Radial patterns of bitumen dykes around Tromen and Auca Mahuida volcanoes, Neuquen Basin, Argentina: evidence for abnormally rapid generation of hydrocarbons in Quaternary times
Résumé
The oil-bearing Neuquén Basin of west-central Argentina contains several hundred dykes or sills of solid hydrocarbon (bitumen). Many of these have been mined. By consensus, the bitumen has resulted from maturation of organic-rich source rocks, especially the Vaca Muerta Fm of latest Jurassic age. To explain the maturation, most authors have invoked regional subsidence, but some have also considered volcanic activity. In the northern part of Neuquén Province, sills are rare and dykes are mostly steep. A number of features argue for a close relationship between the dykes and the main volcanic centres of the area. 1. Except for one bitumen dyke that is radial to Auca Mahuida volcano, and two other dykes along the Rio Colorado, all lie within 50 km of the crater of Tromen volcano. 2. Typically, the dykes are straight and continuous for many km and they crosscut regional folds and faults. 3. The dykes form a radial pattern around Tromen, although many of them are nearly parallel to the current direction of tectonic compression. 4. With distance from Tromen, the dykes tend to splay and to decrease in thickness, from a maximum of as much as 10 metres, to less than 1 cm. 5. In general, the bitumen decreases in maturity with distance from Tromen. 6. On the eastern flank of Tromen, hydrothermal mineral assemblages have formed along a thrust fault, in close proximity to bitumen dykes of high maturity. 7. Where recent thrusting has exhumed Jurassic host rocks, the dykes contain brecciated fragments of source rock, which have moved downward. 8. Where host rocks are Cretaceous or Tertiary, bitumen has moved upward from the source rock. 9. Many of the dykes splay upward on reaching the current land surface, indicating that they have reached dynamic equilibrium. On Tromen volcano, surface lavas are of Pliocene to Quaternary age, whereas, on Auca Mahuida, lavas and sills are Quaternary. If the bitumen dykes indeed formed in association with these volcanoes, they must be young (probably Quaternary). Thus the overall evidence is for rapid recent maturation of the source rock. In part this was due to heat advection by hydrothermal fluids, coming from deep magmatic sources. The bitumen dykes presumably formed in directions perpendicular to the least principal stress, which itself resulted from a combination of tectonic compression, topographic relief, and fluid overpressure. The bitumen is the remaining solid fraction of the hydrocarbons. Oil and gas may have formed in much larger quantities. An outstanding question is, how much remains in subsurface traps, and how much has escaped to the surface or to the atmosphere?