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Finite strain associated with various deformation mechanisms can significantly alter the porosity and permeability of reservoir rocks in overthrust belts. Mechanisms such as pressure solution and cataclasis reduce porosity and permeability, whereas extension fracturing and brecciation increase them. A theoretical derivation of the relationship between finite strain and original and final porosity indicates that little porosity is preserved in rocks whose strain (R) exceeds 1.5. Studies of variations in the relative importance of these mechanisms are used to define regional and local variations in reservoir properties in the central Appalachian overthrust belt. Regional limits of hydrocarbon potential are defined by combining finite-strain data with thermal-maturation (con dont color alteration index or CAI) data. On the basis of these studies, I conclude that the potential for oil production is primarily restricted to the Appalachian Plateau province. The potential for gas production decreases from west to east in the Valley and Ridge and Plateau provinces, with its approximate eastern limit defined by the North Mountain thrust.
Local variations in reservoir properties depend on original depositional and diagenetic fabric of the reservoir, the relative timing of different mechanisms, lithologic and textural characteristics, and structural and lithotectonic position. The possibility of retaining open fractures depends on the relative timing of fracturing and pressure solution. Fractures formed during the main phase of folding are more likely to be sealed by pressure-solved material, whereas fractures formed or reactivated late in the deformation are more likely to remain open. Lithologic units that enhance fracturing and inhibit pressure solution most likely have higher fracture and matrix porosity. Fine-grained argillaceous limestone is more susceptible to pressure solution than orthoquartzite, whereas fractu ing is more intense in brittle, thin-bedded and fine-grained units. Structural positions with high fracture potential include angular fold hinges, fault zones, and the steep forelimbs of folds. The steep forelimbs of folds also are characterized by intense pressure solution and cataclasis, which reduce porosity. Consideration of the above criteria can be useful in evaluating the reservoir potential of prospective structures in the central Appalachian and other overthrust belts.
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