Permian Basin (North America)

• 1. Introduction

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  The Permian Basin of western Texas and far-southeastern New Mexico produced an average of 4.2 million barrels of crude oil every day in 2019. That made it the highest-producing oil field in the United States. Yet the oil is only the newest layer of a much older story. Under the wells lie rocks that record an ancient sea, a living reef that thrived through a long stretch of the Permian, and a slow death by salt. How did one corner of North America collect some of the thickest Permian rocks on Earth? What killed a reef that once flourished there? And who first suspected that oil was hidden below the surface?

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• 2. Anatomy of a Basin

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  More than 86,000 square miles make up the Permian Basin, a region about 250 miles wide and 300 miles long. Its name comes from the Permian geologic period, the last period of the Paleozoic era. The basin holds some of the world's thickest deposits of rocks from that time. Despite its single name, the basin is really a family of smaller basins. The Midland Basin is the largest, the Delaware Basin is the second largest, and the Marfa Basin is the smallest. Between the Delaware and Midland sits the Central Basin Platform, an uplifted block of basement rock capped by carbonate. The Delaware Basin tilts eastward and holds about 25,000 feet of laminated siltstone and sandstone. The Midland Basin tilts the opposite way, to the west. Reef and carbonate shelves, the Eastern and Northwestern Shelves, flank both basins. Inside the Midland Basin runs the Horseshoe Atoll, a westward-tilting chain of reef mounds 175 miles long. It contains 15 significant petroleum reservoirs at depths from 6,099 to 9,902 feet. On the surface, the Texas cities of Midland, Odessa, and San Angelo serve as headquarters for some of the oil work. The first production well in the Horseshoe Atoll, the Seabird Oil Company of Delaware No. 1-B J. C. Caldwell, was completed in 1948.

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• 3. Granite and Ancient Seas

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  Biotite-quartz granite was found at a depth of 3,847 meters beneath the Permian Basin. It belongs to the basement rock of the ancestral Tobosa Basin, a calm marine setting where shallow seas laid down sediment. Some of that basement dates back 1,330 million years and can still be seen in the Guadalupe Mountains. Layered mafic rocks, thought to belong to the Pecos Mafic Igneous Suite, underlie the whole area. They stretch 360 kilometers across the southern United States and have been dated to 1,163 million years ago. In the nearby Apache and Glass Mountains, the basement is metamorphosed sandstone and granite instead. Above it, the Montoya Group formed in the Ordovician as dense, non-porous dolomite and limestone, from 46 to 155 meters thick in the Glass Mountains. The Silurian brought dramatic swings in sea level. The Fusselman Formation, 15 to 50 meters thick, was partly dissolved into karst when the sea dropped. A rising sea then built the Wristen Formation, up to 450 meters of mud, shale, and dolomite. The Devonian added the Thirtyone Formation, with limestone, chert, and shale up to 300 meters thick. In the Mississippian, the Mississippian Limestone added 15 to 52 meters. The Barnett Shale followed at 60 to 140 meters, thickened by sediment that tectonic activity probably shed into the area. All these periods together stacked almost 2,000 meters of sediment.

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• 4. A Collision Splits the Land

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  In the Late Mississippian, the Ouachita Orogeny, a mountain-building episode, folded and faulted the region. The Tobosa Basin broke into three pieces: the Delaware Basin, the Midland Basin, and the Central Basin Platform. The cause was a collision between North America and Gondwana, the landmass of South America and Africa. Old scars guided the break. Rifting in the Cambrian had already left fault zones, and these acted as planes of weakness when the collision came. Folding of the basement then split the basin, with the Delaware to the west and the Midland to the east. Pennsylvanian rocks tell what happened next. The Morrow Formation, deposited in a delta, became an important reservoir of sandstone and shale. The Atoka Formation above it is fossil-rich limestone up to 200 meters thick, and it holds the first reef structures to form in the Delaware Basin. The Strawn Formation, also up to 200 meters thick, shows a marked increase in reef mounds. Its limestone preserves brachiopods, foraminifera, bryozoans, corals, and crinoids. Two more formations, the Canyon and the Cisco, matter most to modern drillers because major oil reservoirs were found in them.

