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.