Page 91 - Geology and Geochemistry of Oil and Gas

TABLE 3.4 66
Types of mechanisms responsible for generating abnormally-high formation pressures (AHFP) (after Chilingar et al., 2002)

Type of changes Description of process

Changes in the rock pore volume TEMPERATURE
Vertical loading (undercompaction) Rate of sedimentation and deposition. High depositional rates in clastic sequences and high shale/sand ratios
(undercompaction)
Massive areal rock salt deposition. Presence of impermeable salt (NaCl) beds. For example, massive salt deposits in
USA, Russia, North Africa, Middle East, North Germany, etc. AND
Paleopressures. Sealed-off reservoir rocks experiencing a depth change due to either uplifting or erosion
Lateral tectonic loading Tectonic activities. Local and regional faulting, folding, lateral sliding, and slipping; squeezing caused by down-
dropping of fault blocks; diapiric salt, sand, or shale movements; earthquakes; etc. The pore volume is reduced by PRESSURE
horizontal tectonic compression of rock
Secondary cementation Cementation. Calcium sulfates, sodium chloride, dolomite, siderite, calcite, silica, etc., may act as sealing barriers
(‘‘pressure caps’’), and directly cause increased pore pressure by decreasing pore space due to crystal grows within IN
closed reservoirs (e.g., NaCl in Markovo oil pool in the Osinskiy Series, Russia) THE
Changes in the volume of interstitial
fluids
Temperature change (aquathermal Thermodynamic effect. Formation temperature increase causes expansion of fluids with consequent increase in the
expansion) fluid pressure SUBSURFACE
Mineral transformation Diagenetic and catagenetic processes. Postdepositional alterations (release of bound water): (1) montmorillonite and
mixed-layer clays altered to illites (smectite dehydration); (2) gypsum to anhydrite dehydration