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Geochimica et Cosmochimica Acta, Vol. 69, No. 22, pp. 5317–5331, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2005.06.026

Stable sulfur isotope partitioning during simulated petroleum formation as determined byhydrous pyrolysis of Ghareb Limestone, Israel

ALON AMRANI,1 MICHAEL D. LEWAN,2 ZEEV AIZENSHTAT1,*

1Casali Institute of Applied Chemistry and Department of Organic Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel2U.S. Geological Survey, Box 25046, MS 977, Denver, CO 80225, USA

(Received January 10, 2005; accepted in revised form June 27, 2005)

Abstract—Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immatureorganic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. Thiswork focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of theirstable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of thesulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H2S(gas). Up to 70% of this organic sulfur is released as H2S(gas) that is isotopicallylighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in 34S during thermal maturationcompared with the initial �34S values. The �34S values of the three main organic fractions (kerogen, bitumenand expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation wasobserved during the experiments. The early released sulfur reacted with available iron to form secondarypyrite and is the most 34S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopicfractionation for the early formed H2S is a result of the system not being in equilibrium. As partial pressureof H2S(gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchangeand isotopic homogenization. Part of the �34S-enriched secondary pyrite decomposes above 300°C resultingin a corresponding decrease in the �34S of the remaining pyrite. These results are relevant to interpretingthermal maturation processes and their effect on kerogen-oil-H2S-pyrite correlations. In particular, the use ofpyrite-kerogen �34S relations in reconstructing diagenetic conditions of thermally mature rocks is questionablebecause formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantityof the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can berecorded by using both sulfur content and �34S of each phase including the H2S(gas). H2S generated inassociation with oil should be isotopically lighter or similar to oil. It is concluded that small isotopicdifferentiation obtained between organic and inorganic sulfur species suggests closed-system conditions.Conversely, open-system conditions may cause significant isotopic discrimination between the oil and itssource kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability

0016-7037/05 $30.00 � .00

of iron in a source rock resulting in secondary formation of pyrite. Copyright © 2005 Elsevier Ltd

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1. INTRODUCTION

The �34S of sulfur-rich kerogen and oil is controlled byulfur incorporation during early diagenesis at relatively lowemperatures (Krein, 1993; Anderson and Pratt, 1995; Amranind Aizenshtat, 2004; Aizenshtat and Amrani, 2004a) andulfur decomposition and reactions during catagenesis at higheremperatures (Aizenshtat and Amrani, 2004b). At the begin-ing of the catagenesis stage, Type-IIS kerogen is rich inhermally unstable, organically bound sulfur (8–12 wt%). Thisow thermal stability is considered a major factor for the earlyeneration of bitumen and heavy oil and the release of largemounts of H2S (Tannenbaum and Aizenshtat, 1984, 1985;ewan, 1985; Orr, 1986; Baskin and Peters, 1992; Aizenshtat etl., 1995; Nelson et al., 1995; Koopmans et al., 1998; Lewan,998). The effect of these processes on the �34S of the organicnd inorganic species in the source rock remains to be deter-ined.Orr (1986) evaluated many oils and their suggested kerogen

recursors from the Miocene Monterey Formation. He con-luded that the maximum difference between the two was only

Author to whom correspondence should be addressed (zeev@vms.

tuji.ac.il).

5317

2‰ enrichment of the sulfur into oil relative to that in theource-rock kerogen, suggesting �34S is a good parameter forource rock to oil correlations. In contrast, selected oils anderogens from the Ghareb Limestone and Monterey Formationnalyzed by Dinur et al. (1980) and Aizenshtat and Amrani2004b) showed larger isotopic variations, with the oils beingnriched in 34S by as much as 10‰. Only a limited number ofxperiments monitoring �34S changes during thermal matura-ion of kerogen have been reported. These previous studiesIdiz et al., 1990; Stoler et al., 2003) were conducted under dryyrolysis conditions in which no expelled oil was produced.oreover, they did not consider a complete mass balance of

ulfur nor the �34S of the inorganic sulfur species except for

2S(gas). Idiz et al. (1990) performed dry pyrolysis experimentsn closed ampoules at 300°C on isolated kerogen from the

onterey Formation. These experiments showed small varia-ions of only 2‰ between the sulfur in source kerogen, bitumennd H2S(gas). Conversely, open-system pyrolysis of a Dead Seaulfur-rich source rock and isolated kerogens at 300 to 500°Chowed larger isotope differences of 10‰ between kerogen,itumen and hydrogen sulfide (Stoler et al., 2003).The present study was conducted to follow the �34S parti-

ioning of the main organic and inorganic sulfur species during

hermal maturation of organic- and sulfur-rich rock of the

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5318 A. Amrani, M. D. Lewan, and Z. Aizenshtat

hareb Limestone, Israel. To better simulate natural thermalaturation, whole rock samples were subjected to hydrous

yrolysis experiments at temperatures ranging between 200 and65°C for 72 h. Organic-and inorganic-sulfur species wereuantified and �34S values measured to establish completeass and isotopic balance of sulfur. These results provide new

nsights on the behavior of sulfur during thermal maturationnd how it partitions into different organic and inorganichases during petroleum formation.

