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| Eagle Ford Group | |
|---|---|
| Stratigraphic range:Cenomanian-Turonian ~96–90 Ma | |
| Type | Group |
| Sub-units | Britton Formation,Arcadia Park Shale Formation,Lake Waco Formation, South Bosque Formation,Boquillas Formation |
| Underlies | Austin Chalk |
| Overlies | Woodbine Formation orBuda Limestone |
| Lithology | |
| Primary | Shale |
| Other | Marl,limestone,sandstone,volcanic ash beds |
| Location | |
| Region | Texas |
| Country | United States |
| Type section | |
| Named for | Eagle Ford, Texas[1] |
| Named by | Robert T. Hill[1] |
| Year defined | 1887 |
 | |
TheEagle Ford Group (also called theEagle Ford Shale) is asedimentaryrock formation deposited during theCenomanian andTuronian ages of theLate Cretaceous over much of the modern-day state ofTexas. The Eagle Ford is predominantly composed oforganic matter-richfossiliferous marineshales andmarls with interbedded thinlimestones. It derives its name fromoutcrops on the banks of the West Fork of theTrinity River near the old community of Eagle Ford,[1] which is now a neighborhood within the city ofDallas. The Eagle Fordoutcrop belt trends from theOklahoma-Texas border southward toSan Antonio, westward to theRio Grande,Big Bend National Park, and the Quitman Mountains ofWest Texas.[2] It also occurs in the subsurface ofEast Texas andSouth Texas, where it is thesource rock foroil found in theWoodbine,Austin Chalk, and theBuda Limestone,[3] and is produced unconventionally inSouth Texas and the "Eaglebine" play ofEast Texas.[4]
The Eagle Ford was one of the most actively drilled targets forunconventional oil and gas in the United States in 2010,[5] but its output had dropped sharply by 2015.[6] 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 firmWood Mackenzie. This strike has been the hardest hit of any oil fields in the world. As of 2016, the spending was, however, expected to increase to $11.6 billion in 2017. A full recovery was not expected any time soon.[7]
Fossils are relatively common in Eagle Ford rocks. FossilizedPlesiosaurs,mosasaurs, Fish,shark teeth,crustaceans,sea urchins,feather stars,ammonites,oysters, clams, and othergastropod shells have all been found there.[8][9][10]
The Eagle Ford rocks were created by the remains of sea life that dropped to the floor of aninland sea (or epeiric sea) that covered much of modern-dayTexas. The Texas shelf during theCenomanian-Turonian was bounded by theOuachita Uplift to the north, the Sabine Uplift to the East, relict reef margins of theStuart City Formation and theSligo Formation to the southeast, and theWestern Interior Seaway to the west. TheEast Texas andSouth Texas regions were divided by an extension of theLlano Uplift 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.[11]
Thebottom waters of the Eagle Ford sea werestarved of oxygen when most of the Eagle Ford material dropped to the sea floor, and this is related to the global OceanicAnoxic Event 2 (OAE2), orCenomanian-Turonian boundary event, although the Texas shelf became that way nearly two million years prior to OAE2.[2] The low-oxygen conditions helped preserve theorganic matter that ultimately generated thehydrocarbons associated with the Eagle Ford in the subsurface.[3] Evidence for anoxia include the high amounts of organic matter, the lack offossils ortrace fossils of the kind of creatures that live on the sea floor, and enrichment in theredox proxiesmolybdenum andvanadium.[2]
After the significant drop insea level (marine regression) associated with deposition of theWoodbine during the Early Cenomanian, the sea level began to rise (marine transgression), allowing for the deposition of Lower Eagle Ford organic-richmarls in South Texas andlimestones of the Terrell Member of theBoquillas Formation inWest Texas starting at about 96 million years ago.[12][13] The rise in sea level eventually drowned the East Texas Woodbineriver deltas, initiating Eagle Ford deposition in East Texas.[14] The initialdeposits, known as the Six Flags Limestone in Dallas and the Bluebonnet Limestone in Waco, arecalcarenites predominantly composed of disaggregated prisms of "Inoceramus" clams andplanktonicforaminifera tests.[15][16]
Following deposition of the calcarenites, a river delta began to prograde from the Ouachita Uplift to the northern East Texas Basin.[14] Although thesandstones andsiltstones from this delta, known as the Templeton Member, were originally placed within the Woodbine Formation,[17] theammonites found within them indicate that they are better associated with the Eagle Ford.[2] In areas unaffected by the Templeton Delta, depositional rates were low, producing a condensed section composed of organic-rich,calcareousmarls, limestones, andvolcanic ash beds in both South Texas and West Texas. Themicrofossils found within the marls are predominantlycoccoliths andplanktonicforaminifera, whereas the limestones contain abundantradiolaria and calcispheres (calcareouscysts produced by somedinoflagellates).Inoceramus fragments andfish bones are also found in these deposits.[2]
