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The Chaco is a vast, very gently southeastward sloping flat plain with a dense cover of semi-arid low brush and small trees which extends from the foot of the Andes Mountains eastward to the Rio Paraguay and from the 18° parallel south into Argentina. It is underlain by Tertiary and Quaternary continental sandstones and conglomerates which bury a pre-Tertiary topography. The only relief features that break the extremely flat surface of the Chaco are occasional isolated hills of pre-Tertiary rocks which project through the Tertiary and Quaternary blanket. In Bolivia only Gondwana and younger rocks are exposed in these hills; in Paraguay, Devonian and older rocks are found.
The Parapeti River comes out of the Andes Mountains at the 20 degree parallel and swings northward across the Bolivian part of the Chaco to disappear in the Izozog swamps at parallel 19. Near the mountains it crosses a major syncline in which over 8,000 feet of Tertiary and Quaternary rocks are preserved. Its northward course runs along the east limb of that syncline and just west of a major east-west trending structural high in the pre-Tertiary rocks which projects into Bolivia from northern Paraguay. This high has been active through geologic time as evidenced by the fact that the basal Gondwana unconformity cuts down into the Devonian rocks eastward onto the high and the unconformity at the base of the Tertiary also truncates the Gondwana progressively deeper toward the east. Along a line just east of the Parapeti, the Tertiary unconformity cuts all the way through the Cretaceous and Gondwana rocks, placing the Tertiary directly on the Devonian.
In Bolivia, all oil production to date is located in the extreme eastern fringe of the Andes Mountains in Devonian and Lower Gondwana sandstones. The Gondwana production is concentrated in the south of the country where suitable Devonian reservoir sands are not developed and where a thick section of impermeable gritty shales is present in the top of the Lower Gondwana. Various possibilities for production in the Parapeti area exist. Except in the very axis of the frontal syncline, the Devonian is within economic reach of the drill. Closed structures are present in the area and are especially attractive east of the Parapeti River where the Devonian lies within 1,000 feet of the surface. There is also the chance of updip pinch-outs and accumulations against the Gondwana unconformity. In the Gondwana, oil may be present in local structural traps within the major syncline, in updip sand pinch-outs or updip against the overlying unconformity.
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