2. METHODS

.1. Sample Description

The rock sample used in this study (930616-9) is an organic-richimestone (15.22 wt% total organic carbon, TOC) containing thermallymmature Type-IIS kerogen (atomic S/C � 0.065). The thermal im-aturity of the samples is indicated by its low Tmax of 402°C, high

ydrogen index of 836, low production index of 0.01, and low bitumen/OC ratio of 45.9 mg/g TOC (Table 1). This sample is a massiveedium-gray chalky limestone collected from a 17-cm interval, 6 m

bove the Mishash phosphorite bed in the Nukhel Efe quarry (a.k.a.otem deposit, Israel) operated by the Pama Co. (31.076917N and5.176853E). The sample is blocky with no signs of saprolite rinds orottled discoloration indicative of pedogenic weathering. The stratig-

aphy and geology of the Ghareb Limestone are reported in Minster etl. (1992) and Shahar and Wurzburger (1967).

.2. Hydrous Pyrolysis

The experiments were conducted in stainless-steel Hastalloy C-276eactors (internal volume: �1025 cm3) with carburized surfaces (Le-an, 1993). In each experiment, 200 g of gravel-sized rock chips

0.5–2 cm) were loaded into the reactor with 400 g of distilled water.alculations based on specific volumes (Lewan, 1993) indicate that thismount of rock would be submerged in liquid water during the entirexperiment. After the reactors were sealed, they were filled with 6900Pa of He and checked for leaks. The pressure was then reduced to 240Pa of He, and the reactor weighed. Temperatures were monitored withype J thermocouples that were calibrated against national standards at80, 300, 320, 340, and 360°C. Temperature and pressure variationsere manually recorded for 20- to 30-min intervals every 10 to 15 h.xperimental temperatures had standard deviations less than 0.5°C,xcept for the dry experiment, which had a standard deviation of 0.8°C.ll experiments were conducted for 72 h, which was measured from

he time the reactors reached the experimental temperature to the timehe heaters were turned off. Experimental temperatures were attainedetween 30 and 60 min. Cooldown time to room temperature took

Table 1. General data describing the experimental temperatures andS1, S2, S3 Tmax, and HI) of the unheated and recovered rocks, and thexperiments.

Reactiontemperature

°C

Runpressureb

kPaTOC

Wt. %S1

mgHC/gRockS2

mgHC/gRock

nheated — 15.22 1.64 127.2100 1538 15.37 1.36 120.6360 5537 14.52 10.05 114.3900 9605 13.01 21.01 86.5120 12639 11.18 22.98 58.2440 16672 7.89 13.20 22.0565 23671 6.38 7.52 6.1140 drya 3330 11.14 29.98 23.81

a Experiment with 200g rock and no addition of water.b Absolute pressure, measured within the last 3 hours of the experim

etween 18 and 24 h. During cool down, a 50°C decrease occurred n

ithin the first 2 h and a 100°C decrease occurred within the first 4.3 h.he dry pyrolysis experiment was conducted as described above butith no added water.

.3. Kerogen Isolation and Hydrous Pyrolysis

Kerogen was isolated from pulverized rock chips by HCl and HFreatments followed by a heavy liquid separation according to therocedure described by Lewan (1986). The elemental analysis of thesolated kerogen gave 68 wt% C, 8.13 wt% H, 1.9 wt% N, 9.57 wt% O,2.05 wt% Stotal, and 11.76 wt% Sorganic. The balance between totalulfur and organic sulfur is pyrite sulfur that was not completelyemoved by the kerogen isolation procedure. We used 40 g of theerogen for the hydrous pyrolysis experiments. This amount approxi-ates kerogen content in 200 g of rock that we normally used in ourhole rock experiments. Because the isolated kerogen is slightly hy-rophobic and difficult to submerge in the reaction water, the weighedmount of kerogen was initially mixed with water using isopropyllcohol as a wetting agent. The isopropyl alcohol was then removed byequential rinsing of the wet kerogen in a filter apparatus with distilledater (Lewan, 1997). The final filtered wet kerogen was then added to

he reactor with 450 g of distilled water.