During the Late Cenomanian the Sabine Uplift along the modern-day Texas/Louisiana border became active, causingerosion of Eagle Ford and Woodbinesediments[18] and deposition within the Harris Delta complex.[19] Clay from this delta reached as far south asDeWitt County, Texas.[14]
Towards the end of the Late Cenomanian, the bottom waters of the Texas shelf and the Western Interior Seaway became oxygenated,[20] which may be related to the sea-level maximum associated with the Cenomanian-Turonian boundary event.[2] Evidence for this oxygenation event, known as the "Benthonic Zone,"[21] include an increase in the abundance of benthic organism fossils andbioturbation, a decrease in redox proxiesuranium, molybdenum, and vanadium, and a reduction in organic matter.[22][23] 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.[10][20][22] Anunconformity occurs throughout East Texas at this level, possibly due to a lack of sediments reaching the basin during the sea-level maximum.[14]
The sea-level began to drop after the EarlyTuronian sea-level maximum; this is most obvious atoutcrops nearLangtry, Texas, where water depths became shallower than 100 ft (30 m). This limestone-rich unit is known as the Langtry Member of theBoquillas Formation. It contains very little organic matter, and abundantsea urchinfossils.[10] TheKamp Ranch Limestone 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 ofInoceramus clams[24] and hasripple marks indicative of shallow-water deposition.[25] 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 inoutcrop.[26][27] 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 overlyingAustin Chalk, although an unconformity is found between the Eagle Ford and the Austin Chalk in both South Texas and East Texas.[2]
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 ofclastic sediments from the Late Cretaceous to the present.[citation needed]
Paul Basinski, the geologist who helped discover the Eagle Ford basin, has been called "one of the fathers of fracking".[28]Petrohawk drilled the first well to unconventionally produce gas from the Eagle Ford in 2008, inLaSalle County, Texas. Oil companies quickly extended the productive area, which stretches from the Texas-Mexico border inWebb andMaverick counties and extend 400 miles towardEast 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 ofcarbonate, which makes it brittle, and it is thus easier to usehydraulic fracturing to produce the oil or gas.[29]
The oil reserves in the Eagle Ford Shale Play were estimated in 2011 at 3 billion barrels.[30] The U.S.Energy Information Administration (EIA) estimated that the Eagle Ford held 50.2 trillion cubic feet (TCF) of unproved, technically recoverable gas. The average well was estimated to recover 2.36 billion cubic feet (BCF) of gas.[31]
In 2011, the Eagle Ford produced an average of 1.14 BCF/day of gas and 211,000 barrels/day of oil andcondensate. In 2012, the Eagle Ford produced an average of 2.43 BCF/day of gas and 566,000 barrels/day of oil and condensate. By the end of 2013, production had skyrocketed to well over 1,000,000BOE/day.[32] In 2013, the Eagle Ford produced an average of 3.73 BCF/day of gas and 975,000 barrels/day of oil and condensate. In 2014, the Eagle Ford produced an average of 4.85 BCF/day of gas and 1,376,000 barrels/day of oil and condensate.
The large increase in tight oil production from the Eagle Ford is one of factors that led to the oil price drop of late 2014.[33] Total production peaked in March 2015 at 2.62 million BOE/day (1.625 million BO/day and 5.75BCF/day).
Eagle Ford production has occurred in 27 counties in Texas.[34] The large area of oil and gas operations of the Eagle Ford 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 ofSan Antonio,Austin,Houston,Victoria,Corpus Christi,Laredo, and neighboring cities.[35]
*EIA estimate includes reserves in the basin in other overlying and underlying strata including the Austin Chalk, Olmos/San Miguel, etc.
The Eagle Ford Formation extends into northern Mexico's Burgos Basin, where it is known as theBoquillas Formation and has an average thickness of 200 metres (660 ft). 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 oftight oil. The national oil companyPemex first began exploring in 2010. Pemex had an exploration program in progress until 2015.[40]
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.[41] Gas drilling in the Burgos Basin, close to the U.S. border, has been hampered by drug gangs.[42] 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.[43]
With the worldwide decline in crude oil prices in 2015, a sharp downturn swept through Eagle Ford play. In January 2015, there were 840 activedrilling rigs 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.[6]
