Geology Reference
In-Depth Information
individual regions tectonic subsidence could dominate. In such a case, a
change from one mechanism to the other results in lengthy coexistence in
the sedimentary section of regionally developed low-density (undercom-
pacted and decompacted) clays with syngenetic AHPP. The relaxation of
compressed fluids, the normalization of their pressure, the preservation
of low density and abnormally high porosity (water-saturation) depend
on the outflow conditions of the pore fluid. They include thickness, min-
eral composition and permeability of the clay series, the extent of its
faulting and fracturing, total contact area between clays and reservoirs,
which can act as clay drainage areas, the extent of completeness of dehy-
dration processes and montmorillonite consummation, tectonic activity
of the area, etc.
Following are diagnostic features of syngenetic
AHPP:
t
Regional abnormally high porosity (low density, elevated
water-saturation) of clay formations independent of struc-
tural conditions and hydrocarbon accumulations in the
underlying sediments.
t
Seismic velocity inversion zones on seismic cross-sections
with chaotic seismic events accompanied by stable negative
gravity, electric and magnetic anomalies.
t
Increase in pore pressure abnormality down the regional dip
and increase in bed thickness and application of geo(neo)
tectonic stress.
t
The association of maximum porosity values with mid-
intervals of clay sequences (at their mostly gravitational
consolidation) and in the contact zone of mobile compe-
tent and incompetent fault-blocks (at their orogen-stress
compaction).
t
Compressed syndepositional water in the clay pore space.
t
The association of intervals with abnormally high porosity/
elevated water-saturation of clay with montmorillonite con-
version zones into hydro-micas.
t
Specific complications in drilling (clay swelling/blow-out
2
in
boreholes plentiful recovery of cuttings and rock fragments;
2
In well drilling, swelling of drilled clays may occur in a borehole. It happens when
the counterpressure from drilling mud is low and when the clays in the section
have high pore pressure (AHPP). In such cases, mud has to be weighed to prevent
a blow-out.