.4. Sampling and Collection of Reaction Products

After the reactor cooled to room temperature, pressure and temper-ture were recorded. To quantify the sulfide species in the water phase,e decreased the reactor’s pressure by releasing the gas into evacuated

tainless-steel cylinders with a wide range of volumes to a final abso-ute pressure of 76–93 kPa (the atmospheric pressure at Denver,olorado is �86 kPa). This collection system was allowed to equili-rate for �1 h before two 51-cm3 evacuated cylinders were used to takeas samples for compositional analysis and H2S precipitation. Exper-ments on the influence of the collection pressure on the sulfur isotopesield negligible differences with a standard deviation (�0.3‰), almostithin the measurement error. The expelled oil generated in the hy-rous pyrolysis experiments was collected from the surface of the watert the end of the experiment with a Pasteur pipette and a benzene rinses described by Lewan (1997). The water recovered from the reactoras filtered (0.45 �m) and an aliquot immediately analyzed for pH andh at room temperature. The remaining water was sealed with ainimum amount of air space and was analyzed for sulfide and sulfateithin 3 d. Rock chips were removed from the reactor and dried in aacuum oven at 50°C for 24 h. The dried rock chips were pulverizednd bitumen was extracted in a Soxhlet apparatus for 72 h with anzeotropic mixture of CH2Cl2 and MeOH. The refluxed solvent wasltered and the bitumen was concentrated by rotary vacuum evapora-

ion and weighed.The first gas cylinder collected from the reactor was purged with

es for 72-h durations, total organic carbon (TOC) and Rock-Eval datac products (bitumen, oil and gas) generated in the whole rock pyrolysis

/gCTmax(°C)

HImgHC/gC

Extractedbitumen

mg/gOrig.TOC

Expelled oilmg/gOrig.

TOC

Total gasmg/gOrig.

TOC

402 836 45.9 0.0 0.0401 785 148.6 0.0 39.4411 788 588.6 3.3 98.6423 665 760.3 54.4 180.7434 521 526.8 150.2 239.8443 280 190.5 388.9 315.4449 96 107.3 417.1 436.9445 214 327.1 0.0 269.4

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itrogen into a AgNO3/ NH4OH solution to precipitate the H2S(gas) as

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5319Stable sulfur isotope partitioning during simulated petroleum formation

g2S. The solution was then filtered and the Ag2S dried in an oven at20°C. The second gas cylinder was subjected to gas analysis per-ormed on a Wasson Modiro HP gas chromatograph using a modifiedersion of the ASTM D2650-88 method to determine mole percent-ges, which were converted to mole and mass units assuming ideal gasehavior at room temperature.The pulverized and extracted rock from each experiment was se-

uentially collected and gravimetrically analyzed by the separationrocedure of Tuttle et al. (1986). The first step is water extraction of theock to remove the water soluble sulfate (SO4

2�(w-rock)). Then the

nely ground 5-g samples were treated in an inert atmosphere with hotN HCl for 30 min to dissolve the acid-soluble sulfates

SO4(acid-rock)) and to form H2S from the decomposition of mono-ulfide minerals (acid-volatile sulfur, AVS(rock)). The same treatmentas applied to the recovered water (20 g) of each experiment touantitatively collect the dissolved H2S(aq) and HS�

(aq) (AVS(aq)) andulfate (SO4

2�(aq)). Stannous chloride was added to samples to prevent

xidation of the evolved H2S by Fe3� which may present in theolution. The H2S was collected as Ag2S and the sulfate as BaSO4.isulfide minerals (mainly pyrite) were reduced in a hot, acidifiedr(II) solution in a nitrogen atmosphere to form H2S that was collecteds Ag2S. This method has been shown to cause minimal damage to therganic matter in comparison to other methods (Acholla and Orr,993). Precision of the sulfur-species analyses was �10%. The residualatter (mostly kerogen and some silicates) was dried and weighed.

.5. Speciation of Elemental Sulfur

Elemental sulfur was determined according to Tuttle et al. (1986) byxtracting three selected samples (unheated, 200°C, 340°C HP exper-ments) with acetone. Only trace amounts of elemental sulfur wereetected, within the measurement error. All the samples were alsooxhlet extracted with addition of Cu0 curls as previously described.ith the exception of the dry-pyrolysis rock, no formation of CuS

ccurred, which indicates the absence of elemental sulfur.

