Rocks with permeabilities of 10−13 to 10−14m2 are considered to be excellent to good reservoirs (Deming et al., 1990; Deming and Nunn, 1991; Thomas and Clouse, 1995). Figure 21.1(a) shows that these values are characteristic of the higher parts of the range of matrix permeability in carbonates and sandstones (e.g., Deming, 1994; Smith and Wiltschko, 1996). Grain size and sorting are the primary control on matrix permeability. For the same lithology, rocks with a larger grain size tend to have a higher permeability than rocks with a smaller grain size (Fig. 21.1b; Prince, 1999). Similarly, wellsorted sedimentary rocks can have a higher permeability than poorly sorted rocks due to a reduction of the pore space in the latter by finer-grained particles occupying the space between larger grains. Matrix permeability is also controlled by pre-orogenic diagenetic processes. An example of contrasting matrix permeability comes from the Painter field in the Absaroka thrust sheet of the Wyoming Thrustbelt. This field contains two facies of the producing Nugget Sandstone; dune and interdune–sand sheet facies (Tillman, 1989). The dune facies, characterized by well-sorted, well-rounded grains and well-connected intergranular porosity, has a porosity, and horizontal and vertical permeabilities of 16.3%, 196 and 13 md, respectively. The interdune–sand sheet facies, represented by alternating fine- and coarse-grained laminae, with poorly sorted, more angular and more tightly packed grains in the finer-grained laminae, has a porosity, and horizontal and vertical permeabilities of 10.2%, 4.5 and 0.67 md, respectively.

However, in thrustbelts, the internal straining that takes place before thrust sheets are developed also influences reservoir permeabilities (e.g., Geiser, 1974; Koyi, 1995, 1997; see Fig. 3.4).