmeridge Clay over-

reservoirs. In these cases, lateral traps which developedare dependent on lithology, thickness (sand:shale ratio),and the amount of fault displacement. Inversion couldinitiate remigration of hydrocarbon into shallower res-ervoirs, but drilling to target these plays suggests thefaulting (which acted previously as conduits for theht 2

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lying the lower Jurassic reservoirs. Structural tilting ofthe blocks during extensional tectonics (Permian toCretaceous) and later by structural inversion (Ceno-zoic) may have contributed to the sealing of the

migration of hydrocarbon) became barriers or seals as aresult of compressional forces (Selley and Stoneley1987) and perhaps cementing of fault traces throughdiagenesis.

Portuguese Margin

Portuguese margin basins

The Lusitanian basin is bounded to the east andwest by emergent Paleozoic highlands that provide thesource of siliciclastic sediments to the basin (Fig. 3).The Berlengas highlands separate the Peniche basinoffshore, to the west from the Lusitanian basin. ThePeniche basin is open to the Atlantic, has no wells, and

has limited proprietary seismic data in the region. It isexpected that the geology progresses from non-marineand littoral environments to shelf, slope, and basinfloor settings. There may be similar basins outboard,west of the Peniche basin.

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Source rock and maturity

The Lusitanian basin geological record containsdifferent units having source-rock potential, includingbasement Paleozoic deep-marine black-shales, and tur-bidites, with hydrocarbon generation and migrationinto Mesozoic reservoirs (Uphoff, 2005; Pena dos Reisand Pimentel, 2010a, 2010b). Lower Jurassic (Sinemu-rian-Pliensbachian) marls and Upper Jurassic(Oxfordian) laminated marly limestones are the twomajor units having high TOC; however, Hettangian(Dagorda Formation), Kimmeridgian (Abadia Forma-tion), and Cenomanian-Turonian (Cacém Formation)should also be considered (Spigolon et al., 2011).

The Lower Jurassic marine marly black shales(Duarte and Soares, 2002) up to one hundred metersthick, are the lower part of the Brenha Group, whichextends across the basin and contains a highly variableTOC (up to 22.5 wt%; Duarte et al., 2012) of kerogenTypes I-II (Spigolon et al, 2010). These homoclinalcarbonate ramp deposits dip to the northwest and arerelated to the initial opening of the Lusitanian basin tomarine influences. Two significant organic-rich maxi-mum flooding surface intervals (Água de MadeirosFormation and Vale das Fontes Formation) have beenstudied in detail, regarding their TOC values, isotopes,palynofacies, etc. (Duarte et al., 2010, 2012; Silva etal., 2010, 2011; Poças Ribeiro et al., 2013).

The Upper Jurassic source rock (Cabaços Forma-tion) is composed of marly limestones deposited inlacustrine to lagoonal and coastal environments(Spigolon et al. 2011; Silva et al. 2013). TOC values indarker layers usually range from 2 to 5 wt%; kerogen

widespread basin-wide, and have no preferential sec-tors (Silva et al., 2013).

Maturation of both source rocks has been mod-eled, based in lithology and thickness well data,calibrated by vitrinite reflectance data (Teixeira et al.,2012, 2014). Both Jurassic source rocks have attainedthe hydrocarbon generation window, although noteverywhere in the basin as a result of the highly hetero-geneous basin’s subsidence and overburden, especiallyin the Late Jurassic.

Non-mature Lower Jurassic source rocks areknown in outcrop, namely at the Peniche, Montemor-o-Velho and São Pedro de Muel sections (Oliveira et al.,2006; Silva et al., 2010; Spigolon et al., 2011; Duarteet al., 2012), but the same units have reached maturityin several exploration wells in the basin (Teixeira et al.,2012, 2014). Also, non-mature Upper Jurassic source-rocks are known in different outcrops, such as CaboMondego or Montejunto (Spigolon et al., 2011),whereas they reached the oil window in nearby wellssuch as SB-1, FX-1 and CP-1 (Teixeira et al., 2012,2014). This situation points to a very important role ofdifferential subsidence along the basin, both in timeand space.

As a general statement, it may be considered thatthe Lower Jurassic source rock is mature for oil in thenorth sector of the basin and mature for gas in the southsector, whereas the Upper Jurassic source rocks may benot mature in the north sector and are mostly mature inthe south sector (Teixeira et al., 2012, 2014).

