Geology Reference
In-Depth Information
Both mechanisms result in the generation in pore volume of clayey
sequences of abnormally high fluid pressures (AHPP).
They are func-
tionally and spatially associated with intervals of preservation and new-
formation, and development of zones of elevated water-saturation-porosity
in clays. This is a statement of an indirect connection between AHPP and
geothermal regime of deep subsurface. For many regions, this in turn
explains the phenomenon of a noticeable increase in formation tempera-
ture and decrease in its gradients within reservoir members underneath
thick clay fluid barriers with the close-to-geostatic abnormal pressure.
Changes in the thickness, water content and areal extent of under-
consolidated and decompacted clay massifs affect heat flow due to the
simultaneous action of two independent factors. One of them occurs in
connection with boundary conditions on contact of rocks with different
heat conductivity. Distortions of heat flow lines at intersection of such
boundaries is determined by bed geometry and their heat-conductivity.
Another one (so-called “porosity effect”) occurs due to a change in heat
conductivity of a sequence as affected by differences in the pore water con-
tent. Natural geothermal conditions tend to equilibrium. For this reason,
the temperature underneath a thermal barrier must be high enough to pro-
vide for heat flow through a bed, which would be equal to the regional aver-
age. That is why the temperature observed at any depth is a direct function
of rock heat conductivity in the entire overlying sequence (Jones, 1975).
Heat accumulated in a reservoir underneath the clay sequence with
AHPP can be conserved for a long geologic time practically until the
moment of pore pressure relaxation and total clay conversion into argil-
lite. It happens due to thermo-insulating effect of the seal and insubstantial
outflow of compressed fluids. The factors affecting maximum temperature
generated in such an environment are the thickness and porosity (water-
saturation) of clay, pore fluid loss rate by the clay, extent of thermal rock
alteration, and areal extent of the reservoir, its lithology and time.
The sedimentary sequences contain swelling clay varieties. They are
subject to endothermal metamorphism accompanied by a partial expen-
diture of the depth heat and causing some decrease of heat flow toward the
surface. This is an additional factor limiting the intensity of heat transfer
up the section and decreasing the amount of heat transferred to the upper
intervals.
Following are examples of this kind:
t
The South Caspian Depression; as previously mentioned,
includes extremely thick montmorillonite clay sequence and
quite depressed temperature regime.
