Biodegradation, as it affects the gasoline compositional ratios considered here, is illustrated by a suite of experiments conducted by the
writer at Texas A&M University (unpublished data). Samples of a Type IIS oil were maintained in sealed flasks with an oxygenated aqueous suspension of
surficial marine sediment from a Gulf of Mexico seepage site. Sterile controls were also maintained. Both were sampled on the first, the 24th and the
125th day of the experiments.
Figure 56 illustrates the effects on the normal alkane profile after 24 days, comparing
n-alkane peak areas, normalized at nC25. Increasingly rapid degradation occurs with decreasing carbon number. Relatively rapid loss is evidenced close to nC10 and at
carbon numbers below 8.
Figure 57 illustrates the effects of experimental biodegradation on the gasoline
Heptane and Isoheptane Ratio values. The angular attitude of the alteration vector compares well with those developed by biodegradation
in nature, as shown in Figure 59 and in Section 12.
BIODEGRADATION IN OILS
DIAGRAMS REPRESENTING HEPTANE AND ISOHEPTANE VALUES
Several of the LHC ratios detailed above (Thompson, 1983) lend themselves to the recognition of biodegradation. Figures 58 through 61
illustrate the effects on U.S. Gulf Coast oils, employing the data of Thompson and Kennicutt, 1990.
Figures 58 and 59 illustrate, respectively, Heptane Values versus Isoheptane Values in suites of
unbiodegraded and biodegraded oils from the U.S. Gulf Coast, selected on the basis of normal or diminished values of the ratio n-C17 /pristane.
The highly branched isoprenoids are resistant to biodegradation.
Figure 59 illustrates biodegraded cases showing alteration vectors closely comparable to that developed experimentally,
shown in Figure 57.
AROMATICITY - PARAFFINICITY DIAGRAMS
The aromaticity-paraffinicity diagram, B vs F, has become a standard test for the occurrence of evaporative fractionation. As shown above,
these variables are also sensitive to, and may reflect, biodegradation, increasing maturity and the extraction of aromatics by water.
Selecting undegraded and degraded suites oils from the U.S. Gulf Coast on the basis of normal or diminished values of the ratio n-C17 /pristane,
Figure 60 illustrates B versus F in unbiodegraded oils. The majority exhibit evaporative
fractionation. Figure 61 compares biodegraded oils which show marked decreases in
both B and F.
THE RATIO OF NORMAL-PENTANE TO ISOPENTANE
The only intercomponent ratios available in standard reservoir fluid analyses which provide a measure of biodegradation involve normal and isopentane,
also normal and isobutane, on the basis of the resistance of branched alkanes.
The mean value of nC5/iC5 in 53 Western Canada Sedimentary Basin reservoir fluids which are known to be undegraded on the basis of reservoir
temperatures never below 80C, is 1.15 +- 0.19
Figure 62 presents values of nC5/iC5 in North Sea oils at various levels of biodegradation as
determined by Vieth and Wilkes (2006). Again, 1.15 presents a satisfactory cut-off for undegraded oils.
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