The Permian Midland Basin & Delaware Basin, North America

The Permian Midland Basin & Delaware Basin, North America

The Permian Basin is a large sedimentary basin in the southwestern part of the United States. The basin contains the Mid-Continent Oil Field province. This sedimentary basin is located in western Texas and southeastern New Mexico. It reaches from just south of Lubbock, past Midland and Odessa, south nearly to the Rio Grande River in southern West Central Texas, and extending westward into the southeastern part of New Mexico. It is so named because it has one of the world's thickest deposits of rocks from the Permian geologic period. The greater Permian Basin comprises several component basins; of these, the Midland Basin is the largest, Delaware Basin is the second largest, and Marfa Basin is the smallest. The Permian Basin covers more than 86,000 square miles (220,000 km2),[1] and extends across an area approximately 250 miles (400 km) wide and 300 miles (480 km) long.

The Permian Basin lends its name to a large oil and natural gas producing area, part of the Mid-Continent Oil Producing Area. Total production for that region up to the beginning of 1993 was over 14.9 billion barrels (2.37×109 m3). The cities of Midland, Texas, Odessa, Texas and San Angelo serve as the headquarters for oil production activities in the basin.

The Permian Basin is also a major source of potassium salts (potash), which are mined from bedded deposits of sylvite and langbeinite in the Salado Formation of Permian age. Sylvite was discovered in drill cores in 1925, and production began in 1931. The 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.

The West Texas Permian Basin

Regional Tectonic history

During the Cambrian–Mississippian, the ancestral Permian Basin was the broad marine passive margin Tobosa Basin containing deposits of carbonates and clastics. In the early Pennsylvanian–early Permian the collision of North American and Gondwana Land (South America and Africa) caused the Hercynian orogeny. The Hercynian Orogeny resulted in the Tobosa basin being differentiated into two deep basins (the Delaware and the Midland Basins) surrounded by shallow shelves. During the Permian, the basin became structurally stable and filled with clastics in the basin and carbonates on the shelves.

Lower Paleozoic passive margin phase (late Precambrian–Mississippian, 850–310 Mya)

This passive margin succession is present throughout the southwestern US and is up to 0.93 miles (1.50 km) thick. The ancestral Permian basin is characterized by weak crustal extension and low subsidence in which the Tobosa basin developed. The Tobosa basin contained shelf carbonates and shales.

Collision phase (late Mississippian–Pennsylvanian, 310–265 Mya)

The two lobed geometry of the Permian basin separated by a platform was the result of the Hercynian collisional orogeny during the collision of North America and Gondwana Land (South America and Africa). This collision uplifted the Ouachita-Marathon fold belt and deformed the Tobosa Basin. The Delaware Basin resulted from tilting along areas of Proterozoic weakness in Tobosa basin. Southwestern compression reactivated steeply dipping thrust faults and uplifted the Central Basin ridge. Folding of the basement terrane split the basin into the Delaware basin to the west and the Midland Basin to the east.

Permian Basin phase (Permian, 265–230 Mya)

Rapid sedimentation of clastics, carbonate platforms and shelves, and evaporites proceeded synorogenically. Bursts of orogenic activity are divided by three angular unconformities in basin strata. Evaporite deposits in the small remnant basin mark the final stage of sedimentation as the basin became restricted from the sea during sea level fall.

Depositional history

The Permian Basin is the thickest deposit of Permian aged rocks on Earth which 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 mya).

Proterozoic

Prior to the breakup of the Precambrian supercontinent and the formation of the modern Permian Basin geometry, shallow marine sedimentation onto to the ancestral Tobosa Basin characterized the passive margin, shallow marine environment. The Tobosa Basin also contains basement rock that dates back to 1330 million years ago (mya), and that are still visible in the present-day Guadalupe Mountains. The basement rock contains biotite-quartz granite, discovered at a depth of 3847 m. 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 360 km into southern USA and has been dated to 1163 mya.

Late 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 2000 m of sediment at the start of the Pennsylvanian Period (323.2 – 298.9 mya).[7] 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 descried 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 46 to 155 m.

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 15 to 50 m, 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 450 m 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 would then drop again, which led to major exposure, erosion, and karstification of these formations.

Late Mississippian–Early Permian

Climatic zones of the Carboniferous-Permian boundary

The collision of North America and Gondwana Land (South America and Africa) during the Hercynian orogeny created the Ouachita–Marathon thrust belt and the associated foreland basins, the Delaware and Midland Basins, separated by the Central Basin Platform. The tectonic activity resulted in the distribution of voluminous siliciclastic sediments into the basins during the Early Pennsylvanian. Siliciclastic sedimentation was followed by the formation of carbonate shelves and margins at the basin flanks in the Early Permian.

