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Comment

Comment on ``PAH refractory index as a source

discriminant of hydrocarbon input from crude oil

and coal in Prince William Sound, Alaska'' by

F.D. Hostettler, R.J. Rosenbauer, K.A. Kvenvolden

A.E. Bence

a,

_{*, W.A. Burns}

a

_{, P.J. Mankiewicz}

a

_{, D.S. Page}

b

_{, P.D. Boehm}

c

a_{ExxonMobil Upstream Research Company, PO Box 2189, Houston, TX 77252-2189, USA} b_{Department of Chemistry, Bowdoin College, Brunswick, ME 04011, USA}

c_{Arthur D. Little Corp., Acorn Park, Cambridge, MA 02140, USA}

Received 25 February 2000; accepted 8 May 2000 (returned to author for revision 12 April 2000)

The recent article by Hostettler et al. (1999) describes a source-dependent refractory PAH index, T/C [C26(R)+C27(S)-triaromatic steranes/1-methylchrysene],

and uses that index to argue that coal is the dominant source of the natural petrogenic hydrocarbon back-ground in the marine sediments of Prince William Sound (PWS), Alaska. However, our more complete sampling of potential sources from the region shows that, based on T/C only, coal is only one of several possibilities. Measurements of vitrinite/kerogen re¯ec-tance, hydrocarbon mass balance calculations, and consideration of other molecular indicators, including the polycyclic aromatic hydrocarbons (PAH) and satu-rate biomarkers, clearly indicate that coal cannot be a dominant source. Inadequate sampling and failure to consider other chemical evidence resulted in the incor-rect interpretation reported by Hostettler et al. (1999). In addition, Hostettler et al. (1999) do not consider the thermal dependency of the T/C index. Consequently, its application to sources having the wide range of matu-rities observed for the region is problematic.

The sources of the natural petrogenic hydrocarbon background in PWS oshore sediments occur in eastern Alaska, in the coastal region of the Gulf of Alaska (GOA) (e.g. Page et al., 1993, 1995, 1996; Hostettler et

al., 1999; Short et al., 1999) (Fig. 1). Hydrocarbons from these sources and ®ne-grained sediments from glacial streams are incorporated into the Alaskan Coastal Current (ACC), transported westward into the Sound and deposited (Page et al., 1993, 1995; Bence et al., 1996). Analyses of 210_{Pb-dated deep cores from}

PWS indicate that this in¯ux of hydrocarbons has con-tinued for more than 160 years.

The speci®c sources of these hydrocarbons are the subject of debate. Page et al. (1996) argue that the sources are a mixture of eroding petroleum source rocks (Tertiary shales) and oil residues from natural seeps. Short et al. (1999) and Hostettler et al. (1999) argue that the source is coal. This Comment summarizes argu-ments against the coal source and cautions against uncritical application of the T/C ratio.

1. Inadequate sampling

Hostettler et al. (1999) used a small sample set to conclude that T/C values support coal as a possible source. The only oil they report on from this region of the Gulf of Alaska (GOA) was Katalla seep oil (fresh and weathered), whose T/C value (11±13) is higher than PWS marine sediments (0±0.2). Hostettler et al. (1999) report that area coals, sampled from the Bering river coal-®eld and coaly particles collected from the beach at Katalla, also have low T/C values (0±0.02). They subsequently

Organic Geochemistry 31 (2000) 931±938

www.elsevier.nl/locate/orggeochem

* Corresponding author.

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conclude that ``this PAH parameter supports the hypothesis that coal is a more likely source of PAH in oshore sediments of PWS''.

Hostettler et al. (1999) do not report data for Yaka-taga-area seep oils or Tertiary petroleum source-rock shales east of the Katalla area (Fig. 1), suggested by others as possible sources (Page et al., 1993, 1995, 1996, 1998; Bence et al., 1996). Both have low T/C values. We measured T/C ratios of 0.01±0.04 for Yakataga oils and 0.001±8 for hydrocarbons extracted from Yakataga-area Tertiary shales (Fig. 2). Thus, coals are only one of several possible sources that might account for the low T/C ratio in PWS sediments. The Hostettler et al. (1999) argument that T/C provides evidence that the source of the back-ground is coal, not oil, is countered by additional samples from our more rigorous ®eld-sampling program.

2. Vitrinite/kerogen re¯ectance

There is strong evidence that the hydrocarbons in PWS oshore sediments are a complex mixture derived from multiple sources, including reworked sedimentary rocks. Re¯ectance data for all kerogens comprising the detrital organic component in PWS oshore sediments have a bimodal distribution (Fig. 3) that covers a wide range of thermal maturity (0.3 to >5%). This distribu-tion con®rms contribudistribu-tions from multiple sedimentary sources, such as those in Fig. 4.