The different geological setting of these twoht 201

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types are variable, but there is a predominance of TypeII-III. Their deposition overlies a major regional Callo-vian unconformity and records the early syn-rifttransgressive interval during the Oxfordian. The pre-vailing paleogeographic conditions were controlled bya north-northeast/south-southwest oriented depression,fed by a fluviodeltaic network from the north-north-east, towards the deepening areas developing more tothe south-southeast (Pena dos Reis et al., 2011). Therichest intervals are not strictly synchronic, but they are

Jurassic organic-rich intervals, has generated geochem-ically distinct types of hydrocarbons. Therefore,detailed studies of oil seeps and oil shows haverevealed the presence of mature oils from both sourcerocks, in different depositional and tectonic settings.Good examples are the presence of Lower Jurassicrelated oils identified in Cretaceous sandstones close todiapir walls at Praia da Vitória and Leiria, as well asUpper Jurassic related oils identified in Oxfordianlimestones close to a diapir wall at Torres Vedras(Spigolon et al., 2010).

Reservoir rock

Several siliciclastic and carbonate units havegood reservoir potential in the Lusitanian basin. Theseinclude Late Triassic alluvial-fan to fluvial red beds(Silves Group), Middle Jurassic (Candeeiros Group)

and Late Jurassic (Montejunto Formation) fracturedcarbonates and biohermal build-ups, Late Jurassic tur-biditic (Abadia Formation), and fluviodeltaic(Lourinhã Formation) sandy lobes and channels. More-

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over, the sedimentary infill of the basin presents at itsuppermost section abundant Cretaceous fluvio-estua-rine sandy deposits (Torres Vedras Group) related tothe breakup unconformities (Dinis et al., 2008).

These Cretaceous units have very good reservoirproperties but have been disregarded as a potential res-ervoir due to their high stratigraphic position and lackof apparent seal. However, these same units areexpected to be present in offshore areas, namely in thedeep offshore Peniche basin where the more distallocation could provide adequate seal facies. Currentexploration in these areas make it increasingly import-ant to understand the reservoir potential. Twoanalogous outcrops have been studied in detail tounderstand facies relationships and heterogeneities –the Ericeira outcrop, 35 km northwest of Lisbon, andthe Crismina outcrop, 30 km west of Lisbon.

The outcrop at Ericeira lies along the westernpart of the outcropping Lusitanian basin. The succes-sion comprises the unconformity-bounded surface ofLate Aptian age (Rey et al., 2006). This unconformityis marked by the abrupt shift from highstand carbon-ates in the Aptian (Crismina Formation) to terrestriallow gradient coastal sediments (Rodízio Formation)comprising red and grey mottled silt and clays and flu-vial channels consisting of fine-grained to pebble-sizedsediments. The coarser grain size may reflect reactiva-

tion of faults along the western margin of the basin(Dinis et al., 2008); basin margin syndepositional faultsare visible in the Ericeira section. The successionabruptly fines up into medium to dark grey shalesbefore the resumption of small channel deposition,marking a forced regression at the base of the Albian.

The section at Crismina, is located along thesouthwestern edge of the Lusitanian basin. The sectionis similar to Ericeira, also representing the Late Aptianbreakup unconformity, but between more distal faciesof the two aforementioned formations (Rey et al.,2006). The outcrop shows an abrupt transition fromshallow marine and lagoonal carbonates in a highstandduring which there was limited siliciclastic sedimentinput to the basin. There is an erosive surface and anabrupt transition to clean white quartzose sands depos-ited in estuarine conditions, with evidence of subtidalchannels and barforms. The succession rapidly shal-lows, and coarser grained sediment and trough crossbedding reflect further progradation of sediment intothe basin and deposition of fluvially derived sedimentssourced from the Paleozoic highlands that bounded thebasin to the west (Dinis et al., 2008). These highlandsseparate the Peniche basin from the most southern sec-tors of the Lusitanian basin (south of the Lousã-Caldasfault) and their erosional remnants are expressed todayas the Berlengas Islands.

