Depositional Paleoenvironment and Petroleum Generation Potential of Limoeiro Formation, Foz do Amazonas Basin, Brazil, through Organic Geochemistry

The Foz do Amazonas Basin is located on the Brazilian Equatorial Margin, bounded by the Guyana-Suriname Basin to the northeast and the Pará-Maranhão Basin to the southeast. Recent discoveries of oil accumulations in basins sharing the same tectonic setting and geological analogy with the Foz do Amazonas Basin have sparked economic interest in the region, which is now considered a new exploration frontier. Cretaceous shales belonging to the Limoeiro Formation occur within the Foz do Amazonas Basin; these are organic-rich and favorable for petroleum generation, making them the basin's primary potential source rock. This study aims to evaluate the depositional paleoenvironment, hydrocarbon generation potential, and thermal maturity of the Limoeiro Formation shales in the Foz do Amazonas Basin through the organic geochemical characterization. To this end, data from 289 samples collected from four wells drilled along the basin platform were used, comprising total organic carbon (TOC) analyses, Rock-Eval pyrolysis, vitrinite reflectance (%Ro), saturated biomarkers, and carbon isotopes, provided by the National Agency of Petroleum, Natural Gas and Biofuels (ANP).The TOC contents in the collected samples range from 0.12% to 4.40%, indicating variations in the conditions of organic matter production and/or preservation; S2 values ​​range from 0.14 mg HC/g rock to 20.01 mg HC/g rock, with hydrocarbon generation potential classified as ranging from low to very good. The organic matter in the Limoeiro Formation samples is classified as types II, III, and IV, based on Hydrogen Index (HI), Oxygen Index (OI), and paleoredox factor (PRF) parameters. The relative distribution of C₂₇–C₂₈–C₂₉ regular steranes and the hopane/sterane (< 4), gammacerane/(gammacerane + C30 hopane), and TPP/(TPP + DIA) (< 0.4) ratios demonstrate that the organic matter input in the evaluated samples is essentially of marine origin with terrestrial contributions—corroborated by stable carbon isotope distribution—and was deposited in a paleoenvironment ranging from suboxic to anoxic, based on PRH values. The samples exhibit a low degree of thermal maturity, as indicated by Tmax (< 440°C) and %Ro (< 0.6) values, as well as the Ts/(Ts + Tm), 20S/(20S + 20R), and C32S/(C32S + C32R) ratios. Geochemical data then indicate that the studied samples from the Limoeiro Formation represent shales with organic matter richness ranging from poor to very good, reflecting variations in the depositional paleoenvironment, which ranges from suboxic to anoxic. They exhibit generation potential (S2) ranging from poor to very good, Type II, III, and IV kerogen, marine organic matter with a terrestrial contribution, and low thermal maturity. Although immature in platform areas, the Limoeiro Formation shales are likely within the generation window in deep-water and ultra-deep-water settings.