Page 110 - Geology and Geochemistry of Oil and Gas

82 WATER

sections on rocks: squeezing-out of water, different heat expansion rates, and
neoformations). Water-head distribution in such basins is inverted. Water-head
decreases from the deepest portion toward the edges. N. B. Vassoyevich (in:
Eremenko and Chilingar, 1996, p. 53) proposed to call such basins ‘‘elision’’ basins.
The water-head in such a basin reflects the potential energy distribution, i.e., an
opportunity for reservoir water to move from the basin center to its periphery. This
can happen, but slowly. The distribution of anomalous pressure, which forms,
follows a certain pattern: higher in the center, decreasing upslope to a total
disappearance at the edges.
Reservoir water is saline, sometimes with the geochemical inversion (salinity
decreases with depth) and the reducing environment. The best oil and gas potential
exists in the ‘‘elision’’ water-head complexes. Oil and gas accumulations, formed
during the formation of stratigraphic sequences, are associated with the periphery of
present-day and ancient depressions, separated by highs and with major swells
present in the way of possible underground water movement (Bars, 1984). This
overconfident statement may well be true but is difficult to prove: comparable
amounts of hydrocarbons are known in basins with different hydrodynamic regimes.
Further evolution in such basins may create reservoirs cut into blocks, which
changes basin parameters. Inasmuch as the individual fault blocks are isolated from
one another, the anomalous pressure in each one is distinct. This also stops or at
least substantially limits the lateral migration. Numerous faults along and across the
strike make the vertical migration within the formation, and sometimes outside
of it, easier. The water composition does not change gradually but rather in a
stepwise manner. Changes in the water composition are easier to identify in the
section rather than in a lateral direction. The communication with the surface occurs
along the basin periphery where the thickness is lower. A typical infiltration drive
arises there.
This kind of a combination/isolated differential drive is quite common but has not
been named yet. Together with the infiltration and ‘‘elision’’ drives, Beletskaya
(1990, p. 34) distinguishes the thermohydrodynamic (deep subsurface) drive. She,
however, does not provide its description. The Mesozoic sequence of the North
Caucasus region is a typical example. Gas accumulations there are encountered at
the transition boundary between the block-isolated (deep subsurface) drive and the
infiltration drive. Before some other basins are studied in this respect, it is difficult to
tell at this time to what extent this phenomenon is common for the basins with
combination/isolated drive.
The appearance (or the preservation) of the infiltration drive is quite possible in
the basins with a typical ‘‘elision’’ drive. Such may be the Paleozoic basins in the
Volga-Ural Province. This, however, does not involve the entire basins. The deepest
portions of basins may preserve the ‘‘elision’’ drive with its characteristic anomalous
pressure. Still, this anomalous pressure is usually significantly lower than that at an
equal depth in typical younger ‘‘elision’’ basins.
At this time there is no technique or reliable parameters available to distinguish
between the preserved infiltration drive and the re-infiltration drive. It is quite
possible, however, to eventually develop such technique.