Late Permian

After the Hercynian orogeny, 4 kilometres (2.5 mi) of sediment filled the rapidly subsiding Delaware and Midland basins. The Midland basin was filled by about 270 mya, as it received the majority of clastic sediment from the Hercynian Orogeny via a subaqueous delta, while the Delaware Basin continued to fill until the late Permian. Sandstones and some deep water, organic rich shales were deposited within the basins while reef carbonates were deposited on the Central Basin Platform and on the shelves of the basins. The extensive reef deposits fringing the Delaware Basin became known as the Capitan Limestone. In the later Guadalupian, the Permian sea retreated, and the basins were capped with evaporite deposits, including salts and gypsum. The deep water shale and carbonate reefs of the Delaware and Midland Basins and the Central Basin Platform would become lucrative hydrocarbon reservoirs.

Generalized facies tracts of the Permian Basin

The Permian basin is divided into generalized facies belts differentiated by the depositional environment in which they formed, influenced by sea level, climate, salinity, and access to the sea.

Lowstand systems tract

Lowering sea level exposes the peritidal and potentially, the shelf margin regions, allowing linear channel sandstones to cut into the shelf, extending beyond the shelf margin atop the slope carbonates, fanning outward toward the basin. The tidal flats during a lowstand contain aeolian sandstones and siltstones atop supratidal lithofacies of the transgressive systems tract. The basin fill during a lowstand is composed of thin carbonate beds intermingled with sandstone and siltstone at the shelf and sandstone beds within the basin.

Transgressive systems tract

These facies results from the abrupt deepening of the basin and the reestablishment of carbonate production. Carbonates such as bioturbated wackstone and oxygen poor lime mud accumulate atop the underlying lowstand systems tract sandstones in the basin and on the slope. The tidal flats are characterized by supratidal faces of hot and arid climate such as dolomudstones and dolopackstones. The basin is characterized by thick carbonate beds on or close to the shelf with the shelf margin becoming progressively steeper and the basin sandstones becoming thinner.

Highstand systems tract

Highstand systems tract facies results from the slowing down in the rise of sea level. It is characterized by carbonate production on the shelf margin and dominant carbonate deposition throughout the basin. The lithofacies is of thick beds of carbonates on the shelf and shelf margin and thin sandstone beds on the slope. The basin becomes restricted by the formation of red beds on the shelf, creating evaporites in the basin.

Evolution and Deposition

This will be the first of a three part series where I will discuss the Permian Basin as well as the similarities and differences in the Midland Basin and the Delaware Basin. This first discussion will cover the evolution and deposition while the following will cover stratigraphy, reservoir quality, and production of this basin.

The Greater Permian Basin (GPB) is one of the largest and most structurally complex regions in North America. This sedimentary basin is comprised of several sub-basins and platforms. It covers an area about 250 miles wide and 300 miles long in 52 counties in west Texas and southeast New Mexico. That’s more than 75,000 square miles! Though it contains one of the world’s thickest deposits of Permian aged rocks, it was actually named after the period of geologic time (Permian: 299 million to 251 million years ago) where the basin reached its maximum depth of 29,000 feet.

Evolution

The evolution of the basin can be attributed to three distinct phases: (1) mass deposition (2) continental collision (3) basin filling. Before the Permian Basin was formed, this region was a broad marine area called the Tobosa Basin. During the Cambrian to Mississippian periods (541 to 323 million years ago), massive amounts of clastic sediments were deposited in this area causing it to form a depression. What we define as the basin today began forming in late Mississippian and early Pennsylvanian (323 to 299 million years ago) when the supercontinents Laurasia and Gondwana collided to form Pangea causing faulting and uplift. While the area was covered by a seaway (figure 1), episodes of faulting, uplift, and erosion (associated with the Marathon-Ouachita Orogeny) as well as different rates of subsidence caused structural deformations in the larger Tobosa Basin that divided it into sub-basins and platforms.

Paleographic time sequence, from youngest to oldest, of the evolution of the Greater Permian Basin, Source: DI 2.0 Paleo Layer

The final process that created the GPB was the filling of the sub-basins with sediments. The Midland Basin, Central Basin Platform, and the Delaware basin are the three main components of the GPB that we know today. Other sections of the GPB include: the Northwest Shelf, Marfa Bain, Ozona Arch, Hovey Channel, Val Verde Basin, and Eastern Shelf.

