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Título

THE SIGNIFICANCE OF CRETACEOUS SOURCE ROCKS FROM THE BRAZILIAN EQUATORIAL MARGIN: UNVEILING THE GEOCHEMICAL SIGNATURES OF A NEW EXPLORATION FRONTIER

Texto do resumo

The exploration of petroleum resources is strategically and economically crucial for various countries, including Brazil, where the Brazilian Equatorial Margin (BEM) has emerged as a new exploratory frontier. This region, encompassing the northern and part of the northeastern margin, consists of a series of predominantly offshore basins, such as Foz do Amazonas, Pará-Maranhão, Barreirinhas, Ceará, and Potiguar, formed due to the breakup of Gondwana. This study discusses the distribution of organic facies, the controls on basin fill, and the influence of regressive and transgressive cycles on source rock formation in the Ceará Basin. Additionally, the deposition and preservation of organic matter and climatic, tectonic, and eustatic changes were evaluated using a geochemical approach to show the significant events that imply source rock generation. Methodologically, we integrated geochemical methods, including total organic carbon (TOC) content, Rock-Eval pyrolysis (RE), calculated vitrinite reflectance (Ro), relative hydrocarbon potential (RHP = S1 + S2/TOC), the index for interpreting the oxygenation conditions, and carbon isotopic data. Four petroleum source-rock units were identified, from oldest to youngest: (1) Mundaú Formation – top of the Rift Sequence (Berriasian-Aptian); (2) Paracuru Formation – Breakup Sequence (Aptian-Albian); (3) Itapajé Member of the Ubarana Formation – Continental Drift Sequence (Albian-Turonian); and (4) Uruburetama Member of the Ubarana Formation – Continental Drift Sequence (Turonian-Maastrichtian). The results indicate good potential for gas generation for the Mundaú Formation. Fault activity during the rift phase favored the source rock preservation. Kerogen type III predominates in shallower portions and probably reflects the contribution from the continent. Good to excellent potential for oil and gas generation is indicated for the Paracuru Formation. Kerogen types I and II predominate, with marine organic matter deposited in reducing conditions. The most promising interval for hydrocarbon generation is limited by the top of the Trairí Member and the top of the Paracuru Formation. Moderate to good potential for oil and gas generation is suggested for the Uruburetama Member. Poor to fair potential for oil generation is indicated for the Itapajé Member. The presence of inert kerogen is related to high-energy, oxidizing depositional environments. The top of the Paracuru Formation correlated with a significant anoxic event characterized by evaporitic facies, maximum RHP values, and maximum flooding surfaces related to transgressive events. This interval suggests the occurrence of the Aptian-Albian Oceanic Anoxic Event (OAE-1b) in the Ceará Basin. In conclusion, understanding sequence stratigraphy and its interaction with organically enriched rocks provides insights into depositional systems. It serves as a tool for regional and local stratigraphic correlations and identifies intervals favorable for preserving organic matter and hydrocarbon generation. These organic geochemical constraints are essential oil and gas exploration guidelines in the Ceará and adjacent basins.

Palavras Chave

Organic geochemistry; Organic facies; Hydrocarbon generation; Oceanic Anoxic Event; Stratigraphic sequence