Sulfur Geochemistry of Pre-Salt Lacustrine Source Rocks: Insights from the Campos and Santos Basins, Brazil

Understanding sulfur compounds in source rocks is essential to the study of petroleum systems and vital for predicting hydrogen sulfide (H₂S) hazards during oil production, which cause severe operational, health, environmental, and oil quality risks. This study integrates inorganic, organic, and stable sulfur isotope data from 42 pre-salt lacustrine source rock samples (21 from three wells in the Campos Basin and 21 from three wells in the Santos Basin). Samples were analyzed using x-ray diffraction (XRD), x-ray fluorescence (XRF), elemental analysis (TOC, TS), Rock-Eval pyrolysis, chromium-reducible sulfur (CRS) extraction, and sulfur isotopic composition in pyrite (δ³⁴Scrs) in order to understand the sulfur cycle in the sedimentary environment, as well as the sources and processes of sulfur incorporation into the inorganic and organic fractions of the rocks. Major elements (Ca, Si, Mg, Fe, Al, K, S, Mn, and Ti) correspond to the presence of calcite, quartz, dolomite, gypsum, muscovite, and pyrite, revealing a predominantly carbonate depositional setting. In both basins, redox-sensitive trace elements (V, Ni) and CRS values (0.15% to 0.76%) indicate persistent reducing conditions that favored organic matter preservation and pyrite precipitation within the lacustrine depositional environment. All samples exceed the minimum TOC threshold for oil generation; values from 0.56% to 10.58% indicate very good to excellent organic matter richness. Rock-Eval pyrolysis classifies the organic matter as type I kerogen (10 samples), type II (22 samples), and type III (10 samples). Thermal maturity (T_max) shows an immature stage for 15 samples, while 27 have reached the early oil generation window. Total sulfur (0.69% to 2.99%) incorporates a significant organic sulfur fraction (0.04% to 0.73%), representing over 40% of the reduced sulfur in some samples; both TS and S_org strongly correlate with TOC. In the Campos Basin, δ³⁴Scrs reveals enriched values (13.09‰ to 23.50‰) compared to contemporary seawater sulfate (~+21‰). In conclusion, the integration of sulfur geochemistry demonstrates that pre-salt reducing conditions promoted excellent organic matter preservation and active sulfurization of labile organic matter across both basins. Furthermore, the low concentrations of terrigenous-proxies (Al, Fe, K, and Ti) compared to Ca reflect a limited clastic influx into the depositional environment. Kerogen types and T_max values indicate significant variations in organic matter accumulation, preservation, and hydrocarbon generation potential among the studied wells. While the strong δ³⁴Scrs enrichment, alongside the observed presence of evaporitic minerals, suggests thermochemical sulfate reduction (TSR) as a possible source of sulfide in the Campos Basin, the imminent acquisition of sulfur isotope data for the Santos Basin will expand this regional comparative model. This will provide a framework to predict fluid souring risks and optimize exploration strategies.