Trap

The traps in the Lusitanian basin are predomi-nantly structural. The Late Jurassic rifting phase andthe Late Cretaceous–Eocene alpine inversion, followed

capping units, is likely to have trapped late Jurassichydrocarbons in fractured limestones, and coarse-grained Kimmeridgian turbidites. Salt movement alsoht 2

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by the Miocene Betic inversion due to Iberia-Africacollision, caused a significant structural complexityand created a tectonic block puzzle and both local sub-sidence and later uplift with movement of salt. Thisstructural framework allowed the contact of differentsource rocks, migration pathways, and reservoir units,sometimes not completely sealed. Some major folding,induced by deep salt doming combined with clay-rich

has had an important role in hydrocarbons accumula-tion, focusing the vertical migration of Early Jurassichydrocarbons upwards into Cretaceous siliciclasticsaround the diapirs. These traps are proven by somehydrocarbons staining and fracture oil seeps associatedwith salt walls related to reactivation of deep basementfaults.

Seal

Fine-grained clays and evaporates of the Het-tangian Dagorda Formation (Palain, 1976) and thickclays of the Maastrichtian Taveiro Formation (Pena dosReis, 2000) are certainly the most effective seals in thebasin. The Dagorda Formation consists of compactedred clays containing variable amounts of gypsum,

halite, and dolomite, up to hundreds of meters thick. Itis intensely deformed in areas closer to basementfaults. The clayey Taveiro Formation, up to 200 mthick, includes sandstone and carbonate, capping theMesozoic areas north of the Lousã-Caldas fault.

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There are several other clay-rich units, but theyrarely act as a perfect seal due to their frequent interca-lation with sandy layers as is the case of the UpperJurassic and Lower Cretaceous units. However, Upper

Cretaceous and Cenozoic shale units may have acted asimportant seals, particularly in more subsiding or distalareas of the basin.

Newfoundland Margin–Grand Banks

Recent discoveries by Statoil in the deep-waterbasins of the Flemish Pass have expanded the explora-tion potential and proven viable petroleum systemsoutside the traditional areas of the Grand Banks whereexploration success has driven development for nearlyfour decades (Fig. 4). The Jeanne d’Arc Basin includesproduction from the Hibernia, Terra Nova, White Rose,and Hebron fields. Hebron has been the latest fielddevelopment with estimates of three billion barrels inplace but of a heavier grade oil (21 degree API)(Enachescu 2006). Baur et al. (2009) suggest that theJeanne d’Arc basin formed as failed rift basin, first by aLate Triassic event and then slow stretching from theLate Jurassic though to the Early Cretaceous, althoughthe latter did not have a significant impact on thermalmaturity. Immediately to the south of the Jeanne d’Arcbasin are the Carson basin and to the east of the Carsonis the Salar basin on along the eastern edge of the

Grand Banks. These basins lie on the present-day slopeof the Grand Banks and began as a network of inter-connected rift basins (Enachescu, 2006), that formedwith the opening of the Atlantic Ocean and break-up ofPangea in the early Mesozoic.

The Mesozoic and Cenozoic sedimentary fill ofthe Carson basin on the eastern Grand Banks of New-foundland has been penetrated by four wells. Wielenset al. (2006) indicate that the Salar and Carson basinsare underlain by thicknesses of earliest Jurassic salt(Argo Formation), thicker on the on the western mar-gins of the basins. They recognize that stratigraphicunits could be correlated between the Carson andJeanne d’Arc basins and that there are clear eustaticoverprints across both basins. In the Jurassic, deposi-tional conditions are inner neritic to marginal marinebased on palynomorph, foraminiferal, and ostracoddata.

Source rock, maturity and migration

In the Jeanne d’Arc basin the Egret Member ofKimmeridgian age is a mature marine source rock con-taining some terrigenous material and having TOCvalues between 4-6% and Hydrogen Index from 100-610 mgHC/ gTOC (Baur et al. 2010). Modelling of the

leum systems of the Mara, Hebron, Ben Nevis,Springdale, and White Rose fields. In the Carson basin,the Mesozoic section includes reservoir and seals butsource rocks similar to the Jeanne d’Arc Basin have notbeen proven (Wielens et al., 2006). Bauer et al. (2009)ht 2

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petroleum systems of the Terra Nova oil field suggests,rather than one source from the Egret Member as previ-ously thought, that there is potential for an additionalkitchen area between the Terra Nova and Hibernia oilfields. This could impact understanding of the petro-

modeled a source rock in the deeper regions of the Car-son basin that would be equivalent to the Egret in theJeanne d’Arc basin and postulate that hydrocarbonscould be generated and be trapped in Lower Cretaceousor Cenozoic reservoirs.

