Hydrogen generation during hydrous pyrolysis of a Brazilian Type-I kerogen

Hydrous pyrolysis (HP) is an experimental maturation technique used to simulate the thermal evolution of source rocks and the associated generation of oil and gas under controlled conditions. Among the generated products, molecular hydrogen (H₂) has received increasing attention, although its generation mechanisms and relationship with source-rock maturity remain poorly understood. This study investigated H₂ generation during HP of an immature Type I kerogen-rich lacustrine-lagoon shale from the Aptian interval of the Araripe Basin, NE Brazil, as a function of thermal maturity. The initial rock had a total organic carbon (TOC) content of 19.73 wt.%, S1 and S2 values of 3.26 and 139.89 mg HC g⁻¹ rock, respectively, a hydrogen index (HI) of 711 mg HC g⁻¹ TOC, confirming its high organic richness, oil-prone character, and low thermal maturity. Rock samples were subjected to HP from 280 to 365 °C for 72 h. Generated gases were quantified by gas chromatography, while solid residues were analyzed by Rock-Eval pyrolysis. H₂ yield increased from 37 µmol g⁻¹ TOC₀ at 280 °C to a maximum of 224 µmol g^-1 TOC₀ at 350 °C, followed by a slight decrease to 190 µmol g⁻¹ TOC₀ at 365 °C. In contrast, the H₂ mole fraction reached a maximum of 6.4% at 320 °C and subsequently decreased. CO₂ was the dominant gaseous product throughout the experiments, increasing from 732 to 2352 µmol g⁻¹ TOC₀ between 280 and 365 °C. Gaseous hydrocarbon generation also increased markedly with thermal maturity. Methane was the dominant hydrocarbon, representing approximately 61–69% of gaseous hydrocarbons, and increased from 112 to 2013 µmol g⁻¹ TOC₀ over the same temperature interval. Gaseous hydrocarbons were predominantly saturated, with olefins representing on average only 0.86% of the hydrocarbons. Thus, while CO₂ and gaseous hydrocarbon yields continued to increase considerably with temperature, H₂ exhibited a distinct evolution, reaching a maximum yield around 350 °C followed by stabilization or a slight decline. The difference between the temperatures of maximum H₂ mole fraction and maximum absolute yield demonstrates that its relative abundance does not directly reflect its absolute generation. The continued increase in both CO₂ and gaseous hydrocarbons, particularly CH₄, progressively increases their contribution to the total gas phase, contributing to the decline in relative H₂ abundance at higher maturity even while its absolute yield remains high. These results demonstrate that Type I organic-rich source rocks can generate measurable H₂ during maturation and reveal distinct temperature-dependent patterns between its absolute yield and relative abundance.