The Permian Basin is a large sedimentary basin in the southwestern part of the United States. It is the highest-producing oil field in the US, producing an average of 4.2 million barrels of crude oil per day in 2019. This sedimentary basin is located in western Texas and far-southeastern New Mexico.

It is named after the Permian geologic period, the final period of the Paleozoic era, as it contains some of the world's thickest deposits of rocks from the period.

The Permian Basin comprises several component basins, including the Midland Basin, which is the largest; Delaware Basin, the second largest; and Marfa Basin, the smallest. The Permian Basin covers more than ,Ball - The Permian Basin - USGS and extends about wide and long.

The Texas cities of Midland, Odessa, and San Angelo serve as headquarters for some of the oil production activities in the basin.

The Permian Basin is also a major source of potassium salts (potash). Potash mines are located in Lea and Eddy counties, New Mexico, and are operated by the room and pillar method. Halite (rock salt) is produced as a byproduct of potash mining.B. R. Alto and R. S. Fulton (1965) "Salines" and "The potash industry" in Mineral and Water Resources of New Mexico, New Mexico Bureau of Mines and Mineral Resources, Bulletin87, p. 299–309.

Components

Delaware Basin

The Delaware Basin is the larger of the two major lobes of the Permian Basin within the foreland of the Ouachita–Marathon thrust belt separated by the Central Basin Platform. The basin contains sediment dating to Pennsylvanian, Wolfcampian (Neal Ranch and Lenox Hills Formations), Leonardian (Avalon Shale), and early Guadalupian times.

The eastward-dipping Delaware basin is subdivided into several formations (figure2) and contains about of laminated siltstone and sandstone. Aside from clastic sediment, the Delaware basin also contains carbonate deposits of the Delaware Mountain Group, originating from the Guadalupian times when the Hovey Channel allowed access from the sea into the basin.

Midland Basin

The westward-dipping Midland Basin is subdivided into several formations (figure 4) and is composed of laminated siltstone and sandstone. The Midland Basin was filled by a large subaqueous delta that deposited clastic sediment into the basin. Aside from clastic sediment, the Midland Basin also contains carbonate deposits originating from the Guadalupian times when the Hovey Channel allowed access from the sea into the basin.

Central Basin Platform

The Central Basin Platform (CBP) is a tectonically uplifted basement block capped by a carbonate platform. The CBP separates the Delaware and Midland Basins and is subdivided into several formations, from oldest to youngest Neal Ranch, Lennox Hills, Abo, Yeso, Glorieta, San Andres, Grayburg, Queen, Seven Rivers, Yates, and Tansill Formations (Figure5). The sequence mainly comprises carbonate reef deposits and shallow marine clastic sediments.

Eastern and Northwest Shelves The Eastern and Northwestern Shelves are composed of shelf edge reefs and shelf carbonates flanking the Delaware and Midland Basins that grade up-dip into siltstones and evaporites. The Eastern and Northwestern Shelves are subdivided into the San Andres, Grayburg, Queen, Seven Rivers, Yates, and Tansill Formations.

San Simon Channel The San Simon Channel is a narrow syncline that separated the Central Basin Platform from the Northwestern Shelf during Leonardian and Guadalupian times.

Sheffield Channel The Sheffield Channel separates the southern margin of the Midland Basin from the southern shelf and the Ouachita–Marathon thrust-belt during Leonardian and Guadalupian times.

Hovey Channel The Hovey Channel is a topographical low located on the southern edge of the Delaware Basin, allowing access to the Panthalassa sea during Guadalupian times. The Hovey Channel was originally an anticline which formed during Precambrian faulting, and was the main source of sea water for the Delaware Basin. The closing of the Hovey Channel towards the end of the Permian Period eventually caused the death of the Permian Reef, as without water being brought in through the Channel, salinity levels rose drastically in the Delaware Basin and the reef could not survive.

Horseshoe Atoll

The Horseshoe Atoll is a westward-tilting arcuate chain of reef mounds long located in the Midland Basin, consisting of of limestone accumulated in the Pennsylvanian and in the Permian, with 15 significant petroleum reservoirs from to in depth.Vest, E. L. Jr., 1970, Oil Fields of Pennsylvanian-Permian Horseshoe Atoll, West Texas, AAPG Memoir 14: Geology of Giant Petroleum Fields, Tulsa: AAPG, pp. 185–186.

The reef complex consists of Upper Pennsylvanian Strawn, Canyon and Cisco limestones, overlain by Lower Permian Wolfcamp sandstones and shales of terrigenous origin prograding northeast to southwest.Vest, E. L. Jr., p. 185. The first production well, Seabird Oil Company of Delaware No.1-B J.C. Caldwell, was completed in 1948.Vest, E. L. Jr., p. 186.

Depositional history

The Permian Basin is the thickest deposit of Permian-aged rocks on Earth; they were rapidly deposited during the collision of North America and Gondwana (South America and Africa) between the late Mississippian through the Permian. The Permian Basin also includes formations that date back to the Ordovician Period, 445 million years ago (mya).

