Dados da Submissão

Título

Geochemical evaluation of the Limoeiro Formation, Foz do Amazonas Basin, Brazil: depositional paleoenvironment and petroleum source rock

Texto do resumo

The Foz do Amazonas Basin covers an area of 268,000 km2 and has a strong correlation with the Sierra Leone-Liberia Basin, in addition to the Guyana-Suriname Basin constituting its uninterrupted natural extension. Its main source rocks are part of the Limoeiro Formation (Cenomanian/Turonian), characterized by neritic and bathyal marine shales. Recent discoveries of oil accumulations in basins geologically similar to the Foz do Amazonas Basin have aroused economic interest in this region. The aim of this study is to evaluate the hydrocarbon generating potential, the quality of the organic matter, thermal maturation and the depositional paleoenvironment of the Limoeiro Formation shales, based on the results of Total Organic Carbon (TOC) and Rock-Eval Pyrolysis analyses. We used data from 144 samples collected in wells APS-31A and APS-44, drilled in shallow waters of the Foz do Amazonas Basin, provided by the National Agency of Petroleum, Natural Gas and Biofuels (BDEP-ANP). The initial results indicate that the TOC content of the Limoeiro Formation samples collected in these two wells ranged from 0.10% to 2.27%, allowing them to be defined as having poor to very good potential for hydrocarbon generation. The insoluble residue (IR) values ranged from 71% to 93% for the samples from well APS-31A, indicating siliciclastic sediment deposition, and for the samples from well APS-44, the IR ranged from 38% to 86%, suggesting carbonate/siliciclastic sedimentation. The S1 values of the samples from both wells are low (<0.50 mg HC/g rock), indicating little natural hydrocarbon generation. The relationship between genetic potential (S1+S2) and TOC reveals a hydrocarbon-generating potential ranging from poor to very good. The maximum pyrolysis temperature values (Tmax < 435°C) indicate that the analyzed section of the Limoeiro Formation is immature in both wells, which is corroborated by the IP values of these samples (< 0.10), and by the vitrinite reflectance values (%Ro < 0.60). The Hydrogen Index (HI) and Oxygen Index show that the kerogen associated with the samples from these wells is predominantly type III, of continental origin and with the potential to generate gas, while there is also type II kerogen, of marine origin at the base of the section analyzed and with the potential to generate oil, as well as type IV kerogen, resulting from reworked or highly oxidized organic matter. The relationship between RI and HI points to a depositional paleoenvironment ranging from anoxic at the base of the Limoeiro Formation to oxic at the top. Based on the parameters analyzed, it can be concluded that the samples from the Limoeiro Formation, in the wells analyzed, have hydrocarbon generating potential ranging from poor to very good and are thermally immature for generation. The kerogen contained in the samples from these wells is type II, III and IV, indicating sedimentation in a mixed environment, ranging from anoxic to oxic, with input from marine and continental organic matter, with potential for generating oil and gas. This study highlights its preliminary nature and suggests future investigations, including petrographic analyses, seismic profile studies and computer modeling, for a more complete understanding of the hydrocarbon potential of the Foz do Amazonas Basin.

Palavras Chave

Foz do Amazonas Basin; Cenomanian-Turonian; Source Rock; Depositional Paleoenvironment; Thermal Maturation