Geological evidence for fluid overpressure, hydraulic fracturing and strong heating during maturation and migration of hydrocarbons in Mesozoic rocks of the northern Neuquén Basin, Mendoza Province, Argentina

Abstract

In the northern Neuquen Basin of Argentina (especially in Mendoza Province), there is strong geological evidence for fluid overpressure in the past. The evidence takes the form of bitumen veins and beddingparallel veins of fibrous calcite (‘beef’). Such veins are widespread in the fold-and-thrust belt of the Malargue area, where bitumen mining has been active for a century or so. So as to collect information on the development of fluid overpressure in this part of the Neuquen Basin, several old mines were visited and studied in the Malargue area. Here the bitumen veins have intruded mainly the Late Jurassic to Early Cretaceous Mendoza Group, but also the Late Cretaceous Neuquen Group. The veins have the forms of bedding-parallel sills or dykes and they are especially thick within anticlines, forming saddle-reefs in several places. Beef veins are also numerous in the Malargue area. They contain bitumen and therefore seem to have formed at the same time as the bitumen veins. Near many outcrops of bitumen and beef, we have found fine-grained volcanic intrusive bodies. The best examples are from the La Valenciana syncline. According to 39Ar-40Ar dating, these bodies are mainly of Mid-Miocene age. More generally, volcanism, deformation and maturation of source rocks seem to have reached a climax in Miocene times, when the subducting Pacific slab became relatively flat.