However, sources having re¯ectance values >1.5% contribute few biomarkers and PAH relative to TOC

because their concentrations in kerogen decrease dra-matically above 1±1.3% (Dzou et al., 1995). Conse-quently, the biomarker and PAH distributions in the oshore sediments largely re¯ect contributions from sources having re¯ectance values < 1.5%. Vitrinites from Tertiary shales associated with active petroleum seeps in the Katalla and Yakataga areas have Ro values from 0.4±1.2% (Page et al., 1999a). Coal and a natural coke from the Bering river area haveRovalues of 1.6 and 5±6%, respectively. They contain few PAH relative to TOC, and negligible biomarkers (Page et al., 1999b).

The re¯ectance measurements indicate that the Ter-tiary shales are generally in the oil-generation window (i.e. re¯ectance values between 0.5 and 1.5%). At those maturities the shales should contain substantial amounts of extractable hydrocarbons (estimates range from 3 to 18% of the solid organic matter depending upon level of maturation) sorbed both by kerogen and by clays and occurring as a separate phase in pores (Dow, 1978; Behar and Vandenbroucke, 1988; Sandvik et al, 1992). Ten Tertiary shales from the Katalla and Yakataga areas average 0.9 wt.% TOC (range 0.3±4.8 wt.%). Their extractable organic matter (EOM) averages 13% of TOC (range 1±88 wt.% of TOC). The re¯ec-tance measurements also show that high-maturity coal particles occur in PWS marine sediments. However, the PAH and biomarkers contained by these coal particles are overwhelmed by the greater PAH and biomarker signal of lower maturity oil and shale components (Page et al., 1999b).

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Fig. 2. Triaromatic sterane/1-methylchrysene ratios for PWS oshore sediments (OSS), potential sources of background hydro-carbons, and eastern GOA glacial stream sediments. K=Katalla area oils. Y=Yakataga area oils.

Fig. 3. Composite re¯ectance values for all detrital kerogens from four PWS oshore sediment samples.

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3. Katalla beach ``coal''

The Katalla ``coal'' reported by Hostettler et al. (1999) is presumably the coaly strand-line sediment from Katalla beach reported by Short et al. (1999). Typical strand-line sediment from Katalla beach is dominated by micaceous minerals (Page et al., 1999a) that give rise to the dark bands photographed and reported by Short et al. (1999). These sediments also contain variable amounts of kerogen, bitumen, rock fragments, coal, and natural coke particles. The organic component appears to be a mixture of high-maturity coal from the Bering river coal®eld, or similar coal deposits, and low-maturity kerogen/bitumen from eroding Tertiary shales (Fig. 4). The PAH pro®le of the strand-line sediments resembles Tertiary shales rather than Bering river coal (Fig. 5). We measured a T/C value of 0.03 for our richest Katalla beach sediment (lat. 60.177_{N, long. 144.588}_{W; total organic carbon=} 11.62%), compared to 0.02 by Hostettler et al. (1999). The triaromatic steranes that result in the 0.02±0.03 T/C values apparently come from kerogens, bitumens, or seep oil residues rather than Bering river coals, which have T/C=0.

4. Mass balance constraints

The fundamental and most serious ¯aw in the appli-cation of T/C by Hostettler et al. (1999) is their failure to observe elemental mass balance constraints imposed by the concentrations of total organic carbon (TOC), total PAH (TPAH), individual PAH analytes, and indi-vidual and total saturate and aromatic biomarkers.

Oshore sediments from PWS have TOC and TPAH concentrations averaging 0.640.18 wt.% (including an unknown fraction of recent organic matter [ROM] that is not a contributor of petrogenic PAH or biomarkers) and 1145470 ng/g, respectively (Figs. 5 and 6). This restricts the maximum contribution of Bering river coal (TOC80%) or seep oil (elemental carbon 80±85%) to <1% by weight. This limits the PAH and biomarker contributions of the coal but not the oils, which contain far higher concentrations of PAH and biomarkers on a TOC basis than the Bering river coal.

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(0.010 vs 7.1 ng/g), C29aaa-steranes (20R) < 0.1% (0.00 vs 3.3 ng/g), and C26(R)+C27 (S)-triaromatic steranes

<0.1% (0.00 vs 0.74 ng/g) (Fig. 7b). Thus, coals con-tribute less than 1% of the petrogenic PAH and bio-markers found in the PWS oshore sediments (Page et al., 1999b). Statistical analyses of PAH and biomarker ®ngerprints independently con®rm this ®nding. These same criteria indicate that the remainder of the back-ground petrogenic PAH and biomarkers come from Tertiary shales and seep oils.

TOC in Katalla beach coaly sediments ranges from 0.12 wt.% (TPAH=11 ng/g) to 11.62 wt.% (TPAH=1900 ng/g). If a Katalla beach sediment con-taining12 wt.% TOC was the sole source of the0.6 wt.% TOC in PWS oshore sediments, the concentra-tions of TPAH, regular steranes, diasteranes, hopanes, and triaromatic steranes in the PWS sediments would be less than 10% of observed levels (Figs. 7a,b).