Reservoir rock

Siliciclastic deposition from highlands border-ing the Jeanne d’Arc basin are the source of themajority of the reservoir prone sediments, which haveporosities ranging between 15-25% and permeabilitiesto 100md (Baur et al., 2009). The formation of trapswas during the Berriasian (140 ma) during depositionof the Hibernia Formation; a second phase occurred

during the Paleocene (53 ma) followed by a constantpattern of migration for the remainder of the Cenozoic(Baur et al., 2009).There are several reservoir and sealpairs. The most effective seal, the Fortune shale unit,deposited during the latest Tithonian, represents atransgressive succession, capping the coarser grainedJeanne d’Arc reservoirs.

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Trap

The formation of fault-bounded anticlinal trapswas during the Berriasian (140 ma) during depositionof the Hibernia Formation; a second phase occurred

during the Paleocene (53 ma) and then with a constantpattern of migration for the remainder of the Cenozoic(Baur et al., 2009).

Seal

There are several reservoir and seal pairs. Themost effective seal, the Fortune shale unit, depositedduring the latest Tithonian, represents a transgressivesuccession, capping the coarser grained Jeanne d’ArcBasin siliciclastic reservoirs.

Enachescu (2006) reports potential petroleumplays in sandstone trapped in fault blocks of the Jeanned’Arc, Hibernia, and Avalon/Ben Nevis units. TheJeanne d’Arc Formation sandstones have been depos-ited during Late Kimmeridgian-Tithonian in stacked

incised valley systems and inner neritic environment.This play has been successful at the Terra Nova andHebron fields and it is expected to extend to other areasin the south-eastern Jeanne d’Arc basin. Oil pay isfound in the Hibernia sandstone reservoir in several ofthe fields surrounding the parcels. Similarly, Avalonand Ben Nevis sandstones contain oil and gas in sev-eral of the fields, and petroleum potential is alsorecognized in the Upper Cretaceous- Late Cenozoicsandstone members.

Scotian Basin

The Mesozoic to Cenozoic Scotian basin was ini-tiated during the Triassic syn-rift to lower Jurassicpost-rift phase on the Atlantic margin; terrestrial silici-clastic sediments and evaporites mark this phase (Fig.5). In the Middle Jurassic, the Abenaki carbonate plat-form developed as an enigmatic succession of platformcarbonates juxtaposed with sands and shale of theSable Delta complex (Eliuk and Wach, 2008). Themajority of the succession is a passive margin basin fillof sand and shale sequences deposited in response toglobal relative changes in sea level. In the later Jurassicand Cretaceous, the Sable and Laurentian deltas pro-duce transgressive and regressive packages of deltaic,

scheduled to end later this decade. The Encana DeepPanuke project (2013) has begun production of gasfrom the Late Jurassic Abenaki carbonate margin andfull production is scheduled for the year-end. A third isthe light oil and condensate production from deltaicand shallow marine reservoirs in the now decommis-sioned Lasmo (later PanCanadian-Encana) Cohasset-Panuke project (1992-99); Panuke directly overlies theDeep Panuke field.

The western Shelburne subbasin is located indeep water and remains essentially untested for hydro-carbons, with but a single well drilled to test a shallow(Cenozoic) stratigraphic anomaly on the deep waterht 2

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shelf margin, and slope deposits (Wade and MacLean1990; Kidston et al., 2002).

There are two active petroleum systems currentlyproducing in the Scotian basin, both gas prone. Thefive field ExxonMobil Sable project (1999-present)produces from siliciclastic deltaic and shallow marinereservoirs having some condensate; production is

Scotian Slope. An untested delta prism is interpreted indeep water on the western margin (Wade and MacLean,1990). The subbasin is on track for future exploratorydrilling following completion by Shell (2013) of a large

(~8100 km2) regional WAP 3D seismic program.

