Page 90 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 71

Crest

Horizontal Lines
Pressure Isobars

1 -- Zone of lower than hydrostatic pressures
2 -- Zone of higher than hydrostatic pressures

River

Fig. 3-1. Pressure distribution in the case of free convection basin water flow caused by non-horizontal water
table. (Modified after Gurevich et al., 1994, fig. 1, p. 69.)

because free and forced convection and pressure distributions associated with them, are
caused by different factors and should be correlated with different parameters.
Geological factors are nearly always combinations of physical and chemical factors.
To avoid confusion it is more convenient to follow the cause-and-effect picture of natural
phenomena and processes and, therefore, to list all factors from a viewpoint of physics
and physical chemistry. A list of theoretically possible pressure generating and pressure
changing mechanisms is presented below.

Free convection of ground fluids

Free convection of ground fluids is caused by non-uniformity of fluid density
unstable in the gravitational field. It is convenient, from a geological and physical point
of view, to identify different cases of such non-uniformity as follows: (1) non-horizontal
groundwater table -- the boundary between water and air that generally follows the
relief of the Earth's surface; (2) vertically and horizontally non-uniform water density
distribution owing to the salinity and temperature non-uniformities; (3) inclusions of oil
and gas -- from bubbles to pools in size- in water-saturated formations.
The first factor can cause subnormal pressures below elevated (recharge) areas
and abnormal pressures below low (discharge) areas (Fig. 3-1). In intermontane areas
deviation from hydrostatic pressure may reach up to 30-50%. In flat areas fluid
pressure distribution caused by non-horizontal relief and water table is rather close to
hydrostatic. Non-uniformity of water density generally causes only minor deviations
from hydrostatic pressures. But oil and especially gas pools in traps of considerable
height can cause noticeable to very high abnormal pressures.