The Upper, Middle and Lower Amazon basins are large elliptical, elongated syneclises located between the Guyana and the Brazilian Precambrian shields, covering a total area of about 850,000 sq km (327,000 sq mi). These basins, or sags, contain multiple cycles of Phanerozoic sediments interrupted by unconformities. Sediments are predominantly Paleozoic with minor Cretaceous covering. Regional, positive elements include upwarps, swells, platforms, structural terraces, and major interbasinal arches, entirely caused by basement tectonics. Crustal stretching also caused widespread rifting and associated basic volcanism. Tectonic modifiers comprise mostly wrenching, within or at basin margins, especially in the Upper Amazon basin, where folds and reverse faults trap large gas a cumulations.

Exploration in the Amazon basins began in 1950, with seismic and gravimetric surveys along major rivers. Based on these surveys, 130 wells were drilled prior to 1971. Because of constraints imposed by poor seismic resolution and also because of massive diabase sills, only regional, positive features and minor, subdued structures were identified and the exploratory drilling results were disappointing. From 1976 to 1984, however, an impressive enhancement on seismic reflection methods combined with airborne magnetometry and gravity maps allowed the detailing of important structural features, and exploration success began.

The ultimate natural gas resources of the Upper Amazon basin are presently rated at 120 billion cu m (4.2 tcf). The Middle and Lower Amazon basins still remain as frontier areas for hydrocarbon exploration.

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