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laboratory a biodegraded crude oil does not mature to normal crude oil, and
is still recognizable). So some change in the water flow must have occurred
during the accumulation of the regressive sequence to bring bacteria into the
system during secondary migration of the biodegraded oil. This is an enuiron-
mental matter related to the proximity of the regressing land, and may there-
fore involve a change in source material.
The Bell Creek field in Montana, U.S.A., studied by Winters and Williams
(1969), is both interesting and perplexing. There are several sandstone res-
ervoirs in the Cretaceous Muddy Formation, and the petroleum source rocks
are considered to be in the enveloping mudstones. The average reservoir depth
is about 1370 m, and their temperatures are in the range 35-41°C, so condi-
tions are well within the tolerances of microbes. Within a single reservoir,
three types of crude oil are found: unaltered crudes, crudes with all normal
alkanes missing, and crudes with normal alkanes missing up to about CIS.
One curious feature is that the zone in which all normal alkanes are missing
separates the zone of unaltered crudes in the south-west from those with up
to CI8 missing in the north-east.
Winters and Williams reported direct evidence of microbial action in the
Bell Creek field. They found that water produced with the oil of some wells,
when cultured, produced aerobic micro-organisms that consumed both n-
butane and n-hexadecade, while others did not. These wells matched the
data of the chromatograms. While this evidence appears to indicate alteration
within the accumulations, there may be another explanation because, as Win-
ters and Williams noted, the microbial activity must take place at the oil/water
interface.
There is another particularly perplexing problem. There are chromatograms
of mudstone extracts that show all the characteristics of biodegradation al-
though the sample came from a position some metres away from the nearest
sand*. It seems physically impossible for bacteria to enter a compacting mud-
stone against the water currents of compaction, highly unlikely that a bacteria
community could survive such an environment for millions of years, and
chemically implausible that biodegradation of the source material led to re-
sults so similar to biodegradation of the products.
Taking all these things into account, the accessibility of bacteria to crude
oil during secondary migration seems to require that biodegraded crude oil
was formed early, and may have had an inherently different composition
from crudes that were not biodegraded in the same field.

* I have not found a published example, but have been shown two in the research labora-
tories of two oil companies.