Page 202 - Petroleum Geology

CHAPTER 9

ORIGIN AND MIGRATION OF PETROLEUM:
GEOLOGICAL AND PHYSICAL ASPECTS

SUMMARY
(1) Petroleum is a product of the diagenesis of fundamental organic com-
pounds in organic matter that accumulated with fine-grained sediment in a
low-energy environment deficient in oxygen.
(2) The energy of petroleum in its source rock is greater than that it will
have when it reaches the accumulation. Each path of migration from source
to accumulation is a path of continuously decreasing energy during migra-
tion. The energy is derived largely from the compaction of the petroleum
source rock - usually a mudstone, but some may be fine-grained carbonates.
(3) Petroleum exists as a separate phase by the end of primary migration,
from source to a permeable carrier bed. Capillary forces then retard migra-
tion, and prevent it when the energy of the migrating petroleum is insuffi-
cient.
(4) Primary migration may be stratigraphically upwards or downwards,
depending on the direction of decreasing energy. Both normally occur in a
compacting mudstone that is intercalated between sandstones or other per-
meable units. The surface dividing upward and downward migration within the
mudstone is a perfect physical and chemical barrier to migration.
(5) Secondary migration is lateral within porous and permeable rock units,
generally towards the land of the time. Petroleum accumulates when it arrives
in a position in which there is insufficient energy to move it further.

INTRODUCTION

The origin and migration of petroleum have been topics of interest for at
least a century, since the early days of the industry, but they are still poorly
understood. There are several reasons for this, but the main one is that the
processes are too slow to model in the laboratory with confidence, and scaling
the model introduces doubts regarding the chemical aspects. Migration through
a mudstone is a very slow process because of the low permeability. We can
speed this up by taking a material of greater permeability - but the material,
and so the chemical composition of it, must be changed. Also, we can accele-
rate the chemical reactions by heating, but it is not certain that in so doing
we get the same reaction as that that would have taken place at a lower tem-