Proterozoic Before the breakup of the Precambrian supercontinent and the formation of the modern Permian Basin geometry, shallow marine sedimentation onto the ancestral Tobosa Basin characterized the passive margin, shallow marine environment. The Tobosa Basin also contains basement rock that dates back to 1,330 million years ago (mya), and that is still visible in the present-day Guadalupe Mountains. The basement rock contains biotite-quartz granite, discovered at a depth of .

In the nearby Apache and Glass Mountains, the basement rock is made of metamorphosed sandstone and Precambrian-aged granite. The entire area is also underlain by layered mafic rocks, which are thought to be a part of Pecos Mafic Igneous Suite, and extends into the southern US. It has been dated to 1,163 mya.

Early to mid-Paleozoic (Late Cambrian to Mississippian)

Ordovician Period (485.4–443.8 mya) Each period from the Paleozoic Era has contributed a specific lithology to the Tobosa Basin, accumulating into almost of sediment at the start of the Pennsylvanian Period (323.2–298.9 mya). The Montoya Group is the youngest rock formation in the Tobosa Basin and was formed in the Ordovician Period (485.4–443.8 mya), and sit directly on the igneous and metamorphic basement rocks.

The rocks from the Montoya Group are described as light to medium grey, fine to medium grained crystalline calcareous dolomite. These rocks were sometimes inter-bedded with shale, dark-grey limestone, and, less commonly, chert. the Montoya Group sequence is made up of carbonate limestone and dolomite which is described as dense, impermeable, and non-porous, and is more commonly found in the Glass Mountains outcrop, with thickness varying from .

Silurian Period (443.8–419.2 mya) During the Silurian Period, the Tobosa Basin experienced dramatic changes in sea level which led to the formation of multiple rock groups. The first of these groups, called the Fusselman Formation, is mostly made up of light grey, medium to coarse grained dolomite. The thickness of this formation varies from , and parts of the Fusselman Formation were also subject to karstification, which indicates a drop in sea level.

The second rock group that formed during the Silurian Period is called the Wristen Formation, which is mud, shale, and dolomite rich rock that reaches a thickness of in some places. Karstification of the Fusselman Formation shows that a drop in sea level occurred, but sea levels rose again during a transgressive event, which lead to the creation of the Wristen Formation. Sea levels then dropped again, which led to major exposure, erosion, and karstification of these formations.

Devonian Period (419.2–358.9 mya) The Thirtyone Formation was developed during the Devonian Period. This formation is characterized by its limestone, chert, and shale beds, some of which had a peak thickness of . this formation had many different types of limestone, including light-colored siliceous, chert-dominated, crinoid-rich, and sandy limestone. The Thirtyone Formation is very similar to the formation of the Mississippian Period, which is likely because there was little to no change in the environment during this time.

Mississippian Period (358.9–323.2 mya) The Mississippian Limestone is the main formation to develop during this Period. This formation, similar to the previously mentioned Thirtyone Formation, is composed primarily of limestone and shale. The limestone beds are described as being "brown to dark brown, micro-crystalline to very finely crystalline, commonly sandy, and dolomitic", while the shale beds are "grey to black, hard, platy, pyritic, organic, and very siliceous". The Mississippian Limestone ranges from in thickness, while generally being thinner towards the southern part of the Tobosa Basin.

The Barnett Shale is the second formation to have developed during the Mississippian Period. It consists mainly of silty brown shale and fine-grained sandstone and siltstone. This Formation was much thicker than the Mississippian Limestone, ranging from . The increased thickness can be explained by increased sedimentation in the area, which was likely caused by tectonic activity in the region.

Tectonic Activity During the Mississippian Period The Ouachita Orogeny occurred during the Late Mississippian, leading to tectonic activity in the region. The subsequent folding and faulting caused by this Orogeny led to the Tobosa Basin being divided into three sections: the Delaware Basin, the Midland Basin, and the Central Basin Platform. The end of the Mississippian Period also led to the beginning of the formation of the modern Permian Reef Complex. The legacy of the early to mid-Paleozoic is almost of sediments that were accumulated due to almost uninterrupted sedimentation.

Late Paleozoic (Pennsylvanian to Permian)

Pennsylvanian Period (323.2–298.9 mya) The Pennsylvanian Period marked the beginning of geological processes that would shape the Permian Basin into what we see today. Rifting events during the Cambrian Period (early Paleozoic) left fault zones in the region. These fault zones acted as planes of weakness for faulting that was later initiated by the Ouachita Orogeny.

These fault zones caused the Tobosa Basin to be transformed, due to tectonic activity, into the Permian Reef Complex, which comprises three parts: the Central Basin Platform, which is encircled by faults, and the Midland and Delaware Basins on either side. Mississippian sediments are absent either due to erosion or nondeposition. Marine shales were deposited in the center of the Delaware, Midland and Val Verde basins, while the basins' periphery saw the deposition of shallow marine, carbonate shelf and limestone sediments.

The Morrow Formation The Early Pennsylvanian Morrow Formation underlies the Atoka Formation. The Morrow is an important reservoir consisting of clastic sediments, sandstones and shales, deposited in a deltaic environment.

Other formations