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 th e sea floor at similar to 1920-1930 m water depth in Atwater Valley (AT) Bl ock 425, near the juncture of the lower slope of the Gulf of Mexico Salt Ba sin and the abyssal plain. The site is in the eastern Mississippi Fan Foldb elt (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 C-2-C-5 hydrocarbon gases of the MFF are isotopically distinct when compared to oth er gases from the upper and middle slope, being strongly depleted in C-13. The biomarkers (m/z = 191 and 217) of oil inclusions from AT 425 gas hydrat e are also distinct when compared to oils of the upper Gulf slope and the S mackover Trend. Biormarkers of AT 425 oil are consistent with a marine sour ce rock deposited in an area of strong siliciclastic influx, with possible higher plant organic matter, The presence of a shale or mudstone source roc k at the downdip, limit of the Gulf raises new questions as to paleogeograp hy during source rock deposition. Emergent highlands immediately to the sou th and cast of the downdip limit of the Gulf of Mexico Salt Basin during th e Mesozoic may explain the occurrence of the shale or mudstone source rock that gave rise to the gas and oil at AT 425. (C) 2001 Published by Elsevier Science Ltd. All rights reserved.