Page 234 - Petroleum Geology

CHAPTER 10

ORIGIN AND MIGRATION OF PETROLEUM:
GEOLOGICAL AND GEOCHEMICAL ASPECTS

SUMMARY

(1) Petroleum is primarily a product of the diagenesis of fundamental or-
ganic compounds contained in organic matter that accumulated with fine-
grained sediment in a low-energy environment deficient in oxygen. The dia-
genesis takes place during burial, under the influences of heat, time and pres-
sure - probably in the presence of clay catalysts. Early diagenesis removes
the more volatile and soluble components, leaving kerogen. Some kerogens
are considered to be the important source material for petroleum.
(2) Mudstones are important source rocks of petroleum, but not all mud-
stones are source rocks. Thicker mudstones are probably more important
than thinner because greater volumes of water must be expelled from them
eventually. Concentrations of residual organic matter may be less in thicker
mudstones than in thinner mudstones, but the total quantity must be large
for large volumes of petroleum to be generated.
(3) Petroleum is probably generated as a separate phase in the source rock,
and migrates as a separate phase in water to the accumulation. During migra-
tion, oil may be altered by removal of the more soluble components (water
washing); and bacterial degradation may remove normal-alkanes in the range
C16-C25, then progressively those with higher carbon numbers.
(4) High-wax crude oils seem to be genetically related to the environment
in which the sediments accumulated. They are associated with sandstone/
mudstone sequences, both transgressive and regressive, of Devonian age and
younger. Not all petroleum provinces have waxy crude oils, and some impor-
tant carbonate provinces appear to have no waxy crude oils.

INTRODUCTION

The advances in petroleum geochemistry during the last few decades have
been spectacular to the point that many regard the basic hypotheses as facts.
This may be so; but the topic of origin and primary migration is so complex
and contains so many apparent anomalies and contradictions that there is
still great danger of error. As before, we shall accept without further scrutiny
that commercial accumulations of oil and gas have their origin in organic
matter that accumulated with fine-grained sediment in a low-energy environ-