Eagle Ford Formation
The Eagle Ford Formation (also called the Eagle Ford Shale) is a sedimentary rock formation from the Late Cretaceous age underlying much of South Texas in the United States, consisting of organic matter-rich fossiliferous marine shale. It derives its name from the old community of Eagle Ford where outcrops of the Eagle Ford Shale were first observed. Due to the sloping nature of the formation, the shale reaches the surface in North Texas while the oil-bearing portion is much deeper in South Texas where the drilling activity occurs. The Eagle Ford Shale is one of the most actively drilled targets for oil and gas in the United States in 2010.
Transgression continued to occur after complete deposition of the Woodbine around 92 million years ago ("mya"). Creation of the Colorado Group, which first created the Eagle Ford Shale, occurred between ~92 and 88 mya. The Eagle Ford is mostly confined within the subsurface but outcrops on west side of Dallas and continues at a 1 degree easterly prograding tilt. The Eagle Ford Shale had sea level depths about 100 meters or 330 feet, and deposited about 20-50 kilometers from the shore. The depositional environment in the lower beds was low energy and slightly anoxic. This anoxic setting of the deeper oceanic waters was a result of increased amounts of CO₂ during deposition in the Cretaceous. The lower section of the Eagle Ford consists of organic-rich, pyritic, and fossiliferous marine shales which mark the maximum flooding surface, or the deepest water during Eagle Ford deposition. The different fauna present in the Eagle Ford suggest the waters were calm and within the photic zone. A small member of the Eagle Ford that consists of a thin limestone unit between shales is known as the Kamp Ranch. A small regressive highstand formed this carbonate layer towards the top of the Eagle Ford, identifiable by high energy features, such as ripple marks from storm generated waves and interbedded carbonaceous siltstones. The overall thickness of the undivided Eagle Ford Group is 200–300 feet thick.
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 ~88mya. A decrease in the effective elastic plate thicknesses caused the basin to subside, as the uplift became increasingly elevated. As a result, an estimated 150m 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 well known East Texas petroleum reservoir, and 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 sediments from the Late Cretaceous to the present.
Eagle Ford Group undivided
North of Hill County, shale, sandstone, and limestone; shale, bituminous, selenitic, with calcareous concretions and large septaria; sandstone and sandy limestone in upper and middle parts, platy, burrowed, medium to dark gray; in lower part bentonitic; hard limestone bed marks base in Ellis and Johnson Counties; locally forms low cuesta; thickness 200–300 feet.
Oil and natural gas
Petrohawk drilled the first well to produce oil and gas from the Eagle Ford in 2008, in LaSalle County, Texas. Oil companies quickly extended the productive area, which stretches from the Texas-Mexico border in Webb and Maverick counties and extends 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 which makes it brittle and easier to use hydraulic fracturing to produce the oil or gas.
The oil reserves are estimated at 3 billion barrels. The US Energy Information Administration 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/day. As of 2013, Eagle Ford production extended into 24 counties in Texas, and as of 20 Sep 2014, injected over $87B into the Texas economy.
The oil and gas operations are clearly 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.
Proved 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
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 200 meters. 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. The national oil company Pemex first began exploring in 2010-2011. Pemex has an exploration program in progress until 2015.
In April 2013, Pemex started producing the nation's first shale gas well, just south of the US border. The well was completed in the equivalent of the Eagle Ford Formation. Gas drilling in the Burgos Basin, close to the US 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, lack of expertise, and because the Mexican company Pemex was investing in oil deposits that yield a higher rate of return.
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