Stratigraphic trap potential lightly tested
Jim Munns
UK Department of Trade and Industry
Sue Stoker
British Geological Survey
The majority of the 465 oil and gas fields and significant discoveries to date on the UK continental shelf (UKCS) are found in structural traps. Of the discovered hydrocarbons, just 12% is found in combination structural/stratigraphic traps and only 5% in stratigraphic traps.
In existing fields and discoveries, the stratigraphic and combination traps occur mainly within Upper Jurassic syn- and post-Jurassic post-rift play fairways. The sheet-like geometry and sand-rich nature of many of the pre-rift reservoirs (Middle Jurassic and older) make stratigraphic entrapment uncommon in these strata. Combination traps in pre-rift plays generally involve major erosional truncation. In contrast, stratigraphic entrapment in deepwater coarse clastics is an important play in the syn- and post-rift play fairways.
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Historically, few stratigraphic traps have been the primary targets of exploration drilling on the UKCS due to the higher level of risk, and greater difficulty in identifying them on seismic data. Interestingly, a significant number of fields and discoveries in stratigraphic and combination traps were found through sheer good fortune during exploration or appraisal drilling to other, generally deeper, structural targets. However, we cannot rely on serendipity for the future, but must find means of identifying subtle stratigraphic traps.
Significantly, sophisticated seismic data analysis, in particular amplitude versus offset (AVO) techniques, has not proved to be entirely reliable on the UKCS, particularly within the Paleogene deepwater sand play on the UK Atlantic Margin. The Buzzard field discovered in 2001 is a large (1.1 Bbbl in place) Upper Jurassic stratigraphic pinchout/dip trap on the margin of the Moray Firth basin. It was found by applying traditional methods of seismic interpretation, and integration of well and other data, leading to the development of a strong conceptual model.
Post-rift stratigraphic potential
Of the UKCS Lower Cretaceous fields and discoveries, 58% are combination or stratigraphic traps. The mass-flow genesis of much of the Lower Cretaceous coarse-clastic sediment means that mounding enhanced by differential compaction provides the mechanism for paleogeomorphic entrapment. Since seismic imaging of Lower Cretaceous sandstones is known to be poor, a sound depositional model must be built from core and other well data to predict sand distribution, thickness, and geometry to identify potential stratigraphic traps.
Five percent of the 465 oil and gas fields and discoveries on the UKCS are found in stratigraphic traps.
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The serendipitous discovery of the Scapa field within a syncline exemplifies the importance of potential traps within syn-sedimentary lows or synclines. This is a trap type that is not typically explored for in the early phase of exploration in a basin but comes to the fore in latter exploration phases such as we are seeing in the North Sea now.
Until recently, all of the UKCS Upper Creta-ceous chalk fields were believed to be structural traps. However, Megson and Hardman have documented dipping oil-water contacts in the chalk at the Joanne, Fife, and Flora fields. In the Danish sector, the Halfdan field is purely a stratigraphic trap and is actually a body of oil within chalk in a Tertiary migration pathway. Recognition of stratigraphic and combination traps within the Upper Cretaceous requires determination of migration entry points into the chalk, estimation of maximum possible migration distance within the chalk from the entry points, and paleostructure maps relating to time(s) of oil emplacement.
Paleogene traps
Half of all UKCS hydrocarbon discoveries in Paleogene reservoirs occur in traps with full or partial stratigraphic entrapment. The majority of Paleogene reservoirs are deepwater sandstones whose geometry commonly lends itself to stratigraphic entrapment. Many traps are possible including:
- Stratigraphic pinch-out traps generally occur where Paleogene sandstones onlap and pinch-out onto the flanks of basin-margin highs
- Paleogeomorphic traps are developed within both the deepwater and coastal-deltaic Paleogene sandstones
- Detached lobes of mass-flow sandstones can exhibit four-way dip closure that results from a combination of sedimentary mounding and post-depositional differential compaction of the encasing mudstones
- Stratigraphic traps formed by incised valley fill mounds with compactional drape, or by compactional-drape enhanced by delta-top paleo-relief occur within basin-marginal late Paleocene-early Eocene coastal- deltaic deposits.
Sophisticated seismic techniques appear to work well on Paleogene reservoirs in the UK North Sea, as they are relatively shallow with minimal structural complexity. The trapping mechanism geometry of the Alba field, a stratigraphic trap of Eocene age has been illuminated by the use of 3D shearwave volume processing using data collected by sea bottom seismic cables.
The Arbroath and Montrose fields in the UK Central Graben contain reservoirs of Paleocene age trapped within simple, four-way dip closures. Seismic attribute analysis has revealed that the Forties Sandstone, the primary reservoir, is highly channelized, and AVO techniques have been used successfully to determine the distribution of oil versus water within these fields (Ahmadi et al, 2003).
