Stepwise pyrolysis-GC-MS for the geochemical characterization of potential source rocks

Source rocks may contain organic-matter and, under suitable conditions, generate hydrocarbons that migrate to reservoir rocks. Pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) with direct sample introduction (DSI) resolves the compounds released under controlled heating: the pyrolyzer reproduces, in minutes, the thermal cracking that generates hydrocarbons in the subsurface, while DSI admits the whole powdered rock, with no prior extraction or fractionation. In the stepwise approach, one aliquot is heated in successive temperature steps, each analyzed separately, separating free and labile compounds from those generated only at higher temperatures, enabling the evaluation of bitumen and potentially generated hydrocarbons from the kerogen. This study assessed how the thermal regime controls the release and transformation of organic compounds in a potential source rock (Type I kerogen, TOC > 25 wt%), simulating the stepwise approach with a single-shot accessory. The pyrolyzer (PY-3030D, Frontier Lab) operated at 100–500 °C with 40 mg of rock. Analysis used a GC-MS (2010Plus-QP2020, Shimadzu); the oven was held at 30 °C (5 min) and raised at 15 °C min^-1 to 320 °C (40 min) with an HP-PONA column (50 m × 0.20 mm × 0.50 µm). The injector was at 300 °C (split 1:25), helium flow 1.05 mL min-1, ionization source (70 eV) at 250 °C. Total ion chromatograms (TIC) and selected ions (m/z 57, 191, 217) supported qualitative analysis. Thermal desorption at 100–150°C yielded low, poorly structured signals, indicating only a small amount of loosely bound, volatile hydrocarbons. At 200–250°C, hydrocarbons associated with the bitumen fraction were volatilized. Primary pyrolysis of kerogen began at 300–400°C, with the main generation interval extending to 500°C. Above 350 °C, the most resistant fractions continued to pyrolyze, yielding broader, less resolved peaks. The integrated area rose markedly from intermediate temperatures, with near-exponential growth of the cumulative area (TIC and m/z 57), reflecting the efficiency of this range in cleaving linear aliphatic chains. Hopanes (m/z 191) showed the most gradual, distributed cumulative growth, contributing across a broader thermal range, whereas steranes (m/z 217) required higher temperatures for efficient release, consistent with their greater structural stability. For this sample, the 300–400 °C interval was the most informative about the generating capacity of the kerogen.Overall, the results demonstrate that stepwise Py-GC-MS provides a temperature-resolved view of source-rock organic matter, distinguishing the release of free and bitumen-associated compounds from the progressive generation of hydrocarbons through kerogen cracking. The distinct thermal responses of aliphatic hydrocarbons and biomarker-related ions further highlight the potential of this approach to assess both hydrocarbon-generating capacity and the accessibility and thermal stability of different organic fractions within the rock.