Bitumen veins and Eocene transpression, Neuquen Basin, Argentina

In the northwestern part of the petroleum-rich Neuquen Basin, veins of soli d hydrocarbon (bitumen) have been reported from over 150 localities. The bi tumen veins are mostly steeply-dipping and show intrusive relationships wit h their wall rocks. The largest are some 8 m wide, 8 km long and probably s everal kilometres deep. The bitumen probably formed in the earliest stages of thermal maturation of organically-rich source rocks. The most likely sou rce rocks for the bitumen, as well as for most of the oil and gas in the Ne uquen Basin, are black marine shales of the Vaca Muerta and Agrio formation s. These were deposited in a rift setting during the late Jurassic and Earl y Cretaceous. According to subsidence models, the source rocks should have reached thermal maturity in the latest Cretaceous or Paleocene and should h ave remained in the oil window ever since. Although the bitumen has not bee n dated directly, the veins probably formed soon after maturation of the so urce rocks, in other words, in the Paleocene or Eocene. The wall rocks of t he veins are mostly the source rocks themselves. Other veins have been empl aced into overlying strata of Early Cretaceous to Paleocene ages or into un derlying sediments of Jurassic age. The bitumen veins have preferred orient ations. The dominant trend is 060 degrees and subsidiary trends are either 000 degrees to 020 degrees or 100 degrees. As in other basins worldwide, th e majority of bitumen veins in the Neuquen Basin probably formed by tensile failure, in orientations perpendicular to the least compressive stress, un der the combined effects of regional tectonics and fluid overpressures. The orientations of the veins are compatible with the direction of oblique con vergence between continental South America and the oceanic Nazca plate in t he Eocene. Many major structures in the Neuquen Basin are also of Eocene ag e and may have formed under right-lateral transpression. (C) 1999 Elsevier Science B.V. All rights reserved.