The Eagle Ford Formation (also called the Eagle Ford Shale) is a sedimentary rock formation from the Late Cretaceous age underlying much of South and East Texas in United States, consisting of organic matter-rich fossiliferous marine shale. It derives its name from the old community of Eagle Ford, now a neighborhood in West Dallas, where outcrops of the Eagle Ford Shale were first observed. Such outcrops can be seen in the geology of the Dallas–Fort Worth Metroplex, and are labeled on images with the label "Kef". The Eagle Ford Shale is one of the most actively drilled targets for oil and gas in the United States in 2010.[1]
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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.
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 since as it is buried below a massive wedge of clastic sediments from the Late Cretaceous to the present.
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.
The Eagle Ford Shale is a hydrocarbon producing formation rich in oil and natural gas fields. The shale play area starts at 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.[2] The oil reserves are estimated at 3 billion barrels with potential output of 420,000 barrels a day.[3]