THE NOBLE GAS AND HYDROCARBON GEOCHEMISTRY OF THE EAGLE FORD SHALE OF TEXAS, USA

The Eagle Ford Shale, with the combined production of oil, condensates, and wet gas, has been a prime example of a major economic success story for unconventional energy development in the US. Despite the economic successes of the Eagle Ford, production has been hampered by the localized occurrence of high CO₂ and H₂S contamination levels. The CO₂ may be derived from mantle-derived gases, carbonate weathering in the crust, or the oxidation of organic matter (e.g., oil). Similarly, H₂S is most commonly derived from either thermal or bacterial sulfate reduction.

Noble gases, including He isotopes, provide an ideal set of inert tracers for evaluating the source and migration of hydrocarbon gases, resolving potential mantle-derived contributions, and the interactions that occur between hydrocarbon fluids and crustal fluids. Noble gases are specifically useful when paired with hydrocarbon molecular and isotopic composition. Nonetheless, to date there has been a dearth of published reports on the noble gas, compound-specific stable isotopes, or H₂S concentrations of the Eagle Ford Shale.

Here we combine noble gases (He-Xe), compound-specific stable isotopes (C₁-C₅), and major gases (CO₂ and H₂S) to differentiate between the potential microbial and thermogenic origins for CO₂ and H₂S. Further, these data provide insight on source and migration of hydrocarbons in the Eagle Ford and their interactions in the crust. Our preliminary results indicate the presence of mantle-derived noble gases and CO₂, correspond with localized normal faulting and increasing thermal maturity. These data suggest that an increased flux of mantle-derived fluids transported along normal faults contributes to the initial flux of CO₂ in this area followed by secondary reactions including thermal carbonate metamorphism and thermal sulfate reduction.