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In highly deformed fold and thrust belts, strain associated with different deformation mechanisms can significantly alter the porosity and permeability of potential reservoir rocks. In general, mechanisms such as pressure solution, intragranular deformation, and cataclasis reduce porosity, whereas extension fracturing increases porosity, and more importantly, permeability. Since these mechanisms are dependent on parameters such as temperature, pressure, deviatoric stress, lithology, and grain size, they can produce significant regional and local variations in reservoir potential. Thus studies of these mechanisms are important for defining regional limits for exploration and, more locally, for evaluating individual prospects.
In the central Appalachians, exploration is presently confined to the Plateau and Valley and Ridge provinces. However, the regional limits of hydrocarbon potential need to be defined, particularly in light of the recent hypothesis by Harris et al in 1981 of subthrust potential underlying the Blue Ridge in the southern Appalachians. This can be done by combining data on strain and deformation mechanisms with conodont CAI data and estimates of displacements on major faults. Penetrative strain axial ratios (R) range between 1.1 and 8.0 in Great Valley carbonates and between 1.6 and 6.9 in Blue Ridge clastics. In the Valley and Ridge, R is usually less than 1.2. For most reservoir rocks, porosity is completely eliminated for R = 1.5, so that little porosity is expected east of the Valley nd Ridge. Conodont CAI data of Epstein et al in 1977 show values approaching 5.0 for the Cambro-Ordovician carbonates and 4.5 for the Silurian-Devonian carbonates along the eastern edge of the Valley and Ridge. These values are at the upper limit for commercial gas production. The North Mountain and Blue Ridge faults, the two major thrusts east of the Valley and Ridge, both have maximum estimated displacements less than 20 km (12 mi), so that subthrust Valley and Ridge-type Silurian-Devonian rocks underlying the Great Valley rocks are fairly limited. Subthrust Cambro-Ordovician carbonates may be more widespread under the North Mountain ramp, but their potential is low because of high CAI and R values. The eastern limit of potential is therefore marked by the North Mountain fault, except or limited areas of subthrust Silurian-Devonian rocks.
The local effects of deformation mechanisms are related to interactions between pressure solution and fracturing, defined by relative timing, lithological characteristics, and structural position. Longitudinal fractures formed early or during the main phase of deformation are often healed or sealed by pressure solution, while transverse fractures, particularly those formed late tend to remain open. Lithological types susceptible to pressure solution, such as impure limestones, are less likely to retain matrix or fracture porosity than pure orthoquartzites. Structural positions with dilational strain, such as fold hinges in competent units, retain more porosity than those undergoing plane strain, such as thinned forelimbs of folds. Brittle deformation zones along splay faults are typic lly sealed through cataclasis. Consideration of these factors can significantly improve the evaluation of prospects in overthrust belts.
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