4. Effect of Biodegradation and Water Washing on Crude Oil Composition C19 and C2o isoprenoids, pristane and phytane)and cyclic 100 Parasit d aromatic hydrocarbons For example, the weight percent of n-paraffins relative to naphthenes and aromatics is approximately 2-15 wt%in the C15+ fraction of biodegraded oils( Figure 1). Gas chro- matograms of whole crude oils show that low molecular weight components,e.g, C1o to C14n-paraffins,are depleted first; the C15+ n-paraffins are then attacked e-g williams et al., 1986). Gas chromatographic patterns of C15+ saturated hydrocarbon fractions(Figure 2)of biode- graded oils contain low amounts of n-paraffins relative to pristane, phytane, and naphthenes. Thus, the loss of n- araffins relative to branched and cyclic hydro conjunction with low API gravity and enrichment in percent sulfur, NSOs, and asphaltenes)is the most er alluded to as an indicator of biod dation Perhaps the focus on the use of the saturated hydro- rbon fraction(e-g n-paraffins, branched paraffins,and naphthenes such as steranes and terpanes)in the applica- Figure 1. Gross Cis, hydrocarbon compositon of tion of oil geochemistry has led to a better understanding and aromatic hydrocarbons. Biodegradation remo aromatic hydrocarbons can also be degraded by bacteria (a)NONDEGRADED OIL (b)SEVERELY BIODEGRADED OIL Pr and Ph= isoprenoids, pristane naphthenes _2557.511256 25}5mzs5h,5aa5如 Retention Time, Minutes Retention TIme, MInutes Figure 2. Effect of biodegradation on the saturated hydrocarbon fraction of crude fraction of a nondegraded oll contains prominent paraffin and branched paraffins, chromatogram of C15+ saturated fraction of a severely biodegraded oil contains primarily naphthenes; the paraffins have been removed.4. Effect of Biodegradation and Water Washing on Crude Oil Composition 49 C19 and C20 isoprenoids, pristane and phytane) and cyclic hydrocarbons (naphthenes and aromatic hydrocarbons). For example, the weight percent of n-paraffins relative to naphthenes and aromatics is approximately 2-15 wt. % in the C15+ fraction of biodegraded oils (Figure 1). Gas chro￾matograms of whole crude oils show that low molecular weight components, e.g., C10 to C14 n-paraffins, are depleted first; the C15+ n-paraffins are then attacked (e.g., Williams et al., 1986). Gas chromatographic patterns of C15+ saturated hydrocarbon fractions (Figure 2) of biode￾graded oils contain low amounts of n-paraffins relative to pristane, phytane, and naphthenes. Thus, the loss of n￾paraffins relative to branched and cyclic hydrocarbons (in conjunction with low API gravity and enrichment in percent sulfur, NSOs, and asphaltenes) is the most common parameter alluded to as an indicator of biodegra￾dation. Perhaps the focus on the use of the saturated hydro￾carbon fraction (e.g., n-paraffins, branched paraffins, and naphthenes such as steranes and terpanes) in the applica￾tion of oil geochemistry has led to a better understanding of the effects of bacterial action on this fraction. However, aromatic hydrocarbons can also be degraded by bacteria. 100% Paraffins AromawcB Figure 1. Gross ds* hydrocarbon composition of crude oils in terms of percent abundance of paraffins, naphthenes, and aromatic hydrocarbons. Biodegradation removes paraffins leaving an oil enriched in aromatic and naphthenic hydrocarbons. ( a > NONDEGRADED OIL f lit Numbered peaks = n-paraffins Pr and Ph = isoprenoids, pristane and phytane naphthenes 2.5 5 -1 1 1 1 1 1 1 r— 1 1 1 — 7.5 IB 12.5 15 17.5 28 22.5 25 27.5 38 32.5 (») SEVERELY BIODEGRADED OIL —i—i—i—i—i—i—i—.—i—i—i—i—i — 2.5 57. 5 IB 12.5 15 17.5 28 22.5 25 27.5 38 32.5 Retention Time, Minutes - Retention Time, Minutes Figure 2. Effect of biodegradation on the saturated hydrocarbon fraction of crude oils, (a) Gas chromatogram of Ci-*. saturated fraction of a nondegraded oil contains prominent n-paraffin and branched paraffins, (b) Chromatogram of $5+ saturated fraction of a severely biodegraded oil contains primarily iiaprithenes; the paraffiiis have been removed