.6. X-Ray Diffraction

Pyrite content was also measured by using X-ray diffraction (XRD)n each residual rock after removing its carbonate minerals with 6 NCl for 2 h. The residual powdered rock was randomly packed in

luminum holders and analyzed with a Philips diffractometer in Bragg-rentano geometry using a Cu tube operated at 40 kV and 20 mA withxed 1-degree divergence slit, graphite-diffracted beam monochroma-

or, scintillation detectors, and NIM-counting electronics. Changes inyrite content of the samples were determined by comparing thentensity of the 1.633-Å pyrite peak to the intensity of the 26.68-Åuartz peak as a decimal fraction. XRD was also used to detect theresence of pyrrhotite in several of the bitumen-extracted residual

Table 2. Sulfur content (mgS/g Total S) of the different sulfur specierror for these results is �10% from the total amount of sulfur in the

Pyrolysistemperature (°C) Oil (OS)

Bitumen(BS)

Kerogen(KS) H

2S(gas)

c AVS(aq)

nheated — 23.2 851.0 — —00 0.0 71.2 654.0 104.3 16.660 1.7 220.2 269.9 319.5 39.700 18.2 162.3 89.4 509.9 54.620 41.4 120.9 79.5 538.1 59.640 86.1 48.0 59.6 566.2 61.365 82.8 29.8 41.4 619.2 54.640 (dry)a 0.0 74.5 135.8 625.8 —40(iso.Kerogen)b 91.1 57.9 66.2 478.5 69.5nheated iso.kerogenb

— — 761.6 — —

a Experiment with 200g rock and no addition of water.b Experiment with 40g isolated kerogen and 450g of distilled waterc The gas quantities were corrected by adjusting the amount of gasd Not detected.

ocks. However, a standard of pyrrhotite (research grade Ward’s u

9V5885) and marbleized Leadville Limestone (Waupaca NorthwoodsLC) showed that pyrrhotite concentrations of less than 1 wt% wouldot be detected by XRD.

.7. Elemental Analyses and Stable Sulfur Isotope Measurements

The expelled oil, extracted bitumen and kerogen (residual matterfter sulfur specification) were subjected to elemental analysis for C, H,, and S using a Carlo Erba 1106 elemental analyzer. Analytical erroras �0.15 wt% for carbon and �0.3 wt% for sulfur. �34S of therecipitated Ag2S, and BaSO4, and the organic samples were measuredy a continuous-flow elemental analyzer connected to a Finnigan /Matelta Plus stable-isotope ratio monitoring mass spectrometer (EA-

rmMS). Sulfur isotope compositions are expressed as per mil (‰)eviations from V-CDT (Vienna Canyon Diablo Troilite) standardsing the conventional delta notation with a standard deviation betterhan 0.3‰ (n � 2). The measurements were directly calibrated againstulfur isotopic standards IAEA-S-1 (Ag2S, �0.3‰) and NBS-127BaSO4,�20.3‰) with a standard deviation less than 0.2‰ (n � 3).

.8. Mass Balance

Mass balance calculations were carried out based on the quantitativeesults for all analyzed sulfur species. The total mass of sulfur in eachxperiment was corrected to the unheated total sulfur value by adjust-ng the amount of S in H2S of the gas phase, because the gas measure-ents have the largest expected error that in some cases exceeded 10%.2S dissolved in the oil could be part of this balance problem. How-

ver, a positive correlation was not observed between H2S mass errornd amount of oil generated. In all cases, the total sulfur mass balancerror was less than 10%. The isotopic mass balance for total �34Sesulting from the calculations of all experiments had a standardeviation of 0.4‰.

3. RESULTS

.1. Character of Initial Sulfur in Unheated Rock

Elemental sulfur could not be detected in the unheatedock. This is in agreement with other studies of this areaDinur et al., 1980). A relatively low pyrite S content of �3t% of total S in the rock was measured (Table 2). Theyrite in this unheated sample is ca.30‰ isotopically lighterhan the organic sulfur (Table 3). This difference is in the

2 hrs hydrous pyrolysis at temperatures noted in table. The maximumed rock.

ock)

TotalAVS SO4(aq) SO4(acid-rock) SO4(w-rock)

Totalsulfates

Pyrite(disulfide)

3 8.3 — 23.2 61.3 84.4 33.12 29.8 18.2 41.4 18.2 77.8 62.99 51.3 16.6 28.1 26.5 71.2 67.96 67.9 21.5 29.8 21.5 72.8 81.16 71.2 23.2 41.4 8.3 72.8 76.25 87.7 28.1 36.4 16.6 81.1 71.21 86.1 28.1 21.5 31.5 81.1 59.62 18.2 — 46.4 36.4 82.8 61.30 74.5 n.d.d n.d. n.d. n.d. 41.4

