Geochemical investigation of the origin of hydrocarbon occurrences in up-dip Wilcox Group reservoirs at Raccoon Bend field, southeast Texas Gulf Coast basin

Raccoon Bend field is a densely-drilled salt dome field in Austin County, Texas located within the southeast Texas Gulf Coast basin. Successful commercial drilling of the Wilcox Group at the Raccoon Bend field has presented a rare opportunity to study the organic geochemical attributes of hydrocarbons in Wilcox-aged reservoir in this basin. Molecular geochemistry of hydrocarbons was integrated with local stratigraphy, subsurface temperature and pressure, and reservoir brine isotope geochemistry to evaluate the source, maturity, and possible migration pathways of the hydrocarbons at Raccoon Bend field. Additionally, rock samples were collected from the Wilcox Group at Raccoon Bend field and were analyzed for total organic carbon, vitrinite reflectance, visual kerogen analysis, and stable isotopes of carbon. Organic geochemical attributes of the rocks, combined with the hydrocarbon geochemistry, were used for oil-source rock comparison at Raccoon Bend field. Geochemical investigation of hydrocarbons at Raccoon Bend field suggests that these oils belong to a single family, and therefore likely sourced from the same rock. Whole-oil gas chromatography and biomarker chemistry suggest that the chemistry of the oils are influenced profoundly by non-marine humic organic matter (Type III kerogen). Raccoon Bend oils correlate with other oils studied from younger reservoirs within the basin. However, there are several geochemical characteristics that suggest that the molecular chemistry of these oils is influenced by the immature coal-rich, high-energy proximal carrier-bed and reservoir rocks where the oils have accumulated. Pr/Ph ratios, n-C15+ distribution, and some saturate biomarker parameters show unusual characteristics for Gulf Coast oils that cannot be explained by source rock facies variations. Rock analyses confirm the Wilcox Group at Raccoon Bend is currently in the ‘early generative window’ and therefore the Wilcox Group is not responsible for the commercial quantities of oil and gas at Raccoon Bend. The low organic carbon content and the dominance of re-worked organic matter in the Wilcox rocks in this area suggest that the Raccoon Bend oils are not likely sourced from the Wilcox Group. It is possible that the Raccoon Bend oils are sourced from laterally more mature, more distal facies of the Wilcox or from older source rocks such as Midway, Austin Chalk, or Eagle Ford.