Gas hydrate and crude oil from the Mississippi Fan Foldbelt, downdip Gulf of Mexico Salt Basin: significance to petroleum system

Structure II gas hydrate, methane–ethane hydrate, and crude oil occur on the sea floor at 1920–1930 m water depth in Atwater Valley (AT) Block 425, near the juncture of the lower slope of the Gulf of Mexico Salt Basin and the abyssal plain. The site is in the eastern Mississippi Fan Foldbelt (MFF), a distinct structural province at the downdip limit of the Gulf of Mexico Salt Basin. The presence of thermogenic hydrocarbons confirms an active petroleum system in the deep eastern MFF. The hydrate-bound C2–C5 hydrocarbon gases of the MFF are isotopically distinct when compared to other gases from the upper and middle slope, being strongly depleted in 13C. The biomarkers (m/z=191 and 217) of oil inclusions from AT 425 gas hydrate are also distinct when compared to oils of the upper Gulf slope and the Smackover Trend. Biomarkers of AT 425 oil are consistent with a marine source rock deposited in an area of strong siliciclastic influx, with possible higher plant organic matter. The presence of a shale or mudstone source rock at the downdip limit of the Gulf raises new questions as to paleogeography during source rock deposition. Emergent highlands immediately to the south and east of the downdip limit of the Gulf of Mexico Salt Basin during the Mesozoic may explain the occurrence of the shale or mudstone source rock that gave rise to the gas and oil at AT 425.