— — — — — 48

e total mass of sulfur in the unheated rock.

s after 7unheat

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pper range for � organic-pyrite for immature samples of this

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5320 A. Amrani, M. D. Lewan, and Z. Aizenshtat

rea (Dinur et al., 1980; Aizenshtat and Amrani, 2004a) asell as other locations of immature sedimentary rocks

Anderson and Pratt, 1995). The very large sulfur isotopicifference might indicate a very early open system forulfate reducing bacteria (SRB), which later became a closedystem resulting in much heavier �34S values of sulfateAizenshtat and Amrani, 2004a). This explanation is sup-orted by the �34S results of sulfates in the present study.ulfate S is only �8% of total S with �34S values rangingetween �5.7 to � 6.7 (Table 3). These relatively low �34Salues could not have resulted from residual marine sulfate�20 � 2‰). The extensive sulfate reduction evident in theormation of Type-IIS kerogen should have left the residualulfate even heavier than the original marine values. The big34S difference between the water-soluble (-5.7‰) and acid-oluble sulfates (�6.7‰) in the unheated rock can be ratio-alized by different times of crystallization. The acid-solu-le sulfate is mostly associated with carbonates, whichrapped sulfate during the earlier stages of formation (Staudtnd Schoonen, 1995) whereas the water-soluble sulfateould be from secondary oxidation of organic sulfur orulfide minerals. This is supported by the similar �34S valuesf water-soluble sulfate and the mineral sulfides (AVS(rock),able 3). Only a small amount of acid-volatile sulfide

AVS(rock)) minerals could be detected (� 0.1% w/w of total). We suggest that the pyrite is formed early, whereas theVS(rock) is formed later with secondary organic sulfur

nrichment. This is contrary to the results reported for theonterey Formation (California, USA; Miocene) by Zaback

nd Pratt (1992). It is possible that the timing of SRBctivity and the closing of the water-sediment interface wereifferent in the case of the Senonian basins, hence the mucheavier range of pyrite �34S values. The bitumen fraction ofhe unheated rock contains 2.4% of the total sulfur in theample with a �34S value that is heavier than the kerogen by‰ (Table 3). In most of the samples from this area (Mishorotem), the bitumen was 34S-enriched relative to the kero-en. However, the �34S values recorded for the variousulfur species of different sites in the Dead Sea area show

Table 3. Sulfur �34S (‰) data values measure for each sulfur specierror is better than �0.3‰ (n�2).

Pyrolysistemperature (°C)

Oil(OS)

Bitumen(BS)

Kerogen(KS) H

2S(gas) AVS(aq)

nheated — 2.6 0.600 — 0.8 0.7 �0.5 �1.360 0.7 0.8 1.0 1.1 1.300 1.6 1.5 1.6 0.2 0.520 1.7 1.4 2.0 0.9 1.040 2.4 1.9 2.6 1.2 0.265 2.3 2.0 2.1 2.0 1.440 drya — 2.3 2.0 0.540(iso.Kerogen)b 1.9 1.2 n.a.c 1.7 1.6nheatediso.kerogenb

— 0.6 — —

a Experiment with 200g rock and no addition of water.b Experiment with 40g isolated kerogen and 450g of distilled waterc Measurement not available.

ide variability (Aizenshtat and Amrani, 2004a) b

.2. Organic Sulfur

Hydrous pyrolysis experiments generate five phases that con-ain organic sulfur: kerogen (KS), bitumen (BS), expelled oil (OS),ater-soluble organic compounds, and sulfur containing gas com-ounds. We assume that the latter two are quantitatively negligi-le, and therefore, they are not addressed in the present study.able 1 shows the variations in these organic phases with increas-

ng experimental temperatures. The amount of bitumen increasesith temperature to 300°C and then decreases as generated ex-elled oil continues to increase with increasing temperatures. Thisitumen-oil relationship has been previously observed in hydrousyrolysis experiments and explained by kerogen initially decom-osing to a polar-enriched bitumen and subsequent decompositionf bitumen to hydrocarbon-enriched oil (Lewan, 1985). With thexception of bitumen above 300°C, the atomic S/C ratio decreasesn all the organic phases with increasing experimental temperatureFig. 1a). The residual kerogen in the lower-temperature experi-ents has the highest S/C and from 340°C the bitumen has the

igher S/C values. The wt% S in each organic phase (Fig. 1b)hows a similar decrease with increasing temperature for the oilnd kerogen but the sulfur content of the oil remains the same origher than the kerogen. Similar results have been reported forydrous pyrolysis experiments on Monterey kerogens (Baskin andeters, 1992; Nelson et al., 1995). The bitumen above 300°Chows an increase in sulfur with increasing temperature and thisncrease explains the essentially constant atomic S/C ratios ob-erved at the higher temperatures (Fig. 1a). Table 2 shows thatore sulfur is retained in the organic matter of the dry pyrolysis

xperiment than in the hydrous pyrolysis experiment at 340°C.his observation has been previously reported and attributed toulfur cross-linking reactions being promoted under dry pyrolysisonditions (Lewan, 1997).

