Page 19 - Origin and Prediction of Abnormal Formation Pressures

2 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

porosity of sediments is high (60-70%), the lithostatic pressure gradient is 0.7 psi/ft.
Only at a depth of about 1.0 km, this variable is about 1.0 psi/ft.
There are many factors that cause abnormal formation pressures, which may be either
less than, or greater than, the pressure resulting from the normal hydrostatic pressure
gradient of the region.

ABNORMAL PRESSURES
Subpressures

In the accumulated experience of the petroleum industry in exploration wells,
abnormally low formation pressures (subpressures or ALFPs) have been encountered far
less than the surpressures (abnormally high formation pressures). In the United States,
ALFPs have been found in Arkansas, some areas of the Appalachian Mountains, the
eastern Colorado plateau and the Oklahoma-Texas panhandle areas. Other locations of
subnormal formation pressures are in central Alberta in Canada, in the Siberian oilfields
of Russia, and the arid regions of the Middle East. Many subnormal formation pressures
have been artificially induced by production of hydrocarbons and water from subsurface
reservoirs, which reduces the formation pore pressure of isolated reservoirs where a
sufficient influx of water does not exist to compensate for the fluids that are withdrawn.
In many cases, this reduction of formation pressure leads to surface subsidence, which
in some cases has resulted in the destructive damage to surface structures. Examples of
subsidence due to fluid withdrawal have taken place in: the Po Delta of Italy; the Bolivar
Coast of Lake Maracaibo, Venezuela; Galveston Bay, Texas; Long Beach, California;
Japan; Taiwan; and other areas (Chilingarian et al., 1995).
The Granite Wash oil-producing formation near Amarillo, Texas, exhibited a forma-
tion pressure of almost one half of the expected normal hydrostatic pressure. Levorsen
(1967) stated that a possible reason may be the fact that the Granite Wash Formation
outcrops in Oklahoma east of the Wichita Mountains at an elevation which is about 1000
ft (305 m) lower than the surface elevation at the producing field in Texas. Subnormal
pressures in the semi-arid areas of the Middle East occur because the water table is
exceedingly deep (several thousand feet in some cases) and the hydrostatic gradient
begins at the water table depth.

Surpressures

Formations containing fluids with abnormally high formation pressures (AHFPs)
have been encountered in all of the continents of the world where exploratory drilling
for hydrocarbons has been conducted. Hunt has noted that AHFPs are present in around
180 basins around the globe. According to Law and Spencer (1998), in the US Gulf
Coast region, for example, there are at least seven stratigraphic units, ranging in age
from Jurassic to Recent, that are abnormally pressured.
These fluid reservoirs are isolated environments or at least the fluid flow out of
the reservoirs is restricted, and the total overburden load is partially supported by the