Evolution of Gas Generation and Secondary Hydrocarbon Cracking During Artificial Maturation of a Brazilian Type-I kerogen

The thermal evolution of organic matter controls the progressive generation of oil and gas in petroleum source rocks. At advanced maturity, gas generation may result from continued kerogen transformation and secondary cracking of previously generated hydrocarbons. Previous hydrous pyrolysis studies on Type I kerogen from the Araripe Basin have characterized petroleum generation within the oil window, prior to extensive secondary gas generation. However, the evolution of this organic matter under advanced thermal conditions and the relative contributions of residual kerogen and secondary petroleum cracking to late gas generation remain less constrained. This study investigates gas evolution during artificial maturation of an initially immature, Type I kerogen-rich lacustrine source rock from the Aptian interval of the Araripe Basin, NE Brazil. The original rock has a total organic carbon (TOC) content of 19.73 wt.%, hydrogen index (HI) of 711 mg HC g⁻¹ TOC, and Tmax of 421 °C, confirming its high organic richness, oil-prone character, and low thermal maturity. High-pressure, high-temperature reactors were used to generate a controlled thermal maturation series by anhydrous pyrolysis. Approximately 8 g of powdered source rock was subjected to 350, 400, 450, 500, 550, 600, 650, and 700 °C for 72 h under autogenous pressure. These temperatures intentionally exceed typical geological conditions to accelerate organic-matter transformation and access advanced maturity stages on laboratory timescales. Generated gases were analyzed by GC-FID/TCD, while original and artificially matured rocks were characterized by Rock-Eval pyrolysis, TOC, elemental analysis, and SEM-EDS. Selected samples were also subjected to solvent extraction, and the recovered organic fractions were pyrolyzed separately to investigate their thermal stability and secondary gas-generation potential. Preliminary results indicate an overall increase in total gas generation with temperature, with yields ranging from approximately 2 to 11 mg g⁻¹ of rock. Hydrocarbons ranging from C1 to C7 were detected in the gas phase, with a clear predominance of C1–C4. Increasing thermal severity produced a progressive shift toward a drier hydrocarbon gas composition, characterized by an increasing relative contribution of methane compared with higher molecular-weight hydrocarbons. Substantial changes were also observed in CO₂ and H₂ with increasing temperature. These results demonstrate the ability of the experimental system to access progressively advanced stages of organic-matter transformation. Integration of residual source-rock properties, gas yields and composition, and extractable organic matter provides a basis for distinguishing the relative contributions of continued kerogen transformation and secondary hydrocarbon cracking during advanced thermal maturation.