Biodegradation in South Texas Eocene oils — Effects on aromatics and biomarkers

Abstract

The Eocene oil producing trend in South Texas provides an excellent area for studying sequential effects of biodegradation on crude oil composition. Many small fields contain oils of a common genetic type which exhibit a broad range of biodegration stages. Gas chromatograms of 26 oils from Eocene Jackson reservoirs in Duval County, Texas, show that n-paraffins are missing in 23 oils, while the other three appear to be undegraded. Among the degraded samples, a progressive increase in degradation is indicated by first a gradual disappearance of isoprenoids, then by disappearance of other prominent peaks in the C 13–C 17 range. Gas chromatography-mass spectrometry analyses were on nine oils, selected on the basis of their gas chromatograms to represent various biodegradation stages. Changes in distributions of geochemically important compound types, including the more common biomarkers, were related to extent of biodegradation. Significant observations include: (1) two- and three-ring aromatics, including benzo- and dibenzothiophenes show selectivity in degradation and disappear during adjacent degradation stages; (2) a series of adamantanes is unaffected; (3) steranes and aromatized steroid hydrocarbons remain undegraded; (4) tricyclic and tetracyclic terpanes are not degraded, but ammong pentacyclic triterpanes, a a slight decrease in C 29 and C 30 hopanes can be seen; and (5) bicyclic and diterpanes are degraded irregularly and less rapidly than aromatics. Aside from an early loss of straight-chain and branched-chain paraffins, biodegradation may follow quite different routes at different locations, particularly with regard to aromatics. Therefore, criteria for correlating biodegraded oils and assessing biodegradation stages on the basis of aromatic hydrocarbon distributions must be carefully selected.