Carbonate Caprock Research Articles

Hydrocarbon Seepage and Salt Dome-Related Carbonate Reservoir Rocks of the U.S. Gulf Coast

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.

Deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate

Crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the Damon Mound salt dome, Brazoria County, Texas. Scanning electron microscopy of calcite-lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment. Solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite. Deposition of pyrrhotite, pyrite, marcasite, sphalerite, and elemental sulfur is attributed to microbial sulfate reduction, and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons. The C 1—C 4 hydrocarbons in pyrrhotite-bearing cap rock samples are a biodegraded residue derived from crude oil. Higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation. Hydrocarbon compositions could be explained by limited aerobic biodegradation, but a component of anaerobic biodegradation cannot be excluded. Some Damon Mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite. This study provides new evidence of a link between crude oil migration, microbial activity, and diagenetic magnetic anomalies in rocks.

Recent hydrocarbon alteration, sulfate reduction and formation of elemental sulfur and metal sulfides in salt dome cap rock

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.