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The Paleozoic basins, covering an area of about 800,000 km2 (309,000 mi2) in the Amazon region, are elongate symmetrical intracratonic synclines filled with as much as 4,000 m (13,000 ft) of sediments, separated by basement uplifts or major arches and located in continental interior areas (as are the United States Illinois, Michigan, and Williston basins). These Amazon basins resulted from an initial crustal thinning followed by rifting with associated ultrabasic intrusions and, finally, cooling and subsidence. Gravity anomalies, coinciding with the axes of the synclines, support this genetic hypothesis.
These synclines were filled during the Silurian-Devonian with one cycle of continental alluvial sediments grading upward to deltaic marine clastics and minor periglacial deposits. A regional unconformity separates the Devonian from the Permo-Carboniferous cycle when, following fluviodeltaic sedimentation, highly restricted marine conditions developed a sequence of evaporite deposits.
Tectonics affected differentially these basins during the Triassic-Jurassic and Early Cretaceous, associated with widespread basic volcanism. A northeast-southwest thrust-fault system, branching southwest, characterizes a compressional orogenic province in the Upper Amazon basin. This compressional province, located in the Jurua River area, constitutes a major structural trend. Adjacent to those faults and extending for over 500 km (310 mi), large natural gas accumulations occur in several domal features. Sandstones of the Permian Monte Alegre Formation, sealed by evaporite strata, are the main reservoir rock. Geologic estimates of natural gas resources are presently rated at 120 billion m3 (4.237 tcf) and exploration follows the productive trend toward the west-southwest.
The Middle Amazon basin, separated from the Upper Amazon by the Purus arch, was affected by lineament-block tectonics, also with associated volcanism and some local mild shearing. Minor domal features of Devonian periglacial Oriximina Formation sandstones comprise small subcommercial oil accumulations. In contrast with the Upper and Middle Amazon basins, the Lower Amazon basin has been the site of rifting since the Permo-Triassic. The rifting was associated with a nearby hot spot that uplifted the eastern part of the basin, forming the Gurupa arch. As a consequence of this uplift, a set of collapse grabens developed in the Lower Amazon basin. Potential reservoir rocks in Middle and Lower Amazon basins are Permian Monte Alegre and Devonian Oriximina sandstones. Major source rocks in al three basins are Devonian Barreirinha black shales. Organic geochemistry data indicate that both Upper and Lower Amazon basins are predominantly gas-prone, whereas the Middle Amazon basin shows potential for oil generation.
Forecasts for the major exploratory trends in the Upper Amazon indicate a good possibility of extending the already discovered natural gas province. In the Lower Amazon basin, further exploration will consist in drilling well-defined structural features identified for the first time by seismic methods, with a possibility of discovering another gas province. Prospects in the Middle Amazon basin are for both oil and gas, but the main problem is identification of adequate structures, as well as stratigraphic traps.
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