Abnormally high pore fluid pressures may be produced as a result of hydrocarbon generation from organic matter (kerogen) contained in "source rocks." Contributing processes include: (1) collapse of rock matrix as overburden-supporting solid kerogen is converted to non-expelled fluid hydrocarbons and (2) volume increases produced by the conversion of solid kerogen to hydrocarbons.

Hydrocarbon generation overpressures exist in the following basins: Williston, Powder River, Anadarko, Delaware, Uinta, and San Joaquin. The phenomenon probably also exists elsewhere. These regions of overpressure represent vertically and laterally restricted "cells" or "pods" in which hydrocarbons are the overpressuring fluid and the only initially producible fluid species present. The pressure cells center around actively generating source-rock units.

Actively generating source rocks within the pressure cells may be characterized by abnormally high electrical resistivities and abnormally low sound velocities. Resistivity increases may be caused by the replacement of conductive pore water with non-conductive hydrocarbons. Low sound velocities may be caused by: (1) the replacement of higher velocity pore water with lower velocity hydrocarbons and (2) the effects of abnormal pressure on porosity enhancement or preservation through dilation or under-compaction.

Hydrocarbon generation overpressures lead to the spontaneous hydraulic fracturing of the source rock and may create associated fracture-type reservoirs. They may also create fractures which propagate upward or downward from the source rocks and control vertical migration through large thicknesses of seemingly impermeable confining strata.

When a source rock ceases to generate, abnormal pressures may decay, resulting in normal or subnormal pressure conditions.

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