AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Radial Patterns of Bitumen Dikes Around Quaternary Volcanoes, Neuquén Basin, Argentina
(1) Geosciences, CNRS and University of Rennes, Rennes, France.
(2) Reasearch Centre, Statoil, Trondheim, Norway.
(3) Physics of Geological Processes, University of Oslo, Oslo, Norway.
(4) Servicio Geologico Minero, Buenos Aires, Argentina.
The Neuquén Basin of west-central Argentina contains three world-class source rocks. Where the basin abuts the Andes, hundreds of veins of solid hydrocarbon (bitumen) crop out. Many of the veins were mined in the past. By consensus, the bitumen has resulted from maturation of organic-rich shale, especially the Vaca Muerta Fm of late Jurassic age. To explain the maturation, recent authors have invoked regional subsidence, whereas early investigators invoked magmatic activity.
During five field seasons, we tracked down the mines, mapped the veins and host rocks, sampled them, and studied their compositions. In Neuquén Province, the veins are mostly sub-vertical dikes. They tend to be straight and continuous, crosscutting regional folds and faults. Almost all of the dikes lie within 50 km of Tromen volcano (4000 m high), although two are close to Auca Mahuida volcano. On both volcanoes, basaltic and andesitic products are of Pliocene to Quaternary age. Although regionally the bitumen dikes track the current direction of tectonic compression (ENE), locally they radiate outward from the volcanoes. Many of the dikes occur near reactivated basement faults, especially at the foot of Tromen. Here the bitumen is high-grade impsonite, whereas elsewhere it tends to be lower-grade grahamite or gilsonite. Also near basement faults, bitumen dikes pass upward into surface caprocks of hydrothermal calcrete. Some dikes or their wallrocks contain hydrothermal minerals (bipyramidal quartz, chalcedony, barite, selenite, sphalerite, or galena). A few bitumen dikes contain fragments of Vaca Muerta shale with characteristic calcite beef. We infer forceful expulsion of source rock. At Curacó Mine, bitumen within beef fragments is of low grade and formed during regional burial (probably in the Cretaceous), whereas bitumen in the vein itself is of higher grade and formed later. Finally, some dikes splay upward near the current land surface, indicating recent dynamic equilibrium.
We conclude (1) that the bitumen dikes of Neuquén Province formed during volcanic activity in the Quaternary, and (2) that heat advection by hydrothermal fluids generated oil, which filled intrusive dikes before solidifying to bitumen. This unconventional hydrocarbon system may have profound implications for regional exploration in the foothills of the Andes.