Page 248 - Petroleum Geology

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Gas chromatograms of crude oil alkanes (Fig. 10-4) indicate the full range
of effects observed in laboratory experiments, from the presence of the full
range of n-alkanes, to suppression,in the range C16-C25, to almost total sup-
pression of the entire range. It is inferred that these changes are due to the
activity of aerobic bacteria. The questions arise, how, where, and when?
The widely accepted view is that biodegradation and water washing occur
in the accumulation after the crude oil has accumulated, as a result of water
flowing through it. Hunt (1979, pp. 382-390) discusses the topicin his chap-
ter on petroleum in the reservoir, and Tissot and Welte (1978, p. 417 and
419) also regard them as alterations that take place in the accumulation.
The evidence of petroleum reservoirs (pp. 156-161) indicates that water

,irflE I i
[BELLSHILL LAKE pHpR
POOLED
01 L i

t - i,

EDGERTON
HEAVY
01 L

FLAT LAKE PH I I
, PR
HEAVY
OIL

PELICAN
I PELICAN PHYTANE
: PRISTANE
HEAVY
01 L /

Fig, 10-4. Progressive biodegradation of crude oil in some fields in western Canada, from
unaltered crude in Bellshill Lake field to severely altered crude in Pelican field. The gas
chromatograms show progressive reduction in n-alkanes and, in Pelican, the elimination
of pristane and phytane (after Deroo et al., 1974).