Page 206 - Petroleum Geology

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at approximately the same rate as the water. Theoretical studies by Brede-
hoeft and Hanshaw (1968) and Smith (1971) have indicated that during sub-
sidence the abnormal fluid potential in thick mudstones can be maintained
for geologically significant periods of time, and that the rates of fluid flow
are very slow indeed. Those products of diagenesis that exist as a separate
phase will move more slowly because of relative permeability and capillary
influences. Products with large molecules may be retained in the mudstone.
We shall postpone consideration of the state of petroleum during primary
migration to the next chapter, where chemical aspects will be discussed; but
whatever the state may be during most of its primary migration, it is very
likely that it exists as a separate phase before primary migration is complete.
The main reason for this belief is that the carrier bed is relatively inert chemi-
cally, compared with mudstones, and the path to the accumulation may be
short.
When petroleum exists as a separate phase in water, two factors affect its
migration: water saturations and capillary pressure. It seems certain that the
flow of two immiscible fluids in mudstone is similar in principle to such flow
in more permeable lithologies. There will be some critical water saturation
above which the petroleum can only exist as discrete globules in the pore
spaces, and that this petroleum is then virtually immobile. There will also be
some irreducible water saturation at which the water is immobile, but at
which the effective permeability to petroleum is close to the intrinsic perme-
ability of the mudstone. Such a state does not mean that the petroleum flows:
for that, the capillary displacement or injection pressure required for the
continuous petroleum phase to move must be less than that existing in the
petroleum phase.
Once the continuous petroleum phase reaches the porous and permeable
carrier bed, the capillary displacement pressure in the latter is very much less
and, as Hubbert (1953, p. 1979) showed, the imbalance of capillary pressure
at each end of the volume occupied by petroleum is sufficient to expel it
(Fig. 9-3). At the water saturation likely for such a continuous petroleum
phase, the effective permeability to petroleum will be relatively large.
The problem of the continuity of petroleum as a separate phase in water is
an intriguing one of some interest. We are faced with seemingly incompatible
alternatives. When the water saturation is so high that petroleum can only
exist as discrete droplets in the pores, the effective permeability to petro-
leum is zero and it cannot migrate. Within the zone of generation, organic
matter is evidently disseminated, so the petroleum generated will also be dis-
seminated. Petroleum must be added to these droplets, or water removed, so
increasing the petroleum saturation and decreasing the water saturation (drain-
age) until a continuous phase is reached. Water will be removed by compac-
tion, so it seems that petroleum disseminated in the pores will only be a tran-
sient condition. For petroleum migration in mudstone, the petroleum must
either be in solution or in a continuous phase at or close to the irreducible