The
authors used three ways in order to analyze the biodegradation rate of the
petroleum in the subsurface reservoirs, which includes the whole oil column
minimum rate assessments, biodegradation rates from oilfield compositional in biodegraded
oil columns, and the examination of mixed degraded and non-degraded fresh oil
circumstances.

           The first method is done by assuming
the biodegradation of the oil is the same along the oil column of zero order
process with respect to the hydrocarbons. The average minimum rate constant can
be obtained through this approach, even though the field-wide rates are lower
compared to the actual rates present in the volume of oil that involve in
degradation.

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           They also analyze the compositional
gradient in hydrocarbons in biodegraded oil columns, where the maximum
degradation rate is regulated by the diffusion rate of the hydrocarbons,
primarily the alkanes to the oil-water contact. The constraint to this method
is the uncertainty on the starting point of the biodegradation, the initial
concentration of the alkanes and the status of the hydrocarbon gradient
relative to the steady state.

           Then, they evaluate the mixed oils as
the relative time scales based on n-alkane degradation and fresh oil charging
can be obtained. This is due to the fact that most oils have more than one
level of degradation as the oils must have formed in the source rock and
migrated to the reservoir rocks, which can cause the mixture of the fresh and
biodegraded oil.

 

·     
The
controls on the composition of biodegraded oils in the deep subsurface – Part
3. The impact of microorganism distribution on petroleum geochemical gradients
in biodegraded petroleum reservoirs

           A 24.5m thick oil column that from a
Canadian heavy oil reservoir is examined through geological, geochemical, and
microbiological analysis in order to evaluate the biodegradation gradients. The
core includes an 8.75m oil-water transition zone (OWTZ), where the oil at this
zone is expected to have the highest intensity of degradation. The authors
conducted this experiment to find out the impact of microorganism distribution
on the degradation rate.

           The viscosity of the dead oil is reported
to be increasing from the top of the oil extending into the OWTZ, exceeding
over 10 McP from only 0.05 McP at 20°C.

The biodegradation of the oil in the column shows a distinct pattern as it
starts with the degradation of alkane and continues with the more resistant
components, but they still occur at a different rate. The assessment using PCR
demonstrates that the maximum cell abundance is located right at the top of the
OWTZ as seen in Figure 3.

           However, this result is obtained from
the quantification of the amount of the bacteria in the same intervals after
the core samples were frozen. Therefore, this is still not the most reliable
data but it is not possible to directly measure the abundance of microbial
populations in the petroleum reservoir. It is confirmed that net rate of
biodegradation is not influenced by the supply of the electron donors but the
authors are still not sure if microbial distribution, nutrient availability or
other undiscovered factors are responsible in this matter.

Most of the papers, including the “Biological activity in the deep subsurface and the origin of heavy oil”, “Crude-oil biodegradation via methanogenesis in subsurface petroleum reservoirs” and “Anaerobic hydrocarbon biodegradation in deep subsurface oil reservoirs” have the same knowledge gap on the actual processes that represent anaerobic biodegradation in deep subsurface petroleum reservoirs. It is an unavoidable limitation because there is no advanced technique that could let us to directly observe all of the processes ongoing over 10m in the ground.  The paper by Bennett et al. is the most recent among all and they used better methods, which reduce the gap in the proximity to the actual processes. This is because the experiments published in the other papers only did analysis on the biodegraded oil sample, while this one is actually done on the drilled core of oil column. The only problem is that they only used an oil column from one place, so the result might be different elsewhere, as a different type of microbial group, would available at the other reservoir and they might behave disparately.  Nevertheless, they succeeded in proving the fact that the biodegradation of oil is the most intense at the oil-water transition zone and they also found out the approximate location by measuring the microbial distribution, the viscosity of the dead oil and the pattern of the component concentrations down through the column. They also realized that the random distribution of microorganisms in natural petroleum reservoirs will cause the process of accelerating the conversion of oil to methane microbially for commercial purpose to be affected to a certain extent.  On the other hand, Jones et al. did a good job at trying to find a possible process that occurs in a petroleum reservoir, which led to the assurance of the hypothesis of the anaerobic biodegradation in the deep subsurface reservoir. They discovered that methanogenesis is the process that fits with the characteristic sequential removal of compound classes as observed in biodegraded petroleum reservoirs, which definitely helps in supporting the hypothesis. In a nutshell, all of the papers have their own series of methods that analyze various components, which contributed to the understanding of the anaerobic biodegradation of oil in petroleum reservoirs, even though there are a few knowledge gaps identified. It is observed that the degradation of the oil in the reservoirs is the most severe in the oil-water transition zone, which corresponds to the area that the microorganisms are distributed the most. However, the effect of nutrients and water availability to the net rates of biodegradation in deep petroleum reservoirs is still undiscovered. It is actually intuitive to say that the increase of water and nutrients in the petroleum reservoirs will increase the net rate of biodegradation.  Laboratory experiments can be done in order to vary the availability of water and nutrients at the oil-water transition zone (OWTZ). The methods used by Bennett et al. are the most practical because multiple aspects including geochemical, geological and microbiological analysis are included in elaborating the effect of microorganism distribution on biodegradation gradients in petroleum reservoirs. More samples should be obtained from various cores drilled in different petroleum reservoirs at different places in order to see the effect on a global scale.  Then, the hydrocarbons at the OWTZ of different oil column can be placed into the microcosm in anoxic condition, together with the microorganisms that are responsible for the biodegradation, which is a method used by Jones et al. Separate microcosms must be set up for the addition of nutrients, water, and nutrients plus water. One microcosm without the addition of any of those is going to be the control flask.  After a period of time, all of the microcosms must be analyzed for the quantification of 16S rRNA genes by qPCR, which is the same approach utilized by Bennett et al. This method will allow us to measure the abundance of microbial populations when experimented with the inclusion of water and nutrients at a different amount. It is anticipated that the increase in nutrients and water available to the microorganisms will result in the increase of microbial population. As a result, the rate of oil biodegradation will increase and lower the quality of the oil. Therefore, it is important to identify the effect of nutrients and water availability to the oil column as the economic consequences can be minimized, where the rate of biodegradation in the petroleum reservoirs is identified. This can lead to the invention of the mechanism that can reduce the rate of oil biodegradation and increase the quality of the oil extracted from the reservoirs. 

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