Page 243 - Petroleum Geology

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the range of mudstone permeabilities reported by Magara (1971, p. 241, fig.
9), and by Bredehoeft and Hanshaw (1968). It therefore seems sound to infer
that the effective diameter of the pore/thoat system is usually larger than
5 nm, and, if the tortuosity of mudstone is greater than that of VYCOR, ap-
preciably greater than 5 nm. It is, of course, also evident that n-decane will
pass through pores of 4 nm diameter, perhaps rather less.
It now appears that primary migration as a continuous, separate, immiscible
phase is entirely possible, irrespective of considerations of structured water,
and that the effect would be of multitudinous, sinuous, vertical streams.
Furthermore, these streams would be largely in contact with mineral grains,
which include clay minerals that are petroleum catalysts, or only separated
from them by a very thin film of adsorbed water. Relative permeability to
water would be close to zero while oil was migrating, and almost all the
compaction energy would be devoted to expulsion of oil.
If this is an important process in primary migration, it suggests that solu-
tion would be restricted to early and late stages.
There is no great difficulty in the expulsion of oil or gas as a separate phase
from the source rock sensu lato into the carrier bed because, as Hubbert
(1953, p. 1979) pointed out, the greater capillary pressure in the finer-grained
material is sufficient on its own to effect expulsion once the continuous
phase reaches the sedimentary interface (Fig. 10-2).
The main difficulty lies, however, between the source rock sensu strict0
and the carrier bed because it is unlikely that true source rock is contiguous
with the carrier bed (Chapman, 1974). The reason for this lies in facies con-
siderations. An alternating sequence of sands and mudstones is essentially
diachronous, sand accumulating in some areas contemporaneously with muds

Fig. 10-2. Capillary pressure tends to expel a continuous oil phase from mudstone once
part of its interface reaches the sandstone. (After Hubbert, 1953, p. 1978, fig, 14.)