This paper analyses the demographics of existing Un

ited Kingdom Continental

 Shelf (UKCS) fields

and discoveries as a means of

 assessing which plays are likely to offer the greatest untapped

potential for stratigraphic traps. The talk is illustrated with examples of proven and untested

stratigraphic traps

Andrews, I.J.

Cameron, T.D.J.

Gray, J.C.

Haile, P.

Stoker, S.J.

NERC Open Research Archive

THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED RESOURCES OF THE UKCSS. J. Stoker1, J. C. Gray2, P. Haile2, I. J. Andrews1 & T.D.J. Cameron11British Geological Survey2Department of Trade and IndustryIntroduction This paper analyses the demographics of existing United Kingdom Continental Shelf (UKCS) fieldsand discoveries as a means of assessing which plays are likely to offer the greatest untappedpotential for stratigraphic traps. The talk is illustrated with examples of proven and untestedstratigraphic traps.As of end 2003, 82% of the oil and gas fields and discoveries on the UKCS have been found instructural traps, 12% have been found in combination structural/stratigraphic traps and only 6% instratigraphic traps (Fig. 1). Current DTI estimates of the UKCS undiscovered reserves rangebetween 4.1 – 8.9 – 21.3 x109 barrels of oil equivalent (BOE) and are based on a prospect mappingapproach (DTI 2004). Complementary to such estimates, the discovery curve for the entire UKCSshows a maturing, but not yet fullymature profile (Munns et al. inpress).As a result of 38 years ofexploration activity, the majority ofthe structural traps have beentested in the UK North Sea.Remaining substantial structuraltraps in the UK North Sea are likely to lie at considerable depth, such that reservoir quality, high-pressure and high-temperature risks render them less attractive to exploration taking into accountcurrent technology and economic considerations. There has been relatively little direct exploration for stratigraphic traps on the UKCS. Severalunexpected major discoveries have been made in stratigraphic and combination traps whilst drillingto deeper, structural targets (e.g. Britannia, Scapa, Highlander). Difficulty in recognizing these onseismic data is a major factor in the higher level of risk and uncertainty associated with stratigraphictraps. Sophisticated seismic data analysis, in particular amplitude-versus-offset (AVO) techniques,has proved highly valuable at the development stage, but has not always been reliable toexploration, particularly in the West of Shetland area (Loizou et al. in prep.). The Buzzard Field, anUpper Jurassic stratigraphic pinch-out/dip trap discovered in 2001, is an outstanding success storycredited to application of traditional methods of seismic interpretation, leading to the development ofa strong conceptual model (Doré & Robbins in press).We have assigned the 660 fields and discoveries (including technical discoveries) found on theUKCS by end 2003 to 43 plays within 15 play groups, and further condensed them into 7 gross playsin order to determine the relative importance of stratigraphic entrapment by play. The majority ofstratigraphic and combination traps occur in association with syn-rift (Upper Jurassic) and post-riftplays. Pre-rift stratigraphic potentialStratigraphic entrapment is relatively rare in Middle Jurassic and older strata, because of the sheet-like geometry and basin-wide distribution of many of the pre-rift reservoirs. Combination traps in thepre-rift plays generally involve major erosional truncation (e.g. Auk Field). Analysis of the existingPage 1 of 5THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED R...05/01/2016mk:@MSITStore:D:\petex2004.chm::/abstracts/003003.htmpre-rift UKCS fields and discoveries shows that there are no purely stratigraphic traps, and 3% of thetraps are combination. The proportions of combination traps within the Palaeozoic, Lower Jurassic-Triassic and Middle Jurassic play groups are 2%, 4% and 3% respectively (Fig. 2). Discovery curve data for the pre-rift reservoirssuggests that the Middle Jurassic play is verymature, or ‘creamed’ (Munns et al. in press). TheTriassic-Lower Jurassic and Permian plays arerelatively mature, whilst the Carboniferous play hasa distinctly immature profile (Fig. 3).Palaeozoic reservoirs span a wide range ofsedimentary environments. Volumetrically, the mostimportant plays are fluvial in the Devonian andCarboniferous, and aeolian in the Permian.Although the future potential for non-structural pre-rift traps appears to be limited, Carboniferous playsin particular offer significant opportunities for thepresence of stratigraphic traps. In the UK SouthernNorth Sea, for example, largely untested potentialremains for exploration of Carboniferous fluvio-deltaic sandstone reservoirs in those sub-Permiantruncation traps where there is no structural closure at base Permian level, but where a Carboniferous intraformational seal performs critical lateral closure. Three important sub-regional seals have been identified within the Carboniferous of the UK Southern North Sea (Munns et al. in press). Syn-rift stratigraphic potentialUpper Jurassic syn-rift reservoirs include both shallow marine sandstones and deep-water mass-flow deposits.  