Abstract: Exploration of Carboniferous System in Oriental Basins of Bolivia

Carlos Salinas, Jaime Oblitas, Carlos Vargas

The Carboniferous in eastern Bolivia is becoming increasingly important because of recent hydrocarbon discoveries in rocks of this age. Five new fields have been found in the last 2 years: three by the Bolivian national oil company (YPFB), one by Tesoro Petroleum, and one by Occidental Boliviana, Inc. Of the total hydrocarbon reserves of the country, 70% is concentrated in Carboniferous sandstone reservoirs. Recent seismic surveys of 24-fold data, totaling 34,000 km, together with these discoveries are responsible for the renewed exploration interest in the area.

The Carboniferous depositional basin, with its Gondwana glaciation-associated sediments, has some unusual aspects. The chaotic deposition of the sediments, the abundant channel-like anomalies, the diagenetic changes, and the Tertiary Andean folding have contributed to the complex nature of hydrocarbon entrapment. Although all of the hydrocarbons discovered in the basin are associated with structural closures, detailed interpretation of seismic records and stratigraphic studies have demonstrated the interrelation between the structural and stratigraphic controls of hydrocarbon accumulation.

The geochemistry of the oils combined with source rock analysis indicates that the source of all hydrocarbons is probably Middle and Upper Devonian marine sediments underlying the Carboniferous. There is evidence for early migration. However, remigration of some of the oil to younger rocks apparently occurred after the Upper Cretaceous sediments were deposited, and a final distribution was reached only during the Andean orogeny. The emplacement and probable pre-Tertiary age of the Izozog and Michicola arches gave the basin its present configuration, modifying the status of the early primary traps. Diagenetic trapping associated with channel-like anomalies detected by seismic work in the Carboniferous sequence is believed to be one of the most important trapping mechanisms that control the considerable reserves expected to be found. The object of new exploration is to test these ideas, in addition to testing the many undrilled structural closures.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii