An integrated approach using stepwise Py-GC-MS and Rock-Eval pyrolysis for oil generation prediction

The assessment of organic matter (OM) thermal maturation is a fundamental factor in petroleum system exploration. Bulk pyrolysis parameters describe the extent of this evolution, but do not resolve which compounds are released at each stage. This study applied stepwise pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) to source-rock samples of the Taubaté Basin/Tremembé Formation previously submitted to different artificial maturation series by hydrous pyrolysis (HP), in order to describe the molecular profile of the organic matter present and generated and correlate it with the parameters estimated by Rock-Eval. The original source-rock (non-pyrolyzed) sample and three HP conditions were evaluated (300°C for 144h, 340°C for 36h, and 340°C for 108h), allowing for continuous monitoring of kerogen transformation. Each rock sample (5 mg) was analyzed in a pyrolyzer (PY-3030D, Frontier Lab) operating in eight stages (100, 200, 300, 325, 350, 400, 500 and 600°C). GC-MS (GC7890A-MS7000C Triple Quad, Agilent^®) analysis used an HP5-MS column (30 m x 250 µm x 0.25 µm), with the oven programmed from 40 to 320 °C at 10 °C min^-1. Data were acquired in full-scan mode (m/z 35-500 Da). Rock-Eval results revealed that the non-pyrolyzed sample exhibited excellent generative potential, featuring a Hydrogen Index (HI) > 700 mg HC/g total organic carbon (TOC) and T_max of ± 434°C. As thermal stress progressed, a reduction in the residual potential (S2 and HI) and an increase in T_max were observed, reaching ± 444°C, consistent with classic petroleum generation kinetic models. The peak of oil generation was identified at 340°C/36h, characterized by an increase in the production index (PI) and the S1 parameter, whereas more severe thermal conditions (340°C/108h) indicated the onset of secondary oil cracking. The experiments using the stepwise Py-GC-MS approach and following the n-alkane related mass fragment at m/z 57 allowed for the thermal partitioning of the aliphatic yield, quantifying the transition of compounds from the high-temperature fraction (≥400°C - kerogen) to the low-temperature fraction (≤300°C - bitumen/free hydrocarbons). Notably, a high linear correlation (r = 0.996) was observed between the high-temperature aliphatic fraction and the HI measured by Rock-Eval. These data confirm the effectiveness of stepwise Py-GC-MS as a reliable tool for simulating thermal evolution of OM in maturation processes and strongly validate bulk parameters, with the added advantage that Py-GC-MS allows for the identification of the compounds generated at each peak (S1 and S2). Although the correlation presented here was established for the n-alkanes, other mass fragments can also reveal the fate, composition and production profile of other hydrocarbon classes in the rock. The integration of these approaches provides a powerful predictive tool for elaborating hydrocarbon generation models, ensuring greater accuracy in basin evaluation and exploration.