Combined Open and Closed System Pyrolysis to Evaluate the Kinetics of Petroleum and Late Gas Generation

Lower Cretaceous source rocks from the Araripe Basin, containing Type I kerogen, exhibit excellent oil generation potential and can be used as analogues for the rift-phase source rocks responsible for a significant portion of the oil generated along the Brazilian continental margin. Previous hydrous pyrolysis studies established reference kinetic parameters for petroleum generation in this lithology, with an activation energy (Ea) of 52 kcal/mol and a frequency factor (A) of 6.4 × 10¹³ s⁻¹ under open-system conditions. As source rocks reach greater depths, lighter hydrocarbons (mostly methane) and inorganic gases become the major products of thermal decomposition of mature kerogen, residual oil, and expelled oil undergoing secondary cracking. This study investigates the kinetic parameters associated with residual gas generation potential as thermal maturity increases in a source rock containing Type I kerogen. The source rock was initially subjected to hydropyrolysis at 355°C for 72 hours to generate oil, and the resulting extracted residual source rock was subsequently pyrolyzed in an open system using Rock-Eval 7S analysis to obtain kinetic parameters (activation energy and frequency factor) for late-stage gas generation. Rock-Eval 7S pyrolysis was carried out at four heating rates (5, 10, 15, and 25 °C/min), in triplicate. Kinetic parameters were determined by non-linear regression using the Multi-Kinetics module, adopting a discrete activation-energy distribution between 40 and 80 kcal/mol, and by the Kissinger method. Both approaches were applied to the original immature rock and to residual rocks from hydrous pyrolysis isotherms previously characterized by routine Rock-Eval analyses. The immature rock yielded an Ea of 53.18 kcal/mol and A of 1.52 × 10¹³ s⁻¹ by the Kissinger method. Multi-Kinetics modeling produced a spectrum with a dominant peak at 52–53 kcal/mol and A of 9.25 × 10¹⁴ s⁻¹. The residual source rock presented an Ea of 50.9 kcal/mol and A of 6.3 × 10¹² s⁻¹ by the Kissinger method. Its Multi-Kinetics spectrum was wider, with a dominant peak at 54–56 kcal/mol, A of 9.25 × 10¹⁴ s⁻¹, and fractions extending up to 68 kcal/mol. These values are consistent with those previously reported for Araripe source rocks, supporting the reliability of the experimental protocol and adopted kinetic approach. As hypothesized, the hydrous-pyrolyzed rock exhibited a broader activation-energy distribution shifted toward higher values, consistent with preferential depletion of the most reactive kerogen fractions and relative enrichment of kinetically more refractory components. The derived kinetic parameters may thus provide insight into the residual cracking behavior and late-stage gas-generating potential. Combining a closed system (hydrous pyrolysis) with an open system (Rock-Eval 7S) provides a promising experimental approach for investigating the late-stage gas-generating potential of residual source rocks with increasing thermal maturity.