Page 242 - Petroleum Geology

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(1979, p. 207) is that this process is unlikely to be significant.
Separate phase. The conceptual simplicity of generating liquid or gaseous
hydrocarbons directly from source material, and evidence that the liquid or
gaseous state exists by the beginning of secondary migration, make primary
migration as a separate phase attractive. The difficulties are largely mechani-
cal, and these have been the main reason for regarding migration as a separate
phase unlikely if not impossible. If disseminated organic matter generates
disseminated droplets of oil, the capillary displacement pressure required
to move these from one pore to another is far greater than that available in
the pore water. Displacement of oil under these conditions must await me-
chanical forces during the reduction of pore volume during compaction (Hob-
son, 1954, pp. 78-80). The problem is reduced to an assessment of possible
processes of concentration, because oil in a continuous, separate phase requires,
as we saw in Chapter 8 on reservoirs, far less work for its movement.
Dickey (1975) suggested that the pore water in a source mudstone is largely
structured water that behaves mechanically more as a solid than as a liquid.
Under these conditions, the oil saturation relative to movable water may be
high enough to form a continuous phase.
Mudstones are seen under electron microscopes (Dickey, 1975, p. 340,
fig. 2) to consist of dominantly platy fragments that are usually slightly de-
formed but lying generally in the bedding planes. Permeability of mudstones,
though difficult to measure with precision, appears to be appreciably aniso-
tropic, with lateral permeability much greater than transverse. We infer that
the pore shapes are also “flat”, with smaller transverse dimensions (“vertical”)
than lateral. We assume that during compaction the loss of porosity is achieved
largely by reduction of the vertical pore dimension.
As an oil droplet forms, it will tend to occupy the position that minimizes
its potential energy, but the spherical shape that also minimizes its potential
energy can no longer be maintained once the diameter of the droplet reaches
the minimum, vertical, dimension of the pore: it is thereafter distorted, which
requires energy. As the droplet grows, any pore throat that comes to be on
the upper or lower, flattened surfaces of the droplet will tend to be entered;
and if the pore throat diameter is larger than the vertical dimension of the
pore, it will enter the throat rather than grow in the pore. It is therefore
possible that the anisotropy in the mudstone induces an anisotropy in oil
concentration that favours vertical continuity of oil phase at water satura-
tions that are larger than those required in isotropic pores. The water will
tend to concentrate in pore space away from throats, analogous to the pen-
dular rings in reservoir rocks.
We concluded on p. 158 that the thickness of an adsorbed water film on
VYCOR at room temperature is not thicker than 1 nm because the flow of
water, acetone, and n-decane obeyed Darcy’s law through pores about 4 nm in
diameter. The intrinsic permeability of VYCOR was found to be about
cmz (Chapman, 1981, p. 65), or darcies, which is near the lower end of