Figure 1c shows the effect of experimental temperature onhe �34S of the organic species. It is clear from the results andn accordance with organic maturity parameters (Table 1) thats thermal maturity increases, the organic sulfur phases arenriched in 34S. Except for the unheated sample, the bitumen islways lighter than the residual kerogen, and the expelled oil

72 hrs hydrous pyrolysis at temperatures noted in table. Measurement

k)

TotalAVS SO4(aq) SO4(acid-rock) SO4(w-rock)

Totalsulfates

Pyrite(disulfide)

�4.6 6.7 �5.7 �2.3 �29.9�0.7 �5.7 1.3 �5.5 �1.9 �25.2

0.4 �6.1 2.9 �5.6 �2.2 �23.60.5 �5.9 3.6 �5.3 �1.8 �21.60.7 �5.9 1.5 �5.1 �1.6 �22.70.2 �5.7 2.5 �5.4 �1.9 �23.60.1 �5.5 5.5 �5.4 �2.5 �27.8

�0.5 — 0.9 �5.7 �2.0 �25.41.5 — — — — �29.3

— — — — — �29.2

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5321Stable sulfur isotope partitioning during simulated petroleum formation

34S differences between the various organic species are verymall (less than 1‰).

The overall change between the �34S of oil and the organiculfur in the kerogen of the unheated sample is about � 2‰.his small value was also observed in dry pyrolysis experi-ents of sulfur-rich kerogens from the Monterey Formation

Idiz et al., 1990).

.3. Sulfides

As shown in Table 2, the amount of sulfides in the gas phaseH2S(gas)) is about one order of magnitude higher than the otherwo sulfide phases (AVS (aq) and AVS (rock)). H2S (gas) has the

Fig. 1. Organic sulfur in residual kerogen, bitumen, and oil as aunction of hydrous pyrolysis temperature. (a) Atomic S/C atomicatios. (b) S wt%. (c) �34S (‰).

ighest mass of all the other sulfur species above 300°C. There s

s a steep increase in the amount of H2S (gas) from 200 to 300°Cnd a gentler increase at the higher temperatures (Fig. 2a). The34S of the H2S (gas) shows a broken trend of 34S enrichmentith increasing temperature (Fig. 2b). 34S enrichment occurs

rom 200 to 260°C and abruptly decreases at 300°C beforeontinuing to increase at the higher temperatures. This overall4S enrichment is consistent with the �34S trend of the organiculfur, but the H2S (gas) is generally lighter than the organiculfur species. AVS (aq) increases with increasing temperatureo 340°C and decreases at 365°C (Fig. 3a). �34S of the AVS (aq)

ncreases steeply between 200 and 260°C and then unsystem-tically fluctuates between 0.2 and 1.4 ‰ at the higher tem-eratures (Fig. 3b). Relatively small amounts of AVS (rock)

xist in the unheated rock (Table 2) and no significant changeccurs until 340°C. At this temperature, the amount ofVS (rock) begins increasing (Fig. 3a), which suggests forma-

ion of metal sulfides like FeS. Pyrrhotite or other FeS-crystal-ine forms was not detected in the recovered rocks using XRDnalyses. However, pyrrhotite is not detectable in concentra-ions equal to or less than 1 wt%. According to the amount ofVS (rock) in our samples, the maximum amount of pyrrhotite

hat could form is less than 0.3 wt%. Therefore, the XRDesults do not exclude the presence of pyrrhotite. The initial34S of AVS (rock) is �4.6‰. This value is lighter than thenitial organic sulfur (�0.6‰) and much heavier than theyritic sulfur (-29.9‰). There are big fluctuations in the �34Salues of AVS (rock) as a function of the experiment tempera-ures with no apparent trend (Fig. 3b). Mass balance calcula-ions were made (Eqn. 1) for total AVS forms (i.e., AVS(rock)

AVS (aq)). The amounts of this summation (AVS(total))ncrease with increasing temperature. The �34S of the AVS(total)

ncreases steeply to 320°C and then remains relatively constantt higher temperatures (Fig. 3b).

