Permeability is a critical factor in the maintenance of abnormal fluid pressures. The permeability of "tight" geologic materials is difficult to measure, particularly in situ. Both the laboratory and in-situ methods require special techniques when the permeabilities are less than 10^-10 cm/sec (10^-7 darcys).

Permeability can be measured: (1) on rock samples in the laboratory; (2) in situ, using well-test procedures; and (3) on a regional scale, using a hydrodynamic analysis of the entire system.

Special techniques have been used to measure permeabilities in the Cretaceous Pierre Shale in South Dakota. Comparison indicates that the regional permeability exceeds the local permeability by two to three orders of magnitude. This suggests that secondary features, probably fractures, control the regional permeability.

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