Page 207 - Petroleum Geology

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Fig. 9-3. Capillary pressure alone can expel a globule of oil from a mudstone into a sand-
stone once its interface reaches the sandstone. (After Hubbert, 1953, p. 1978, fig. 14.)

water saturation, at which the effective permeability to water is very small or
zero, but the effective permeability to petroleum close to the intrinsic perme-
ability. Migration of petroleum in solution may require less work than that
as a separate phase, but the whole migration path must be considered. If we
are correct in assuming that petroleum exists as a separate phase before pri-
mary migration is complete, then that part becomes virtually impermeable to
water, impeding water movement from “upstream” (Chapman, 1972; Hed-
berg, 1974).
Primary migration takes place in a more rigorous chemical environment
than secondary migration, and during it, the fluids are subjected to more severe
physical changes. In this connection, upward migration must be distinguished
from downward migration. Upward migration above the insulating surface
may take all fluids from relatively high pressures to relatively low on expul-
sion into the overlying carrier bed (the exact amount depending on the rate
of upward movement in a subsiding and compacting mudstone). Downward
migration into an underlying carrier bed may involve little or no pressure
change. Upward migration may involve little temperature change, while
downward migration will be to higher temperatures. These influences will be
amenable to analysis when we can identify with confidence the source rocks
of petroleum accumulations, and so compare the crude oils and gases that
have migrated upwards with those that have migrated downwards.
Petroleum migrating as a separate phase almost certainly comes into con-
tact with mineral grain surfaces, or is separated from them by a very thin
film of adsorbed water, as it does in the reservoir when the water saturation
is sufficiently low. Several common clay minerals are known petroleum cata-