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Subsurface water exists in a dynamic state, and the condition of hydrodynamic flow, or potential for flow, is indicated by abnormal fluid pressures. Excess pressures indicate updip flow of water and transfer of hydrocarbons from source rock to reservoir in stratigraphic or structural traps. Deficient pressures most commonly indicate downdip flow, reinforcement of capillary-pressure barriers, and enhanced oil columns in stratigraphic traps. The principles of hydrodynamic flow can be applied in either case for prediction of sites for petroleum accumulation; furthermore, a quantitative estimate can often be made of the amount of oil or gas trapped.
Excess pressures in the Gulf Coast Tertiary section are generated by several mechanisms: (1) nonequilibrium compaction, (2) clay transformation, (3) aquathermal pressuring, and (4) hydrocarbon
generation. These mechanisms operate at different depths and temperatures, but may, in some cases, operate together to produce hydrodynamic flow. The result of flow is development of secondary porosity by dissolution of grains or cement and by hydrocarbon migration. Oil and gas accumulate in local, low-potential volumes of reservoir rock in which the pressures can be either slightly or greatly in excess of normal hydrostatic pressures.
Deficient pressures in Rocky Mountain basins are caused by uplift, exposure, and recharge of aquifer systems by meteoric waters. Downdip hydrodynamic flow results in oil columns of unusual height in which the oil trapped by flow greatly exceeds that which can be trapped by capillary-pressure barriers alone. These basins may have locally excess pressures due to clay transformation, or hydrocarbon generation, or locally deficient pressures due to gas blockage in fine-grained rocks.
The principles of flow are well established but not widely applied, and there is a need for better documentation of causes for abnormal pressures and the effects of flow. Knowledge of fluid-pressure regime can often be determined from relatively few points of subsurface control for a better understanding of fluid-migration history. Such knowledge is essential to oil and gas exploration in both poorly tested and mature basins.
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