On the geologic map of Kentucky, Ordovician and Silurian rocks are surrounded by a thin ring of Devonian strata (410 to 360 million years ago) shown in red. Devonian strata consits of limestones and dolostones and a thick deposit of dark gray to black shale. The limestones are mined in the Louisville area. They sometimes contain abundant fossils, as at the Falls of the Ohio in Louisville, Kentucky (shown in picture above). Thick, dark gray to black shales are the dominant Devonian strata in many areas of Kentucky. The color of the shales comes from organic material trapped in the rock. During the Late Devonian, muds were deposited beneath a sea that covered most of the eastern United States. Organic material in the muds was trapped. When the organic-rich sediments were buried deeper beneath the surface, pressure and temperature converted some of the organic material in the rock to liquid form, called oil, and into gaseous form, as natural gas. The largest gas field in Kentucky, with an estimated reserve of many billions of cubic feet of gas, is the Big Sandy Gas Field located in eastern Kentucky. The gas reservoir is in Devonian shales buried deep beneath the surface. Much of the oil found in Kentucky was originally in the Devonian shales, but migrated to other rocks where it is found today.
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