Whole-Rock Molecular and Biomarker Characterization of Tremembé Formation Outcrop Samples by Stepwise Analytical Pyrolysis (Py-GC/MS)

Pyrolysis coupled to gas chromatography-mass spectrometry (Py-GC/MS) allows direct molecular analysis of source (or potential source) rocks without sample preparation, determining hydrocarbon presence and abundance through desorption of free hydrocarbons or pyrolysis of kerogen equivalent to S1 or S2 Rock-Eval parameters, respectively, depending on temperature. However, in single-temperature experiments both phenomena may occur simultaneously, challenging to establish whether the hydrocarbons detected were already present as free bitumen or were generated from kerogen. Moreover, molecular characterization, particularly of biomarkers, can be hindered by occlusion or strong retention in the rock matrix, so that they are not released under given experimental conditions. In this work, original rock sample and post-hydrous-pyrolysis rock residues generated by different temperature–time conditions (300–340 °C, 36–144 h) from organic-rich lacustrine shales from Oligocene–Miocene Tremembé Formation were subjected to Py-GC/MS experiments. A micro-furnace multi-shot pyrolyzer coupled to a GC/MS-MS triple quadrupole was operated in a stepwise mode where a sample aliquot was heated in consecutive increments of temperature from 100 to 600 °C. Therefore, each step probes the components remaining from the previous condition, releasing free and labile compounds at lower temperatures and those from kerogen or occluded in the matrix at higher ones. The objective was to determine which classes each thermal domain releases (n-alkanes, hopanes, steranes and alkylbenzenes) and how their distribution and abundance vary with artificial maturation. A single desorption step at 300 °C of the original rock sample yielded virtually no hopanes or steranes. The stepwise series results indicated that these biomarkers were present but not free: hopanes were detected in stepwise series from 325 to 500 °C, whereas steranes required even higher temperatures and, throughout the series, remained at very low relative abundance compared to hopanes, reflecting lacustrine organic matter input. A C₁₃–C₃₈alkylbenzene series was also identified in relatively high abundance; as fatty acids and alcohols had been proposed as precursors. Therefore, alkylbenzenes may relate to those precursors preserved during deposition of the Tremembé Formation. In the post-hydrous-pyrolysis rock residues, similar biomarker distributions were observed at 300 °C, with a higher total hopane area than that in the original sample, in which biomarkers were not released by desorption, indicating possibly occlusion or strong matrix retention of these compounds, and also the effect of previous hydrous-pyrolysis enhancing hydrocarbon generation and expulsion. The n-alkanes shifted from generation-dominated (400–500 °C) to desorption-dominated (200–300 °C), consistent with Rock-Eval results. The technique revealed how biomarkers are hosted and how accessible they become with the increase of temperature.