Eighteen core samples of shale from the Eocene Wilcox formation in the Gulf Coast of Texas were extracted in organic solvents and digested in hydrofluoric acid in order to examine the bitumen and kerogen fractions. The kerogen was found to be largely terrigenous in origin with stable carbon isotope values ranging between -26 and -29 ppt (PDB). Smectite dehydration in the area begins at less than 1 km depth (60°C). At the depths of initial petroleum generation (1.5 km and 85°C), expandable layers comprise only 30% of the illite-smectite clays. It therefore does not seem likely that water expelled during smectite dehydration can mobilize significant amounts of hydrocarbons.

Geopressuring of pore fluids occurs toward the end of the main stage of petroleum generation, below 3 km. In the gas zone, hydrocarbon concentrations increase between 3.6 and 4.6 km from a low of 7 mg/gC to 15 mg/gC. Two samples in this region showed anomalously high concentrations of hydrocarbons (100 mg/gC). These observations may indicate subsurface migration of hydrocarbons, perhaps in a methane and carbon dioxide-rich fluid phase. In addition to facilitating hydrocarbon migration, carbon dioxide generated from kerogen may also be involved in the precipitation of ankerite cement in sandstones below 2.5 km.

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