The distribution and isotopic composition of sulfur in

Geochemical Investigations in Earth and Space Science: A Tribute to Isaac R. Kaplan q The Geochemical Society, Publication No. 9, 2004 Editors: R.J. Hill, J. Leventhal, Z. Aizenshtat, M.J. Baedecker, G. Claypool, R. Eganhouse, M. Goldhaber and K. Peters

The distribution and isotopic composition of sulfur in solid bitumens from Papua New Guinea M. Ahmed1, S. A. Barclay1, S. C. George1, B. McDonald2 and J. W. Smith1,3 1 CSIRO Petroleum, PO Box 136, North Ryde, NSW 1670, Australia CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670, Australia 3 CSIRO Energy Technology, PO Box 136, North Ryde, NSW 1670, Australia

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Abstract—Previous geochemical and petrographic studies have only partly revealed the history and origins of solid bitumens from the East Papuan Basin, Papua New Guinea. In particular, the alteration phenomena, the distribution of hydrocarbon components and the increasing yields of pyritic sulfur indicate major effects of biodegradation and reduction of sulfate to sulfide. Sulfur isotope analysis of the three main sulfur fractions (elemental, sulfate and pyritic) showed these to cluster closely around a d 34S value near 225‰. An unlimited source of non-marine sulfate of constant isotopic composition is visualized. However, one clear trend in the data is the increase in the 34S content of pyrite as abundance increases. Such an isotopic relationship might well occur where, on reduction of a pool of sulfate, an early loss or reduction of 32S species would result in an increase in the 34S contents of the residual sulfate and product pyrite. Where the oxidation of pyrite to sulfate and sulfur occurs, the relatively greater 34S content of the elemental sulfur is consistent with previous data.

INTRODUCTION

PRELIMINARY geochemical and petrographic studies (Barclay et al., 2003) suggest that solid bitumens in the Late Cretaceous Pale and Subu sandstones from the Subu-1 and Subu-2 wells, Aure Scarp, Papua New Guinea (Fig. 1), were derived by biodegradation of migrating crude oil. The crude oil possibly migrated up faults and was altered to varying degrees to form the reservoir bitumens when it reached a fluctuating water table. Two separate oil charges from marine source rocks have been identified based on the molecular geochemistry of the solid bitumens. One (family A) is from a strongly terrestrially influenced marine source rock that may well be Jurassic. A second oil charge (family B) is from a calcareous-influenced marine source rock with an abundance of prokaryotic organic matter input and a less oxic depositional environment than for the family A solid bitumens. Since changes observed in the hydrocarbon distribution with increasing degree of biodegradation correspond with changes in the distribution of sulfur forms, it was expected that the sulfur isotopic data might reveal general reaction pathways. SAMPLES AND GEOCHEMICAL CHARACTERISTICS

Three sandstone samples containing solid bitumens were selected for analysis (CN409, CN457 and CN250). Gross compositional data together with some selected biomarker parameters for these samples are provided in Table 1. The first two samples are family B solid bitumens and are characterized by significant amounts of 28,30-bisnorhopane (BNH), 29,30BNH and 2a-methylhopanes, consistent with sourcing from a more reducing marine rock with a calcareous influence (Mello et al., 1988; Subroto et al., 1991; Table 1). These two samples also have a higher maturity, as indicated by methylphenanthrene indices (MPI, Radke and Welte, 1983) consistent with the peak of the oil window. On the other hand the third sample, CN250, is a family A member. This sample does not contain BNHs and has a lower content of 2a-methylhopanes, but it contains high amounts of diterpanes that suggest coniferous organic matter input, in common with other family A solid bitumens. Sample CN250 has a lower MPI (Table 1), indicating a low maturity overprint from indigenous organic matter in the Pale Sandstone. 51

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Fig. 1. Location map of the Subu-1 and Subu-2 well sites in Papua New Guinea.