These mass balance constraints show that area coals and coaly sediments, because of their low concentra-tions of PAH, saturate biomarkers and aromatic bio-markers relative to TOC, cannot be signi®cant contributors of background hydrocarbons to PWS

o-shore sediments. Seep oils and some of the Tertiary shales, because of their high concentrations of PAH, saturate biomarkers, and aromatic biomarkers relative to TOC, do not suer from this critical TOC mass bal-ance constraint.

5. Other molecular indicators

The bulk T/C ratio of a sediment sample containing contributions from a mixture of sources is a function of the T/C ratios and relative proportions of each source. Quanti®cation of source contributions cannot be done using a single ratio, such as T/C. This requires addi-tional constraints provided by high quality quantitative chemical analyses of multiple PAH and biomarker compounds and statistical analysis of those data (Page et al., 1999b). A single ratio of peak intensities, as used by Hostettler et al. (1999), is insucient.

Hostettler et al. (1999) ignore compositional data other than T/C that argue against coal as the dominant source of PAH in PWS marine sediments. For example, Short et al. (1999) report that Bering river coals from Queen Vein and Carbon Ridge have ratios of C2-dibenzothiophene to C2-phenanthrene (DPI) of 0.36± 0.59 respectively, compared to 0.11±0.21 for PWS sedi-ments. These values are consistent with our analyses (Table 1). If coal were the dominant source of PAH in PWS, the DPI ratio in PWS sediments would have been higher. Instead, PWS sediments have DPI values similar to seep oils (0.08±0.14) and to Tertiary shales (0.09± 0.40) rather than to area coals.

Other molecular indicators also argue against coal as the dominant contributor to the hydrocarbon back-ground. For example, the biphenyl concentration in Bering River coal is quite high (Fig. 5), but is low in oils, shales and PWS marine sediments. Further, the ratio of 4-methyldibenzothiophene to 1-methyldibenzothiophene re¯ects thermal maturity (Dzou et al., 1995). Queen Vein coal has a value of 39 (Table 1), indicating high thermal maturity, while values for seep oils (1 to 10) and Tertiary shales (3 to 22) re¯ect lower maturity. PWS sediment values range from 5 to 7.

6. Limitations of T/C

Hostettler et al. (1999) are probably correct in their assumption that the long chain triaromatic steranes in the numerator of the T/C are relatively stable in the weathering environment. However, they ignore the published observation that, with increasing thermal maturity, the long-chain triaromatic steranes [i.e. C26

-cholestane (20R) and C27-ergostane (20S)] are less stable

than the-short chain triaromatic steranes (MacKenzie et al.,1981; Peters and Moldowan, 1993).

Fig. 5. PAH distributions of Queen Vein coal (Bering river coal®eld), Tertiary shale from the Katalla formation and Katalla beach coaly sediment. P2=sum of dimethylphenan-threne+dimethylanthracene isomers. D2=sum of dimethyldi-benzothiophene isomers.

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With increasing maturity, the ratio of long-to-short chain triaromatic steranes decreases as the longer, thermally less stable chains break (usually at the aromatic ring or at the ®rst methyl group). Methylchrysenes on the other hand are thermally very stable, although they too will crack and lose their methyl group at the very high thermal maturities observed locally in the region. Consequently,

over the range of thermal maturities observed for hydrocarbon contributors to PWS sediments (i.e. Ro=0.3 to6%), T/C varies due to dierences in both thermal stability and source facies. Hostettler et al. (1999) do not consider the thermal dependency of T/C when they apply it to the hydrocarbon sources in PWS oshore sediments.

Table 1

Selected molecular parameters in PWS sediments and in the potential sources of the hydrocarbon background

PWS

6.1680.815 2.824 4.026 9.814 6.237 1.266 2.7±21.7 5.62 34.8

a _T=C

26(R)+C27(S) triaromatic steranes. b _{C=1-methylchrysene.}

c _{DPI=C2-dibenzothiophene/C2-phenanthrene.}

d _{4-mDBT/1-mDBT=4-methyldibenzothiophene/1-methyldibenzothiophene.}

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Fig. 7. Quantitative concentrations of TPAH and C26(R)+C27(S)-triaromatic steranes available when each source is limited to

con-tributing the average TOC value of 0.6 wt.% observed in PWS oshore sediments. K=Katalla area oils. Y=Yakataga area oils. KB=Katalla Beach sediment. BR=Bering river coal.

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7. Conclusion

A number of petrogenic sources, each with a range of chemical properties due to varying organic facies and thermal maturity, contribute to the natural hydrocarbon background in PWS oshore sediments. Identi®cation and quanti®cation of the relative contributions of these sources is a dicult task that can be accomplished only through carefully planned ®eld sampling and analytical programs (Page et al., 1999a, 1999b). The following elements must be incorporated into those programs: (1) a thorough understanding of the geology and physical geography of the region being studied, (2) sampling of the major probable contributors, (3) precise quantitative analysis of those samples for total organic carbon, the PAH, and the saturate and aromatic biomarkers, (4) statistical analysis of the chemical data and (5) applica-tion of elemental mass balance constraints. Hostettler et al. (1999) do not adequately address these elements and the results do not support their conclusions.

Associate EditorÐJ. Curiale

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