Page 99 - Origin and Prediction of Abnormal Formation Pressures

80 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH

amplitude and frequency of the subsidence-uplift oscillations, and rate of temperature
increase.
Generally, in areas where subsidence and sediment accumulation occurred prior to the
Cretaceous period and was followed by an uplift or stabilization, clays are consolidated
and processes other than compaction play major roles in abnormal pressure generation
and maintenance. No correlation between pressure and porosity or other compaction
parameters can be expected in such formations and areas. In some areas where clays
are within or interbedded with carbonate formations a reverse correlation was observed
(Fertl and Chilingarian, 1989; Chilingarian et al., 1992).

Upward fluid migration

In areas where there are lithological 'windows' in seals and/or where faults and
fractured zones are present, vertical migration can play a very important and sometimes
major role in the origin and maintenance of abnormal pressures. Vertical migration of
fluids can create pressure abnormality by several mechanisms.
Good local vertical connection between two or several beds within a structural trap
can create a high oil or gas pool (piercing several formations) and, thus, a high pressure
at the top of the pool. This, actually excessive hydrostatic pressure can exceed the
geostatic one in shallow traps. Such cases were encountered in Azerbaijan, California
and other intensively faulted regions. Bourgoyne (1994) emphasized the danger of these
shallow high-pressure pools.
Vertical and subvertical columns of gas migrating through formations are encountered
in regions of active recent or current tectonic activity and currently continuing vertical
migration of hydrocarbons. In such cases, abnormality may even decrease upwards
due to dissipation of pressure caused by penetration of gas laterally from the column
into permeable beds. Pressure typically decreases laterally from the column outward.
Vertical migration and pressure distribution of this kind mostly occur in consolidated
deeper formations with reduced lateral permeability and increased capacity to form
fractures.
Upward gas migration along the high-permeability fractured zones can greatly
increase pressure by the piezo-convective effect. In some cases of localized zones of
upward gas migration, this effect may make a major contribution.
Origin of abnormality in areas of intensive upward fluid migration is a combina-
tion of a free convection (hydrostatic including) component and a forced convection
component. The whole geologic section down to and often including basement rocks
is undergoing mechanical deformations and geochemical transformations, including oil
and gas generation. This increases compression of fluids and, therefore, pressure. Faults
and fractured zones provide paths for mostly localized upward fluid migration within
the whole thickness of a sedimentary basin. It is difficult and most often impossible
to determine what factor of free and forced convection contributed most heavily in an
observed pressure distribution. On one hand, fluid is compressed and being 'squeezed'
from its previous position along the direction of decreasing potential. On the other hand,
distribution of pressure in the column of migrating upward fluid always has a hydrostatic
component depending on fluid density.