Geology Reference
In-Depth Information
West Apsheron Lokbatan-Puta-Kushkhana prospect; in Central Apsheron,
Surakhany, Karachukhur, Bibieybat; in East Apsheron, Buzovny-Mashtagy,
Kala, Zyrya, Shakohova Kosa; in South Apsheron Trough, Djanub).
Calculations indicate that the pressure gradient between the basal portion
of PT and underlying clay reaches 20-50 MPa (Durmishyan, Muradian
and Rachinsky, 1973; Ovnatanov, 1962; Rachinsky, 1990).
Ubiquitous development and preservation in the underlying Miocene
clays during long periods of time syngenetic AHPP close to geostatic pres-
sure combines with regional development of almost hydrostatic pressure in
the adjacent PT-KT reservoirs. It indicates, despite significant pressure gra-
dients between them, a reliable hydrodynamic separation between Miocene
clays and overlying permeable section intervals. It is caused by limited scale
at the discharge energy of the compressed pore fluids and, thus, determines
the absence of regional hydraulic communication between them.
Otherwise, as the calculations show (Gurevich, 1980), the pore pres-
sure at contact with reservoirs should have been hydrostatic. It needs to
be added that basal reservoirs (in the Kala and Kirmaki formations) usu-
ally are low-permeability, variable and lens-shaped. This does not facilitate
pressure redistribution in the reservoirs and would certainly provide for
regional AHFP conservation in them if its origin were due to the commu-
nication with underlying clays. Besides, near contact of “undercompacted”
clay with reservoirs, favorable conditions are usually created for second-
ary silica, calcite, sulphates, etc., crystallization. That determines drastic
decrease of their permeability in such areas. Also, as a result of low clay
permeability, the release of pore fluids by the compacting clay into the
adjacent reservoirs may occur only in their contact zone. Thus, not the
entire clay volume but only its limited contact portion is converting after
some geologic time into a compact hydraulic barrier.
The lack of abnormal pressure in Kala, Podkirmaki and Kirmaki res-
ervoirs between the structures and their local manifestations only within
anticlines (Apsheron Peninsula) attests to the small scale of the synge-
netic AHPP filtration relaxation (“dispersion”) in the basal PT-KT zone
by way of pressure transmission into the adjacent reservoirs along its
entire contact with clays. Even when the indicated zone contacts with
high-permeability Podkirmaki Formation (which has significant piezo-
conductivity in the Central Apsheron), a relatively high pore pressure is
preserved in it. This is supported, in particular, by the drilling results at
the Karachukhur prospect (northwestern portion of the anticline) and
Ramany. There the tubing blowouts, sticking and other complications
occurred on penetrating the basal PT intervals and underlying deposits
(Ovnatanov, 1962).
