Page 39 - Origin and Prediction of Abnormal Formation Pressures

22 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

TABLE 2-1
Types of mechanisms responsible for generating abnormally high formation pressures (AHFP)

Type of changes Description of process
Changes in the rock pore volume
Vertical loading Rate of sedimentation and deposition. High depositional rates in clastic
(undercompaction) sequences and high shale/sand ratios (undercompaction).
Massive areal rock salt deposition. Presence of impermeable salt
(NaC1) beds. For example, massive salt deposits in U.S.A., Russia,
North Africa, Middle East, North Germany, etc.
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 horizontal tectonic compression of rock.
Secondary cementation Cementation. Calcium sulphates, 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 growth within closed reservoirs (e.g., NaC1 in Markovo oil
pool in the Osinskiy Series, Russia).
Changes in the volume of interstitial fluids
Temperature change Thermodynamic effects. Formation temperature increase causes
(aquathermal expansion) expansion of fluids with consequent increase in the fluid pressure.
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.
Hydrocarbon generation Conversion of organic material/kerogen to petroleum. Generation of
oil and gas from kerogen (maturation) results in a significant
increase in pore volume.
Decomposition of Breakdown of hydrocarbons. About 2- to 3-fold volume increase
hydrocarbons (thermogenic) caused by breakdown of hydrocarbon long-chained molecules into
shorter-chained molecules. Such reactions generally occur at depths
below 2 to 4 km and temperatures greater than 70 ~ to 120~
Thermal cracking of organic molecules is initiated at temperatures
of 120 ~ to 140~ depending upon the depth of sediments. At
temperatures greater than 180~ almost all the hydrocarbons are
converted to methane.
Migration of fluids Gas migration. Upward migration of hydrocarbon gases from lower to
upper horizons along faults. This can result in overpressuring of
upper horizons.
Changes in fluid pressure (hydraulic head); movement of fluids
Osmosis Osmosis. Contrasts in the brine concentration of formation fluids can
induce the transfer of fluids across a semipermeable membrane. On
regional basis, e.g., San Juan Basin, New Mexico, Western
Sedimentary Basin, Canada, San Joachim Valley, California. US.A.
Gulf Coast, and Paradox Permian Basin, !llinoi% U.,~I.A.
Fluid pressure head Piezometric fluid level. Effect of regional potentiometric surface, e.g.,
artesian water system. Examples would include the Artesian Basin,
Florida, U.S.A., Great Artesian Basin, Australia, and North Dakota
Basin, U.S.A.
Structure of permeable reservoir. Pressure transmission to shallower
part of reservoir. Large anticlines, steeply dipping beds, etc.