Under certain conditions, living microorganisms (primarily
bacteria, but also yeasts, molds, and filamentous fungi) can alter
and/or metabolize various classes of compounds present in oil, a
set of processes collectively called oil biodegradation.
Biodegradation affects oil spills and surface seeps. Furthermore,
as first noted more than 30 years ago, biodegradation also alters
subsurface oil accumulations (e.g., Winters and Williams, 1969).
Shallow oil accumulations (< 80 oC reservoir
temperature) are commonly found to be biodegraded to some degree.
In fact, the vast majority of the world's petroleum is severely
biodegraded oil in the shallow, super-giant Orinoco and Athabaska
tar sands in Venezuela and Canada, respectively (e.g., Demaison,
1977). Smaller, but still giant, accumulations of biodegraded oil
occur elsewhere throughout the world (Roadifer, 1987). This article
- effects of oil biodegradation on oil fluid properties (API
gravity and viscosity),
- effects of oil biodegradation on oil geochemistry,
- conditions necessary for oil biodegradation, and
- current estimates of the rates at which these processes destroy
oil under surface and subsurface conditions.
Effects of Oil Biodegradation
Biodegradation gradually destroys oil spills and oil seeps by
the sequential metabolism of various classes of compounds present
in the oil (e.g., Bence et al., 1996). When biodegradation occurs
in an oil reservoir, the process dramatically affects the fluid
properties (e.g., Miiller et al., 1987) and hence the value and
producibility of an oil accumulation. Specifically, oil
- raises oil viscosity (which reduces oil producibility)
- reduces oil API gravity (which reduces the value of the
- increases the asphaltene content (relative to the saturated and
aromatic hydrocarbon content)
- increases the concentration of certain metals
- increases the sulfur content
- increases oil acidity
- adds compounds such as carboxylic acids and phenols
For example, in a set of genetically related oils from Oklahoma,
Miiller et al. (1987) found the following changes in oil properties
with increasing levels of biodegradation:
|Moderately Biodegraded Oil
|Moderately Biodegraded Oil
Vertical and lateral variations in oil properties (e.g., API
gravity and viscosity) caused by spatial variations in the extent
of oil biodegradation can be mapped throughout a field using a
variety of geochemical tools. During field development, such
techniques allow the targeting of "sweet-spots" (areas of less
degraded oil) within an accumulation that has been affected by
biodegradation (see Predicting
Fluid Properties). Such vertical and lateral
variations in the extent of biodegradation typical fall into two
- Variations due to distance from the oil-water contact. Because
biodegradation occurs at or near the oil-water contact (e.g., Head
et al., 2003), biodegraded oil columns commonly are compositionally
graded, with the most biodegraded oil occurring near the oil-water
- Variations due to the "pulsed" nature of reservoir filling.
Since the time scale of biodegradation (discussed below) is often
similar to the time scale of reservoir charging (e.g., Larter et
al., 2003), a biodegraded oil column may consist of a mix of oil
that arrived first in the reservoir (here called the "primary
charge") and subsequent pulses of oil that arrived later (here
called the "secondary charges" to the accumulation). The primary
charge may be more biodegraded than the secondary charge, since the
primary charge has been subjected to in-reservoir biodegradation
for a longer period of time. Therefore, depending on the migration
pathways into the reservoir, vertical variations in the relative
abundance of "primary" vs. "secondary" charges may cause vertical
variations in the oil fluid properties (e.g., API gravity and
During oil biodegradation, oil fluid properties change because
different classes of compounds in petroleum have different
susceptibilities to biodegradation (e.g., Goodwin et al., 1983).
The early stages of oil biodegradation are characterized by the
loss of n-paraffins (n-alkanes or normal alkanes) followed by loss
of acyclic isoprenoids (e.g., norpristane, pristine, phytane,
etc.). Compared with those compound groups, other compound classes
(e.g., highly branched and cyclic saturated hydrocarbons as well as
aromatic compounds) are more resistant to biodegradation. However,
even those more-resistant compound classes are eventually destroyed
as biodegradation proceeds. Larter et al. (2005) estimates that
heavily degraded oils have typically lost on the order of 50% of
Peters and Moldowan (1993) proposed a 1-10 scale on which the
extent of biodegradation of an oil can be ranked based on the
analysis of the oil geochemistry (e.g., using the presence or
absence of various biomarkers that have different susceptibilities
to biodegradation, with "1" indicating very early degradation
(partial loss of n-paraffins) and "10" indicating severely degraded
The early stages of oil biodegradation (loss of n-paraffins
followed by loss of acyclic isoprenoids) can be readily detected by
gas chromatography (GC) analysis of the oil. However, in heavily
biodegraded oils, GC analysis alone cannot distinguish differences
in biodegradation due to interference of the unresolved complex
mixture (UCM or "hump") that dominates the GC traces of heavily
degraded oils. Among such oils, differences in the extent of
biodegradation can be assessed using gas chromatography-mass
spectrometry (GC-MS) to quantify the concentrations of biomarkers
with differing resistances to biodegradation. The UCM present on
the GC trace of a heavily degraded oil does not affect this GC-MS
Conditions Under Which Biodegradation Can Occur
Oil biodegradation by bacteria can occur under both oxic and
anoxic conditions (e.g., Zengler et al., 1999), albeit by the
action of different consortia of organisms. In the subsurface, oil
biodegradation occurs primarily under anoxic conditions, mediated
by sulfate reducing bacteria in cases where dissolved sulfate is
present (e.g., Holba et al., 1996), or methanogenic bacteria in
cases where dissolved sulfate is low (e.g., Later et al., 2006,
Bennett et al, 1993).
