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In the Folded Appalachians of the south most of the petroleum interest and all of the oil and gas production have been within or close to a unique structural feature, the Cumberland overthrust block. This large mass includes parts of Virginia, Kentucky, and Tennessee. Displacement of the block toward the northwest is about 11 mi along its southwest edge in Tennessee and about 4 mi along its southeast edge in Virginia. Rocks of all systems from Cambrian to Pennsylvanian are involved, totaling approximately 20,000 ft in thickness. Oil has been produced from rocks of Ordovician age and gas from rocks of Devonian and Mississippian ages. Other targets exist.
Two reasons are adduced to account for the fact that hydrocarbon accumulations persist in rocks subjected to such extensive displacement. First, gravity sliding rather than tectonic lateral stress seems best to explain the structural relations from surface to basement. Thus, the "overthrust" rocks were under little if any more compressional stress during faulting than before it, and they have not been compacted, recrystallized, or cemented to an extent precluding effective porosity. Second, the Pine Mountain fault, which underlies the block, is a bedding plane fault throughout most of its extent, but locally it ramps upward from one weak zone to another. Four weak zones that favored localization of the fault plane are known. Only at, and northwest of, the ramps are older rocks piled o younger, resulting in abrupt thickening of the sedimentary section.
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