These three samples exhibit different degrees of biodegradation, as shown by the total ion chromatograms (TIC) of their aliphatic and aromatic hydrocarbon fractions (Figs. 2 –4). Aliphatic hydrocarbons of CN409 are characterized by a unimodal distribution of C11 – C34 nalkanes (Fig. 2a). Alkylphenanthrenes are the most abundant class of compounds in the aromatic hydrocarbons of this sample, with substantial amounts of alkylnaphthalenes, alkylfluorenes and polycyclic aromatic hydrocarbons (Fig. 2b). Such a composition may be considered as characteristic of unaltered hydrocarbons. Aliphatic hydrocarbons of CN457 are characterized by a bimodal distribution of C8 – C32 n-alkanes, which exhibit signs of biodegradation. There has been a depletion of n-alkanes over the entire range, leaving two prominent humps of aliphatic unresolved complex mixtures (UCM) (Fig. 3a). Similarly, the aromatic hydrocarbons are also characterized by a UCM hump with few substituted and unsubstituted naphthalenes and phenanthrenes, and dominant polycyclic aromatic hydrocarbons (Fig. 3b). These signatures indicate that CN457 has been partly altered by biodegradation (level 4 of Volkman et al. (1984) and Trolio et al. (1999)). The TIC of the aliphatic hydrocarbons of CN250 shows the near complete removal of n-alkanes, two humps of UCM and large amounts of relatively resistant steranes (Fig. 4a). Such a molecular composition indicates that the aliphatic hydrocarbons of Table 1. Gross compositional and some selected geochemical parameters for three sandstone samples containing solid bitumens from the Subu boreholes, East Papuan Basin

EOM (mg)/kg of rock Aliphatic hydrocarbons (EOM%) Aromatic hydrocarbons (EOM%) Polars þ asphaltenes (EOM%) 19NIP/C30ab C312aMe/(C312aMe þ C30ab hopane) 28,30-BNH/C30ab hopane 29,30-BNH/C30ab hopane MPI

Unaltered (Fig. 2) Subu-1, 142.95 m Pale Sandstone CN409

Biodegraded (level 4) (Fig. 3) Subu-2, 257.27 m Subu Sandstone CN457

Biodegraded (level 5) (Fig. 4) Subu-1, 88.5 m Pale Sandstone CN250

747 3.3 36.6 60.1 0.25 0.58 0.04 0.13 1.17

141 18.1 27.1 54.8 0.06 0.75 0.03 0.11 1.13

1299 5.3 36.3 58.3 0.34 0.28 ,0.01 ,0.01 0.86

EOM, extractable organic matter; 19NIP, 4b(H)-19-isopimarane; BNH, bisnorhopane; MPI, methylphenanthrene index (1.5 £ [3-MP þ 2-MP]/[P þ 9-MP þ 1-MP]).

The distribution and isotopic composition of sulfur in solid bitumens from Papua New Guinea

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Fig. 2. Total ion chromatograms for the sandstone sample (CN409, Subu-1, 142.95 m) showing (a) the distribution of aliphatic hydrocarbons and (b) the distribution of aromatic hydrocarbons. Numbers refer to n-alkane chain length. Pr, pristane; Ph, phytane; DMNs, dimethylnaphthalenes; TMNs, trimethylnaphthalenes; MFl, methylfluorene; P, phenanthrene; MPs, methylphenanthrenes; and DMPs, dimethylphenanthrenes.

CN250 have been altered by biodegradation (level 5). Relatively higher abundances of polycyclic aromatic hydrocarbons compared to alkyl-substituted hydrocarbons in this sample provide consistent evidence for its alteration (Fig. 4b). EXPERIMENTAL

The analytical scheme for determination of sulfur distribution and isotopic compositions, adapted from Smith and Batts (1974), is shown in Fig. 5. Sample splits (50 – 100 g) of the three crushed rock samples (Table 2) were Soxhlet extracted under reflux with dichloromethane and methanol (93:7) for 72 h. The total extract, which consisted of extractable organic matter (EOM) and elemental sulfur, was separated by reflux with bright Cu turnings. More Cu turnings, activated in dilute HCl and pre-cleaned with methanol, were added to the sample flask containing the total extract and refluxed, until the brightness persisted on further additions of Cu turnings. The CuS resulting from direct elemental

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Fig. 3. Total ion chromatograms for the sandstone sample (CN457, Subu-2, 257.27 m) showing (a) the distribution of aliphatic hydrocarbons and (b) the distribution of aromatic hydrocarbons. Abbreviations are defined in Fig. 2. UCM, undifferentiated complex mixture; MNs, methylnaphthalenes; Bp, biphenyl.

