Hydrogen distributions during thermal maturation of organic-rich sedimentary rocks: Generation potentials and influencing factors of hydrogen-bearing gases revealed by pyrolysis

Abstract

Hydrogen derived from organic-rich sedimentary rocks plays important roles in conventional and green energy applications, energy utilization, and atmospheric pollution. Although thermal simulation experiments have preliminarily revealed the mechanisms of hydrogen generation, the understanding of its migration from source rocks and accumulation processes remains scarce, which severely constrains exploration practices. This study conducted high-pressure gold-tube (closed-system) pyrolysis at 600 °C to investigate three series of source rocks with different maturities, obtained by artificial maturation. The work examined the amount of hydrogen generated during pyrolysis of source rocks. We carefully quantified hydrogen in its multiple phases and evaluated the influencing factors of hydrogen-bearing gases and H₂ generation potential. Firstly, organic hydrogen (TOH) contents for up to 36.2–72.2% of total hydrogen (TH) in low-mature original source rocks. When thermally evolved to a high maturity (Easy %Ro 1.98), a significant reduction in TOH is observed (by 62–76% of the original TOH), while the inorganic hydrogen (TIH) remains relatively stable. Moreover, when source rocks evolved to Easy %Ro 4.45, hydrogen is primarily converted into CH₄ (up to 40.8% relative to the TH of original source rocks), much higher than H₂ (up to 1.2%). In contrast, in high‑sulfur source rocks, due to the competition of sulfur for hydrogen, the percentage of hydrogen in H₂S generated (5.8%) exceeds that of H₂. Secondly, the yields of H₂ and CH₄ exhibit a strong positive linear correlation with HI values of Rock-Eval analysis, whereas H₂S yields also depend on the source rock's sulfur content. For type I kerogen with significantly higher HI, the H₂ yield is greater than that of type II. However, for samples classified as type II kerogen, the HI value alone does not fully indicate the magnitude of H₂ yield. The thermal maturation process alters the organic matter structure, thereby changing the H₂ generation characteristics of different source rocks. Moreover, H₂ and CH₄ generation is influenced by the pyrolysis conditions. Open-system pyrolysis generally exhibits higher H₂/CH₄ molar ratios than closed-system pyrolysis, which may be due to much higher temperatures adopted in open-system pyrolysis, as well as the occurrence of hydrogenation reactions in closed systems. Finally, hydrogen distribution analysis indicates that source rocks retain significant H₂ generation potential even with thermal evolution to a post-mature stage (Easy %Ro > 4.45), with a maximum yield of approximately 7.4–25.5 mg/g TOC upon complete conversion of residual organic hydrogen to H₂. Notably, post-mature source rocks, initially having type II kerogen rich in condensed aromatic structures, demonstrate a higher residual potential for H₂ generation compared to those having type I kerogen. This study helps to refine the estimation of the H₂ generation potential in source rocks.