Geology Reference
In-Depth Information
The fluids squeezed out in the process of salt compaction have signifi-
cant syngenetic AHPP's. These AHPP's cause an increase in formation pres-
sure in the adjacent reservoirs to the abnormal level approaching in some
cases the geostatic (lithostatic) ones. Besides, as the salt is highly plastic it,
affected by gravitational load from the overburden, by basement motions
or tectonic stresses, may invade the dislocations of the adjacent clastic or
carbonate rocks facilitating the generation in them of a high abnormal-
ity due to compression of the fluid-saturated draping formations. A result
of these processes is a wide commonality of abnormal pressures directly
in the salt sequences and in the salt dome-draping clastic and carbonate
intervals.
Obviously, maximum abnormal pressures will be recorded in perme-
able lenses inside the salt sequences. The reason is that the possibility for
pressure relaxation there is highly restricted because of practically total
hydraulic isolation of these lenses from the discharge zones and areas.
Also favorable are conditions for AHFP generation in the lower section
of the post-salt and in the subsalt complexes. There are certain patterns
controlling in general the pressure gradient distribution in the sediments,
which include evaporite formations. They are:
t
In the post-salt complex, gradients rather drastically
decrease up the section.
t
In the reservoir members (lenses) within the salt sequence
the gradients are practically constant and almost match the
lithostatic level.
t
In the sub-salt complex they decline down the section at a
slow rate (in case the interval is close to the basement rocks
the pressure gradients in the sub-salt sediments may remain
almost constant within the entire section).
The generation of AHPP and AHFP in sedimentary formations is
closely associated with tectonic and neotectonic processes. It is to a sig-
nificant degree their derivative and materializes in different lithofacies at
different stages of a region's geologic evolution. Thus, the emergence of
syngenetic pore pressure in clay sequences and of formation pressure in
reservoirs occurs mostly at the immersion evolution stage of relatively
young mobile belt basins, epiplatform orogens, etc. The formation of
epigenetic modifications may occur practically in any basin tectonotype
(young and old geosynclines, platforms, orogens, aulacogens, etc.).It is
most commonly associated with inversion stage of tectogenesis and is
