EMPLACEMENT OF BITUMEN (ASPHALTITE) VEINS IN THE NEUQUEN BASIN, ARGENTINA

Veins of solid bitumen (asphaltite) have been commercially exploited i n the Neuquen basin, Argentina, for over 100 yr. Veins are up to 5 m w ide and several kilometers in length, over a region of 15,000 km(2). T hese veins were emplaced in fractures both parallel and at high angles to bedding, in close proximity to their source rocks in the Vaca Muer ta and Agrio formations (Late Jurassic-Early Cretaceous). Two or more phases of bitumen emplacement can be recognized in several localities; structures bearing viscous oil are younger than structures having sol id bitumen. Bitumen emplacement was vigorous and caused brecciation an d spalling of the host rocks. The bitumen was also viscous, and suppor ts rock debris ranging in size from sand grains up to meter-scale slab s. Brecciation, bedding-parallel injection, and wall rock impregnation suggest high fluid pressures during emplacement. High fluid pressure may have been engendered by substantial hydrocarbon generation from ri ch source rocks in a low-permeability sequence, and probably caused th e fractures into which the bitumen migrated. The bedding-parallel vein s facilitated decollement during thrusting that took place during and after bitumen emplacement. The timing of emplacement relative to thrus ting and oil migration constrains bitumen emplacement to the Eocene-Ol igocene.