OLIGOCENE TO HOLOCENE HYDROCARBON MIGRATION AND SALT-DOME CARBONATES,NORTHERN GULF-OF-MEXICO

The geochemistry and geology of Damon Mound salt dome in Brazoria Coun ty, Texas provides an insight to the timing of hydrocarbon migration a nd carbonate caprock development in the Houston salt-dome province. Re servoir rocks flanking the salt dome received a charge of crude oil fr om deeply buried Eocene source rocks during Early Oligocene-Early Mioc ene time. The light deltaC-13 of the carbonate caprock at Damon Mound (-24.8 to -31.7 parts per thousand PDB) is consistent with an origin f rom bacterial oxidation of crude oil with similar deltaC-13 (-27.1 to -28.8 parts per thousand PDB). A minimum weight estimate of carbonate caprock at Damon Mound is 32.7 x 10(6) metric tons, suggesting bacteri al oxidation of about 34.2 x 10(6) barrels of crude oil to form the ca rbonate. Late Oligocene-Early Miocene erosional exposure of the carbon ate caprock provided a hard substratum to initiate the development of a coral reef in an otherwise unfavourable mud-dominated environment. A lthough the bulk of the reef carbonate displays normal marine deltaC-1 3, some late carbonate cements show light deltaC-13 values (-22.1 to - 29.6 parts per thousand) from bacterial hydrocarbon oxidation. Buried carbonate caprocks and coral reef facies serve as carbonate reservoir rocks over Gulf of Mexico salt domes, lending support to the hypothesi s that intense hydrocarbon oxidation can, in itself, give rise to some carbonate reservoir rocks. Ongoing hydrocarbon migration from deeply buried Cretaceous source rocks and the development of embryonic carbon ate caprocks over shallow salt domes on the present Gulf of Mexico con tinental slope offer analogies and contrasts to the processes that gav e rise to carbonates at Damon Mound in the geological past.