The Importance of the Stratigraphic Trap
DTI staff from Licensing, Exploration and Development have contributed to a new collection of papers (April 2006) published by the Geological Society ‘The Deliberate Search for the Stratigraphic Trap’. The collection of papers draws upon contributions that examine current industry perceptions of stratigraphic trap exploration and the technologies, tools and philosophies employed in such exploration, given the changing industry environment. The specific DTI contributions (authored by Joy Gray, Peter Haile, Richard Milton-Worssell and Nick Loizou in collaboration with BGS staff) are summarised below. The full list of papers and authors, and details of how to purchase the volume are available on the Geological Society website.
The Importance of Stratigraphic Plays in the Undiscovered Resources of the UK Continental Shelf
This paper analyses the statistics of existing United Kingdom Continental Shelf (UKCS) fields and discoveries as a means of assessing which plays are likely to contain the greatest untapped potential for stratigraphic traps. Current Department of Trade and Industry (DTI) estimates put the maximum discovered ultimate recovery of the UK at 50 billion barrels of oil equivalent (BBOE); estimated technically recoverable undiscovered resources are between 4.1 to 21.3 BBOE, based on a prospect mapping approach.
As of end 2003, 82% of the oil and gas fields and discoveries on the UKCS have been found in structural traps; 12% have been found in combination structural/stratigraphic traps and only 6% in stratigraphic traps. The majority of stratigraphic traps and combination traps occur in association with syn-rift (Upper Jurassic) and post-rift deep-water plays. There has been relatively little direct exploration for stratigraphic traps until recently, and a number of the major discoveries in stratigraphic traps were found by chance.
Few substantial untested structural traps remain in the UK North Sea except at considerable depth with associated risks. DTI and BGS estimate that perhaps 50% of the UKCS undiscovered resources are located in stratigraphic or combination traps, principally within syn- and postrift deep-water sandstone plays.
Figure 1: (a) Discovery curve and discovery history histogram for UKCS fields and discoveries. (b) Number of exploration wells drilled on the UKCS per year and drilling success rate (percentage of offshore exploration wells that have proved technical successes). (c) Proportion of 660 UKCS fields and discoveries in structural, stratigraphic and combination traps. (d) Proportion of UKCS in-place discovered volumes in structural, stratigraphic and combination traps.
Lower Cretaceous Deep-Water Sandstone Plays in the UK Central Graben
Up to the present, exploration of the UK Lower Cretaceous deep-water sandstone play has been confined largely to the Moray Firth basins. The Lower Cretaceous of the Central Graben area has been modelled previously as predominantly shale-prone, and hence unattractive to exploration. There is a growing realisation that this may not be the case. Since seismic imaging of Lower Cretaceous sandstones is known to be poor whether hydrocarbon-bearing or water-wet, a robust depositional model must be constructed from well and regional geological data in order to predict sandstone distribution and geometry, and hence to aid identification of potential hydrocarbon traps.
Of the hundreds of wells drilled in the Central Graben area that targeted deeper Jurassic–Triassic reservoirs, virtually all have been located on the flanks of the graben, or on intra-graben highs. However, 71 of these wells have proved sandstones or traces of sandstone within the Lower Cretaceous, giving grounds for optimism that more substantial deep-water sandstone developments may be present within the graben depocentres. Twenty-six leads have been identified within these depocentres; most of these are located within stratigraphic traps in interpreted detached basin floor fans.
West of Shetland Revisited: The Search for Stratigraphic Traps
The West of Shetland area has scope for the stratigraphic entrapment of hydrocarbons at various Jurassic to Palaeogene stratigraphic levels. Mapping and identification of such traps requires a fundamental understanding of the regional geology, the study of analogues and source kitchens, and a thorough approach to trap validation.
Since 1982, 47 exploration wells have been positioned on Paleocene prospects with a significant stratigraphic component, but few have found hydrocarbons – many failing as a result of poor trap definition and overconfidence in the predictive use of amplitude anomalies. Hydrocarbon sourcing of many of the failed prospects was also poorly constrained. Few amplitude-related stratigraphic features could be tied with confidence to a viable source kitchen. The presence of a regional seal is a prerequisite ingredient for a successful Paleocene play. Many remaining undrilled, subtle prospects rely on a stratigraphic trapping component, and high-quality 3D seismic data are seen as an essential search tool.
Examples of undrilled prospects are presented from the Paleocene of the northern Faroe-Shetland Basin and the Mesozoic of the East Solan Basin and Corona Ridge.