Geochemical characterization of potentially petroleum source rocks and the origin of natural gas in the Foz do Amazonas basin

The Foz do Amazonas Basin, in the northwestern Brazilian Equatorial Margin, extends along the Amapá and Pará coasts over ~268,000 km². It is geologically related to the Guyana-Suriname and Sierra Leone-Liberia basins, formed during Gondwana breakup. Despite growing exploration interest, regional geochemical data remain scarce, requiring further evaluation of its petroleum source rock potential. This study used data from the Paleocene–Eocene Amapá and Travosas formations from wells 1-APS-45B-AP and 1-BP-3-APS (BDEP/ANP), including TOC, Rock-Eval pyrolysis, calculated vitrinite reflectance, GC-MS, and GC-IRMS analyses. In the Paleocene–Eocene interval of the Amapá Formation, TOC values range between 0.5% and 1.3%, indicating poor to moderate organic matter (OM) contents. Rock-Eval pyrolysis data show moderate free hydrocarbon values (S1), low generation potential (S2 < 2 mg HC/g rock), hydrogen index (HI) values lower than 300 mg HC/g TOC, and Tmax temperatures below 440 °C throughout the analyzed interval. Biomarker data from organic extracts were used for paleoenvironmental and thermal maturity interpretations of the Amapá Formation. The Hopane/Sterane ratio indicates a marine environment (<4.0), as do the relative proportions of %C27, %C28, and %C29 steranes, which suggest a contribution from marine organic matter. The Pristane/Phytane ratio indicates reducing conditions (<1), while the TS/(TS+TM) ratio suggests thermal immaturity of the samples. Therefore, the Amapá Formation likely acts as a petroleum reservoir for hydrocarbons generated by potentially richer and deeper source rocks, such as the Cassiporé/Codó (Aptian–Albian) and Limoeiro (Albian–Cenomanian) formations. The geochemical characterization of the Travosas Formation (Paleocene–Eocene) reveals a significant contrast between the analyzed intervals. While the Miocene–Oligocene interval presents modest TOC values (0.5% to 1.21%) and HI < 300 mg HC/g TOC, the Eocene interval demonstrates greater organic matter preservation, with TOC values reaching up to 8% (excellent content) and HI values higher than 300 mg HC/g TOC, indicating Type II/III and III kerogen, favorable for oil and gas generation. This organic enrichment at greater depths (~5200 m) is accompanied by progressive thermal evolution, evidenced by Tmax values above 440 °C and calculated vitrinite reflectance values exceeding 0.6%, indicating that deeper intervals reached the maturity required for petroleum generation. The genetic classification of natural gas contained in samples collected from the Travosas Formation indicates a mixed signature composed of biogenic and thermogenic gases. The predominance of δ¹³C values ranging between –50‰ and –60‰ supports this interpretation, indicating that thermogenic hydrocarbons generated in deeper and more mature sections migrated to shallower stratigraphic zones, where they became associated with biogenic gas.