Page 211 - Petroleum Geology

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ability to oil at the tail is probably less than it is at the head, so there may be
a tendency for the oil stream to attenuate. Any oil left behind as a discon-
tinuous phase will be swept up by the next front. All fronts (if there are
more than one) will follow the paths of local minimum potential energy on
the upper surface of the carrier bed (like an inverted river drainage system),
so progress towards the trap will tend to bring migration paths together, and
the accumulation will be fed by one or more streams of oil. A detailed con-
tour map of the top of the carrrier bed would indicate the possible paths.
Natural carrier beds are rarely homogeneous or isotropic, so we must con-
sider briefly the main effects of heterogeneity and anisotropy on migration
paths.
When two immiscible liquids occupy a single pore, the pressure in the wet-
ting liquid is slightly less than that in the non-wetting liquid, and the non-
wetting liquid occupies the position that minimizes its potential energy. The
difference of pressure across the liquid/liquid interface is the capillary pres-
sure. We are not concerned here with isolated drops of oil in pores, but with
a continuous network of oil through the pore space in a definite volume of
the carrier bed. We are concerned, therefore, with the macroscopic upper
water /oil interface.
The magnitude of the capillary displacement pressure is a function of the
radii of curvature of this interface within each pore along the macroscopic
interface, such that the smaller the radius of curvature the greater the capillary
displacement pressure. The radii of curvature in the smaller pores are less
than those in the larger pores, and the capillary displacement pressures re-
quired for displacement through the smaller pores are greater than those
for the larger pores. The migrating oil occupies the larger pores preferentially,
because these are the paths of least work.
Heterogeneities and anisotropy in carrier beds are generally related to
bedding, and so affect the upward migration of oil across the bedding. Migra-
tion across a graded carrier bed in which the grain size, and so the pore size,
increases upwards is facilitated by the decreasing capillary displacement pres-
sure required and the increasing pressure available within the oil. If the oil is
a bubble that is large compared to a single pore, the imbalance of capillary
pressure at the leading and trailing surfaces impels the bubble upwards. Grading
in the opposite sense retards migration. Beds of alternating fine and coarse
grain that are not horizontal lead to refraction of the migration path up-dip
(Hubbert, 1953, p. 1972, fig. 10). In the coarser beds, the flow path will
tend to deviate updip: in the finer, more vertical.
Similarly, the lateral migration will also be along paths of least resistance,
favouring the larger pores and perhaps by-passing the smaller.
The water in which petroleum migrates will not always be at rest, but also
moving along an energy gradient to positions of lower energy.