Page 247 - Petroleum Geology

There is a lack of substantiating literature on subsurface microbial processes,
as distinct from assertions that they take place based on comparisons between
details of crude oil compositions and those resulting from laboratory degrada-
tion by bacteria.
The facts appear to be these (see Davis, 1967): Microbes are 0.2-2 pm in
smallest dimension, and they require oxygen, mineral nutrients and water for
their existence, generally attaching themselves to a solid surface. They repro-
duce by cell division, and leave cell walls when they die. Their motility is
random, with a bias towards lower temperatures and pressures. They can live
at temperatures up to 75°C at least, and some survive pressures of 170 MPa
(25,000 psi) corresponding to hydrostatic water pressures at depths to about
3.5 km. Aerobic bacteria at the surface can oxidize crude oil (to alcohols.
ketones and acids). They attack n-paraffins in the range CI6-Cz5 first, and
may later attack the higher range and other components. They convert low
molecular weight hydrocarbons to higher molecular weight hydrocarbons,
creating long-chain insoluble waxes around C40. Anaerobic bacteria are not
known to utilize hydrocarbons with this result, but Bailey et al. (1973b)
have suggested that they do so more slowly than aerobes. Cell walls have not
been found in oil reservoirs.
It is not known how long a bacterium lives, but Davis (1967, p. 51) reports
that of the bacteria found at a depth of 2m in the sediment of lake Biserovo,
Russia, 70% were dead. This suggests a negligible life-span from a geological
point of view. It is not the life span of the individual but the true life span of
a viable community of bacteria that is important.
Bacteria were found in artesian water from the Carrizo Formation (Eocene)
of south Texas from a depth of 1280 m (4200 ft) at 58°C (136°F) (Davis,
1967, p. 167). This well is 45 km from the outcrop of the aquifer, and the
velocity of the water flow is estimated to be between 15 and 30 m/yr. If
bacteria were introduced into the sand in its intake area, and were transport-
ed at the maximum water-flow rate, the community would have to survive at
least 1500 years to be produced from this, the deepest well. It is curious,
though, that. no bacteria were found in a nearby well.
The motility of bacteria is random, of the order of centimetres a year,
with a tendency to follow a decreasing temperature gradient and a decreasing
pressure gradient. Because passage into the subsurface is against these tenden-
cies, and because their tendency to cling to solid interfaces and their size
would also be retarding influences, it is unlikely that a bacteria colony would
reach the deepest Carrizo well in less than 5000 years. During this time, an
adequate supply of nutrients would have to be maintained.
With reservations, therefore, concerning the longevity of microbe commu-
nities, there is no known parameter that would prevent their existence at
depths of one or two kilometres, given the right conditions. Movement of
ground water from the surface to the migrating or accumulating crude oil there-
fore seems to be an essential condition for biodegradation of the crude oil.