Geochemical and isotopic data for methane and ground water indicate that gaseous hydrocarbons in Eocene aquifers in east-central Texas form by bacterial processes. The δ13C values of methane from five wells in the clay-rich Yegua and Cook Mountain Formations range from -71‰ to -62‰. Methane from ten wells in the cleaner sands of the Sparta and Queen City Formations have δ13C values between -57‰ and -53‰. The carbon isotopic difference between methanes from the Yegua and Sparta aquifers is comparable to the isotopic difference in sedimentary organic matter from outcrops of the units, suggesting substrate control on the δ13C of bacterial methane. Hydrogen isotopic compositions of methane from the aquifers are similar, averaging -181‰. This high value suggests methane production predominantly by CO2 reduction.

The δ13C of dissolved inorganic carbon (DIC) in high bicarbonate waters increases from about -20‰ to 0‰ with increasing DIC. Mass-balance calculations indicate that the DIC added to the ground water has δ;13C values as high as 10‰. This 13C-enriched carbon is predominantly derived from CO2 production by fermentation and anaerobic oxidation reactions combined with CO2 consumption by CO2 reduction. This process is responsible for high bicarbonate contents in these and probably other Gulf Coast ground waters.

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