interaction, together with excess Cu, was collected by filtration and combined with the insoluble residue from the initial solvent extraction (Fig. 5). The mix of insoluble carbonaceous and mineral matter and CuS was transferred to a flask purged with N2 and reacted with cold 10% HCl in a stream of nitrogen. The H2S generated was carried in the N2 stream through a water scrubber to remove traces of HCl and then into a 0.1 M AgNO3 solution, where it was collected by precipitation as Ag2S. Extraction of dissolved sulfide continued until the AgNO3 solution cleared and precipitation was complete. The precipitate was weighed and saved for isotopic analysis of acid volatile sulfide (fraction #5; Fig. 5). Next, the HCl mixture remaining from the initial separation was heated to boiling and the H2S generated was collected as Ag2S as before. The precipitate was weighed and saved for isotopic analysis of elemental sulfur (fraction #1; Fig. 5). Finally, the hot HCl mixture was filtered. Original soluble sulfate contained in the filtrate was recovered by the addition of BaCl2 and precipitated as BaSO4. The precipitate was dried and weighed, and saved for isotopic analysis of soluble sulfate (fraction #3; Fig. 5). Material insoluble in the hot 10% HCl was regarded as being comprised of pyrite and carbonaceous remnants (including insoluble bitumen). On reaction with 10% HNO3 the pyrite was directly oxidized to sulfate in solution and recovered as BaSO4 as above. The precipitate


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Fig. 4. Total ion chromatograms for the sandstone sample (CN250, Subu-1, 88.5–88.6 m) showing (a) the distribution of aliphatic hydrocarbons and (b) the distribution of aromatic hydrocarbons. Abbreviations are defined in Figs. 2 and 3.

was dried and weighed, and saved for isotopic analysis of pyritic sulfate (fraction #2; Fig. 5). No attempt was made to differentiate isotopically between sulfur derived from pyrite and that from carbonaceous material. These were co-precipitated. The analysis of the iron and sulfur contents in aliquots of the total HNO3 solubles and differences in the Fe/S ratios of these from the theoretical ratios of Fe/S in pyrite allow an estimation to be made of the possible contribution of sulfur from carbonaceous material. Calculations suggest that in the highly degraded bitumens these contribution may be 20% and in the least degraded bitumen it is , 10%. Insoluble carbonaceous material remaining at this stage was converted to sulfate by fusion with Eschka mixture (MgO þ Na2CO3, 2:1) in a crucible at 8008C for 2 h. The filtrate was acidified, heated and sulfate was precipitated from the solution by the addition of BaCl2 to form BaSO4. The precipitate was dried and weighed, and saved for isotopic analysis of sulfur in insoluble bitumen þ carbonaceous residue (fraction #4; Fig. 5). The EOM remaining in the elemental-sulfur-free solvent extract solution was recovered by careful evaporation of the solvent in a nitrogen stream. The total EOM was combusted with oxygen in a Parr Bomb and the sulfur retained as BaSO4. The yields of EOM by solvent extraction were too small to support combustion in the Parr Bomb, so 1 ml of hexane was added to promote combustion. Even with such additions, gravimetric yields of BaSO4 were less than 1 mg and no isotopic measurements were achieved on this organic sulfur (EOM) fraction (fraction #6; Fig. 5).

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Fig. 5. Analytical scheme for determination of sulfur isotopic compositions (adapted from Smith and Batts, 1974).

Solid sulfur compounds (Ag2S and BaSO4) were converted to SO2 for isotopic analysis by modified versions of the methods of Kaplan et al. (1970) and Bailey and Smith (1972), respectively. Ag2S was oxidized to SO2 by reaction with Cu2O at 10508C. BaSO4 was directly decomposed to SO2 by heating at . 17008C in a quartz tube (Bailey and Smith, 1972). Isotopic determinations were carried out on a Finnigan MAT 252 IRMS, calibrated by a series of IAEA standards. Results are presented as d 34S relative to Canyon Diablo Troilite, with a precision of ^ 0.2‰. The yields and isotopic composition of all fractions are included in Table 2. The mass of sulfur in each fraction expressed as their percentage of the original rock mass is directly calculated from the mass of BaSO4 or Ag2S. Table 2. Sulfur contents and isotopic compositions for three sandstone samples containing solid bitumens from the Subu boreholes, East Papuan Basin Unaltered (Fig. 2) Subu-1, 142.95 m Pale Sandstone CN409

