--> Geochemistry and Organic Petrography of Aptian-Albian Source Rocks in the Araripe Basin, Northeastern Brazil

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Geochemistry and Organic Petrography of Aptian-Albian Source Rocks in the Araripe Basin, Northeastern Brazil


The Mid-Cretaceous is a key interval for oil and gas exploration in Brazilian basins, with tens of billions of barrels of oil discovered. The northeastern Brazil interior Araripe Basin, correlative to the prolific marginal basins, was formed by the reactivation of Precambrian basement structures related to the rift phase. A geochemical characterization of Aptian-Albian organic shales aimed at the determination of their generation potential and correlation with coeval Equatorial Atlantic source rocks. Ten samples of organic shales from the 3 members of the Santana Formation were analyzed. XRD analyses indicate quartz, smectite and calcite as the essential mineralogy, with subordinate gypsum, pyrite, illite and kaolinite. The lower Crato Member samples comprise 87% amorphous organic matter (AOM; aggregated, flaky, yellow-brown and granular under white light, and yellow fluorescent under UV excitation), 11% phytoclasts (opaque, woody, equidimensional to pseudomorphic) and 2% palynomorphs. The intermediate Ipubi Member samples contain 68% AOM (mostly granular, highly reworked and oxidized), 15% phytoclasts (opaque, fungi, thin and fibrous tissues) and 7% palynomorphs. The uppermost Romualdo Member samples contain 68% AOM (translucid, granular, preserved), 26% phytoclasts (elongated, dark brown, fungi) and 6% palynomorphs. There is a progressive upwards decrease in AOM and increase in phytoclasts. The few palynomorphs are basically spores and Classopolis pollens, associated with gymnosperms, indicative of arid climates. TOC varied from 1 to 29.5 wt%, reflecting facies and stratigraphic variations, but with overall excellent hydrocarbon generation potential. Kerogen is mostly type II, indicating a mixed lacustrine and marine origin, with two Ipubi samples of type I (lacustrine) kerogen. HI varies from 2.1 to 243.1 mg HC/g, indicating reasonable to excellent generation potential. Pyrolysis Tmax of 361 to 433 degrees C, spore coloration index 3-4, and vitrinite reflectance of 0.29-0.6% Rrandom suggest immature to marginally mature organic matter. The intervals with better generation potential are stratigraphically located in the high-accommodation systems tract at the base of the Ipubi Member, correlative to the Trairi and Ponta do Tubarão Layers in the Ceará and Potiguar Basins, respectively. Araripe Basin Aptian-Albian organic shales reveal important information for the exploration of coeval Equatorial Margin petroleum systems.