Carrie L. Decker¹, Roger Sassen², Joel S. Watkins¹
(1) Texas A&M University, College Station, TX
(2) Texas A & M University, College Station, TX

Abstract: Implications of complex hydrocarbon charge history in Ship Shoal 274/293, Gulf of Mexico

Ship Shoal 274/293 is a highly compartmentalized field that shows geological and geochemical evidence of a recent complex oil and gas charge history. Crude oils show a wide range of API gravities consistent with the range from black oil to condensate, but appear to be derived from a single source system, probably mixed Upper Jurassic and Lower Tertiary. The variable oil properties are inferred to result from multiple phases of hydrocarbon migration into the field accompanied by remigration during the field's structural evolution. Crude oils were emplaced in reservoir compartments by migration from large-displacement, salt-cored normal fault conduits and associated small-displacement faults, and some were altered by biodegradation. Pervasive evaporative fractionation occurred during a late charge of unaltered thermogenic gas focused from the most recently active normal fault, and was accompanied by multiple phases of remigration of altered oils among compartments. Remigration was focused along one set of recently active small-displacement faults. Geological evidence suggests that all hydrocarbon charge in the field occurred within the last 250,000 years, while geochemical evidence indicates that charge did not occur directly from the source rock, but underwent multiple episodes of parking and remigrating through a sequence of downdip traps. Recent gas charge is suggestive of undrilled downdip hydrocarbon reservoirs in the Ship Shoal 274/293 drainage area. This complex charge history is inferred to be characteristic of much of the thermally mature Gulf outer shelf, but does not necessarily reflect charge history throughout the less thermally mature Gulf slope.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana