Yang Yanan¹, Yang Chengyu¹, Liu Xiaoqiang¹, Zhao Yuhan¹, Yan Bo¹
(¹ College of Petroleum, China University of Petroleum-Beijing at Karamay, Karamay 834000, Xinjiang, China)
Abstract: Highly evolved natural gases in deep to ultra-deep basins (e.g., Sichuan, Appalachian, Western Canada) often display isotopic patterns that conventional thermal cracking cannot explain: methane δ¹³C heavier than residual kerogen or solid bitumen, accompanied by relatively light methane δD and CO₂ δ¹³C. This coupled carbon–hydrogen isotope anomaly suggests that late-stage gas generation in deeply buried systems may involve processes beyond simple kerogen or oil cracking.
Hydrogen availability is a key constraint for hydrocarbon generation in overmature source rocks. With increasing maturity, kerogen loses hydrogen and becomes carbon-rich; highly evolved kerogen commonly has an H/C ratio below 0.8, whereas methane has an H/C ratio of 4. Continued methane generation therefore requires an additional hydrogen source. Basin fluids, especially water-derived hydrogen, may provide this source under deep burial conditions.
To test this possibility, preliminary hydrous pyrolysis experiments were conducted using a dual-gold-capsule system. The fluid redox state was buffered by magnetite–hematite, nickel–nickel oxide, cobalt–cobalt oxide, and molybdenum–molybdenum trioxide assemblages, covering a range from sedimentary-basin-like conditions to more reducing, H₂-rich fluids.
The results show that hydrocarbon yield increases with fluid reducing capacity. The generated gas is methane-dominated, with C₁/C₁⁺ values above 0.98, resembling highly evolved natural gases. As reducing conditions strengthen, methane δ¹³C becomes lighter and CO₂ δ¹³C becomes heavier. Under weakly reducing conditions, methane δ¹³C can be heavier than residual kerogen, reproducing a key isotope anomaly observed in natural deep gas.
Although methane δD was not measured in this preliminary work, involvement of external hydrogen-bearing fluids implies that methane hydrogen isotopes should be influenced by the reacting fluid as well as by organic matter. This offers a plausible explanation for the coexistence of heavy methane δ¹³C and light methane δD in some overmature reservoirs.
These findings indicate that organic–inorganic interactions may constitute an important methane-forming pathway in deep basins. The coupled carbon–hydrogen isotope anomaly may therefore serve as a diagnostic marker for identifying this process and should be considered when evaluating highly mature source rocks and interpreting the origin of highly evolved natural gas.
Keywords: Deep strata; Highly evolved natural gas; Isotopic anomaly; Hydrous pyrolysis; Organic–inorganic interaction
Author Profile (corresponding author):
Yang Yanan, male, born in 1990, lecturer, mainly engaged in oil and gas geochemistry research. E‑mail: yny@cupk.edu.cn.