Shallow marine sandstones such as the Piper and Fulmar formations are mostly disposed in structural traps (e.g.Piper and Fulmar fields), and there is only limited potential for further stratigraphic discoveries in syn-rift shallow marine reservoirs.However, the geometry and lateral distribution of syn-rift deep-watermass-flow deposits are highly conducive to at least partial stratigraphic entrapment, since they areinterbedded with mudstones of the Kimmeridge Clay Formation. Of the existing fields anddiscoveries with syn-rift reservoirs, 24% are located in stratigraphic and combination traps (Fig. 4).Taking into account only deep-water syn-riftreservoirs, the proportion of both stratigraphic andcombination traps is 48%. Discovery curve data indicate that the UpperJurassic deep-water play is not yet mature (Fig. 5).Despite the profile flattening during the 1990’s,discoveries made in the last three years show arejuvenation of the play.Most of the syn-rift deep-water sandstone combination traps were initially drilled as structural traps(e.g. South Brae), but subsequently proved to be larger than pre-drill prognosis, due to closureenhancement by an element of stratigraphic pinch-out.Many of the Upper Jurassic deep-watersandstone reservoirs that occur within entirely stratigraphic pinch-out traps are located above theflanks of an underlying structural trap, and were discovered through serendipity (e.g. Hot Lensreservoirs at Tartan and Highlander fields).Page 2 of 5THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED R...05/01/2016mk:@MSITStore:D:\petex2004.chm::/abstracts/003003.htmTraps formed by combination of dip closure and stratigraphic pinch-out may occur in both intra-basinand basin-margin settings. Typically, such traps are subtle and may not be directly identifiable fromseismic data, thus requiring the development of a well-grounded conceptual model. On the southernmargin of the Moray Firth rift basins, the giant Buzzard Field represents the first major discovery in abasin margin pinch-out play (Doré & Robbins in press). Buzzard is the largest oil discovery in theNorth Sea since 1984, and its size clearly demonstrates that those more subtle, complex trapsremaining to be found are not necessarily smaller than existing structural traps. Munns et al. (inpress) recognized leads with a similar up-dip pinch-out geometry elsewhere in the Moray Firth riftbasins. Post-rift stratigraphic potentialUKCS fields and discoveries within post-rift reservoirs contain the largest proportion of stratigraphicand combination traps (51% overall), with the greatest ratio of stratigraphic/combination to structuraltraps recorded from the reservoirs of Lower Cretaceous age (Fig. 6). Despite broadly post-rift style basin development during the Early Cretaceous, deep-watersandstones continued to be deposited in response to local extensional faulting and inversion,probably under a strike-slip tectonic regime (Copestake et al. 2003). Seventy-six percent of UKCSLower Cretaceous fields and discoveries occur in combination or stratigraphic traps (Fig. 6).Discoveries within the Lower Cretaceous play are almost exclusively limited so far to the Moray Firtharea of the Central North Sea between CaptainField in the west and Britannia Field in the east.Additional undrilled potential on this play existswithin the Central Graben to the south (Milton-Worssell et al. in prep., Morgan & Went 2003), andthe Viking Graben to the north (Oakman in press). Twenty-five percent of UKCS Upper CretaceousChalk fields have an element of non-structuraltrapping (Fig. 6). Partial stratigraphic entrapment, asevidenced by tilted oil-water contacts, is due to acombination of post-emplacement structuration andvery low lateral permeability preventing remigration.There has been little exploration on this play inrecent years, but our improved understanding of theunusual trapping mechanisms involved offers hopefor future exploration opportunities.The majority of Paleogene reservoirs are deep-water sandstones whose geometry commonly lendsitself to stratigraphic entrapment. Hence, half of all UKCS Paleogene hydrocarbon discoveries occurin traps with full or partial stratigraphic entrapment (Fig. 6). Purely stratigraphic traps are mostcommonly detached lobes of mass-flow sandstones with 4-way dip closure resulting from acombination of sedimentary mounding and differential post-depositional compaction of the encasingmudstones (e.g. Frigg Field).Sophisticated seismic techniques appear to work well on Paleogenereservoirs in the UK North Sea. However, in the West of Shetland area, the use of AVO as ahydrocarbon predictor in Paleogene plays has been less successful (Loizou et al. in prep.). In theNorth Sea Eocene deep-water sandstone play, almost all of the fields and discoveries occur instratigraphic or combination traps. Mounded geometries are common within palaeogeomorphic traps(e.g. Frigg and