Page 204 - Petroleum Geology

181

HYDRODYNAMIC ASPECTS OF MIGRATION

We have already seen in earlier chapters that fluids, if they move, move
from positions of greater energy or potential to positions of lesser energy
or potential. Movement involves loss of energy. It is therefore axiomatic that:
(1) the energy of petroleum in its source rock is greater than that it will have
when it reaches the accumulation; and (2) each path of petroleum migration
from source to accumulation is one of continuously decreasing energy during
migration.
These axioms are independent of any hypothesis concerning the nature
and state of petroleum during migration. If any part of the total migration
path involves transport in solution in water, then the paths of water migrc-
tion are, of course, those of petroleum migration and the same constraints
apply to both water and the petroleum. In any part of the migration that
takes place as a separate phase from water, the path of petroleum migration
may diverge from that of the associated water, and capillary forces may act
in such a way that migration of one phase may be prevented or retarded by
the other. But no situation can arise in which either phase can move to a
position in which it will have greater energy.
The commonest source rock is considered to be a mudstone. It must be
recognized that a mudstone will rarely be entirely source rock: the source
will naturally be part of a mudstone. If the physiographic environment of a
sand was not favourable for the preservation of organic matter, it is unlikely
that the contiguous mudstone facies was favourable for its preservation be-
cause that would involve the coincidence of physical and chemical criteria.
The concept of diachronous rock units suggests that at any moment of time
during the accumulation of a sequence of mudstones and sandstone, for
example, some areas accumulate mud and others sand; and part of the area
of mud accumulation may also accumulate and preserve a significant organic
content (Fig. 9-1). There will then be a source facies within the mudstone
facies that will be both laterally and vertically discontinuous.
As this mudstone subsides, and more sediment accumulates on top of it, it

Fig. 9-1. Source rock is a facies that is not contiguous with the sandstone facies. (Sand-
stone, dots; source mudstones, dashes.) View as map or section.