Structural differences between the Delaware Basin, Central Platform, and Midland Basin, source: Kelly et al. “Permian Basin – Easy to Oversimplify, Hard to Overlook”

Deposition

The Midland and Delaware sub-basins share mutual characteristics such as age and lithology, but depths, nomenclature, and development vary throughout the GPB. The sub-basins rapidly subsided, while the platform remained at a higher elevation. This resulted in areas having very different water depths and depositional environments. The basins accumulated terrigenous clastics that are associated with deep water environments, whereas coarse grains associated with shallow reef environments were deposited along the platform. Differences in sedimentary depositions and tectonics initiated stratigraphic discontinuities between the two sub-basins.

The Midland Basin

The eastern Midland Basin accumulated large amounts of clastic sediments from the Ouachita orogenic belt during the Pennsylvanian (323 to 299 million years ago). As these sediments were deposited, they formed a thick subaqueous deltaic system that consumed the basin from east to west. During the Permian period, the delta system was covered with floodplains and was nearly filled by the Middle Permian.

The Delaware Basin

The western area of the GPB, the Delaware Basin, was a structural and topographical low that provided an inlet for marine water during most of the Permian. Minor sedimentation was received from the low coastal plains that surrounded the basin. While the Midland Basin was almost full of sediment by the Middle Permian, the Delaware became host to reefs built by sponges, algae, and microbial organisms. These organisms, along with the deep water inputs supplied by the Hovey Channel (figure 3), promoted carbonate buildups that formed a higher elevation area which separated the shallow water and deep water deposits.

Permian Map: The Hovey Channel supplied the Delaware Basin with deep water sediment, while the Midland Basin was restricted by carbonate reefs of the Central Platform

Depth also had an important impact on the way sediments were deposited in the basin. The Delaware Basin is approximately 2,000 feet deeper than the Midland Basin (figure 4), thus causing the sediments to experience nearly twice as much pressure during burial. This is a leading factor in the stratigraphic discontinuities between the two sub-basins.

Depth map of the Delaware Basin, Central Platform, and Midland Basin

Delaware Basin

The Delaware Basin is a geologic depositional and structural basin in West Texas and southern New Mexico, famous for holding large oil fields and for a fossilized reef exposed at the surface. Guadalupe Mountains National Park and Carlsbad Caverns National Park protect part of the basin. It is part of the larger Permian Basin, itself contained within the Mid-Continent oil province.

Exposed and buried parts of Capitan Reef

Geology

By earliest Permian time, during the Wolfcampian Epoch, the ovoid shaped subsiding Delaware Basin extended over 10,000 square miles (26,000 km²) in what is now western Texas and southeast New Mexico. This period of deposition left a thickness of 1600 to 2200 feet (490 to 670 m) of limestone interbedded with dark-colored shale.

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 (Wolfcamp Formation), Leonardian (Avalon Shale), and early Guadalupian times. The eastward-dipping Delaware basin is subdivided into several formations and contains approximately 25,000 feet (7,600 m) of laminated siltstone and sandstone. Aside from clastic sediment, the Delaware basin also contains carbonate deposits of the Delaware Group, originating from the Guadalupian times when the Hovey Channel allowed access from the sea into the basin.

A narrow outlet that geologists call the Hovey Channel periodically supplied new seawater from the Panthalassa Ocean to the west. The somewhat smaller and shallower Midland Basin was just east and the much smaller Marfa Basin was to the southwest. All three basins were south of the equator, north of the Ouachita Mountains of mid-Texas, and part of the northern continent Laurasia. Structurally the Delaware, Midland and Marfa were foreland basins created when the Ouachita Mountains were uplifted as the southern continent Gondwana collided with Laurasia, forming the supercontinent Pangea in the Late Carboniferous (Pennsylvanian). The Ouachita Mountains formed a rainshadow over the basins, and a warm shallow sea flooded the surrounding area. On the other side of the equator, the Ancestral Rocky Mountains formed a large mountainous island.

The Delaware Basin temporarily stopped subsiding in the Leonardian Epoch at the start of the mid-Permian. Small banks along its margin developed along with small discontinuous patch reefs in the shallow water just offshore. The first formation that resulted was the Yeso and consists of alternating beds of dolomitic limestone, gypsum, and sandstone. The sediments responsible for creating the Yeso were deposited in nearshore areas that graded into the carbonate banks of the Victorio Peak Formation in the deeper waters. Thin-bedded limestones of the Bone Spring Formation accumulated as limy ooze in the stagnant deepest part of the basin.

Subsidence of the Delaware Basin restarted later in the mid Permian and by the Guadalupian Epoch of the upper Permian the patch reefs had grown larger. Sediments deposited close to the shore are now the cherty dolomites of the San Andres Formation while deposition a little further out forms the quartz sandstone and scattered patch reefs of the Brushy Canyon Formation.

