Abstract
Natural gases from deep to ultra-deep Ediacaran–Cambrian carbonate reservoirs in the Sichuan Basin exhibit complex geochemical signatures, including widespread isotope reversals, inconsistent with conventional single-source maturation models. These features reflect the combined effects of multi-source mixing and intense post-emplacement thermal alteration. Integrated geochemical data and quantitative modeling constrained by thermal simulation experiments indicate that the gases are multi-source mixtures in which external oil-cracking gas provides the dominant charge (typically 40–60%), with additional contributions from in situ source rock-derived thermogenic gas. In the most thermally mature reservoirs, extreme isotopic signatures result from intense fluid-mediated alteration, characterized by near-complete ethane cracking (δ13C2 of up to –23.6‰) and hydrogen isotope rollover (δ2H-CH4< –155‰) caused by methane-water interaction. Quantitative modeling results further show that diffusive loss, while significant for the gas volume, played a subordinate role in the isotope fractionation. In this study, we established a multi-stage genetic model for these gases, in which an initial mixing event was subsequently overprinted by intense, in situ geochemical reactions.
Paper Information:
Ziqi Feng, Chenzhao Quan, Fang Hao*, Huayao Zou, Mingxiang Yang, Haohao Wang, Xiang Piao. Geochemical origins and post-emplacement alteration of natural gases in ultra-deep Ediacaran–Cambrian reservoirs, Sichuan Basin. Geochimica et Cosmochimica Acta, 2026, 427: 76-89.
