Austin Chalk stratigraphic column in Texas

The Eagle Ford Group (also called the Eagle Ford Shale) is a

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

rock formation A rock formation is an isolated, scenic, or spectacular surface rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock formation'' can also refer to specific sedime ...

deposited during the

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...

and

Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded b ...

ages of the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

over much of the modern-day state of

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

. The Eagle Ford is predominantly composed of

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

-rich

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

iferous marine

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

s and

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part ...

s with interbedded thin

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

s. It derives its name from

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

s on the banks of the West Fork of the Trinity River near the old community of Eagle Ford,Hill, R. T. (1887) The topography and geology of the Crosstimbers and surrounding regions in North Texas: American Journal of Science, v. 33, no. 196, pp. 291–303. which is now a neighborhood within the city of

Dallas Dallas () is the third largest city in Texas and the largest city in the Dallas–Fort Worth metroplex, the fourth-largest metropolitan area in the United States at 7.5 million people. It is the largest city in and seat of Dallas County ...

. The Eagle Ford

belt trends from the

Oklahoma Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...

border southward to

San Antonio ("Cradle of Freedom") , image_map = , mapsize = 220px , map_caption = Interactive map of San Antonio , subdivision_type = Country , subdivision_name = United States , subdivision_type1= State , subdivision_name1 = Texas , subdivision_ ...

, westward to the

Rio Grande The Rio Grande ( and ), known in Mexico as the Río Bravo del Norte or simply the Río Bravo, is one of the principal rivers (along with the Colorado River) in the southwestern United States and in northern Mexico. The length of the Rio G ...

Big Bend National Park Big Bend National Park is an American national park located in West Texas, bordering Mexico. The park has national significance as the largest protected area of Chihuahuan Desert topography and ecology in the United States, and was named after ...

, and the Quitman Mountains of

West Texas West Texas is a loosely defined region in the U.S. state of Texas, generally encompassing the arid and semiarid lands west of a line drawn between the cities of Wichita Falls, Abilene, and Del Rio. No consensus exists on the boundary betw ...

.Denne, R. A., Breyer, J. A., Callender, A. D., Hinote, R. E., Kariminia, M., Kosanke, T. H., Kita, Z., Lees, J. A., Rowe, H., Spaw, J. M., and Tur, N. (2016). Biostratigraphic and geochemical constraints on the stratigraphy and depositional environments of the Eagle Ford and Woodbine Groups of Texas: "in" Breyer, J. A. (ed.), The Eagle Ford Shale: A renaissance in U.S. oil production, AAPG Memoir 110, pp. 1–86. It also occurs in the subsurface of

East Texas East Texas is a broadly defined cultural, geographic, and ecological region in the eastern part of the U.S. state of Texas that comprises most of 41 counties. It is primarily divided into Northeast and Southeast Texas. Most of the region co ...

and

South Texas South Texas is a region of the U.S. state of Texas that lies roughly south of—and includes—San Antonio. The southern and western boundary is the Rio Grande, and to the east it is the Gulf of Mexico. The population of this region is about 4.96 ...

, where it is the

source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...

for

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

found in the Woodbine, Austin Chalk, and the

Buda Limestone The Buda Limestone is a geological formation in the High Plains and Trans-Pecos regions of West Texas and in southern New Mexico, whose strata date back to the Late Cretaceous. Pterosaur remains are among the fossils that have been recovered ...

,Surles, M. A. (1987) Stratigraphy of the Eagle Ford Group (Upper Cretaceous) and its source-rock potential in the East Texas Basin: Baylor Geological Studies Bulletin 45, Waco, Texas, 57 p. and is produced unconventionally in

and the "Eaglebine" play of

. The Eagle Ford was one of the most actively drilled targets for unconventional oil and gas in the United States in 2010, but its output had dropped sharply by 2015.Jennifer Hiller, "Hard Times Hit Eagle Ford," "

San Antonio Express-News The ''San Antonio Express-News'' is a daily newspaper in San Antonio, Texas. It is owned by the Hearst Corporation and has offices in San Antonio and Austin, Texas. The ''Express-News'' is the third largest newspaper in the state of Texas, with ...

," January 3, 2016, pp. 1, A20 By the summer of 2016, Eagle Ford spending had dropped by two-thirds from $30 billion in 2014 to $10 billion, according to an analysis from the research firm

Wood Mackenzie Wood Mackenzie, also known as ''WoodMac,'' is a global research and consultancy group supplying data, written analysis, and consultancy advice to the energy, chemicals, renewables, metals, and mining industries. In 2015, the company was acquire ...

. This strike has been the hardest hit of any oil fields in the world. The spending was, however, expected to increase to $11.6 billion in 2017. A full recovery is not expected any time soon.

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s are relatively common in Eagle Ford rocks.

Vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

s that have been found in the Eagle Ford include plesiosaurs,

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on ...

teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

, and teeth from

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s and other

Invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...

s found in the Eagle Ford include

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) o ...

s, swimming

crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...

inoceramid The Inoceramidae are an extinct family of bivalves ("clams") in the Class Mollusca. Fossils of inoceramids are found in marine sediments of Permian to latest Cretaceous in age. Inoceramids tended to live in upper bathyal and neritic environments ...

clams, and other

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

shells.Donovan, A. D., T. S. Staerker, A. Pramudito, W. Li, M. J. Corbett, C. M. Lowery, A. M. Romero, and R. D. Gardner (2012) The Eagle Ford outcrops of West Texas: Understanding heterogeneities within unconventional mudstone reservoirs: GCAGS Journal, v. 1, p. 162–185.

Depositional environment

The Eagle Ford was deposited in an inland sea (or epeiric sea) that covered much of modern-day

. The Texas shelf during the

was bounded by the Ouachita Uplift to the north, the Sabine Uplift to the East, relict reef margins of the Stuart City Formation and the Sligo Formation to the southeast, and the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...

to the west. The

and

regions were divided by an extension of the

Llano Uplift The Llano Uplift is a geologically ancient, low geologic dome that is about in diameter and located mostly in Llano, Mason, San Saba, Gillespie, and Blanco counties, Texas. It consists of an island-like exposure of Precambrian igneous and m ...

known as the San Marcos Arch. Primary basins active during Eagle Ford deposition were the East Texas and Brazos Basins in East Texas and the Maverick Basin in South Texas. The bottom waters of the Eagle Ford sea were

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

during most of Eagle Ford deposition, which is related to the global Oceanic Anoxic Event 2 (OAE2), or Cenomanian-Turonian boundary event, although the Texas shelf became anoxic nearly two million years prior to OAE2. The low-oxygen conditions were conducive for preservation of the

that ultimately generated the

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s associated with the Eagle Ford in the subsurface. Evidence for anoxia include the high amounts of organic matter, lack of evidence for

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

organisms (

s or

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

s), and enrichment in the

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

proxies

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...

and

vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...

. After the significant drop in

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...

(

marine regression A marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land. Evidence of marin ...

) associated with deposition of the Woodbine during the Early Cenomanian, the sea level began to rise (

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

), allowing for the deposition of Lower Eagle Ford organic-rich

s in South Texas and

s of the Terrell Member of the Boquillas Formation in

starting at about 96 million years ago. Gardner, R. D., M. C. Pope, M. P. Wehner, and A. D. Donovan (2013
“Comparative stratigraphy of the Eagle Ford Group in Lozier Canyon and Antonio Creek, Terrell County, Texas”
GCAGS Journal, v. 2, p. 42-52. Wehner, M., R. Gardner, M. M. Tice, M. C. Pope, A. D. Donovan, and T. S. Staerker (2015) Anoxic, storm dominated inner carbonate ramp deposition of Lower Eagle Ford Formation, west Texas: Unconventional Resources Technology Conference, San Antonio, Texas, USA 20-22 July 2015, doi:10.15530/urtec-2015-2154667 The rise in sea level eventually drowned the East Texas Woodbine

river delta A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...

s, initiating Eagle Ford deposition in East Texas.Denne, R. A., and Breyer, J. A. (2016) Regional depositional episodes of the Cenomanian-Turonian Eagle Ford and Woodbine groups of Texas: "in" Breyer, J. A. (ed.), The Eagle Ford Shale: A renaissance in U.S. oil production, AAPG Memoir 110, p. 87-135. The initial deposits, known as the Six Flags Limestone in Dallas and the Bluebonnet Limestone in Waco, are

calcarenite Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), carbonate grains. The grains consist of sand-size grains of either corals, shells, oo ...

s predominantly composed of disaggregated prisms of "

Inoceramus ''Inoceramus'' (Greek: translation "strong pot") is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus '' Pteria''. They lived from the Early Jurassic to l ...

" clams and

planktonic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

tests. Following deposition of the calcarenites, a river delta began to prograde from the Ouachita Uplift to the northern East Texas Basin. Although the

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

s and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

s from this delta, known as the Templeton Member, were originally placed within the Woodbine Formation, the

s found within them indicate that they are better associated with the Eagle Ford. In areas unaffected by the Templeton Delta, depositional rates were low, producing a condensed section composed of organic-rich,

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

s, limestones, and

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

beds in both South Texas and West Texas. The

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

s found within the marls are predominantly

coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as '' Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere' ...

s and

, whereas the limestones contain abundant

radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...

and calcispheres (calcareous

cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...

s produced by some

dinoflagellate The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

s). ''

'' fragments and

bones are also found in these deposits. During the Late Cenomanian the Sabine Uplift along the modern-day Texas/

Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...

border became active, causing

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

of Eagle Ford and Woodbine

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

s and deposition within the Harris Delta complex. Clay from this delta reached as far south as

DeWitt County, Texas DeWitt County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 19,824. The county seat is Cuero. The county was founded in 1846 and is named for Green DeWitt, who founded an early colony in Texas. His ...

