Page 202 - Geology and Geochemistry of Oil and Gas

TIME OF FORMATION OF HYDROCARBON ACCUMULATIONS 171
the time of formation of accumulation and at present, and Z f and Z 0 are the gas
compressibilities at the time of formation of accumulation and at present.
The calculation is conducted in two stages. At the first stage, the ratio T f Z f /T 0 Z 0
is assumed to be equal to 1 due to the unknown value of the numerator. The time of
formation of accumulation is determined from the stratigraphic section at depth H f .
At the second stage, a more accurate H f value is determined taking into account the
T f and Z f values as determined from the previous H f calculation.
This method was refined by K. M. Marchenko, V. A. Kirov, V. A.
Chakhmakhchev, etc. (in: Eremenko and Chilingar, 1996), but the imprecision due
to the ‘‘trial and error’’ approach has not been eliminated. Most acceptable results
were obtained when determining the accumulation age in several closely positioned
accumulations within the same productive formation (especially, if they had been
formed according to the differential entrapment principle).

9.4.6. Saturation Pressure Technique
The gas-saturation pressure method has been proposed by Gussow in 1953. The
assumption is that the gas-saturation pressure at the time of formation of
accumulation has been hydrostatic. With increasing overburden pressure, the
reservoir pressure increased, whereas the saturation pressure remained the same. The
time of formation of accumulation is determined using the depth of reservoir, where
the reservoir pressure is equal to the saturation pressure. The determinations are
usually made using cross-sections.
A drawback of this technique is the assumption of equality between the saturation
and reservoir pressures at the time of formation of accumulation. As previously
mentioned, at least 40% of the liquid hydrocarbons formed are initially under-
saturated.
The accumulations begin to be destroyed at the moment they begin to form. Their
very existence is a temporary phenomenon related to the dominance of accumulation
processes over those of destruction. Gas diffusion is associated with all gas
accumulations. The calculations by V. A. Sokolov and V. F. Linetsky (personal
communication, 1994) showed that any commercial gas accumulation (at a depth of
1,000 m and initial pressure of 10 MPa) would be destroyed through diffusion of gas
within a few million years. Apparently, gas accumulations are preserved due to their
replenishment. The effect of gas diffusion is much weaker in oil accumulations, but
still may cause substantial alterations in oil composition.
Eruptions cause significant destruction of oil and gas accumulations. This is
evidenced by the mud volcanoes (Buryakovsky, 1993; Buryakovsky et al., 2001) and
other massive oil and gas seeps associated with faults (Khilyuk et al., 2000). As
shown in Chapter 3, the fault zones are often less permeable than the adjacent
reservoirs. A fault may act as a seal in one location and may be highly permeable in
another location along its length. Also, permeability of fault zone can change with
time depending on the geologic conditions (including thermodynamic and
geochemical). Finally, the oil composition can change within the weathering zone
or the accumulation can be totally destroyed by erosion.