Page 232 - Origin and Prediction of Abnormal Formation Pressures

TECTONICS AND OVERPRESSURED FORMATIONS 205

Based on the mathematical model proposed by Anikiev (1971), porosity values for
the shales at different depths can be estimated as follows:

4~ - 4~(1.0 - 0.25/3D) (8-1)
where ~b D is the shale porosity value at depth D" qS~h is the shale porosity at the surface
sh
(~35%); and r is the irreversible rock compaction factor (/~ -- 27 • 10 -5 MPa-1).
For example, the amount of shale water (0.13 m 3) expelled at 3000 m, where
porosity decreased by 13%, is well over the capacity of the regionally developed
Semilukskiy reservoirs. That is why the pore water expelled from the Buregskiy
shales was sufficient to create overpressure conditions in both the Semilukskiy and
Voronezhskiy reservoir sequences, provided these reservoirs were hydraulically sealed
(Zavgorodniy and Pakhol'chuk, 1985).
As pointed out by Zavgorodniy and Pakhol'chuk (1985), the areal distribution of the
overpressures is related to the thickness of the Buregskiy shale (Fig. 8-8; Table 8-1). The
highest overpressures (i.e., with an abnormality factor of 0.2-0.4; see Table 8-1) are in
the easterly portion of the northern structural-tectonic zone where the Buregskiy shale
thickness is between 40 and 60 m (e.g., South Rechitskaya and Krasnosel'skaya fields),
whereas to the west the shale thins out to 5-10 m with a corresponding decrease in the
abnormality factor in the range of 0.06-0.14. East of Barsukovskaya and Vetkhinskaya,
oil fields with several promising subsalt structures have been defined with an anticipated
overpressure abnormality factor as high as 0.3-0.4. Basically these potential reservoir
rocks occur in deeply buried and hydraulically sealed fault blocks. On the other hand,
no significant overpressures are anticipated within the subsalt sedimentary complex in
the highly faulted part of the central structural-tectonic zone of the Pripyatskiy Deep.

CONCLUSIONS

The following generalized conclusions as to the overpressure environments in the
Pripyatskiy Deep were made by Zavgorodniy and Pakhol' chuk (1985).
(1) Overpressured rocks do not occur above 3000 m.
(2) Below 3000 m, overpressures may exceed hydrostatic pressure by as much as
40%. They are caused by 'compaction' water from shales and marls which entered the
carbonate reservoirs.
(3) Overpressures in the subsalt Semilukskiy and Voronezhskiy carbonates are con-
trolled by the thickness of associated Buregskiy shales and the degree of hydrodynamic
sealing of the carbonate reservoirs. Highest overpressures are most likely in the eastern
region of the Pripyatskiy Deep where the thickness of Buregskiy shales is maximum
(30-60 m).
(4) Intersalt rocks exhibit significant overpressures only within the Vasilevichskiy
Depression, i.e., the deepest portion of the Pripyatskiy Deep. Elsewhere, such overpres-
sures are only encountered locally and are insignificant.
(5) Overpressures occur only in the presence of good sealing of reservoirs.
In the opinion of the writers, the greater the degree of overcompaction in shales,
mudstones and/or marls due to the tectonic activity, the greater is the overpressure