Abnormal Formation Pressure: A Review

Abnormal formation pressure is defined as any pressure that deviates from the normal hydrostatic gradient. In order for these pressures to form and be preserved, a near-seal is required. The seal is not necessarily impermeable but may be a low permeability formation, such as shale, or a fault-related barrier. The possible causes of abnormal formation pressure are: 1) compaction of sediments, 2) tectonic activities, 3) temperature changes, 4) osmosis, 5) diagenesis, 6) methane generation, and 7) buoyancy. Temperature changes due to decay, diagenesis, and changes in burial depth of the formation are one of the two most important causes of abnormal formation pressure. The other important cause is compaction pressure related to the weight of overburden with increased depth of burial. If the pore fluids cannot escape, they will support a greater proportion of the total overburden stress and become abnormally highly pressured. Tectonic activities such as faulting as well as removal of overburden by erosion also play an important role in either raising or lowering pore fluid pressure. Osmosis, diagenesis, methane generation, and buoyancy are all additive to the overall effects of pressure and temperature.