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cannot flow upwards through an oil accumulation, and so cannot carry bac-
teria from the aquifer into the accumulated oil. Water washing is limited to
the period of actual accumulation, prior to the acquisition of its irreducible
water saturation, while water is being displaced by oil in the trap. Since the
latter applies to all oil accumulations, variations of effect would be limited
to variations of water composition and temperature, and their capacity to
take hydrocarbons into solution. Bacterial degradation of a complete accu-
mulation after accumulation (or a significant thickness of an accumulation
after accumulation) could only result from bacteria trapped in the pendular
rings at grain contacts. Hydraulic continuity does not exist here: bacteria
could not move beyond the pendular ring in which they were trapped, and
the requirements of oxygen and nutrients for a community would not be satis-
f ied .
If conditions favourable for biodegradation exist at the time of oil accu-
mulation, they also exist during at least the later stages of secondary migra-
tion. Indeed, the passage of an oil stream through water in which there are
aerobic bacteria is clearly the most favourable condition for both bacterial
degradation and water washing. The movement of oil through the water
would expose it to larger bacterial communities, the cell walls of which would
remain in the carrier bed (if not transformed). Time is not a constraint be-
cause Jobson et al. (1972) and Bailey et al. (1973b) found in the laboratory
that dramatic changes took place in days. Long secondary migration paths
would enhance water washing.
It is possible that microbes could penetrate an accumulation for a short
distance towards the level of irreducible water saturation, and so degrade the
oil near the oil/water contact. The presence of such “tar mats” is evidence
that the water quality has changed since the oil accumulated, otherwise the
whole accumulation would have been degraded.
Likewise, accumulations that result from asphalt seals near the surface
(and there are some very large ones, such as Lagunillas, Venezuela, and Co-
alinga, California) are probably due to the arrival of migrating oil in an en-
vironment favourable to degradation. Migration ceases when degradation im-
mobilizes the oil, leaving a gradation to unaltered crude oils down-dip.
In a regressive sequence with multiple reservoirs and sources, the shallowest,
heavy oil sometimes shows evidence of biodegradation (see, for example,
Hunt, 1979, pp. 388-391). Paraffinic, waxy oils are found in the deeper res-
ervoirs. Hedberg’s (1968) tabulation of crude oils from which he inferred a
causal association between high-wax crude oils and terrigenous source materials
shows a positive association between wax content and API gravity -the lighter
the oil, the more wax - with appreciable wax content, in general, in crudes
of greater than 30-35”API (s.g., 0.88-0.85). This seems to lend support to
the hypothesis of biodegradation of the heavier crudes at shallow depth,
with the removal of paraffin waxes. But the deeper reservoirs were once
shallow, and were not degraded (Connan et al., 1975, showed that in the