Permian Basin
- Also called:
- West Texas Basin
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Permian Basin, large sedimentary basin in western Texas and southeastern New Mexico, U.S., noted for its rich petroleum, natural gas, and potassium deposits. Owing to its economic importance, it is one of the most well-studied geologic regions of the world. Deposits of the Permian Basin are featured in Guadalupe Mountains National Park.
Although named for the Permian Period (299 million to 251 million years ago) of geologic time, the origins of the basin can be traced back much earlier to Precambrian tectonic events occurring from about 1.3 billion to 850 million years ago. During the early Paleozoic Era, the basin was covered by a shallow seaway. During late Paleozoic times, more structural deformation occurred during the Alleghenian, Ouachita, and Marathon orogenies (mountain-building events) which formed the complex structures and basins that are collectively known as the Permian Basin.
The Permian Basin is made up of three component parts: the eastern Midland Basin, the Central Basin Platform, and the western Delaware Basin. These structures existed throughout the Carboniferous, Permian, and Triassic periods. The two basins were rapidly subsiding features that accumulated sediments, including black, organic-rich, deepwater materials that would become petroleum source rocks in later geologic time intervals. The Central Basin Platform remained relatively high in elevation and accumulated deposits of coarse-grained, shallow-water sediments and saw the development of small reefs. These reefs became good reservoirs for petroleum. The Permian Basin is bounded on the east, north, and west by cratonic platforms which also received shallow-water marine, eolian (wind-driven), and fluvial (stream-driven) sediments. To the south a structural fore-deep (marine trench) basin, the Val Verde Basin, which formed in advance of the Marathon orogenic belt, connected the Midland and Delaware Basins around the southern end of the Central Basin Platform.
As a result of tectonic and sedimentary differences, the Pennsylvanian and Permian histories of the Midland and Delaware basins differ significantly in detail. During the Pennsylvanian times of the Carboniferous Period (about 318 million to 299 million years ago) and the early part of the Permian Period, the Ouachita segment of the orogenic belt contributed large amounts of sediments from clastic (eroded and broken) rock. These were deposited as a thick, subaqueous delta system that gradually filled the Midland Basin from east to west. During the Permian Period, floodplain and red bed deposits covered much of this delta and now contain records of important fossil vertebrate fauna—amphibians, reptiles, and synapsids (mammal-like reptiles). In contrast, the Delaware Basin received relatively minor amounts of sediment from the low coastal plains. By the beginning of the Middle Permian (Guadalupian) Epoch some 271 million years ago, the Midland Basin was nearly filled with sediments, while the Delaware Basin continued to receive deepwater inputs.
As the Middle Permian Epoch continued, the Delaware Basin became rimmed with luxurious, reeflike, carbonate buildups. Unlike modern reefs, these were likely built by sponges, algae, and microbial organisms, although bryozoans (colonial “moss animals”) and brachiopods (lamp shells), which stabilized large amounts of carbonate sediment, were also found. These reef deposits formed the Capitan Limestone, which is 600 metres (about 1,970 feet) thick and dramatically exposed in Guadalupe Mountains National Park. During Permian times the reefs formed a region of higher elevation that separated marine and nonmarine shallow-water deposits of the platform from organic-rich deepwater deposits of the basin.
As the Late Permian (Lopingian) Epoch (about 260 million to 251 million years ago) progressed, the marine entrance to the Delaware Basin became restricted and the entire region became evaporitic, which promoted the deposition of thick sequences of salts and gypsum. These salts have been mined and used primarily for industrial purposes. Because evaporites have very low levels of permeability, the thick evaporite deposits in New Mexico are now the site of the Waste Isolation Pilot Plant, which is designed to store low-level radioactive waste in the subsurface for 10,000 years.
More than half of the world’s petroleum derived from Permian times has come from the Permian Basin. Although some older strata are also productive, the majority of the oil and gas obtained from the Permian Basin has been recovered from Permian rocks. Though oil and gas were already being extracted from the Permian Basin by the mid-1920s, most of the petroleum production activity has taken place since the 1950s. As a result, for a large part of the 20th century the Permian Basin played a significant role in the economic development of the state of Texas, including such towns as Midland, Odessa, and Marathon. Petroleum geologists often use the Permian reef system as a model for the exploration of other petroleum source and reservoir rocks.
Potash (potassium-rich evaporite minerals) and sulfur deposits are two other economically important resources in the Permian Basin. About 85 percent of the American production of potassium has come from one potash district near Carlsbad, New Mexico. Uranium minerals dated to Permian times are also present in the region.
Due to different erosion rates exhibited by different sediments, the modern topography of the Permian Basin probably reflects the bathymetry (underwater topography) of the basins when they were submerged during Permian times. The resistant reefal limestones today form a 600-metre (about 1,970-foot) rim around what was the Delaware Basin. The difference in elevation between Guadalupe Peak on top of the reef and the Salt Basin is about 1,600 metres (1 miles).