Page 208 - Petroleum Geology

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lysts, and it is unlikely that a significant film of water separates the two effec-
tively in the mudstone during primary migration.
Indirect evidence of petroleum as a separate phase during primary migra-
tion is given by abnormal pressures and abnormally high resistivities in parts
of mudstones in some areas (such -as the Bakken Shale of the Williston basin,
U.S.A., reported by Meissner, 1978).
We visualize primary migration ending and secondary migration beginning
over a large area of the interface between the mudstone that contains the
petroleum source rock and the carrier bed.

Secondary migration

In spite of the fact that no secondary migration paths have been recognized
with confidence and reported, the conceptual difficulties are not as great as
those with primary migration. In the first place, we have petroleum seepages
in many parts of the world; and secondly, when we put an oil well onto
production, oil demonstrably flows through the reservoir to the well. The
main difficulty concerns the role of water movement, so we shall begin with
a discussion of secondary migration in an aquifer in which the water is at rest.
We shall also simplify the discussion by assuming oil migration in an isotopic,
homogeneous, granular, water-wet carrier bed; and regard oil as incompressible,
without gas in solution. The principles apply to gas.
Movement in the final stages of primary migration seems to require the
petroleum to be in a continuous phase through the pore space, so it is inferred
that it remains as a continuous phase initially (at least) in the carrier bed.
Considering upward migration from the mudstone interface first, there is
some critical vertical dimension to the oil that will enable it to move upwards
under the force of gravity (buoyancy) against the resisting forces, chiefly
capillarity. The upward pressure due to buoyancy increases relative to the
ambient water pressure by (p - po)g per unit of elevation above the carrier
bed interface from which the oil is emerging. Hobson (1954, p. 73) and others
have estimated this critical vertical dimension to be of the order of a few
metres at most in typical carrier and reservoir rocks.
We can measure the capillary displacement pressure required to move the
oil front from one set of pores to the next, so the critical dimension can be
estimated from:

Ah0 = Pi/@ -PO) g- (9.1)
If we take 10 kPa (1.5 psi) as being a representative maximum carrier bed dis-
placement pressure, and 200 kg m- as a representative difference of oil and
water mass densities, then:
aho = 104/(ZO0 X 9.8) = 5 m.
The oil being less dense than the water, the macroscopic water/oil interface