Geology Reference
In-Depth Information
manifestations in specific geologic environments. Factual data from a
number of regions (South Caspian Depression, Indolo-Kuban Trough,
etc.) show the following: the zones of either practical absence or intense
manifestation of syngenetic pore and epigenetic formation pressure are
unfavorable conditions for the formation and preservation of hydrocar-
bon accumulations. Absent or low AHPP gradients in clays describe the
environment of the completed realization of the energy potential in the
oil-gas-generating sequence. High gradient values between clays and res-
ervoirs indicate obstructed discharge from the incompetent intervals of
a geo-fluid-dynamic system and, for this reason, a substantially limited
probability of commercial saturation in the adjacent reservoirs. Maximum
gradient and formation pressure abnormality factor values in reservoirs
pertain to the conditions with limited natural reservoir capacity. Input in
them of even small fluid volumes results in a drastic increase in formation
pressure.
Unfavorable for the preservation of accumulations appear to be envi-
ronments where low AHPP gradient values and low abnormality factors
exist in clays overlying reservoirs. In clays, this indicates declining (up to a
total loss) sealing properties. In reservoirs, this results in elevated ground
water mobility facilitating hydrocarbon wash-out from traps.
Typical examples of negative environments for the formation of oil
and gas occurrences maximum AHPP zones are local highs in the cen-
tral and southern Baku Archipelago (South Caspian Depression). There,
in the crestal tectonically and hydrodynamically isolated horsts of struc-
tures with intense AHFP (abnormality factors 1.7-1.3), commercial oil and
gas accumulations are absent and saturation is sporadic. Interestingly, in
these cases productive are less isolated peripheral fault-blocks and areas
of the structures with much lower AHFP. For this reason, negative results
over the structural crests (in conditions of intense maximum AHFP) do
not always negate their potential. Moreover, in some cases the analysis of
AHFP manifestations can indicate possible favorable zones of hydrocar-
bon accumulations as it was in the Baku Archipelago.
The reviewed options of the oil and gas occurrence forecast are based
mostly on the premise of regional hydraulic connectedness of intervals
with AHPP with the adjacent reservoirs. In the real natural environments
of the geologic section, cases are common where, together with such form
of connectedness, communications are provided through the prevailing
role of faults. In those cases, fluid migration paths are mostly regional and
smaller faults. In this sense, the indicated faults, which have quite signifi-
cant contact surface, are a fluid-conducting medium.