. Towards the end of the Late Cenomanian, the bottom waters of the Texas shelf and the Western Interior Seaway became oxygenated,Eldrett, J. S., D. Minisini, and S. C. Bergman (2014) Decoupling of the carbon cycle during Ocean Anoxic Event 2: Geology, v. 42, p. 567–570. which may be related to the sea-level maximum associated with the Cenomanian-Turonian boundary event. Evidence for this oxygenation event, known as the "Benthonic Zone," include an increase in the abundance of benthic organism fossils and

bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a ...

, a decrease in redox proxies

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

, molybdenum, and vanadium, and a reduction in organic matter.Denne, R. A., R. E. Hinote, J. A. Breyer, T. H. Kosanke, J. A. Lees, N. Engelhardt-Moore, J. M. Spaw, and N. Tur (2014) The Cenomanian-Turonian Eagle Ford Group of South Texas: insights on timing and paleoceanographic conditions from geochemistry and micropaleontologic analyses: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 413, p. 2-28.Boling, K. S. and Dworkin S. I. (2015) Origin of organic matter in the Eagle Ford Formation: Interpretation, v. 3, p. SH27–SH39. This oxygenation event marks the boundary between the Lower and Upper Eagle Ford in West Texas and the subsurface of South Texas. In general, Upper Eagle Ford rocks deposited during the Cenomanian-Turonian boundary event (OAE2) contain much less organic matter than Lower Eagle Ford rocks, which is the reverse of organic matter trends seen in the global ocean. An

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

occurs throughout East Texas at this level, possibly due to a lack of sediments reaching the basin during the sea-level maximum. The sea-level began to drop after the Early

sea-level maximum; this is most obvious at

s near Langtry, Texas, where water depths became shallower than 100 ft (30 m). This limestone-rich unit is known as the Langtry Member of the Boquillas Formation. It contains very little organic matter, and abundant

s. The

Kamp Ranch Limestone The Kamp Ranch Limestone is a geologic formation in Dallas County, Texas. It preserves fossils dating back to the Cretaceous period. The most notable is ''Dallasaurus'', a squamate that was the first of the aquatic mosasaurs. See also * List o ...

is found above the unconformity in the Dallas area. It is similar to the older Six Flags and Bluebonnet Limestones, as it is predominantly composed of disaggregated prisms of ''

'' clams and has

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''u ...

indicative of shallow-water deposition. As the sea level continued to fall during the Late Turonian, deltaic sediments originating from the Ouachita Uplift prograded into the northern East Texas region. These sandstones are known as the Sub-Clarksville Delta in the subsurface and the Bells Sandstone in

. In the South Texas subsurface, the age equivalent unit to the Langtry Member is more calcareous than the underlying Upper Eagle Ford rocks, making them difficult to distinguish from the limestones of the overlying Austin Chalk, although an unconformity is found between the Eagle Ford and the Austin Chalk in both South Texas and East Texas.

Eagle Ford unconformity

In the Cretaceous after the Woodbine and Eagle Ford formations were deposited, the Sabine Uplift started to become elevated again due to its reactivation ~88 mya. A decrease in the effective elastic plate thicknesses caused the basin to subside as the uplift became increasingly elevated. As a result, an estimated 150 m of uplift over the Sabine region caused the eastern parts of the Woodbine and Eagle Ford formations to have a subaerial exposure, which eventually resulted in their easterly erosion. Deposition of the Austin Chalk after this erosional occurrence caused a sealing of the East Texas petroleum reservoir and the creation of a middle Cretaceous unconformity. Currently, the Sabine Uplift is in the subsurface, and the middle Cretaceous unconformity is not seen because it is buried below a massive wedge of

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

sediments from the Late Cretaceous to the present.

Oil and natural gas production

Gas Production from Eagle Ford 2000-2013

Petrohawk drilled the first well to unconventionally produce gas from the Eagle Ford in 2008, in LaSalle County, Texas. Oil companies quickly extended the productive area, which stretches from the Texas-

Mexico Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...

border in

Webb Webb most often refers to James Webb Space Telescope which is named after James E. Webb, second Administrator of NASA. It may also refer to: Places Antarctica * Webb Glacier (South Georgia) * Webb Glacier (Victoria Land) * Webb Névé, Victor ...

and

Maverick Maverick, Maveric or Maverik may refer to: History * Maverick (animal), an unbranded range animal, derived from U.S. cattleman Samuel Maverick Aviation * AEA Maverick, an Australian single-seat sportsplane design * General Aviation Design Bure ...

counties and extend 400 miles toward East Texas. The play is 50 miles wide and an average of 250 feet thick at a depth between 4000 and 12,000 feet. The shale contains a high amount of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

, which makes it brittle, and it is thus easier to use

hydraulic fracturing Fracking (also known as hydraulic fracturing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of bedrock formations by a pressurized liquid. The process involves the high-pressure injection of "fra ...

to produce the oil or gas. The oil reserves in the Eagle Ford Shale Play were estimated in 2011 at 3 billion barrels. The U.S.