Source rock and maturity

Precisely identifying all source rocks on the Sco-tian Margin is a significant challenge with a mix ofknown and suspected intervals (OETRA, 2011). TheLate Triassic rift syn-rift successions remains unprovenbut hold potential for Type I kerogens from lacustrinesources facies. Similarly, there are thick, late pre-riftevaporate deposits (Hettangian Argo Formation) that in

addition to creating numerous structural configurationsand traps may cap a hypersaline shallow marine suc-cession (Type II). A lower Jurassic (Pleinsbachian-Toarcian) earliest post-rift marine source rock (Type II)is postulated but remains unproven. Potential exists fora Middle Jurassic Verrill (Callovian) source interval(Type II-III), although due to limited data its extent and

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thickness is unknown. Better known is the Late Juras-sic to Late Cretaceous Verrill Canyon Formationassumed to be the source of most hydrocarbonsencountered so far (gas). Its main source intervals arein the Tithonian (Type II-III), Valanginian (Type III),

and Aptian (Type III); terrigenous detritus from theSable Delta provides significant organic content. Notethat the source of the high gravity Cohasset-Panukeoils, and those of other oil shows and discoveries (e.g.,Penobscot field), remain unknown.

Reservoir distribution

Understanding the linkages between shelf sedi-ment capture/delivery, the role of shelf margin deltas,sea level, and slope processes is critical to detectingreservoir rock distribution in deep and ultra-deep water(Mosher et al. 2010). Thick Jurassic and Cretaceousfluviodeltaic to shallow marine sands of the Mic Mac,Missisauga, and Logan Canyon formations are com-mon on the Scotian Shelf, and depending on locationhave very high sand:shale ratios. Better ratios are foundin the middle and distal portions of the respective for-mations. The deep water Scotian Slope demonstrates a

history of canyon and channel cut and fill and sedimentmass transport. Significantly, results from a recent(2000-2004) phase of deep-water exploration (2D and3D seismic) and drilling has confirmed the margin is aby-pass zone, as coarse siliciclastics have been trans-ported into the salt-dominated region down slope(Kidston et al., 2007; Deptuck, 2010; 2011a; 2011b).These processes link to relative sea level in combina-tion with sediment volume, seismicity, and othercausative factors.

Trap

Although the Sable Delta represents an activepetroleum system associated with many significant gasshows in the Upper Jurassic to Lower Cretaceous delta,economic hydrocarbon accumulations have been lim-ited (structural trap risk). Incised valleys, cut into shelfdeposits during sea level lowstands, are recognized on3D data and indicate potential reservoirs when cali-brated to well data. The dominant (and mostsuccessful) trap styles are rollover anticlines in which

sand-shale ratios range from 15-30% and there is lim-ited crestal faulting (Richards et al., 2008). Potentialexists for stratigraphic traps (but seal risk is high due toan overall high net-to-gross section). Several trapsstyles are set up by the movement of the underlying lat-est Triassic-earliest Jurassic salt, particularly in deepwater, but only a few have been tested and most by asingle well: only one subsalt play has been drilled to-date.

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Within the Sable subbasin, variable scale (fieldto regional) transgressive events developed “tongues”

of the Verrill Canyon Formation shales that provideexcellent seals to the reservoirs in the Sable Delta.

Discussion

Reservoir distribution

Shelf margin deltaic sequences are difficult tocorrelate in the subsurface (Fig. 6). Delta lobe switch-ing contributes to stratigraphic complexity. Numerouspermeability baffles and barriers create complex reser-voir heterogeneities. For example, an extensivenetwork of non-marine channels and incised valleyscut into deltaic and shelf deposits during the fallingstage and lowstand systems tracts at multiple strati-graphic levels in the Sable Delta complex.Progradation of the Sable Delta to the shelf edge is

constrained by localized accommodation controls fromdifferential mobilization of underlying salt. Shelf mar-gin sediments can be trapped at the margin or maycontribute directly to downslope fans.

A significant issue in recent hydrocarbon explo-ration in the deep water on the Scotian and Moroccanmargins is the detection of reservoir rock. Existingmodels of deep-water sedimentation have underesti-mated the links between shelf and slope sedimentationand the roles of sea level, salt tectonism, and canyon

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formation as sediment transport pathways. Mass failureand along-slope sediment transport processes are alsosignificant processes in passive continental margindevelopment. The consequence of these sedimentaryprocesses is the inherent complexities of shelf to slopesedimentation patterns and movement of potential res-ervoir rock to greater depths than previouslyanticipated.

