Unconventional Deepwater Hydrocarbon Traps

Dave Steele
Shell International Exploration and Production, Houston, TX

Deepwater discoveries totaling 183 fields and 54BBOE reserves and SFR were reviewed to determine the hydrocarbon habitat of giant fields, the structural setting, hydrocarbon volumes in various traps - particularly in unconventional stratigraphic traps, and the common controls of unconventional traps. There are 38 fields defined as giants, fields with greater than 500MMBOE. The giants dominate the reserve distribution and account for 71% of the total deepwater reserves and are equally split between oil and gas. Approximately two-thirds of the reserves come from only 3 countries: Australia, Brazil, and Nigeria. The geologic setting in each of the countries is completely different with regard to basin type. Statistical representation of giant fields largely represent the geologic setting of the 3 main basins the non-mobile substrate play of the NW Shelf in Australia, the inner fold-belt of the salt mobilized Campos basin, and the inner fold-belt of the shale mobilized Niger basin. Giant discoveries in outer fold belts account for only 6% of the total reserves but may be expected to grow given they are only recently being explored.

Deepwater giant fields trap styles are surprisingly conventional- anticlines and faulted anticlinal traps account for 60% of the giant field reserves. This is surprising because deepwater reservoirs, deposited by gravity currents, are largely deposited in lows and frequently show complex depositional geometries. Combination structural stratigraphic traps are more unconventional and account for an additional 31% of giant field reserves. Channels over large anticlinal noses are the most common combination structural stratigraphic trap type. Pure stratigraphic traps pinch outs on monoclines or along synclinal flanks or along unconformities are rare and account for 4% of the total reserves. Other trap types that make a combined total of 5% of the total giant field reserves are salt flanks and reefs.