Page 89 - Geology and Geochemistry of Oil and Gas

ORIGIN OF ABNORMAL FORMATION PRESSURES 65
classifications of origin of AHFP are presented in Table 3.4 and those of ALFP, in
Table 3.5.
Abnormally-high formation pressure (or overpressure) may be created by several
mechanisms including:
(1) Stress-related mechanisms (disequilibrium compaction – vertical loading stress)
and tectonic stress (lateral compressive stress).
(2) Fluid volume increase mechanisms, i.e., temperature increase (aquathermal
expansion), mineral transformation – water release due to mineral diagenesis
(gypsum to anhydrite transformation, montmorillonite-to-illite transformation),
hydrocarbon generation (kerogen maturation – oil or gas generation), oil and
bitumen to gas cracking.
(3) Fluid movement and buoyancy mechanisms, i.e., osmosis and hydraulic head.
(4) Hydrocarbon buoyancy (density contrasts).
Some of the above listed mechanisms are responsible for creating large amounts of
AHFP in sedimentary basins. The most common mechanisms, however, are
disequilibrium compaction due to rapid loading of fine-grained sediments in
relatively young basins, and the volume expansion associated with gas generation.
Mineral transformation (dehydration) may also create AHFP in rocks. Where part
of the rock matrix is converted into free fluid (e.g., oil and gas during kerogen
maturation or water as a result of montmorillonite-to-illite transformation), the pore
fluid will tend to assume that portion of the overburden previously carried by the
rock matrix. 6
Gas generation seems to be a viable mechanism to create regional AHFP by
increasing pore pressure. Gas may generate from gas-prone source rocks or can
result from the oil to gas cracking.
Other mechanisms, which rely on increased volume of pore fluids, include
smectite-to-illite transformation, oil generation, and gypsum-to-anhydrite transfor-
mation. AHFP’s can be created locally as a result of these processes. The local effect
may be additive to other simultaneously acting mechanisms, e.g., regional
disequilibrium compaction.
Osmosis may create minor amounts of AHFP if conditions are favorable. Shales
do not act as ideal membranes and the amount of AHFP is limited, even at high
salinity contrasts (see Chilingar et al., 2003).
Abnormally low formation pressure (or underpressure) as a geological
phenomenon is less well known than AHFP. There are several natural events
presumably capable to create the ALFP, including:
(1) Differential discharge – groundwater flow
(2) Differential gas flow
(3) Rock dilatancy
(4) Osmosis
(5) Thermal effects

6
Total overburden load (p t ) is supported by the grain-to-grain stress (effective stress) (p e ) plus the pore
(fluid) pressure (p p ): p t ¼ p e þ p p .