Published 1995 as part of Memoir 62
Copyright © 1995 The American Association of Petroleum Geologists.  All Rights Reserved.
 

The San Bernardo ("Bernárdides") structural province is a multiply deformed belt transecting the peri-Andean segment of the Argentine Patagonia. It is distinctly separate from the fold and thrust belt along the western South American continental plate. The structured zone encompasses a NNW-SSE trending band about 600 km long and 100 km wide. The area includes faults and folds that involve Precambrian-middle Paleozoic basement, upper Paleozoic-Jurassic terrestrial to marine sedimentary and volcanogenic wedges, and Cretaceous nonmarine fill of the intracratonic San Jorge basin. The Cretaceous cover is dominated by discontinuous, narrow, box-shaped folds associated with east- and west-verging reverse faults. Oil finds are restricted to the low-lying unbreached segment between the Senguerr and Deseado rivers where anticlinal structures developed by contractional reactivation of preexisting normal and strike-slip faults.

Oil generation is attributed to amorphous, largely algal-derived organic matter (TOC, 1-3 wt. %) formed in brackish to alkaline stratified lakes. Modeling suggests that oil generation occurred from 110 to 30 Ma. Low-gravity oil (15-25° API) resulted from biodegradation and washing. The reservoir comprises alluvial, channel, and meander belt facies and multistory sandstone sheets. Stacked pay intervals are separated by shales, which limit interconnectedness in the fields. Porosity loss is due to authigenic zeolites and devitrified glass by-products in a volcaniclastic grain framework. Traps were formed in the Miocene by compression and inversion of Jurassic half-grabens, expressed in local pop-ups, folds, and weakly inverted structures. Local uplift has resulted in erosional removal and breaching of some traps. Hydrocarbon migration was facilitated by a normally charged system and vertical drainage during the first phase of migration. The sandstone and tuffaceous shale generally impeded migration. Late inversion processes favored hydrocarbon scattering.