Page 221 - Petroleum Geology

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Discussion of the role of faults in petroleum migration will be postponed
to Chapter 11, but the same principles apply to such migration across faults
from one block to another: the pressure in the oil at the fault must exceed
the injection pressure required to pass the fault. There is evidence that this
can happen; but the evidence of fault traps is that faults can seal significant
quantities of oil behind them.
It cannot yet be claimed that use of the hydrodynamic approach hasled
to significant oil discoveries that would not have been found taking the usual
hydrostatic approach; but, by the same token, there may be large oil fields
waiting to be found in positions that would not be credible under hydrostatic
assumptions. Where sufficient data are available, such maps should be drawn
because the possible rewards far exceed the labour involved. Furthermore,
such maps put some constraints on the directions in which the source rocks
of accumulations may lie because they can only lie “upstream” of the pos-
sible migration paths.
Of course, such statements are more easily made than justified. They as-
sume that the hydrodynamics of the area has not changed since oil genera-
tion, and that the structural relief was much the same. Each area must be
judged on its own merits, and it can commonly be assumed that the structural
relief was no greater at the time of oil generation and migration.

Rates of secondary migration

We do not have reliable field data from which the rates of migration to in-
dividual fields can be determined, but a few sums indicate the order of mag-
nitude required.
A giant oil field with 500 million barrels of recoverable oil has about 1.5 X
lo9 bbl of oil in place - about 250 X lo6 m3. There are Pliocene/Miocene
giants (Halbouty et al., 1970, p. 504, table I), so such quantities cannot take
longer than 10-15 m.y. to accumulate. Let us assume that it took one million
years to accumulate 250 X lo6 m3, that is, 250 m3/yr. This is rather less
than 0.7 m3/day, so a single migration path of 1 m2 cross-sectional area
would require a flow of 0.7 m/day, or about 10 pm/s. Migration over a distance
of 10 km would involve a transit time of about 40 yrs.
Consider a gas-free crude oil with kinematic viscosity (v) equal to 6 X loe6
m2/s flowing in a carrier bed in which the effective permeability to oil is
100 md (100 pm’) and the effective porosity is 20%. Assuming that the critical
vertical dimension of the oil is exceeded by only one metre, the gradient of
total head can be estimated from eq. 9.3 to be about 0.2 in static water for
po/p = 0.8. So, from Darcy’s law:

40 = k, (g/v)Ah/l
= 30 pm/s.