Tar sandstones in the Paraná Basin of Brazil: structural and magmatic controls of hydrocarbon charge

There are tar accumulations in Lower Triassic sandstones along the eastern margin of the Paraná Basin. The tar was derived from Permian source rocks that matured along the basic sills of the Serra Geral continental flood basalt cover. This paper exams the habitat of tar accumulation, integrating fieldwork with morphostructural data, digital elevation modelling, gravity and airborne magnetic data, remote sensing, and radiometric dates that constrain the timing of maturation and migration. The focus of this work is the Jacu structural complex, a large annular drainage anomaly, 20 km in diameter, that owes its origin to reactivation of pre-existing faults. Interpretation of the structural framework is based primarily on morphostructural analysis and the distribution of gravity anomalies. The Jacu structure consists of a central low-lying fault block that is surrounded by several structural highs, and is attributed to the interaction of NE- and NW-striking fault zones. The principal basement faults that encapsulate this structure were invaded by the dolerite dykes that fed the widespread flood basalt cover during the Early Cretaceous. An 40Ar/39Ar analysis directly dates this episode of magmatism to 134±2 Ma. Tar sandstones are restricted to the lower Pirambóia Formation and are present around the Jacu structure, especially near the structural highs or close to basic dikes. Morphostructural interpretation suggests the occurrence of tar sandstones at intermediate positions too, between the structural highs and lows. The most likely explanation for these tar accumulations relates oil maturation and migration to the intrusion of Lower Cretaceous Serra Geral sills into the Upper Permian black shales of the Irati Formation, and subsequent migration along faults and dyke walls into eolian sandstone reservoirs of the Triassic Pirambóia Formation. Not only did the basic dykes locally facilitate migration, but there were also places where they were barriers to lateral migration. Furthermore, the interdune and fluvial facies of the Pirambóia Formation formed local seals and barriers to vertical migration.