ABSTRACT

Although most salt dome-related oil production in the Gulf Coast is from clastic reservoirs, oil has been produced from Tertiary carbonate reservoir facies within (1) carbonate cap rocks over salt domes, and (2) the Heterostegina zone coral reef facies of the Anahuac Formation. The only known surface exposure of both carbonates occurs over the Damon Mound salt dome, near Houston, where oil and thermogenic gas have been produced from early Oligocene to early Miocene reservoirs flanking the dome crest. The origin of carbonate cap rock with extremely light ¹³C values at Damon Mound is related to long-term microbial oxidation of crude oil and thermogenic gas that commenced during or prior to the early Oligocene. Exposure of the salt-dome cap rock by erosion offered a shallow-water carbonate hardground that favored localized development of the late Oligocene-early Miocene Heterostegina coral reef facies. Carbonate cements with extremely light ¹³C values in the reef facies suggest that hydrocarbon migration continued during reef development, simultaneous with downward development of the carbonate cap rock by replacement of anhydrite. Processes similar to those that operated at Damon Mound appear to explain the origin of carbonates in cold hydrocarbon seeps of the Gulf continental slope. Generalization of these observations suggests that intense hydrocarbon seepage could result in development of carbonate reservoir facies in other oil basins characterized by siliciclastic sediments and shallow salt.