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The Fort Payne Formation (Osagian), which extends from the type section in Alabama to Indiana, has a wide range of carbonate and clastic lithologies. In the subsurface of Scott County, Tennessee, where the Fort Payne currently produces oil and gas, it consists of a carbonate platform and associated Waulsortian mound sequence. This sequence may be divided into two lithologic units: (1) a cherty dolostone with remnant evaporites, and (2) a fossiliferous limestone. Each of the units is divisible into several microfacies.
At the beginning of Fort Payne deposition, an erosional surface existed on the underlying Chattanooga Shale. A transgression brought about intertidal to shallow subtidal conditions. Lithologic unit 1, deposited in sabkhalike condition, was draped over the Chattanooga, thus preserving its topography. Further transgression brought about more nearly normal-marine conditions and colonization of the area by crinoids and bryozoans. They served to baffle and trap fine-grained carbonate material into mud lenses. Multiple lenses coalesced into the Waulsortian-type mounds of lithologic unit 2, up to several kilometers long and 25 m high. Subsequent subaerial exposure resulted in solution and development of secondary porosity in favorable grainstone types. Other early diagenetic effects included partial collapse of the mounds to form fractures during dewatering. Later diagenetic effects included emplacement of petroleum in reservoirs now producing oil and gas.
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