Liya Feng 1,2, Chengsheng Chen 1, Yunpeng Wang 1, Yun Li 1, Xuan Yang 1,2, Shiwen Deng 1,2, Qiang Wang 1, Zheng Zhou 3
(1. State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.
2. University of Chinese Academy of Sciences, Beijing, 100049, China.
3. Lancaster Environmental Centre, Lancaster University, Lancaster, LA14YQ, UK.)
Abstract: Soil hydrogen (H2) anomalies in sedimentary basins hold significant implications for potential deep energy resources and geodynamic processes. The Sanshui Basin, located along the continental margin of South China, is characterized by extensive Cenozoic mafic volcanic rocks, fault systems, and active groundwater systems, providing favorable conditions for natural hydrogen generation and migration. Previous soil gas surveys have identified significant H2 anomalies, but their origins and deep migration processes remain poorly constrained. We collected 7 soil gas samples from high H2 flux zones in the central basin and its margin along main faults. Gas compositions and isotopes (δ13CCO2, δ2HH2, 3He/4He, 20Ne/21Ne/22Ne) were analyzed to trace H2 sources and migration mechanisms. The results show that H2 concentrations in the soil gas reach up to 1437 ppm. Measured 4He and 20Ne significantly exceed atmospheric levels, and the 4He/20Ne and Ne isotopic compositions, after correction for air and air saturated water (ASW) contributions, documented the groundwater dissolution-degassing process. Measured 3He/4He ratios (0.54-0.92 Ra) indicate a predominantly crustal origin, with an estimated crustal contribution of 94.6–98.7%. The δ13CCO2 values range from -21.86‰ to -11.71‰, combined with CO2/3He ratios, these values indicate that CO2 is mainly derived from inorganic carbon released by water-rock interaction in mafic rocks, mixed with CO2 from organic sediments. Partial CO2 dissolution and calcite precipitation occurred at formation temperatures of 50-150°C. δ2HH2 values in samples (-799.52‰ to -676.19‰), together with H2/3He systematics, indicate a predominantly crustal origin and are consistent with H2 generated by serpentinization reported worldwide. Based on these integrated geochemical constraints, we establish a genetic model for natural hydrogen generation and soil H2 anomaly formation in the Sanshui Basin. Soil H2 anomalies are mainly generated by water-rock interaction (serpentinization) between Cenozoic mafic rocks and groundwater, rather than by active mantle degassing or biological processes. H2 and He are transported to shallow depths via groundwater as a common carrier and undergo phase partitioning due to differences in solubility. He preferentially exsolves into the gas phase and migrates rapidly along fault zones, while H2 remains predominantly in the dissolved phase, migrating with the hydraulic gradient and being released to the surface through shallow strata. This study reveals the genetic and groundwater-mediated migration mechanisms of soil H2 anomalies in the Sanshui Basin and provides new insights and geochemical tracers for studying natural hydrogen systems in analogous intracontinental rift basins.
Keywords: Natural hydrogen; Soil gas; Noble gas isotopes; Water-rock interaction; Groundwater-mediated migration; Sanshui Basin
Author Profile:
Liya Feng, Female, PhD student, mainly engaged in natural hydrogen and noble gas geochemistry research. E-mail: fengliya@gig.ac.cn
