The geological evolution of the Paraguayan Chaco

The main objective of this study is to reconstruct the geologic evolution of the Paraguayan Chaco. The Paraguayan Chaco occupies an area of approximately 240 thousand km^, and is part of a string of modem foreland basins to the east of the Andean Belt. Only 38 wells, mostly shallow stratigraphic tests, were drilled. Over 5,000 kilometers of seismic reflection lines were interpreted.

Three main geologic events were observed in the Paraguayan Chaco: (1) Deposition of a Paleozoic sedimentary section in a relatively stable platform with a predominantly eastern source area; (2) rifting of this Paleozoic section during the Cretaceous to form the Pirizal Subbasin; and (3) deposition of continental and marine sediments in a foreland basin setting. The Paleozoic section penetrated is mostly marine, and ranges from Lower Ordovician to Upper Permian. It contains a Devonian to Carboniferous westward regressing sequence of marine, transitional and continental facies. The main structural features consist of subbasins, arches, and subvertical faults. Paleozoic tectonic events occurred during the Devonian, Carboniferous (Late?) and possibly Late Ordovician to Early Silurian. Later events include the Early Cretaceous rifting of the Pirizal Subbasin and an Eocene event. Thick Paleozoic marine shales generated oil and gas. High temperature gradients are associated with arches, resulting in gas prone settings. Areas distant from the arches have lower temperature gradients and should produce oil.

The sedimentary fill of the Pirizal rift basin is mostly of continental origin, with sea incursions during the Late Cretaceous (Maastrichtian)-Paleocene and Miocene. Two stages were identified in the evolution of the Pirizal Subbasin: (1) Rifting of the Paleozoic sequence, and the accumulation of continental sediments in an asymmetric graben bordered by normal down to the basin faults; and (2) a sag phase with sediments overlapping the graben borders. The subsidence of the basin flanks during the sag phase resulted in a symmetrical distribution of the sedimentary section of this phase. The Late Cretaceous- Paleocene marine sediments have good source potential and generated hydrocarbons. This study relates the Paraguayan Chaco with the better known Bolivian and Argentinian Subandean Belts and adjacent regions of both countries.