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• 5. The Capitan Reef

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  Permian-aged rocks make up 95% of the outcrops in the Permian Basin today. During that period, the supercontinent Pangaea began to break apart, and the basin sat on its western edge, within 5 to 10 degrees of the equator. The planet was shifting from icehouse to greenhouse, and melting ice at the South Pole raised sea levels. The Guadalupian Epoch took its name from the Guadalupe Mountains, because reef building peaked then. The Capitan Reef grew in warm water of about 20 degrees Celsius, clear and shallow, with salinity between 27 and 40 parts per thousand. Upwelling mixed fresh marine water with oxygen-poor water from the basin floor, and the mix fed the reef with nutrients. Erect sponges with rigid skeletons built much of it, alongside red algae and microbial micrite that trapped sediment. One prominent builder was the sponge family Guadalupiidae, which appeared in the Glass Mountains in the mid-Permian and reached the Delaware Basin by the late Permian. Once the reef reached sea level, it spread sideways. Later it climbed fast enough to keep pace with rising seas, and in places it pushed out almost 50 kilometers. A global slump called the end-Guadalupian reef crisis ended this golden age, yet over 100 Guadalupian reefs remain worldwide, the most from any Permian epoch.

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• 6. Death by Salt

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  The Hovey Channel, a low gap on the southern edge of the Delaware Basin, once carried ocean water into the basin from the Panthalassa sea. It began as an anticline, a fold formed during Precambrian faulting. As Pangaea broke up, tectonic change choked off the exchange of sea water through that channel. Passages between the Delaware and Midland Basins also narrowed. Salinity climbed, the late Permian grew hotter, and the reef could not survive. Evaporite growth on the basin floor suggests a stratified water column that was both oxygen-free and sulfidic. The salt left behind is the Castile Formation, nearly 1,300 meters of gypsum, anhydrite, limestone, and salt from the Lopingian Epoch. Its thin laminae range from 1 millimeter to 10 centimeters, and they are thought to track the basin's salinity year by year. Evaporites in the back reef region also show that the climate had turned hot and arid. The reef system lay buried until tectonic activity of the Laramide orogeny exposed it in the Mesozoic. The deep-water shale and carbonate reefs of the basins and the Central Basin Platform would go on to become lucrative hydrocarbon reservoirs.

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• 7. Finding the Oil

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  In 1917, University of Texas geology professor J.A. Udden speculated that the Marathon Fold, tied to the Marathon Mountains, might extend northward. Geologists R.A. Liddle and J.W. Beede elaborated the idea in 1918. They thought the structure might trap oil, and test drilling began in the eastern basin, guided by that theory and known oil seeps. W.H. Abrams documented oil reserves in Mitchell County, West Texas, in 1920. The first commercial well opened in 1921 in the Westbrook Oil Field at a depth of 2,498 feet. Survey crews could not study the basin's interior because it lacked outcrops, so they first imagined it as bowl-shaped. Discoveries followed quickly: Big Lake in 1923, then World and McCamey in 1925, then Hendrick and Yates in 1926. They came from random drilling or surface mapping, and seismographs and magnetometers later helped find anomalies. By 1924, companies such as Gulf Oil and The Texas Company had set up regional geological offices. Distances were long, pipelines were scarce, and deep tests were costly, so wells stayed shallower than about 5,000 or 6,000 feet until 1928. That year, the No. I-B University discovery well struck oil at 8,520 feet in the Ordovician formations of Big Lake. Fields such as Harper, Goldsmith, Foster, Keystone, Means, Wasson, and Slaughter came in the 1930s. World War II made oil urgent enough to justify deep drilling, and reservoirs turned up in every formation from the Cambrian to the Permian. Embar, TXL, Dollarhide, and Block 31 were among the wartime finds. Potash had its own story: Udden found it in the region in the late 1800s. Its value rose in World War I, when the United States could no longer import it from Germany. By the mid-1960s, seven potash mines operated on the New Mexico side.

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• 8. Boom and Burn-Off

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  In 1966, the basin produced 600 million barrels of oil and 2.3 trillion cubic feet of gas, together worth $2 billion. Gas pipelines and refineries pushed output steadily higher. By 2018, the basin had produced more than 33 billion barrels of oil and 118 trillion cubic feet of natural gas. That equals 20% of US crude oil production and 7% of US dry natural gas production. Many believed output peaked in the early 1970s. Then horizontal drilling and hydraulic fracturing opened tight oil shales such as the Wolfcamp Shale, and production rose dramatically. The Energy Information Administration estimated that proven reserves still held 5 billion barrels of oil and about 19 trillion cubic feet of gas. A price came with the boom. By October 2019, executives said they had been cutting back on flaring, which burns natural gas. Drilling had far outpaced pipeline construction, so flaring grew, and so did venting of gas straight into the air. Both practices are legal under state law. Gas was so cheap that smaller companies with pipeline capacity chose to flare it rather than pay to ship it. Satellite data show that 3.7% of the gas produced is lost in leaks, equal to the use of 7 million Texas homes. Most of the methane emitted comes from a small number of sources.

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