Hydrodynamic forces in deep sedimentary basins reflect gradients in hydraulic head produced by gravitational and molecular forces. The relative contribution of these fluid-driving forces cannot be measured, so observed gradients in head are explained in terms of gravitational forces only. This is reasonable in practical reservoir mechanics because pressures in excess of the gravitational forces will rupture the confining beds and release fluid.
Hydrodynamic regimes in deep sedimentary basins can be described in terms of the depth-pressure gradient, as (1) the hydropressure zone, (2) the geopressure zone, and (3) the zone of transition between them. In the hydropressure zone the aquifer systems are open to the atmosphere, and the fluid pressure at any depth reflects roughly the weight of the superincumbent fluid column. In the geopressure zone the aquifer systems are cut off from the atmosphere, and fluid pressure at any depth reflects part, or all, of the weight of the superincumbent rock column. In the transition zone, few field data are adequate to define the conditions.
These pressure zones are associated with geothermal and formation-water salinity regimes, both of which are keyed to water flux. Confinement (mainly water) in the geopressure zone causes thermal buildup and pressure increases owing to thermal expansion of fluids and rock matrix as well as dehydration of expandable clay minerals. Thermal diagenesis of clay minerals and kerogen converts solids into fluids, reducing the load-bearing strength of the rocks and increasing the interstitial fluid pressure. Progressive loading further increases fluid pressure in the clay shales, and in the adjacent sand-bed aquifers which causes an influx of shale water. The flush of these low-salinity waters upward through interlayered aquifers of different salinity results in osmotic forces.
As molecular forces exceed geopressures, fluid loss provokes fault movement, and the shear zone of a growth fault continues to propagate downward into the massive underlying marine shales until the dehydration process is completed.
Associated with these interacting regimes are phase changes of fluids, changes in mineral and hydrocarbon solubility, and chemical reactions triggered by temperature, pressure, water chemistry, and clay-mineral catalysis. Mass transfer of soluble rock constituents from depth and their upward redistribution as precipitates (cements), as liquids (saline solutions and oil), and as gases (mainly methane) thus occur.