THE UTILITY OF NOBLE GASES IN UNDERSTANDING HYDROCARBON STABLE ISOTOPE REVERSALS

The growth of oil and gas production in the last two decades from unconventional reservoirs such as tight shales and sands has shown that reversals in the stable carbon isotopic signatures (δ¹³C-methane > δ¹³C-ethane) of well gases are prevalent 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 cause of 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 data 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.