Page 246 - Petroleum Geology

221

are hard to produce). Hedberg pointed out that high-wax crudes are not ubi-
quitous; large and important petroleum provinces such as the Middle East,
Mexico and West Texas - all mainly carbonate provinces - did not appear to
have high-wax crude oils. To this observation we add that there seems to be
no preference for regressive or transgressive sequences of mudstonesand-
stone lithologies, but in regressive sequences, the heaviest, shallowest crude
oils are non-waxy. These heavy oils are typically associated with fresh to
brackish formation waters, and their properties have been attributed to water-
washing and bacterial degradation in the reservoir.
The causal association between terrestrial contribution and high wax con-
tent of crude oils seems established beyond reasonable doubt. We therefore
conclude that when high-wax crude oils are found in sandstones that accu-
mulated in continental, paralic or near-shore marine environments, their source
may be stratigraphically associated with the accumulation, and, for the hydro-
dynamic considerations of mudstone-sandstone sequences discussed on pp.
61-62, we infer that they are intimately associated, with short primary migra-
tion paths.

ALTERATION OF CRUDE OIL AFTER GENERATION : WATER-WASHING AND
BIODEGRADATION

There are no great difficulties in understanding secondary migration as a
separate phase in water, and we shall assume that this is the state of secondary
migration and address ourselves to the matter of possible transformations of
crude oil during its passage through the carrier bed, and within the accumu-
lation. As always, catalytic transformations are possible when the water satu-
ration is sufficiently small, but the most important processes for the geologist
are water washing and biodegradation.
Crude oil flowing past water, and water flowing past crude oil, can result
in the removal of some of the more water-soluble components. In general,
hydrocarbons of low molecular weight are more soluble than those of higher
molecular weight, so the general effect of water washing is to increase the
density, decrease the API gravity, of the crude oil. According to Hunt (1979,
p. 390), hydrocarbons up to CIS or even higher can be removed. In the ex-
treme, an asphaltic, heavy, unproducible oil is left.
A very similar, perhaps indistinguishable effect is due to microbial action.
Both seem to take place under similar geological conditions, so the two pro-
cesses are commonly taken together.
The role of microbes, of which bacteria are probably the most important,
in the transformation of crude oil is almost certainly important, but difficult
to assess because of the lack of reliable data. This is due mainly to the enor-
mous difficulties of avoiding contamination during the taking of subsurface
samples. Bacteria are normally present in surface waters and drilling fluids.