Yongqi Ruan¹, Dongya Zhu², Quanyou Liu¹, Mingming Jiang¹, Tao Luo¹

(¹School of Earth and Space Sciences, Peking University, Beijing 100871, China; ²Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 102206, China)

Abstract: Serpentinization is widely regarded as the dominant geological mechanism of natural hydrogen generation, and its hydrogen-generating capacity is under joint kinetic and thermodynamic control: the reaction rate governs hydrogen production per unit time (kinetic control), whereas the total hydrogen a system can release depends on its inventory of oxidizable Fe²⁺ (thermodynamic control). Conversion efficiency and hydrogen generation potential, the two quantities central to the assessment of natural hydrogen systems, correspond respectively to these two controls. In nature and in the laboratory, however, serpentinization rarely proceeds to completion: cumulative hydrogen yield does not reach the ceiling set by thermodynamics and remains limited by reaction rate and reaction extent, so that the two controls are coupled. Previous work has largely addressed the effect of individual variables on hydrogen generation, without separating their influence on reaction rate from that on cumulative yield under the combined action of multiple parameters (temperature, pressure, grain size, water-to-rock ratio, salinity and initial pH), and without establishing which factors dominate each response or how that dominance shifts with conditions. We therefore compiled 123 sets of experimental and thermodynamic-modelling data on serpentinization published between 1970 and 2026 and trained a multi-parameter random forest model, using SHAP (SHapley Additive exPlanations) analysis to quantify the relative contribution of each parameter. Reaction rate and cumulative hydrogen yield are controlled by different parameters, and the relative roles of kinetics and thermodynamics change as reaction proceeds: once normalized for grain size, reaction rate is governed mainly by pressure and temperature (combined importance ~94%) and peaks at 250–300 °C, whereas cumulative yield is governed mainly by grain size, water-to-rock ratio and salinity (~73%). The control exerted by grain size weakens as reaction approaches completion, indicating a shift from kinetic limitation in incompletely reacted systems to thermodynamic control by the total oxidizable Fe²⁺ once reaction is sufficiently advanced. Natural hydrogen systems can accordingly be divided into a “rate-dominated” type, sustained by high reaction rates, and an “accumulation-dominated” type, enriched through long-term net accumulation. The former occurs mainly in tectonically active settings and is typified by ultramafic-hosted hydrothermal systems at mid-ocean ridges, such as the Rainbow field on the Mid-Atlantic Ridge, for which exploration criteria emphasize heat flow and deep-seated migration pathways. The latter is expected in intracratonic settings such as greenstone and ophiolite belts, where the volume of reactive rock, the persistence of water recharge and sealing capacity become decisive; the configuration of reservoir, fault-controlled migration and dolerite seals at the Bourakebougou hydrogen field in Mali provides a geological analogue. By distinguishing rate-based from yield-based measures of hydrogen generation during serpentinization and defining the controlling factors and conditions of application of each, this study offers a complement to hydrogen generation potential assessments based on lithological assemblages.

Keywords: Natural hydrogen; Serpentinization; Machine learning; Kinetic–thermodynamic coupling; Hydrogen generation potential

Author Profile (first or corresponding author):

Yongqi Ruan, male, MSc candidate, whose research focuses on the genetic mechanisms and accumulation conditions of natural hydrogen and helium. E-mail: 2401210198@stu.pku.edu.cn