Stephen T. Sweet1,
Alexei V. Milkov1,
Debra A. Defreitas1
(1) Texas A & M University, College Station, TX
Abstract: Significance of gas hydrates and oil seeps to Mississippi Fan Foldbelt Petroleum System
Rapid thermogenic seepage in the eastern Mississippi Fan Foldbelt is consistent with geologically recent migration in a petroleum system with poor trapping efficiency. We recovered intact orange gas hydrates with inclusions of free crude oil from seep sites in Atwater Valley Block 425, using piston cores to obtain samples from about 193- meters water depth. The unusual structure II gas hydrates contain non-biodegraded methane, ethane, propane, isobutane, normal butane, and isopropane. The 13C values of all the hydrocarbon gases from the gas hydrate are significantly lighter than typically encountered in other non-biodegraded structure II gas hydrates from Mississippi Canyon and Green Canyon, suggesting differences in maturity or organic facies. For example, mean 13C of ethane is –38.0% PDB. Whole-oil chromatography of oil inclusions shows biodegradation, with episodic recharge of fresh oil. Whole-oil 13C is –27.0% PDB. Across the middle to lower slope, oil seep biomarkers show a range of signatures consistent with carbonate to marl or clastic source facies. The oil from Atwater Valley Block 425 shows a signature in the mart to clastic range. the seep sites are near the downdip limit of the Gulf of Mexico Salt Basin, a zone of abrupt geologic change. Properties of hydrocarbon gases and crude oil are consistent with an early mature to mature Upper Jurassic source rock in an area with a clastic influx, raising questions as to the paleogeography during source rock deposition.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana