Page 227 - Geology and Geochemistry of Oil and Gas

196 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

Fig. 10.12. Conceptual cross-sections of hydraulically trapped oil and gas accumulations. (a) Next to the
conductive faults. (b) At the anticlinal crest. (c) In monoclines within the areas of changing reservoir
properties. (d) In monoclines underneath the stratigraphic uncomformities. (e) At structural noses of
monoclines. (f) Near the reservoir shale-out boundaries. 1 – Piezometric surface; 2 – reservoir; 3 – shales; 4
– direction of water movement; 5 – oil and gas accumulations.

Chapter 4) or the Savchenko’s (1977) equation could be used:

Dh ¼ Dp =gðr r Þ (10.1)
norm water oil
where Dh is the amount of shift at the edge of accumulation, Dp norm is the
difference in normalized pressures, r water and r oil are the density of water and
oil, respectively, and g is the gravitational acceleration.
A barrier (facies change, stratigraphic unconformity, and fault) often turns out to
be a barrier due to the presence of pressure difference across it, rather than because
of the appearance of an impermeable barrier in the way of fluid movement.
Fig. 10.12 shows possible relations of the positions of the piezometric surface
(normalized pressure head) and the positions of the oil–water contact. The necessary
condition for the preservation of hydraulically trapped accumulation next to a fault
is a higher potential head of the water next to the fault zone than that of the
productive formation (the surplus pressure is included). This condition may exist if,
for instance, there is a communication along the fault between the accumulation and
the reservoir with AHFP (overpressure).
In monoclines, the accumulations can be preserved (see Fig. 10.12c,e and f) if the
potential head decreases downdip in locations where the dip increases (see Fig.
10.12e) or the dip of the piezometric surface decreases (see Fig. 10.12f). The latter is
possible when the reservoir–rock properties change (i.e., capillary forces enter into
play). The oil–water contact can close onto themselves (but crossing the structural
contour lines on the top of the reservoir) or can about the trapping barrier. (The
contributions of Plotnikov, 1976; Gattenberger, 1984; and Mikhaylov, 1984, on
hydrodynamic traps are noteworthy.)