In the middle of the Cretaceous, rifting slowed orceased and wider continental shelves developed. Lat-eral facies relationships along these shelf systemscould create stratigraphic pinch-outs to reservoir conti-

nuity in addition to faults that could act as transmissiveconduits or barriers, creating further reservoir compart-mentalization. Condensed sections and transgressiveintervals of shales, diastems, hiatal surfaces, firmgrounds, and hard grounds (Ruffell and Wach, 1998)formed that created potential significant barriers andbaffles to permeability and overall reservoir perfor-mance. These surfaces were apparent in outcrops in theWessex and Channel basins, in the Jeanne d’Arc, andeven in the mid-Cretaceous oil sands of WesternCanada.

Tectonic influences on sedimentation, migration and maturation

Sinclair et al. (1994) and Shannon et al. (1995)demonstrate there are tectonic influences on basin infilland reservoir architecture. In the Wessex basin, thereare minor unconformities and non-sequences that aredue to eustatic changes and variable rates of local tec-tonic subsidence. These subsequently have beensuperseded in the Late Jurassic and Early Cretaceoustimes by a major unconformity associated with lateCimmerian tectonism, cutting the Mesozoic sequencein southern England, the Lusitanian basin, GrandBanks, and the Scotian basins. This period of extensiveerosion is referred to as the late Cimmerian unconfor-mity. The late Cimmerian unconformity has formed inan extensional setting by the combined effects of iso-static footwall block uplift and a contemporaneouseustatic lowering. This has produced a syn-extensionalor early post-extensional isostatic disequilibrium thatcan be recognized throughout southern England. Only

western margins of the Wessex basin, oversteppingprogressively older sediments. In turn, the LowerGreensand is succeeded by the Gault and Upper Green-sand. The Gault marks the second mid-Cretaceousmarine transgression. The dark grey mudstone of theGault oversteps the Lower Greensand to lie uncon-formably on Lower Paleozoic strata of the Londonplatform. The Portsdown anticline and to a lesserdegree the Isle of Wight fault act as structural controlsto sedimentation and have restricted the influence ofthe Boreal Sea from the north and the Tethys Sea to thesouth and east. Abundant Upper Jurassic clasts in peb-ble beds of the Lower Greensand Group suggestcontemporaneous erosion of shore lines along the mar-gins of the Cretaceous depositional basin (Ruffell andWach, 1991). In the Lower Cretaceous, no sediments ofpre-Albian age are preserved north of the fault whereAlbian Carstone sediments rest unconformably onht 2

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in small areas of rapid basin subsidence are the effectsof the unconformity minimized, but the extent of this isnot clearly known.

The deposition of the Lower Greensand in south-ern England marks the end of the late Cimmerianevent. Only in areas of rapid crustal subsidence are theerosional effects minimized. These areas are the centralpart of the Weald and Channel basins (Chadwick,1986). The Lower Greensand thins and pinches out tothe north against the London platform and along the

Jurassic. The Late Cimmerian unconformity was com-

pared between the Jeanne d’Arc Basin of the GrandBanks and the Outer Moray Firth in the North Sea bySinclair and Riley (1995). Early in the exploration ofthe southern Grand Banks, dry wells in the basin wereattributed to an unproven source rock and breaching ofthe traps at the basal Aptian (Avalon) unconformity(Enachescu, 2006).

Basin accommodation space, shifting depocenters and inversion

In the outlier basins examined in this study, thereis evidence of shifting basin depocenters initially con-trolled by local and regional tectonic events. Early riftbasins are filled with evaporite and red shales (Fig. 6).

As basins fill, eustatic controls overprint the tectonicevents, accommodation space is diminished, and depo-sitional environments reflect slower rates of sedimentinflux into a basin, often associated with erosion and

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denudation of surrounding highlands sourcing sedi-ment into the basin. At the end of the basin cycle, thestudied basins have been inverted in response to Alpineorogenic activity and compression along the easternAtlantic margin, particularly during the Oligocenethrough Miocene. It is interesting that in the latest Cre-taceous through early Cenozoic there is thermalevidence of uplift along the Scotian margin (Grist etal., 1995; Grist and Zentilli 2003).

