Page 100 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 81

Pressure distribution caused by an upward fluid migration in areas with multiple
faults may vary laterally very significantly and is less predictable especially in the early
stages of exploration. Some tectonic blocks can have a very high pressure, whereas the
pressure in adjacent blocks may be much lower due to the difference in the vertical
connections with the source of high pressure.
Taking into account insufficient information on geology and pressure distribution
in such areas at the exploration stage, it may be advantageous to use some average
values of pressure for correlation with geologic and physical parameters. The following
characteristics may be recommended for correlation.
(1) For rock and fluid capability to produce an increase in pressure: thickness
of sedimentary basin, degree of consolidation and transformation of sedimentary and
basement rocks, and presence and relative proportion of source rocks.
(2) For external influence: temperature gradient and rate of its increase with depth,
length and frequency of linear faults, and gradient of neo- and recent tectonic differential
movements.

Correlation between porosity and pressure

Next, one should discuss the correlation between porosity and pressure distributions.
Such a correlation may have a cause-and-effect nature even if pressure deviations from
the free convection pressure distribution were caused by the compaction only and there
was no fluid flow. From the hydrodynamics viewpoint, pressure distribution in a fluid
seeping through a rock formation will depend on the permeability distribution. In a case
of steady flow, i.e., when there are no changes in time, a certain local correlation will
exist between pressure and permeability along the fluid flow path. In a case of active
processes, for example, compaction and upward fluid migration, these distributions may
be more complex, but still some correlation between permeability and pressure distri-
butions in poorly permeable rock formations may still exist. As there is a correlation
between permeability and porosity (if lithology is uniform) and correlation between
permeability and pressure distributions, there is a correlation between pressure and
porosity distributions. For example, if a high-porosity formation lies on a low-porosity
formation and is overlain by another low-porosity formation covered by sands up to
the surface, high pressure may be present in the high-porosity formation, lower poor-
porosity formation, and abnormal pressure in the upper poor-porosity formation will
decrease upward to the hydrostatic one at the boundary between the upper poor-porosity
formation and sands. Definitely, as pressure decreases, porosity increases upward in
each formation. Thus, a certain correlation may always be found between the two.
The above-presented theoretical analysis is next applied to a regional pressure
distribution in Azerbaijan, in the South Caspian Basin. Researchers of this region
believe that pressures here fully correlate with porosity and, thus, almost completely
base pressure evaluations on well logging data. As discussed above, hydrodynamic
theory indicates that pressure distribution should heavily depend on the permeability
distribution and that a correlation between porosity and pressure may exist or not.
Carstens and Dypvik (1981) showed this very convincingly using empirical data. If such
a correlation exists, it is an indirect correlation mostly based on correlation between