Rapid subsidence of the basin started in the middle Guadalupian. Patch reefs responded by rapid (mostly vertical) growth, resulting in the Goat Seep Reef. Three facies developed:

• Sediments deposited in a lagoon, forming the sandstones and dolomites of the Queen and Grayburg Formations.
• Sponge and algae skeletons accumulated near the Goat Seep Reef to become the Getaway Bank.
• Quartz sand laid down further in the basin became the Cherry Canyon Formation.

Subsidence of the basin stopped for good by the later part of the Guadalupian. Capitan Reef was the largest in the basin, and it rapidly grew 350 miles (560 km) around it. The facies were:

• Fine-grained sand and carbonates deposited near the shore became the dolomites and sandstone of the Carlsbad Group.
• Offshore accumulations of sand and limey ooze in the basin were lithified into sandstone and limestone belonging to the Bell Canyon Formation.
• The Capitan Formation consists of Capitan Reef and is made of reef limestone.

Capitan Reef was built primarily from calcareous sponges, encrusting algae such as stromatolites, and directly from seawater as a limey mud. In stark contrast, Cenozoic (current era), Mesozoic (age of the dinosaurs), and even middle Paleozoic (well before the Permian) reefs are mainly composed of corals.

Sea level dropped as the late Permian glaciation intensified and locked increasing amounts of water in distant ice caps. Sedimentation continued to fill the Delaware Basin into the Ochoan Epoch of the upper Permian, periodically cutting the basin off from its source of seawater. Part of the resulting brine became the deep-water evaporites of the Castile Formation. The Castile consists of 1/16 inch (1.6 mm) thick laminae of alternating gray anhydrite and gypsum, brown calcite, and halite. As the salt concentration increased, halite and potassium-rich salt precipitated from the briny body of water on its margin and on nearshore areas. This salt layer covered an increasingly large area as the water level dropped, forming the Salado Formation.

The Delaware Basin was filled at least to the top of Capitan Reef and mostly covered by dry land before the end of the Ochoan Epoch. Rivers migrated over its surface and deposited the red silt and sand that now constitute the siltstone and sandstone of the Rustler and Dewey Lake Formations. A karst topography developed as groundwater circulated in the buried limestone formations, dissolving away caverns which were later destroyed by infill and erosion.

Uplift associated with the Laramide orogeny in the late Mesozoic and early Cenozoic created a major fault along which the Guadalupe Mountains were thrusted into existence. The range forms the tilted upthrown part of the system and the Salt Flat Bolson forms the downfallen block. Capitan Reef limestone was exposed above the surface with the 1000-foot-high (300 m) El Capitan being its most prominent feature. Other large outcrops compose the Apache Mountains and Glass Mountains to the south.

Streams eroded the softer sediment away, lowering the ground level to its current position. Acidic groundwater excavated caves in the limestone of the higher areas and eroded sediment helped fill any remaining Permian-aged caves. Unlike most other caves in limestone, in this case the acid was likely derived from hydrogen sulfide and sulfide-rich brines that were freed by tectonic activity in the mid-Tertiary and mixed with oxygenated groundwater, forming sulfuric acid. Carlsbad Caverns and nearby modern caves started to form at this time in the groundwater-saturated phreatic zone. Due to the semiarid climate, the karst topography that was created lacks the characteristic depressions, sink holes, pits, and solutional fissures on the surface. Mass wasting such as landslides further reduced topographic relief.

Additional uplift of the Guadalupe Mountains in the Pliocene and early Pleistocene epochs enlarged Carlsbad Cavern and nearby caves. Parts of the major caves emerged from the saturated phreatic zone into the vadose with temporary periods of repose during which additional solutional excavation occurred in the phreatic zone. These pauses in emergence are thought to be responsible for creating the different levels in Carlsbad Caverns. Precipitation of carbon dioxide-rich water that infiltrated into the cavern created speleothems, especially in the humid parts of the Pleistocene. Speleothems found in the "Big Room" of Carlsbad were dated using electron spin resonance dating and were found to be 500,000 to 600,000 years old. This indicates that the Big Room level was dry by that time.

The soft and easily eroded gypsum of the Castile Formation was removed, exposing the Guadalupe Escarpment. Additional erosion intersected the upper part of Carlsbad Cavern and other caves, forming their entrances. Drying of cave air has reduced the growth rate of speleothems and encouraged the development of nodular travertine ("cave popcorn").

Midland Basin

The westward-dipping Midland Basin is subdivided into several formations and is composed of laminated siltstone and sandstone. The Midland Basin was filled via a large subaqeuous 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.

References:

• Permian Basin (North America), Wikipedia
• Delaware Basin, Wikipedia
• Leslie Sutton, 2014, The Midland Basin vs. the Delaware Basin – Understanding the Permian, info Drillinginfo.com