Abnormal high pressure usually occurs in reservoirs that are sealed by surrounding rocks shuch as chales or by faults that do’nt allow fluids to escape the reservoir. During burial, the sedimentary rocks compact by expelling fluids from the pore spaces. In an abnormally high pressure reservoir, the fluids have been prevented from being expelled by the surrounding rocks, an the rock or lithostatic pressure, which increases about 100 psi/100 ft depth, is transferred to the fluid pressure.
Abnormal high-pressure reservoirs have pressure between hydrostatic and lithostatic pressures. Reservoirs witth abnormal high pressures have relatively low seismic velocities and low densities and are in contrast t abnormal high pressure reservoirs.