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Geochemical origins and post-emplacement alteration of natural gases in ultra-deep Ediacaran–Cambrian reservoirs, Sichuan Basin
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DOI：10.1016/j.gca.2026.07.008.png)
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Abstract
En 中文
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.
Keywords:
Deep-ultra deep
Isotope reversal
Rollover
Mixing
Carbonate reservoir gases
Sichuan Basin
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