Guillemot fields), and remobilized or injected channel sands provide a locallyimportant Eocene deep-water sandstone play (e.g. Alba Field).The Paleogene discovery curve has a maturing profile (Fig. 7).SummaryEighty-one percent of existing UK Continental Shelf (UKCS) fields and discoveries are located within Page 3 of 5THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED R...05/01/2016mk:@MSITStore:D:\petex2004.chm::/abstracts/003003.htmstructural traps, 6% occur within purely stratigraphic traps, and 12% lie within combination structural/stratigraphic traps. The majorityof stratigraphic and combination traps occur in association with syn-rift (Upper Jurassic) and post-riftplays. Proportions of stratigraphic and combination traps within pre-rift, syn-rift and post-rift playsaverage 3%, 24% and 51% respectively. Lower Cretaceous fields and discoveries, almostexclusively deep-water sandstone plays, demonstrate the highest proportion of stratigraphicentrapment of all, with 76% located within stratigraphic and combination traps. This is closelyfollowed by Paleogene deep-water plays at 50%, and Upper Jurassic deep-water plays at 48% ofsuch traps. Many of the major discoveries in stratigraphic traps were found by chance, since there has beenrelatively little direct exploration for stratigraphic plays. Plotting the UKCS discovered data on a sizedistribution chart shows that the structural traps give rise a rather mature, lognormal-type distribution,whereas the stratigraphic and combination traps describe a more irregular distribution that hints atsignificant yet-to-find potential (Fig. 8). The discovery curve for the entire UKCS has a maturing signature. However, discovery curves forindividual plays provide evidence for resurgence, especially in the Carboniferous, Upper Jurassicdeep-water, Lower Cretaceous, Upper Cretaceous and Paleogene plays.Exploration on the UKCS hasmoved into a new era of primarilysearching for subtle stratigraphicplays within both stratigraphic andcombination traps. Deep-watersandstone stratigraphic plays withinthe syn- and post-rift sequencesoffer the greatest potential forsubstantial new resources. Of theforecast 4.1 to 21.3 x 109 BOErecoverable UKCS yet-to-find (DTI2004), we estimate that at least 50%is located within stratigraphic traps.ReferencesCopestake, P, Sims, A.P., Crittenden, S., Hamar, G.P., Ineson, J.R., Rose, P.T. & Tringham, M.F. 2003. LowerCretaceous. In: Evans, D., Graham, C., Armour, A., and Bathurst, P. (eds), The Millennium Atlas: petroleumgeology of the central and northern North Sea. Geological Society, London, 191-211.Department of Trade and Industry Oil and Gas Information. 2004. http://www.og.dti.gov.uk/information/bb_updates/chapters/reserves_index.htm Doré, G. & Robbins, J. In press. The Buzzard Field. In: Doré, A.G. & Vining, B. (eds) Petroleum Geology:North-west Europe and Global Perspectives-Proceedings of the 6th Petroleum Geology Conference, GeologicalSociety, London.Loizou, N., Andrews, I.J. & Stoker, S.J. In prep.. West of Shetland revisited – the search for stratigraphic traps.In: Allen, M., Morgan, R., Goffey, G. & Walker, I. (eds) The Deliberate Search for the Stratigraphic Trap –Where are we now?. Geological Society, London, Special Publication.Milton-Worssell, R., Stoker, S.J. & Cavill, J.E. In prep. Lower Cretaceous deep-water sand plays in the UKCentral Graben. In: Allen, M., Morgan, R., Goffey, G. & Walker, I. (eds) The Deliberate Search for theStratigraphic Trap – Where are we now?. Geological Society, London, Special Publication.Morgan, R. & Went. D. 2003. The Seismic Characterisation of Lithology using AVO: A Window onto LowerCretaceous Sands and their Trapping Potential in the Central Graben. Abstracts, The Deliberate Search for theStratigraphic Trap – Where Are We Now? Geological Society, London.Munns, J.W., Gray, J.C., Stoker, S.J., Andrews, I.J. & Cameron, T.D.J. In press. The remaining hydrocarbonpotential of the UK Continental Shelf. In: Doré, A.G. & Vining, B. (eds) Petroleum Geology: North-west Europeand Global Perspectives-Proceedings of the 6th Petroleum Geology Conference, Geological Society, London.Page 4 of 5THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED R...05/01/2016mk:@MSITStore:D:\petex2004.chm::/abstracts/003003.htmOakman, C.D. In press. The Early Cretaceous Aptian Play of the Central and Northern North Sea – Atlanteandrainage models and enhanced hydrocarbon potential. In: Doré, A.G. and Vining, B. (eds) Petroleum Geology:North-West Europe and Global perspectives – Proceedings of the 6th Petroleum Geology Conference,Geological Society, London.Presented at PETEX 2004, UK Petroleum Systems SessionPage 5 of 5THE IMPORTANCE OF STRATIGRAPHIC PLAYS IN THE UNDISCOVERED R...05/01/2016mk:@MSITStore:D:\petex2004.chm::/abstracts/003003.htm

The importance of stratigraphic plays in the undiscovered resources of the UKCS

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The importance of stratigraphic plays in the undiscovered resources of the UKCS

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