In the dry pyrolysis experiment, �34S of H2S (gas) andVS(rock) are lighter than in the hydrous experiment (Table 3).ith more of the sulfur being retained in the rock under dry

onditions (Table 2) as a result of more cross-linking, it isxpected that the �34S of H2S (gas) and AVS(rock) would beeavier under hydrous conditions. �34S of H2S (gas) andVS(rock) of the isolated-kerogen hydrous experiment are re-

pectively heavier by only 0.3 and 0.4 ‰ than those of thehole-rock hydrous experiment at 340°C (Table 3). This dif-

erence is within the measurement error.

.4. Sulfates

Figure 4a shows the quantitative distribution of sulfate spe-ies vs. experimental temperature. Except for the unheated and65°C samples, the SO4

2�(w-rock) and SO4

2�(acid-rock) are es-

entially mirror images with no systematic trends at the inter-ediate temperatures. The amount of SO4

2�(aq) remains essen-

ially constant from 200 to 260°C, but increases steadily at theigher temperatures. The total amount of sulfates shows aentle decrease from the unheated sample to 260°C, a constantoncentration from 260 to 320°C, an increase to 340°C, and aonstant concentration to 365°C. The unheated �34S values ofhe different sulfate species vary significantly from �5.7‰ forO4

2�(w-rock) to � 6.7‰ for SO4

2�(acid-rock) (Table 3). As

hown in Figure 4b, the water-soluble forms of sulfate have

imilar �34S values that suggest they have a similar source.

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5322 A. Amrani, M. D. Lewan, and Z. Aizenshtat

here is essentially no change in their �34S with increasingemperature. In contrast, acid-soluble sulfate �34S values areonsistently heavier and change dramatically with temperature.lthough this change is not systematic with temperature there

s a good negative correlation between the �34S and amount ofO4

2�(acid-rock) (Fig. 4c). This suggests addition of 34S-de-

leted sulfur species into SO42�

(acid-rock) crystalline form. Nosotopic exchange with other sulfur species is suggested be-ause the total sulfate mass-balance calculation shows smalluctuations within a standard deviation of � 0.3‰. The only

ransformations were between the sulfate species with additionr depletion of isotopically lighter water-soluble sulfates ando recrystallization of the initial acid-soluble sulfate. There-ore, we conclude that no significant thermochemical sulfateeduction (TSR) occurred within the source rock under ourxperimental conditions. The dry pyrolysis experiment hasimilar results to the hydrous experiment at the same temper-ture (i.e., 340°C) with no change in the amount and isotopicalues of total sulfates.

.5. Pyrite

Disulfide minerals are referred to as pyrite in this paperecause XRD analyses detected no other disulfide minerals.s expected (Anderson and Pratt, 1995; Aizenshtat andmrani, 2004a), pyrite is the most 34S-depleted species

elative to all other sulfur species. The �34S of pyrite in thenheated rock is �29.9‰ (Table 3). With increasing exper-

Fig. 2. H2S in the gas phase (H2S(gas)) as a function of hydrousyrolysis temperature (°C). (a) Amount of H2S(gas) in mgS/g Total S,here Total S refers to total sulfur in the original unheated rock. (b)

34S (‰) values of H2S(gas).

mental temperature (Fig. 5a), pyrite becomes isotopically T

eavier to a maximum of �21.6‰ at 300°C followed byecreasing 34S values at the higher temperatures. However,t does not return to the initial �34S value of the pyrite in thenheated rock. Because pyrite is the most 34S-depleted spe-ies in the system, this change in �34S may be the result ofi) isotope exchange with heavier sulfur species or (ii) ad-ition of heavier species of sulfur cleaved off the organicatter during the hydrous pyrolysis experiments to form

econdary pyrite. The first explanation (i) is ruled out be-ause the nonpyritic sulfur species should be 34S depletedccording to mass balance calculations, however, these sul-ur species become 34S enriched (Table 3). The secondxplanation (ii) is more likely and is supported by thencrease and decrease in the amount of pyrite that parallelshe changes in �34S of the pyrite (Fig. 5a and 5b). The pyriteuantity positively correlates with the �34S values (Fig. 5c).his suggests secondary pyrite formation from sulfur species

hat are isotopically heavier than the initial pyrite at theower temperatures and then, as the temperature rises above00°C, pyrite starts to decompose. This is supported by theatio of pyrite to indigenous quartz determined by XRDnalysis in the recovered rocks (Fig. 5d). The results show aimilar trend to that in Figure 5 with the exception of peakecondary pyrite formation occurring at 320°C and not at00°C, as shown by the quantitative and isotopic analyses.his discrepancy is considered to be a result of the semi-uantitative character of the XRD method. The dry pyrolysis

Fig. 3. Acid-volatile sulfur species in the recovered water (AVS(aq)),cid-volatile monosulfides in the recovered rock, (AVS(rock)), and sum-ation of acid-volatile sulfur in the water and the rock (AVS(total)) asfunction of hydrous pyrolysis temperature (°C). (a) Amount in mg S/g

otal S in original rock. (b) �34S (‰) values.

