Bitumen veins and Eocene transpression, Neuquén Basin, Argentina

Abstract

In the northwestern part of the petroleum-rich Neuquén Basin, veins of solid hydrocarbon (bitumen) have been reported from over 150 localities. The bitumen veins are mostly steeply-dipping and show intrusive relationships with their wall rocks. The largest are some 8 m wide, 8 km long and probably several kilometres deep. The bitumen probably formed in the earliest stages of thermal maturation of organically-rich source rocks. The most likely source rocks for the bitumen, as well as for most of the oil and gas in the Neuquén Basin, are black marine shales of the Vaca Muerta and Agrio formations. These were deposited in a rift setting during the late Jurassic and Early Cretaceous. According to subsidence models, the source rocks should have reached thermal maturity in the latest Cretaceous or Paleocene and should have remained in the oil window ever since. Although the bitumen has not been dated directly, the veins probably formed soon after maturation of the source rocks, in other words, in the Paleocene or Eocene. The wall rocks of the veins are mostly the source rocks themselves. Other veins have been emplaced into overlying strata of Early Cretaceous to Paleocene ages or into underlying sediments of Jurassic age. The bitumen veins have preferred orientations. The dominant trend is 060° and subsidiary trends are either 000° to 020° or 100°. As in other basins worldwide, the majority of bitumen veins in the Neuquén Basin probably formed by tensile failure, in orientations perpendicular to the least compressive stress, under the combined effects of regional tectonics and fluid overpressures. The orientations of the veins are compatible with the direction of oblique convergence between continental South America and the oceanic Nazca plate in the Eocene. Many major structures in the Neuquén Basin are also of Eocene age and may have formed under right-lateral transpression.