Sequence stratigraphy and organic geochemistry: An integrated approach to understand the anoxic events and paleoenvironmental evolution of the Ceará basin, Brazilian Equatorial margin.

An organic geochemical investigation combined with sequence stratigraphy was performed in the Ceará Basin, an offshore basin located in Northeastern Brazil. The information available from 30 well logs (gamma-ray, resistivity, density), besides geochemical (TOC, and pyrolysis indexes) and isotopic (δ13C) data, aided the preparation of a dataset for this study. The application of sequence-stratigraphic methods helped classify and correlate seismic and organic facies. Four key petroleum source-rock units were identified, from the oldest to the 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 geochemical characteristics of the Mundaú Formation (high total organic carbon (TOC), hydrogen index (HI), relative hydrocarbon potential (RHP = (S1 + S2)/TOC)) point to a typical transgressive sequence. Six transgressive-regressive (T-R) cycles were recognized in the entire Paracuru Formation. The best geochemical marker is related to the top of the Paracuru Formation. This stratigraphic unit can be correlated to a major anoxic event and is the best source rock of this basin. Evaporitic facies found in this top section, maximum RHP values (anoxic conditions), and maximum flooding surfaces related to transgressive events characterize this interval. Moreover, the wide spatial cover of organic-rich rocks, carbon isotopic data, and the recognition of favorable characteristics for anoxia in other basins of the Equatorial Margin are suggestive of the Aptian-Albian Oceanic Anoxic Event (OAE-1b) occurring in the Ceará Basin. The Ubarana Formation represented by the Uruburetama Member and the Itapajé Member yields the least promising source rocks. However, high TOC values suggest the occurrence of the late Cenomanian–early Turonian Oceanic Anoxic Event (OAE-2), when organic-rich strata started to deposit in deep-water regions. The predominance of a regressive interval in the Uruburetama Member points to oxic or sub-oxic conditions. Additionally, the correlations between the Brazilian Equatorial Margin and its African counterpart, and the organic geochemical characterization allied to the definition of depositional systems for these regions proved to be useful for oil exploration. Flowchart representing the steps of this research. A) Database components. B) Data interpretation. C) Relative hidrocarbom potential (RHP) variations revealing changes in oxic conditions at depth and correlations with gamma-ray (GR) and lithofacies (Facies) stacking patterns. D) Sections showing the results obtained from data interpretation. E) A simplified model for the Paracuru Formation. [Display omitted] • Stratigraphic evolution, organic geochemistry, and Cretaceous Anoxic Events (CAE) were investigated. • Well-log correlations, redox conditions, and modeling to investigate anoxic events occurring in the Ceará Basin. • Relative sea-level fluctuations in 3rd- and 4th-order sequence-stratigraphic contexts are presented. • Relative hydrocarbon potential (RHP) as a geochemical index that reflects oxygenation conditions. • Anoxic markers were defined in the organic-rich, transgressive systems tracts (TST) and condensed section (CS) strata. [ABSTRACT FROM AUTHOR]