Oligocene to Holocene hydrocarbon migration and salt-dome carbonates, northern Gulf of Mexico

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The geochemistry and geology of Damon Mound salt dome in Brazoria County, Texas provides an insight to the timing of hydrocarbon migration and carbonate caprock development in the Houston salt-dome province. Reservoir rocks flanking the salt dome received a charge of crude oil from deeply buried Eocene source rocks during Early Oligocene-Early Miocene time. The light δ 13C of the carbonate caprock at Damon Mound (−24.8 to −31.7‰ PDB) is consistent with an origin from bacterial oxidation of crude oil with similar δ 13C (−27.1 to −28.8‰ PDB). A minimum weight estimate of carbonate caprock at Damon Mound is 32.7 × 10 6 metric tons, suggesting bacterial oxidation of about 34.2 × 10 6 barrels of crude oil to form the carbonate. Late Oligocene-Early Miocene erosional exposure of the carbonate caprock provided a hard substratum to initiate the development of a coral reef in an otherwise unfavourable mud-dominated environment. Although the bulk of the reef carbonate displays normal marine δ 13C, some late carbonate cements show light δ 13C values (−22.1 to −29.6‰) 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 hypothesis 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 carbonate caprocks over shallow salt domes on the present Gulf of Mexico continental slope offer analogies and contrasts to the processes that gave rise to carbonates at Damon Mound in the geological past.