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Roger Sassen1,
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