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The porosity ϕ of a rock is the pore volume per unit volume. The pore space is generally filled with salt water except near the surface, where air may be present, and in petroleum deposits, where the pore spaces contain oil and/or gas.

Pore spaces are usually sufficiently intercon-nected so that the fluid pressure approximates that of a fluid column extending to the surface; this is normal pressure. The weight of the rock column exerts an overburden pressure. The differential, effective, or net pressure on the rock ma-trix is the overburden pressure less the interstitial fluid pressure. However, if there is no communi-cation between the pore spaces and the surface, the interstitial fluid pressure may be greater than the normal pressure causing the differential pressure (and velocity) to be lower than usual for a given depth—a situation known as overpressure.

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