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Unconventional Deepwater Hydrocarbon Traps


Steele, David, Shell Houston, Houston, TX


Deepwater discoveries totaling 535 fields and 90BBOE ultimate recoverable reserves 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 60 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 4 countries: Australia, Brazil, Angola, and Nigeria. The geologic setting in each of the countries is different with regard to basin type and petroleum system. 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-belts of the salt mobilized Campos and Lower Congo basins, and the inner fold-belt of the shale mobilized Niger basin. Unlike the other basins, Lower Congo basin fields have smaller mean volumes but can be clustered by development projects to form giants. 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. However, charge modeling indicates in the outer fold belts indicate marginal charge of proven source rocks in the some of the proven oil basins and trap styles frequently are not simple anticlinal traps. Consequently there is great incentive to diversify the exploration program in search of giants.

Three operators dominate ownership of the deepwater giant fields; Petrobras, Chevron Texaco, and Exxon. Petrobras’ giant Campos Basin fields account for 15 BO – 25% of the total giant field reserves.