Yang Guangkun1,2, Liu Chenglin1,2, Chen Jianfa1,2, Fan Liyong3, Kang Rui3, Wang Haidong1,2, Liu Peng1,2, Yang Zixiang1,2, Zhang Xue1,2
(1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China; 2College of Geosciences, China University of Petroleum, Beijing 102249, China; 3Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, Shaanxi, China)
Abstract: Driven by global carbon neutrality goals, natural hydrogen (H2) has attracted increasing attention as a zero-carbon energy source. The Ordos Basin is a major cratonic sedimentary basin in China, where the genesis of hydrogen associated with Paleozoic natural gas is complex and difficult to discriminate effectively using a single indicator; therefore, multi-isotope synergistic constraints are urgently needed.
This study is based on 86 natural gas samples from 9 gas fields/blocks in the Ordos Basin, covering the main gas-bearing strata of the Ordovician Majiagou Formation and the Carboniferous-Permian Shanxi and Shihezi formations. Compositional analysis was performed on all samples, and selected samples were further analyzed for carbon and hydrogen isotopes of methane (δ13C-CH4, δ2H-CH4), hydrogen isotopes of H2 (δ2H-H2), and noble gas isotopes (e.g., 3He/4He). The results show that: (1) The H2 content in the Paleozoic strata of the study area ranges from 0.008% to 0.350% (average 0.046%), generally exhibiting a “basin-margin high, basin-interior low” planar distribution pattern. The average hydrogen content in the Upper Paleozoic is slightly higher than that in the Lower Paleozoic. (2) 3He/4He ratios (R/Ra = 0.018-0.036) indicate an overwhelmingly dominant crustal helium contribution. δ13C-CH4 values (−39.4‰ to −27.6‰) and δ2H-CH4 values (−198‰ to −168‰) are characteristic of a thermogenic cracking gas origin. δ2H-H2 values are consistent with the ranges of organic matter pyrolysis and water-rock reactions.
Integrated multi-indicator constraints suggest that natural hydrogen in the study area is predominantly derived from thermal decomposition of organic matter, with possible contributions from basement water-rock reactions. Contributions from water radiolysis and mantle degassing are negligible. The “basin-margin high” distribution pattern spatially coincides with basement fault zones at the basin margins, suggesting that faults may have served as migration pathways for H2 generated by processes such as water-rock reactions. The “basin-interior low” pattern indicates a dominance of organic matter thermal decomposition, which is widespread but of low abundance, limiting the prospects for industrial accumulation. These insights provide new perspectives for natural hydrogen exploration in cratonic basins: while acknowledging the contribution from organic matter pyrolysis, greater focus should be placed on tracing hydrogen sources generated by water-rock reactions associated with mafic-ultramafic minerals such as serpentinite. This has direct implications for delineating favorable H2 accumulation zones and optimizing exploration strategies.
Keywords: Natural hydrogen; Ordos Basin; Multi-isotope constraints; Genetic mechanism
Author Profile (first author):
Yang Guangkun, Male, Ph.D. candidate, mainly engaged in natural gas geochemistry research. E-mail: yang.guang.kun@qq.com.
