Page 231 - Origin and Prediction of Abnormal Formation Pressures

204 G.V. CHILINGAR, W. FERTL, H. RIEKE AND J.O. ROBERTSON JR.

By definition, abnormal formation pressures (i.e., overpressures) are characterized
by conditions where the Pres/Pahyd ratio by more than twice exceeds the standard
deviation value. In other words: while below 3000 m the anomalous values approach
20%, the excess anomalous deviation below 3500 m ranges from 20% to 40% (e.g.,
Vetkhinskaya, Krasnosel'skaya, Barsukovskaya, and South Rechitskaya fields). Only in
three oil fields slight overpressures (<15%) were encountered above a depth of 3000
m. Similar observations have been made for the intersalt sediments by Zavgorodniy and
Poroshin (1981).
Whereas magnitude and frequency of overpressures in the subsalt sedimentary
section increase in an easterly direction, abnormal formation pressures in the intersalt
complex are only encountered within the southeastern portion of the area, i.e., in wells
drilled along the periphery and the central portion of the Vasilevichskaya Depression
(Fig. 8-8).
The compilation of subsurface pressure data in Table 8-1 in several fields and wildcat
wells, located within the northern structural-tectonic zone of the Pripyatskiy Deep,
allows the following observations:
(1) overpressures are mainly encountered below the depth of 3000 m;
(2) formation pressures increase with depth and exceed hydrostatic pressures by 30 to
40% between 3500 and 5000 m;
(3) these overpressures occur in oil-bearing as well as water-bearing rocks with no
significant pressure differentials being observed across oil-water contacts.
It is postulated that compaction of the Buregskiy shales expelled shale water primarily
into the good Semilukskiy carbonate reservoirs and only to a very small extent into the
marginal Voronezhskiy reservoirs.
Potential reservoir rocks within the intersalt complex, on the other hand, are invaded
by the 'compaction' waters from shales and marls only in the areas of major subsi-
dence. Zavgorodniy and Pakhol'chuk (1985) refer in particular to the Vasilevichskaya
Depression with depth to the intersalt complex in excess of 5 km. Depth and thickness
of shales, presence of adjacent permeable reservoir rocks, extent of the hydrodynamic
isolation (i.e., sealing) of reservoir rocks, the tectonic history, etc., all are major factors
in the origin, distribution, and magnitude of overpressures.
Variations of electric resistivity (p, ohm m) with depth as observed in the Buregskiy
shales were studied by Zavgorodniy and Pakhol'chuk (1985) to investigate the degree
of shale compaction (Fig. 8-9). The Buregskiy shales have been encountered over a
wide depth range, i.e., from 1249 m in well Novodubrovskaya No. 1 to 5080 m in
well Svetlogorskaya No. 1 with a corresponding significant resistivity change from
2.2 to 129 ohm m. Core samples above 4100 m clearly showed the Buregskiy interval
to be composed of shales (e.g., wells Nos. 1, 23, and 26 in the Ozershchinskaya
field; and well No. 8 in the Barsukovskaya field), or a combination of shales and
mudstones (e.g., well No. 18 in the Ozershchinskaya field; and wells Nos. 15, 22, 25,
28, and 34 in the Barsukovskaya field). At and below 4100 m, the horizon is composed
predominantly of mudstones (e.g., wells Nos. 9, 10, 11, 16 in the East Pervomayskaya
field). These lithological alterations are the result of diagenetic and catagenetic processes
accompanied by the expulsion of pore water which then entered adjacent Semilukskiy
and Voronezhskiy reservoir rocks.