Page 214 - Petroleum Geology

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PO P
h, = ___ h -2. (9.5)
P -Po P -Po
Thus, if there is enough data to map h and z, we can map h,. We shall return
to this equation.
Clearly, the projection of the direction of water movement on a horizontal
surface is the same as that for oil movement ,while the oil is migrating with-
out constraint from the cap rock. Both oil and water equipotential surfaces
are inclined in the direction of water motion, but by different amounts. If
the water flow is directly downdip, there is some critical dip that equals the
“dip” of the oil equipotential surfaces and is normal to the migration path of
the oil through the water. A small volume of oil under a cap rock at this critical
dip would not move.
Huljbert (1953, pp. 1986-1987) has shown that this critical dip, Be, is
given by:

P dh
tan6, = ~ -
P-Po dx
where dh/dx is the slope of the water’s potentiometric surface as given, for
example, by the contour interval divided by the distance separating two con-
tours on the potentiometric surface (not the hydraulic gradient, in which the
length is measured along the aquifer). The coefficient p /(p - p o) indicates
that the heavier the oil, the steeper the slope of the oil’s equipotential sur-
faces and the critical dip. If the oil’s density equals that of the water, oil can
only accumulate by capillary effects because there is no gravitational effect
that will accumulate it.
Gas has a more nearly vertical migration path across a carrier bed by virtue
of its smaller density relative to oil and water. The consequences of these
effects separately on oil and on gas may well not be the same as the combined
effect. If gas alone would take the path G in Fig. 9-7, and oil alone the path
0, it is most unlikely that these would be the paths if both were migrating
simultaneously. Some intermediate path depending on saturations would be
more likely.
Once restrained by the upper surface of the carrier bed, further migration
follows paths of local minimum potential. These will not in general be in-
dicated by a detailed contour map of the interface between the cap rock and
the carrier bed; but if the contours were drawn relative, not to sea level but
to a surface at the critical dip, such a map would indicate the possible paths
of migration.
To get an idea of the magnitudes of the critical dip or slope 6,, consider
a potentiometric surface with a slope of which is about the steepest
regional slope of the Great Artesian basin of Australia. This is a slope of
about 3 min of arc. The mass density of most crude oils falls in the range
750400 kg m- 3, so the amplifying factor ranges from four to ten, and the