SECTION 6,
BIODEGRADATION
  • INDICES OF BIODEGRADATION

    Elevated values of C1/C2 and diminished values of P4/P8 are proposed as hallmarks of evaporative fractionation, provided that these changes are not due to biodegradation, making it necessary to distinguish the effects of the two processes. Evidence of the generation of significant volumes of methane by in situ biodegradation of reservoired oil is presented by Jones et al. (2008). Biogenic methane could contribute to the gasification of oil, raising C1/C2 ratios.

    The only ratios which characterize biodegradation in commercial PVT analytical data are n-butane/isobutane and n-pentane/isopentane, as branched alkanes are more resistant to biodegradation (Thompson, 1983). Mechalas et al. (1973) observed the preferential initial destruction of n-alkanes in the n-C10 region, resulting in a widening “valley” in the n-alkane or pseudo-component profile, eventually resulting in low values of nC4/iC4 and nC5/iC5. Examples of progressive "valley" development in U.S. Gulf Coast oils were illustrated in Figure 7 where an inset table shows the values of phytane/n-C18, a conventional index of biodegradation. These values are highly, inversely, correlated with the ratio [n-C10] / [n-C10*] ( r = 0.99), measuring development of the "valley", where [n-C10] is the observed concentration of n-decane and [n-C10*] is the calculated original concentration, based on a slope evaluation. Figure 23b, Case 376, exhibits a significant valley centered on P10, maximizing at P12, and addresses the question of the effects of simultaneous biodegradation and evaporative fractionation. Despite the substantial valley, values of nC5/iC5 or nC4/iC4 are in the modal range observed in undegraded oils quoted in Table 2, suggesting that widening of the valley has not yet affected n-C5 and n-C4.

    To evaluate the tabulated suite of Western Canadian PVT data for biodegradation, values of nC4/iC4 and nC5/iC5 were assessed in reservoirs presently at temperatures equal to, or greater than, 80 degrees Celsius (80C) relying upon the finding of Wilhelms et al. (2001) that reservoirs are sterilized by attaining this temperature upon burial and remain so, precluding biodegradation. The Western Canada Sedimentary Basin has undergone uplift and erosion since the Oligocene, with the removal of up to 2000m of overburden (Deroo et al., 1977). Of the 177 analyses of known reservoir temperature, 53 are above 80C, ensuring that they are sterile, have never been cooler, and are reasonably postulated to be undegraded. The instances of elevated C1/C2 levels observed in these oils are therefore attributable to high maturity, or fractionated thermogenic gases. Mean values of nC4/iC4 and nC5/iC5 in oils above 80C and in cooler reservoir suites are presented in Table 2.

    In an inverting basin, reservoirs presently at temperatures below 80C could have previously attained this temperature, subsequently cooled, and still contain undegraded oils (Wilhelms et al., 2001). A trial examination of data representing oils in reservoirs presently at temperatures in the interval 40C to 80C is also evaluated in Table 2. There is no statistical evidence of decreased values of nC4/iC4 and nC5/iC5 in the cooler subset, taken as critical evidence that there are very few biodegraded oils included.

    Complicating the task of recognizing biodegradation using nC4/iC4 and nC5/iC5 ratios is the fact that such are diminished by the admixture of gas/condensate. Mean values equal to 1.77 and 1.07, respectively, were measured in a "global" suite of 178 PVT analyses of gas-condensates (author's unpublished data). In the suite of 535 PVT oil analyses the mean values of these ratios are 2.57 and 1.59; the modal ranges are 2.7 - 2.9 and 1.0 - 1.2.

  • BEAUFORT SEA WELLS

    Combined Evaporative Fractionation and Biodegradation

    PVT analyses of three reservoir fluids representative of Beaufort Sea oils are illustrated in Figures 24a through 24c.

    They exhibit characteristics of substantial biodegradation and of postulated evaporative fractionation, identical in most aspects to reservoir fluids of the Gulf of Mexico shelf. Convincing evidence of evaporative fractionation in the Beaufort Sea region is provided by gas chromatograms (not illustrated) of fluids from several wells. An available gas-condensate is strongly enriched in methylcyclohexane, toluene and xylene in an otherwise conventional n-alkane profile, yielding the following gasoline-range compositional parameters: B, toluene/n-heptane, 1.38; F, n-heptane/methylcyclohexane, 0.69. These values are characteristic of evaporative gas-condensates on the evidence of the diagnostic B versus F diagram of Thompson (1988). An oil of 32.9 API gravity exhibits a secondary maximum at P7, is strongly depleted in the entire C2 to P7 region, yet shows no evidence of biodegradation, having a value of nC5/iC5 equal to 1.24 and no evidence of a P10 valley, features compatible only with fractionation. Isotopic analyses provide values of del13C(methane) as heavy as -40.0 per mil, indicative of an elevated level of maturity and dryness. Gases of this nature are capable of removing C3 - Pn oil components.

    Comparing Case 376, Figure 23b , and Case 421, Figure 24a , both are biodegraded, the latter to a greater degree, evidenced by values of nC4/iC4 and nC5/iC5 which are reduced to 1.09 and 0.76, respectively. Furthermore, the "valley" is deeper and extends to P15, though the secondary maximum remains at P8. Figure 24b , Case 426, extends analogous observations: biodegradation increases, the P5 - P15 region is greatly depleted. Figure 24c represents extreme effects of light end loss in Case 424. The three cases clearly exhibit biodegradation, but also aspects compatible with evaporative fractionation, as C1/C2 increases and E7 decreases progressively from Case 376. However, the evidence internal to the PVT analyses is ambiguous. Indications of associated evaporative fractionation can only be drawn with certainty from additional analytical information.

    An interesting feature of Case 424 is revealed in its gas chromatogram. No n-alkane peaks are evident, merely a full suite of isoprenoids plus numerous unidentified compounds projecting from a large unresolved complex base. The PVT pseudo-compound data include both resolved and unresolved material, evidently subdivided by standard n-alkane elution time data. It is noteworthy that the sum of these two types of material still yield the exponential progression of concentrations shown in Figure 24c.

    Characteristics and selected engineering data relating to the oils represented in Figs. 21 through 24 are summarized in Table 3.


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