Geochemical and Seismic Constraints on the Role of Mud Diapirs in Petroleum Migration and Accumulation in the Recôncavo Basin, NE Brazil

ABSTRACT In the Recôncavo Basin, Northeast of Brazil, mud diapirs have been reported for decades, but their role within the basin's petroleum system remains poorly constrained. Here, we combine 2D seismic interpretation, field observations, and organic geochemical analyses of rocks and oil seeps to investigate the relationship between diapirism and hydrocarbon migration in the southern Recôncavo Basin. Oil samples were collected from natural seeps along the unconformity between the Cinzento Diapir (Valanginian–Hauterivian) and the overlying Marizal Formation (Aptian), whereas potential source rocks were sampled 10 km south of the diapir (outcrop D‐01). For the rock samples, bulk geochemical parameters indicate total organic carbon (TOC) contents of up to 4.0 wt% and hydrogen index values exceeding 700 mg HC/g TOC in the upper stratigraphic interval, pointing to substantial preservation of organic matter. However, both T max and biomarker data consistently indicate that the organic matter remains in an immature to early‐mature thermal stage to samples collected in the outcrop. Stable carbon isotope compositions (δ 13 C org = −26‰ to −31‰) and biomarker assemblages—dominated by long‐chain n ‐alkanes ( n ‐C 25 – n ‐C 31 ), abundant C 27 regular steranes, methylsteranes, tetracyclic polyprenoids, and gammacerane—indicate a lacustrine depositional setting with variable salinity and microbial reworking of algal organic matter. The petroleum sample exhibited a hydrocarbon biomarker configuration consistent with thermally evolved material (petroleum), including higher T s / T m ratios, increased tricyclic terpane abundances, and a predominance of thermally stable hopanes and steranes. Despite this maturity difference, biomarker correlations—such as the predominance of C 23 tricyclic terpane, elevated tetracyclic polyprenoids, comparable gammacerane/hopane ratios, and similarities in regular steranes distribution—suggest that the collected oil was sourced from organic facies similar to those observed at outcrop D‐01, potentially deposited in deeper basin depocenters. Seismic interpretation of the Cinzento Diapir reveals listric fault geometries and chaotic internal facies consistent with diapiric ascent and fault‐controlled deformation. These features define potential vertical migration conduits linking mature source intervals to shallow reservoirs and outcrop seeps. The combined geochemical and seismic evidence supports a model in which mud diapirs in the Recôncavo Basin could have acted as important pathways for hydrocarbon migration and accumulation that occurred along structural discontinuities, thereby exerting a first‐order control on the spatial distribution of petroleum occurrences in this early Cretaceous rift system.