Page 189 - Geology and Geochemistry of Oil and Gas

158 FORMATION OF HYDROCARBON ACCUMULATIONS

equalization). Quantitatively, diffusion can be described by Fick’s law for gases:

wAbðC 1 C 2 Þ
Q ¼
h
where Q ¼ the volumetric rate of diffusion, w ¼ the diffusion coefficient, A ¼ the
cross-sectional area, b ¼ the solubility or sorption coefficient, and h ¼ the bed
thickness. The bðC 1 C 2 Þ=h is called the gradient of concentration.
At least two indirect ways for diffusion to participate in the formation of
hydrocarbon accumulations have been proposed:
1. The water within a source rock is being saturated with liquid hydrocarbons. This
water, upon moving into a reservoir rock, with a different thermodynamic and
geochemical environment, releases the dissolved hydrocarbons. The dissolution is
possible if the water is (1) alkaline, (2) CO 2 -saturated, and (3) heated to 120–1501C.
A high concentration of hydrocarbons within the source rocks and their ability to
saturate the water at lower temperatures (o120–1501C) are in doubt. Besides, in
such a case the hydrocarbon-saturated water would be observed in the oil and gas
basins, especially next to the source rocks, which is not the case.
2. Thermal diffusion (Geodekian et al., 1984) results in fractionation of the
diffusion flow and in the concentration of heavier hydrocarbons near the cooler
edge. In the opinion of the writers, however, the thermal diffusion is in effect a
superposition of different phenomena: the thermal fractionation (separation) and
diffusive equalization of fractions. Thus, the diffusion proper equalizes the
composition. As far as the thermal fractionation is concerned, it is widely used in
refinery operations.

9.3. PRIMARY ACCUMULATION AND FREE PHASE MIGRATION (‘‘SECONDARY
MIGRATION’’)

A concept of the active liquid oil migration from the source rocks was extensively
discussed. It is not simply the squeezing-out of separate oil droplets, which is a
complex process in a natural environment. The concept assumes the release of liquid
phase as streams (Savchenko, 1977). Through this migration mechanism the
accumulation can occur already within the source rocks (possibly via the film
mechanism as described above). The major objection to this migration mechanism is
the absence in nature of source rocks (even assumed ones) with a high concentration
of residual oil.
Shklovskiy (in: Eremenko and Chilingar, 1996) developed a flow model for an
infinite cluster. Experimental studies by Efros (1982) indicated that the flow
threshold exists at about 15–20% of residual oil saturation. If oil and water are
present in a good reservoir rock, the relative permeability to oil tends to zero at the
oil saturation of 15–25% (depending on the oil and water viscosity under reservoir
conditions). Even in very good reservoir rocks, at least 25% of the oil is left behind
after production. In poorly permeable rocks (usually, the assumed source rocks
belong in this group) the residual oil saturation is over 70%.