Although subsurface oil biodegradation does NOT require oxygen,
it does require certain essential nutrients (e.g., nitrogen,
phosphorus, potassium), which can be provided by
dissolution/alteration of minerals in the water leg (Larter, et
Empirically, it has been noted that biodegraded oil
accumulations occur in reservoirs that are at temperatures less
than 80oC (e.g., Connan, 1984; Barnard and Bastow,
1991). At higher temperatures, it appears that many of the
microorganisms involved in subsurface oil biodegradation cannot
exist. However, not all oil accumulations at temperatures less than
80oC are biodegraded. Wilhelms et al. (2001a, 2001b)
proposed an explanation for this observation: those authors
suggested that if an oil reservoir has been heated to more than
80oC at any point since its deposition, then, even if
uplift later reduces the temperature to below 80oC, the
"paleopasteurization" or "sterilization" of the reservoir that
occurred at the higher temperature will have killed the organisms
needed for oil biodegradation to occur after the basin uplift.
Therefore, oil reservoirs that have experienced significant uplift
may contain non-degraded oil, despite the currently shallow depth
and low temperature of the reservoir. Apparently, "recolonization"
of such "sterilized" reservoirs by bacteria is typically unable to
occur. Wilhelms et al. (2001a, 2001b) supported this model with a
variety of case studies of uplifted or "inverted" basins from the
USA, North Africa, the Barents Sea, and the Wessex basins.
Because subsurface oil biodegradation does NOT require oxygen,
and can occur at temperatures up to 80oC, in-reservoir
biodegradation can occur even at many thousands of feet below the
surface (e.g., Parkes et al., 1994), as long as the geothermal
gradient is sufficiently low, and nutrients (e.g., nitrogen,
phosphorus, potassium) are available through the water leg.
Rates of Oil Biodegradation
Larter and Aplin (2003) and Larter et al. (2003) suggested rates
of 10-6 to 10-7 /year for anaerobic
in-reservoir oil degradation at 60oC, and
10-2 to 10-1 /year for anaerobic oil
degradation at the earth's surface.
The rate of petroleum biodegradation in the subsurface appears
to be limited by available nutrients and temperature and NOT by the
carbon source (e.g., Larter et al., 2001, 2003, 2006). Hence, the
size of the water leg (which impacts nutrient delivery) impacts
degradation rates. Larter et al. (2003) calculated that the:
"actual fluxes of hydrocarbons being destroyed in oilfields
around 40-70 are around 10-4 kg/m2/year of
oil-water contact area with reservoir temperature controlling the
actual degradation flux value".
Larter et al. (2006) proposed biodegradation rates in the range
of 10-3 to 10-4kg petroleum per m2
of oil-water contact (OWC) per year for fresh petroleum in clastic
reservoirs, with the highest values occurring at temperatures
It should be remembered that an oil accumulation may be affected
by multiple oil alteration processes in addition to biodegradation
(e.g., Milner et al., 1977). For example, water washing,
multi-stage oil charging, and evaporative fractionation can all
affect the composition and fluid properties of an oil accumulation
that is undergoing biodegradation. Oil geochemistry analyses can be
used to decipher which combination of these alteration processes
has affected the physical properties of an oil.
Understanding oil alteration processes, such as those listed
above, provide the key to predicting spatial variations in fluid
properties (API gravity and viscosity) within an oil field or
within a series of stacked petroleum-bearing intervals (e.g.,
McCaffrey, 1996; Smalley et al., 1996; Koopmans et al., 2002). For
information on how oil geochemistry (oil fingerprinting) is used
during field development to predict vertical and lateral variations
in fluid properties, see our article on Predicting
Fluid Properties. Alternatively, contact
OilTracers LLC by e-mail or by
telephone at 214-584-9169.
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