Page 94 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS ']

Due to the heterogeneity of formations, in the majority of regions, permeability
distribution on the regional scale is much nearer to case (2) than case (1). One of the
causes of such lateral heterogeneity is facies change: it is well known, for example,
that at the top of anticlinal structure, formed simultaneously with sedimentation, sand
content is the highest and decreases with distance from the top of structure. Beyond the
anticline, clays may dominate. Thus, permeability in the center of anticlinal structure
may be several orders of magnitude higher than that beyond its boundaries. Still another
cause of low regional average permeability is the presence of faults impermeable or
poorly permeable across their planes. In many regions hydrodynamic environments and
hydrogeochemical zonality are distinctly broken by faults into separate blocks.
Antonellini and Aydin (1994) thoroughly investigated porosity and permeability
distribution in fault zones. They stated that in porous sandstones "deformation bands
have porosity about one order of magnitude lower than the surrounding host rock and,
on average, a permeability three orders of magnitude less than the surrounding host
rock. The wall rock in proximity to slip planes can have permeabilities more than seven
orders of magnitude less than the pristine sandstone". They also stated that capillary
pressure within deformation bands is estimated to be 10-100 times larger than that
in the surrounding host rock, and concluded that as a result of all this, "deformation
bands and slip planes can substantially modify fluid flow properties of a reservoir and
have potential sealing capabilities with respect to a non-wetting phase". It is also true
for the wetting phase water. Therefore, the lateral permeability of formations for a
long-distance flow may be orders of magnitude less than it is often assumed and, thus,
regional lateral flow of underground water and migrating hydrocarbons may be, at least,
highly limited.
At the same time, vertical permeability regionally and during geological time is much
higher than that measured from cores. In some areas with rather consolidated rocks,
a kind of honeycomb pattern of hydrodynamic environment was observed. Vertical
permeability formed by fractures depends strongly on tectonic and other rock-deforming
activities. For example, it was observed that moon earth-tides noticeably affect the width
and permeability of fractures and cause cyclicity in the yield of springs from fractured
rocks. Tectonic activity is a cyclical process and vertical permeability of formation
reaches its peak value at peaks of tectonic deformation. These periods, however short
they could be, can provide the major portion of vertical fluid migration that pulsates
together with tectonic pulsations.
For the origin and maintenance of abnormal pressure, the existence of poorly
permeable and sufficiently thick formations above the zone, where factors of the forced
convection act, is the most important feature of permeability distribution.

PRESENTATION OF PRESSURE AS THE ADDITIVE SUM OF TWO COMPONENTS

It is obvious from the above that free and forced convection and, accordingly, their
individual contributions to pressure distribution are caused by two specific groups
of factors, different both physically and geologically. To obtain a correct statistical
correlation between the pressure and various physical and geological parameters and