Abstract
Geochemical analysis of hydrocarbons and scanning electron microscopy (SEM) provide insight to recent alteration of crude oil, microbial sulfate reduction and mineral deposition in carbonate cap rock at Damon Mound, Brazoria County, Texas, U.S.A. Previous work showed that intense aerobic biodegradation of hydrocarbons occurs in shallow cap rock where S 0 and metal sulfides are not abundant. The present study focuses on some deeper carbonate cap rock characterized by more limited oxygen availability. The C 1-C 4 hydrocarbons in deeper cap rock are a biodegraded residue of hydrocarbons derived from crude oil. Alteration of higher-molecular-weight crude oil components is best explained by the effects of water washing accompanied by some biodegradation. SEM of cavities in carbonate cap rock shows microbes in association with globular masses of solid crude oil and sulfate minerals. Late-forming minerals in cavities include S 0, pyrrhotite, pyrite, sphalerite and aragonite. The presence of S 0 and sulfide minerals indicates that microbial sulfate reduction is associated with limited hydrocarbon biodegradation in this deeper cap rock environment. Similar processes could explain the origin of some S 0 and metal sulfides at Damon Mound in the geologic past.
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