Syn-rift stratigraphic potential
The most exciting potential for stratigraphic and combination traps lies within the Upper Jurassic syn-rift play. The earliest Upper Jurassic sandstones are deltaic to shallow-marine sediments that pre-date the main rifting phase. Upper Jurassic syn-rift clastics are predominantly deepwater mass-flow deposits, whose lateral distribution and geometry is highly conducive to partial stratigraphic entrapment. These clastics are interbedded with the Kimmeridge Clay Formation, a world-class oil source rock that has generated much of the oil in the North Sea.
Most of the syn-rift combination traps were initially drilled as structural traps, but have proved to be larger than pre-drill prognosis because closure is enhanced by an element of stratigraphic pinch-out. Many of the Upper Jurassic deepwater sandstone reservoirs within pinch-out traps are located above an underlying structural trap.
Predicting the distribution and pinch-out of such reservoirs is critical to this play, and relies on the development of a well-grounded conceptual model, since they are typically poorly resolved on seismic data. The increasing use of long offset seismic data with the attendant increase in bandwidth is proving an exciting tool to aid in the definition of the geometry of subtle traps.
Pre-rift stratigraphic potential
The potential for stratigraphic traps within UKCS pre-rift plays is limited with the likeliest plays occurring in the Southern Gas basin. Lower Permian basin-margin stratigraphic pinch-out traps are under-explored and may be important in the future.
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The main reservoir in the Southern Gas basin is the Rotliegend sandstone that contains high quality dune facies that interdigitates basinward into sabkhas and lacustrine shales. This sets up the potential for significant stratigraphic traps.
Proportion of trap types for each principal play fairway in UKCS fields and discoveries.
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A number of the gas discoveries in Carbonifer-ous reservoirs occur where the trap is a combination of dip and erosional truncation beneath the base Permian unconformity, top-sealed by overlying Permian mudstones and evaporites.
Considerable potential remains for exploration of Carboniferous fluvio-deltaic sandstone reservoirs in such sub-Permian erosional truncation traps. Depending on the trap geometry, Carboniferous intraformational top or bottom seals would be required, and proven intraformational sealing intervals have been recognized at three levels within the Upper Carboniferous.
Summary
Stratigraphic traps account for only 5% of existing UKCS fields and discoveries, and they have been infrequently targeted when compared to structural traps up to now. A significant number of stratigraphically trapped fields were discovered serendi-pitously during drilling of other targets.
The spectacular discovery of the Buzzard field in 2001 has marked the beginning of a new era of active exploration for stratigraphic traps. Of the forecast 4-25 Bboe UKCS undiscovered reserves, it is estimated that 50-75% is located within stratigraphic traps.
Stratigraphic trap targets are most important within Upper Jurassic syn-rift and Cretaceous to Paleogene post-rift plays. Deepwater sandstones with limited lateral distribution constitute the principal reservoir in these plays. Successful exploration for such targets will rely on:
- Well-grounded conceptual models for reservoir distribution required to predict trap configuration
- Well-constrained use of seismic techniques such as AVO analysis and long offset seismic where appropriate to map trap geometry and pinpoint drilling locations. ;
References
Ahmadi, Z.M., Sawyers, M., Kenyon-Roberts, S., Stanworth, C.W., Kugler, K.A., Kristensen, J., and Fugelli, E.M.G. 2003. Paleocene. 235-259 in The Millennium Atlas: Petroleum geology of the central and northern North Sea. Evans, D., Graham, C., Armour, A. and Bathurst, P. (editors and coordinators). London: The Geological Society of London.
Doré, G. 2002. The Buzzard field – an overlooked North Sea giant. Extended abstracts, PETEX 2002 CD-ROM.
Garrett, S.W., Atherton, T., and Hurst, A. 2000. Lower Cretaceous deepwater sandstone reservoirs of the UK Central North Sea. Petroleum Geoscience, Vol. 6, pp. 231-240.
Law, A., Raymond, A., White, G., Atkinson, A., Clifton, M., Atherton, T., Dawes, I., Robertson, E., Melvin, A., and Brayley, S. 2000. The Kopervik fairway, Moray Firth, UK. Petroleum Geoscience, Vol. 6, pp. 265-274.
Megson, J. and Hardman, R. 2001. Exploration for and development of hydrocarbons in the chalk of the North Sea: a low permeability system. Petroleum Geoscience, Vol. 7, pp. 3-12.
Editor's Note: For information, contact: Jim Munns, senior geoscientist, Promote UKCS, UK Department of Trade and Industry. Tel: 44 (0)20 7215 5079; email: [email protected].
