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Integrating Noble Gas Geochemistry to Better Understand Hydrocarbon Stable Isotope Reversals

Abstract

Oil and gas production has seen a dramatic increase in the last two decades largely due to the exploitation of unconventional reservoirs such as tight shales and sands. As new unconventional plays are explored, well gases displaying reversals in the stable carbon isotopic signatures (δ13C-methane > δ13C-ethane) are more frequent, particularly in high maturity, low porosity formations. In conventional petroleum systems, the thermocatalytic cracking of kerogen to oil and gas fractionates the stable isotopic values in the hydrocarbon molecules in predictable manners (δ13C-CH4 < δ13C-C2H6 < δ13C-C3H8). Despite recent works, the mechanism that results in the δ13C-C2H6 rollover and reversal (i.e., gradually becoming more depleted than δ13C-CH4) with increasing thermal maturity is still largely uncertain. Additionally, the typically high production yields from natural gas wells with “reversed” gases have led to an increased interest in identifying and understanding these phenomena. Present models that attempt to explain these isotopic reversals include: aerobic or anaerobic oxidation of hydrocarbons, Rayleigh fractionation, diffusive fractionation, mixing with abiotic or mantle-derived methane, mixing of gases with different thermal maturities, and secondary cracking of heavier hydrocarbons. However, these processes may affect the δ13C values in similar or ambiguous manners. Therefore, we try to address these questions by integrating noble gas isotopic geochemistry in addition to stable isotopes. Noble gases represent inert, external tracers that are unaffected by microbial or redox processes, can be used to identify oxidation, and the atmospherically derived isotopes (20Ne, 36Ar, 84Kr) can be used to understand gas-to-water volumes and migration. Further, they have well-understood abundances and production in the hydrosphere and crust, and the temperature-controlled release of radiogenic noble gases from mineral grains into pore fluids can help understand hydrocarbon evolution. We present hydrocarbon molecular (C1/C2+) and stable isotopic compositions (δ13C-CH4, δ13C-C2H6), and noble gas isotopic (e.g., 3He/4He, 4He/40Ar*, 20Ne/36Ar) composition from well gases in the Appalachian Basin and the Fort Worth Basin that display both normal stable isotopic compositions and reversals. Our data suggest reversed gases were produced from closed systems and retain greater amounts of the ASW components compared to conventionally produced gases.