Energy Information Administration The U.S. Energy Information Administration (EIA) is a principal agency of the U.S. Federal Statistical System responsible for collecting, analyzing, and disseminating energy information to promote sound policymaking, efficient markets, and publ ...

estimated that the Eagle Ford held 50.2 trillion cubic feet of unproved, technically recoverable gas. The average well was estimated to produce 2.36 billion cubic feet of gas. In the first six months of 2013, the Eagle Ford produced 2.69 billion cubic feet of gas and 599,000 barrels of oil and condensate per day; the oil production represented an increase of 51% over the average for 2012. By the end of 2013, production had skyrocketed to over 1,000,000

BOE BOE, BoE or Boe may refer to: Abbreviations, acronyms or initialisms * Bank of England, the central bank of the United Kingdom * Bank of English, a representative subset of the 4.5 billion words COBUILD corpus * Barrel of oil equivalent, a unit ...

/day. As of 2013, Eagle Ford production extended into 24 counties in Texas. Analysts expect that $30 billion will be spent on further developing Eagle Ford in 2015. The large increase in tight oil production is one of the reasons behind the price drop in late 2014. The oil and gas operations are visible on nighttime satellite photos of the United States, appearing as a diffuse bright patch about two hundred miles long, between the more concentrated lights of San Antonio, Corpus Christi, and neighboring cities.

Proven reserves (US)

* US EIA, 2010: 2.5 trillion cubic feet of gas * US EIA, 2011: 1.25 billion barrels of oil, 8.4 trillion cubic feet of gas * US EIA, 2012: 3.37 billion barrels of oil * US EIA, 2019: 4.30 billion barrels of oil * US EIA, 2020: 3.25 billion barrels of oil

Mexico

The Eagle Ford Formation extends into northern Mexico's Burgos Basin, where it is known as the Boquillas Formation and has an average thickness of . Total organic content (TOC) is estimated to average 5%. Technically recoverable hydrocarbons are estimated to be 343 trillion cubic feet of shale gas and 6.3 billion barrels of

tight oil Tight oil (also known as shale oil, shale-hosted oil or light tight oil, abbreviated LTO) is light crude oil contained in unconventional petroleum-bearing formations of low permeability, often shale or tight sandstone. Economic production from ...

. The national oil company

Pemex Pemex (a portmanteau of Petróleos Mexicanos, which translates to ''Mexican Petroleum'' in English; ) is the Mexican state-owned petroleum company managed and operated by the Mexican government. It was formed in 1938 by nationalization and expr ...

first began exploring in 2010. Pemex had an exploration program in progress until 2015. In April 2013, Pemex started producing the nation's first shale gas well, just south of the U.S. border. The well was completed in the equivalent of the Eagle Ford Formation. Gas drilling in the Burgos Basin, close to the U.S. border, has been hampered by drug gangs. One Mexican industry expert said that Mexico was unlikely to develop the Eagle Ford because of lack of pipeline infrastructure and lack of expertise and because the Mexican company Pemex was investing in oil deposits that yield a higher rate of return., Emily Pickrell
"Mexico unlikely to tap its Eagle Ford Shale, experts say"
''Houston Chronicle'', October 31 2013.

Hard times strike

With the worldwide decline in crude oil prices in 2015, a sharp downturn swept through Eagle Ford play. In January 2015, there were 840 active

drilling rig A drilling rig is an integrated system that drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill wa ...

s in Texas as a whole; by the end of the year, 321. Within the Eagle Ford play, the decline during these twelve months was from 200 to 76 rigs. The oil price decline rendered it uneconomical to drill sub-optimal wells. Particularly hard hit in the decline were the oil-field workers in South Texas.

Ownership

, Chesapeake Energy lease held 600,000 acres in the field, selling an unspecified interest to CNOOC Limited for US$2 billion.

References

External links

The University of Texas at Dallas

Geosciences Department

*
Eagle Ford Info
{{coord missing, Texas Shale formations of the United States Upper Cretaceous Series of North America Cretaceous geology of Texas Geologic groups of Texas Cenomanian Stage Turonian Stage Oil fields in Texas Oil-bearing shales in the United States