Page 228 - Geology and Geochemistry of Oil and Gas

ACCUMULATIONS BASED ON THE TYPES OF TRAPS 197
Neither of the described types, however, owes its existence to the hydraulic forces
exclusively. They can exist only under condition of the combined interaction of
several different forces: (1) hydraulic and gravity, (2) hydraulic and capillary, or (3)
hydraulic+capillary+gravity forces. The effect of hydraulic forces is commensurate
with that of gravity and capillary forces.
VI Gas accumulations in synclines or in monoclines devoid of structural highs. Ex-
amples of such accumulations have been presented by Masters (1979) and
Perrodon (1984). There is a gas accumulation in the Deep Basin monocline in
Alberta, Canada. The latter accumulation resides in the Mesozoic sandstone,
which is more than 3 km high (the thickness of individual gas intervals is
10–150 m). The sandstone is water-saturated updip the gas accumulation, with
an improvement in petrophysical properties. The gas reserves here are nearly
11.3 TCM. The gas accumulation of Milk River Field (Canada), with 250 BCM
of reserves, is another similar example. The gas accumulation of San Juan Field
(USA) resides in the Mesozoic sandstone in the synclinal part of the structure,
with reserves of 700 BCM. The sandstone is water-saturated over the flanks. The
porosity and permeability within the gas-saturated portion are 14% and 1 mD,
respectively, whereas in the water-saturated portion, f ¼ 25% and k ¼ 100 mD.
To explain this phenomenon, the following two explanations may be suggested:
(1) A rapid gas generation is currently occurring in the Mesozoic sandstones of
the Milk River and San Juan fields at a temperature of 85–921C. This gas is
entering the reservoir at a higher rate that it is being removed from the
reservoir.
(2) The reservoir rocks are hydrophobic (Bolshakov, 1986). Capillary forces
move the gas into the reservoir with finer pores and keep it there.
The authors of this book believe that both explanations are questionable, because
of the high surplus pressure and high capillarity within the water-saturated portions
of the accumulation. In the case of the first explanation above, if the gas-saturated
reservoirs are intercommunicated and form a single accumulation, the surplus pres-
sure in its upper portion should be around 30 MPa. If f ¼ 25% and k ¼ 100 mD, the
effect of capillary forces would be negligible and insufficient for retaining the ac-
cumulation. Also, if the porosity and permeability values are low along the axis of
syncline, a high rate of gas input would be doubtful.
The two explanations above assume that the gas is retained due to the change in
rock properties. In the first explanation the fast gas input from clays is assumed (and
from a larger area than the gas escape area). The second explanation subscribes to
the action of capillary forces in hydrophobic rocks. It is quite possible, however, that
the fluid properties change together with the changes in the reservoir–rock prop-
erties. A prolific gas generation occurred (and may still be occurring) in the
Mesozoic sequence. The gas dissolves in water as soon as it is formed. Thus, the
gas-saturated water enters the reservoir. In this case, the surplus pressure within
the upper portion of the accumulation would be tens of times lower than that for
the gas accumulations [the difference would be (r r ) in one case and
water gas
(r r ) in the other case]. Updip, the pore size increases (as indicated
water emulsion
by the higher porosity and permeability), the capillary pressure and temperature