Clumped and conventional isotopes of natural gas reveal basin burial, denudation, and biodegradation history

Abstract

Formation and post-genetic alteration of hydrocarbons provide insights into the dynamic and complex geologic, hydrologic, and microbial history of shallow crustal environments. Clumped isotopologues of methane (e.g., Δ13CH3D) have emerged as a proxy for constraining methane formation temperatures in sedimentary basins. However, unrealistically high apparent temperatures and microbial cycling of methane necessitate further investigation into how the generation and biodegradation of hydrocarbons may modify methane clumped isotopologue signatures. This study analyzed and modeled the clumped isotopologues of methane, in addition to traditional gas isotopes, to provide new insights into the origin, thermal maturity, migration, and biodegradation histories of hydrocarbons in the Paradox Basin in the Colorado Plateau. The basin was deeply buried in the geologic past and has been recently incised, leading to rapid denudation, enhanced meteoric circulation, and microbial activity. δ13CCH4 and CH4/ΣC2+ ratios suggest that most natural gases in various reservoirs throughout the basin are thermogenic in origin with variable thermal maturities. However, signatures suggestive of anaerobic oxidation of ethane and propane, and secondary microbial methane generation, exist. In the northeastern part of the basin, Δ13CH3D values in reservoirs above and below the Paradox Formation source rocks are consistent with thermodynamic equilibrium, indicating that the thermally mature hydrocarbons equilibrated at ≥160 °C during maximum burial over 30–80 Ma. Disequilibrium Δ13CH3D values of natural gas in Paradox Formation reservoirs along the southwestern margin of the basin suggest the presence of low-maturity hydrocarbons consistent with the region’s shallower burial history. Models of Δ13CH3D values based on the exchange rate of hydrogen isotopes between methane and water and the basin thermal history support that meteoric recharge and microbial activity, following incision/denudation over the past few million years, promoted anaerobic oxidation of hydrocarbons (particularly ethane and propane), biodegradation of crude oil, and generation of secondary microbial methane in shallow reservoirs.