Elemental Pyritic SO4 Eschkab Acid volatile Soluble bitumen a

Biodegraded (level 4) (Fig. 3) Subu-2, 257.27 m Subu Sandstone CN457

Sulfur (wt/wt%)

d34Sa (‰)

Sulfur (wt/wt%)

d34S (‰)

Sulfur (wt/wt%)

d34S (‰)

0.04 0.28 0.25 0.13 n.d. ,0.1

224.7 230.6 228.6 221.5 n.d. n.d.

0.06 1.45 0.42 0.04 n.d. ,0.1

220.1 226.5 223.8 217.2 n.d. n.d.

0.31 3.8 0.37 0.38 n.d. ,0.1

220.9 219.2 226.2 214.7 242.5 n.d.

d 34S values reported relative to Canyon Diablo Troilite (CDT). Represent insoluble bitumen and carbonaceous residues. n.d. ¼ not determined. b

Biodegraded (level 5) (Fig. 4) Subu-1, 88.5 m Pale Sandstone CN250


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RESULTS AND SUMMARY

The pyritic sulfur, elemental sulfur and soluble sulfate have a relatively narrow spread of values (2 19.2 to 2 30.6‰). The residual sulfur in the organic matter (both in insoluble bitumen and any carbonaceous residue present) is isotopically heavier (Table 2), whilst the one measurement on acid volatile sulfide is isotopically very light. No isotopic measurements on sulfur in the EOM could be made, due to the small amount of sulfur (, 0.1 wt%) recovered (Table 2). There is a trend for pyritic sulfur and elemental sulfur to be more abundant and isotopically heavier with greater biodegradation (Table 2). The average d 34S values approximating 2 25‰ for all the sulfur forms (weighted averages are 227.9, 225.5 and 219.5‰ for CN409, CN457 and CN250, respectively) suggest a single nonmarine sulfur source in an open system. However, the isotopic data give very little indication of which sulfur form, pyritic or sulfate sulfur, is the sulfur source, and which represents a final product. In this respect the increase in the 34S content (represented by the increasingly positive d 34S values) and yield of pyrite with increasing degrees of biodegradation is consistent with an early reduction of the more active 32S-enriched sulfate to 32S-enriched sulfide (Kaplan and Rittenberg, 1964). The continuation of this reaction leads to pyrite with an increased 34S content. The high concentration of elemental sulfur associated with the pyrite in the most biodegraded core section is best explained as a co-product of the oxidation of pyrite to ferrous sulfate (Chou, 1990; Stock and Wolny, 1990). The data reported here may represent progression of the reaction to an unusually marked degree. This same isotopic enrichment of elemental sulfur in 34S relative to sulfate sulfur (Table 2), as observed earlier and demonstrated by Smith and Batts (1974) in their study of the forms of sulfur in oxidized high sulfur Australian coals, is found again. The origin of the light acid volatile sulfide was not pursued as it was present in only one sample, in low abundance, and was isotopically far removed from the bulk of the data. The source of the sulfur is uncertain. It may have originated from deeply buried evaporites that have not yet been penetrated by any well intersection, or it may have originated from the oxidation of pyritic sulfur external to the Subu sequence. Sulfate-rich porewaters migrated into the Subu sequence where crude oil already present in the Pale and Subu sandstones provided a nutrient source for sulfate-reducing bacteria to form solid bitumen and pyrite. Evidence from textural and petrographic observations also suggests that the pyrite precipitated as a by-product of biodegradation (Barclay et al., 2003). The more abundant and isotopically heavier pyritic and elemental sulfur in the more biodegraded samples is consistent with pyrite formation during biodegradation. Once pyrite had been formed, it underwent some diagenetic alteration (oxidation) to form FeSO4 and elemental sulfur. Acknowledgements—The expertise of Ken Riley of CSIRO Energy Technology in completing all the Parr Bomb experiments is gratefully acknowledged. We thank InterOil for providing the samples and permission to publish these results. We are grateful for the comments of journal reviewers, Baruch Spiro and Zeev Aizenshtat, which helped to improve the manuscript.

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