oa Hydrocarbon Source Potential and Depositional Environment of the Pirara Formation - Tacutu Basin, Brazil

The Tacutu Basin, formed during the Mesozoic as part of the rifting related to the opening of the North and Central Atlantic, developed as a half-graben within the Guiana Shield. The Pirara Formation marks the rift phase of this evolution and stands out for its significant hydrocarbon generation potential. Although its depositional environment remains partially uncertain due to the lack of outcrops, recent studies—based on core samples from two wells—have provided crucial insights. Four main lithofacies were identified: mudstone, siltstone, marlstone, and evaporite. Mudstone is particularly rich in organic matter, with Total Organic Carbon (TOC) values ranging from 2% to 8%, reaching up to 13% in some intervals—levels considered excellent for hydrocarbon source rocks. The organic material is mainly composed of phytoclasts, palynomorphs, and amorphous organic matter. Geochemical analyses, including Rock-Eval pyrolysis, further confirm the strong petroleum potential of the formation. Although evaporites indicate hypersaline conditions, no marine influence was detected in the palynological data. These results support a “playa lake” depositional setting during the Late Jurassic to Early Cretaceous. Altogether, the high TOC values and favorable geochemical characteristics make the Pirara Formation a highly prospective target for hydrocarbon exploration in the Tacutu Basin.