Hydrocarbon-Bearing Late Paleozoic Glaciated Basins of Southern and Central South America

Abstract

Although glaciated basins are usually associated with nonproductive, poorly sorted strata, hydrocarbons occur in several late Paleozoic glaciated basins of central and southern South America. In Bolivia, the Chaco-Tarija basin has commercial production from more than 30 fields in glacially influenced submarine channel systems (Palmar, Santa Cruz, and Bermejo fields) that accounts for about 60% of current national reserves. Correlative deposits in Argentina host the Campo Duran and Madrejones oil fields. In Brazil, the Parana basin has significant but as yet subcommercial gas shows in thick marine turbidite sandstones of the glacially influenced Itarare Group. The Chaco-Parana basin of Argentina is one of the largest onshore targets for exploration in South America, but it is virtually untested. Glacially influenced foreland basins of Argentina (Tepuel and Paganzo-Maliman) contain complex glacigenic stratigraphies of interbedded tillites and poorly prospective sandstones. In contrast, the glacially influenced marine infills of intracratonic basins in Brazil (Parana), Bolivia, and Argentina (Chaco-Tarija and Chaco-Parana) contain thick sequences of pebbly mudstones and regionally extensive reservoir quality sandstones. The key to the occurrence of good reservoirs and associated trapping mechanisms in these intracratonic basins is the interplay of sediment supply, regional tectonics, and relative sea level changes. Glacial scouring of extensive cratons by ice sheets resulted in the delivery of huge volumes of glaciofluvial sand to deltas. Structural control of drainage patterns on the craton by basement lineaments resulted in persistent sediment sources and depocenters. Frequent earthquake activity along reactivated basement lineaments resulted in downslope mass flow of deltaic sediments and the deposition of thick, amalgamated sand turbidites (reservoirs). Pebbly mudstone seals most likely record higher relative sea levels, resulting from basin subsidence, and deposition from suspended sediment plumes and icebergs. Source rocks are provided by Devonian and Permian shales. This model may be applicable to other parts of Gondwana that contain thick, prospective sandstones in glacially influenced intracratonic basins.