Depocenters “shift” through the stratigraphicsuccession in response to tectonic and eustatic controlscontrolling accommodation space within the basins.Identifying the primary and secondary controls withina basin on sediment influx and distribution can help inthe prediction of sediment conduits and fairways forthe distribution of reservoir quality sediments.

During early Wessex Basin development, the rateof Permo-Triassic sedimentation kept pace with base-ment subsidence (Chadwick, 1986). Compactionbecame a factor later in the basin development as load-ing allowed sediment accommodation to exceed therate of basin subsidence. For example, fluvial condi-tions were maintained during the deposition of theWealden Group in the beginning of the Lower Creta-ceous, despite rapid basin subsidence because ofabundant sediment supply from the erosion of the prox-imal massifs.

Lowering of sea level in the latest Jurassic toEarly Cretaceous created two distinct depocenters sep-arated by the London-Brabant massif. The northernbasin was characterized by relatively slow subsidenceand low rates of sediment infill. In contrast, southern

The Variscan fold belts across the Wessex basinbegin as thrust or reverse faults (Whittaker, 1985). Thedepth to the top of the Variscan basement on the Isle ofWight ranges from 1400-1600m north of the mono-cline, to 2000m on the southwestern side, deepening to2200m on the central and southeastern area of theisland. This deepening of accommodation space to theisland’s south-central area coincides with the thickestdeposit of Lower Greensand sediment in this region.

A half-graben may have formed during thePermian (Chadwick, 1986) to late Cretaceous. TheChalk facies shows evidence of reworking on the mar-gins of the Central Downs and possible hard groundsnear the top of the strata, in the middle of the CentralDowns; combined with thinning of the Chalk strataacross the Central Downs, this suggests syntectonicactivity. Movement is concentrated during the Meso-zoic with an interval of relative quiescence during themid to late Cretaceous. Mesozoic movement was fol-lowed by inversion at the end of the Cretaceous(Stoneley, 1982), which resulted in further develop-ment of steeply folded strata as a result of movementalong pre-existing structures in the Paleozoicbasement.

The basins distributed along the conjugate mar-gin of the Central Atlantic, although with similarities,also demonstrate some marked differences; for exam-ple the location of the (relatively younger) AtlasMountains along the northwest African coast comparedto the Appalachians. What is the significance of thegreater distance from North America to the Mid-Atlan-tic Rift compared to the distance from Africa and theht 2

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England received significant sediment supply from theerosion of nearby massifs, and coupled with rapid sub-sidence rates, the basin was rapidly filled.

breaks with the anomalies on both sides of the Atlan-tic?

Evidence of active petroleum systems–Oil seeps on basin margins

Oil seeps within the Lusitanian and Wessexbasins are stratigraphically located in the upper Meso-zoic, usually Cretaceous and Cenozoic strata. Theaccumulations and active and paleo-oil seeps reflectpast and present petroleum systems that appear to havemigration pathways associated with faults bounding

the basin margins. Seeps are present in Cretaceous sed-iments at Mupe Bay and in Cenozoic sediments atHenigstbury Head. In the Lusitanian basin, there arepaleo- or active oil seeps at Vale Furado (Nazaré) andLeiria. Copyri

g

Conclusions

The key elements in producing effective petro-leum systems (Fig. 6) in the Central Atlantic conjugatemargins are the presence of source rock and reservoir.Trap formation and migration are less of a risk as activetectonics occurred along the margin. The Wessex(Channel) and Lusitanian basins are appropriate out-crop analogs for basins along the Atlantic margin thathave relatively complex geological histories, multiplesources of sediment into the basin, and restricted depo-sitional settings to deeper marine settings. TheLusitanian basin is a good analog for basins that transi-tion from continental to deep basin marine settings.There is more carbonate in the Lusitanian basin, per-haps reflecting more southern latitudes of the basinduring the Mesozoic compared to the Wessex andChannel basins. The Lusitanian basin provides analogsfor potential source rock facies, although maturity ofthese is unlikely with the burial history of the basin lessthan 500m.

Depocenters “shift” throughout the stratigraphicsuccession in response to tectonic and eustatic controlscontrolling accommodation space within these basins.Identifying the primary and secondary controls withina basin is dependent on sediment influx. Sediment con-duits and fairways control the distribution of reservoirquality sediments. Eustatic controls in the Portugalbasins appear to be less of a factor compared to otherbasins along the margins, perhaps due to a greater tec-tonic overprint.

What new exploration concepts and play typesare possible? A confirmed petroleum system is present

be sourced from updip deltaic systems; e.g., the Sableand Morocco delta systems. These deeper water reser-voirs are often encased by excellent seal rocks andwithin the fetch of condensed sequences that may formpotential source rocks.

Deciphering forcing functions, sediment path-ways and depositional processes will improveexploration models for passive clastic margins and sug-gests that exploration must move to deeper waterwhere shelf-equivalent rocks are transported anddeposited. These constraints can be expected in similardepositional settings in other margin basins. If there issalt influence in the basin, new play concepts can begenerated with new trap configurations and the poten-tial for trapping sediment.

There is a need to define basin-wide unconformi-ties with greater precision. A number of unconformitiesare not resolvable on seismic and may be interpreted asone unconformity. These unmapped unconformities aresignificant and mark the potential for downdip trans-port of reservoir quality sediments in to the basin.Seismic data can help predict reservoir but seldom aidsin defining reservoir quality. There is growing ten-dency for plate reconstructions to rely solely onsubsurface data, particularly seismic and derivatives ofthe seismic data used for modeling. John Dewey (1983)reflected on his research and attributed many of theconcepts he developed on plate tectonics, to outcropstudies he completed in Ireland and Nova Scotia. Wepropose that we need to keep looking at the rocks to beable to discern many of the complexities along the mar-ht 2

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in the Flemish Pass basin on the Newfoundland Marginwhich should decrease exploration risk in the adjacentOrphan basin to the north. Exploration potential isoften contingent on the presence of source rock. Wecan point to downdip deeper water reservoirs that may

gins. The Wessex-Channel and Lusitanian basinsprovide excellent outcrops to examine these anddevelop new analysis of petroleum systems for petro-leum exploration and development of fields and basinsof the Central Atlantic margin.

Acknowledgments

It is with great appreciation that we thank the fol-lowing individuals for discussions throughout the yearsboth in the field and at conferences, particularly theConjugate Margins conferences: David E. Brown, IanDavison, Paul J. Post, Iain Sinclair, Ian Atkinson,Michael Enachescu, Webster Mohriak, Bernardo Teix-eira, Stephen Hesselbo, and Alastair Ruffell. David E

Brown provided valuable comments on the manuscript.Carla Dickson, Trudy Lewis, Kristie McVicar, and Car-los Wong assisted with the preparation of this paper.We would like to thank Jim Pindell and Brian Horn asconference organizers and particularly Norm Rosenwho each year continues to shepherd through a timelyconference confronting key issues facing the industry.

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Figure 1. Location of several sedimentary basins formed by the rifting and sea-floor spreading that began in the Late Triassic, leading tothe opening of the Atlantic Ocean (modified from Tankard and Balkwill, 1989; Decourt et al., 2000, among others).Copyri

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Figure 2. Petroleum systems chart of the Wessex basin (based on Underhill and Stoneley, 1998; Cox et al.,1999; Hopson, 2005, among others).

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Figure 3. Petroleum systems chart of Central Portugal (based in Azerêdo et al., 2003; Rey et al., 2006; Witt,1977, Pais et al., 2012, among others).

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Figure 4. Petroleum systems chart of the Grand Banks- Jeanne d'Arc basin (based on Grant and McAlpine,1990; Sinclair et al., 1994; and Enachescu, 2006).

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Figure 5. Petroleum systems chart of the Scotian basin (after Wade and MacLean, 1990).

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Figure 6. Simplified lithostratigraphic schemes of the Scotian Shelf basins (after Wade and MacLean, 1990),Jeanne d'Arc basin (Grant and McAlpine, 1990), Central Portugal (based in Azerêdo et al., 2003; Rey et al.,2006; Witt, 1977, Pais et al., 2012, among others), and Wessex basin (based on Underhill and Stoneley, 1998;Cox et al., 1999; Hopson, 2005, among others).

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