Tar sandstones in the Paraná Basin: a study of the occurrences in the Anhembi Structural High

Abstract

There are tar accumulations in Early Triassic sand-stones along the eastern margin of the Paraná Basin. The aim of this research work is to explain the tar sandstone occurrences in the Anhembi High. The Anhembi High, where permian mudstones outcrop in a Triassic sandstone area, is a Jurassic-Cretaceous structure related to the reactivation of pre-existing basement faults. Tar was derived from Permian source rocks that matured along the basic sills of the continental flood basalt cover of Serra Geral Formation. This paper examines the habitat of tar accumulation and inte-grates fieldwork with morphostructural data, digital elevation model, gravity and airborne magnetic data and remote sensing imagery. The interpretation of the structural framework is based primarily on the morphostructural analysis, the distribution of gravity and magnetic anomalies and on a structural field survey. The Anhembi High consists of three blocks with a N50W-trending major axis. Tar sandstone outcrops occur around the Anhembi structure, on its low-lying block and are restricted to the lower Pirambóia Formation. The morphostructural interpretation suggests the occurrence of tar sandstones at intermediate positions, between the structural highs and lows. The most likely explanation for these tar accumulations relates oil maturation and migration to the intrusion of Early Cretaceous Serra Geral sills into the Late 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 constituted barriers to lateral migration. Furthermore, the interdune and fluvial facies of the Pirambóia Formation formed local seals and barriers to vertical migration.