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Eagle Ford Group
The Eagle Ford Group (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.〔("Eagle Ford Shale - South Texas - Natural Gas & Oil Field" ''OilShaleGas.com'' )〕
== Depositional environment ==
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 the 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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