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

This study of the Chaco basin is based on field studies of outcrops and on exploration data. The Chaco basin covers 246,725 km² of western Paraguay and consists of several depocenters or subbasins, each with a unique tectonostratigraphic record. In the northwest, the Curupaity and Carandaity subbasins contain a well-developed Paleozoic succession. In contrast, Mesozoic subsidence was marked in the southern Pirity and Pilar subbasins and in the shallow Bahia Negra platform and San Pedro low to the east. These depocenters are separated by structural highs. Uppermost Proterozoic-Recent sedimentary sequences are present in the Chaco basin.

The subsidence history of the Chaco basin is recorded in four major unconformity-bounded sequences. Northwest- and northeast-oriented structural lineaments of Eocambrian Brasiliano origin controlled the patterns of subsidence. Mesozoic extensional tectonics related to the opening of the South Atlantic reorganized the structural pattern of the Chaco basin; this episode is expressed in a system of half-grabens. Cenozoic Andean orogenesis imposed the final structural readjustment and established the Chaco area as a modern foreland basin.

Upper Devonian marine shales and Upper Cretaceous shales and carbonates are the primary source rocks for hydrocarbons. The principal reservoir zones are Carboniferous channel sandstones in the Curupaity and Carandaity subbasins and stratigraphic and structurally controlled sandstone reservoirs of Mesozoic age in the Pirity subbasin.