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5323Stable sulfur isotope partitioning during simulated petroleum formation

xperiment showed slightly lower pyrite formation and �34Sn comparison with the hydrous experiment at the sameemperature.

.6. Sulfur Mass Balance

The isotope mass balance was calculated according to theeneral equation

Fig. 4. Sulfate in the recovered water (SO4(aq)), water-soluble sulfaterom the rock (SO4(w-rock)) and acid-soluble sulfate in the rockSO4(acid-rock)) and total sulfate as a function of hydrous pyrolysisemperature (°C). (a) Amounts in mg S/g Total S in original rock. (b)34S (‰) values. (c) �34S (‰) of acid-soluble sulfate as a function ofts amount in mg S/g Total S in original rock.

�T � x�SO4� y�pyrite � z�organic. . . (1) l

here � values are in ‰ for each sulfur species, x,y,z. . ., whichepresent the mass fractions of sulfur in each form. The calcu-

Fig. 5. Disulfite (pyrite) as a function of hydrous pyrolysis temper-ture (°C). (a) �34S (‰) values. (b) Amounts in mg S/g Total S inriginal rock. (c) �34S (‰) of pyrite as a function of amount of pyriteg S/200 g rock). (d) Semiquantification of pyrite relative to indigenousuartz by X-ray diffraction.

ated results showed very similar �34S values for all experi-

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5324 A. Amrani, M. D. Lewan, and Z. Aizenshtat

ents with an average of �0.9 � 0.4‰. This compares wellith the unheated whole rock �34S value of �0.7 � 0.2‰.The distribution of all sulfur species during the hydrous

yrolysis experiments is characterized mainly by the sharpecrease of sulfur content in the kerogen and the formation ofarge quantities of H2Sgas during this process (Fig. 6).

The major loss of sulfur is from the kerogen, which contains85 to 4 wt % of total sulfur in the rock. Most of this lost sulfur

ccurs as H2Sgas that accounts for as much as �62 wt % of theotal sulfur and 70 wt% of the organic sulfur (365°C). Thisassive loss of sulfur is responsible for the �34S changes in all

ulfur species (except sulfate) during thermal alteration. Theest of the sulfur is divided between the organic bonded sulfur,yrite, sulfate and AVS (Fig. 6). The AVS fraction increasesith increasing experimental temperature up to 9 wt %. Sulfateis �8 wt % of total S with very small fluctuations as the

xperiment temperatures increase. The pyritic sulfur fractiontarts at 3.3 wt % and peaks at 300°C reaching 8.3 wt % of totalock sulfur. The bitumen sulfur increases up to 22 wt % at60°C, and then decreases to 3 wt % at the highest pyrolysisemperature (365°C). The expelled oil is formed mostly fromecomposition of the bitumen. The expelled oil sulfur reacheshe highest value at 340°C (8.6 wt %) and decreases slightly to.3 wt % at 365°C.

4. DISCUSSION

.1. Isotopic Equilibrium: Open or Closed System

The present set of hydrous pyrolysis experiments was con-

Fig. 6. Sulfur content in each sulfur species in mg S/g Total S inriginal rock as a function of hydrous pyrolysis temperature (°C) andhe unheated sample.

ucted in a closed thermally controlled system. Hence, reactive c

ulfur species may equilibrate under in situ conditions (See-ald, 2001). These reactions in turn can lead to isotopic ho-ogenization of the different sulfur species in the system.uring our experiments, high amounts (Table 2) of sulfides areroduced and maintained through several reversible processeshat include different sulfur species and phases. The mostbvious equilibrium reactions in our system are between

2S(gas) and AVS(aq) as follows:

H2S(gas)^H2S(aq) (2)

H2S(aq)^HS(aq)� � H(aq)

� (3)

n each of our experiments, we produce three very differentressure and temperature conditions. One is during hydrousyrolysis, where both experimental temperature and pressurere maintained for 72 h. The second is during cooling of theeactor (at least 18 h) resulting in decreasing temperature andressure. The third is after cool down when pressure decreaseshile collecting the generated gases (76–93 kPa; see Experi-ental). In each stage, there are different equilibrium constants

nd therefore, different partitioning between the gas and aque-us phases. Because we did not perform in situ measurementse deal with the overall changes resulting from these three

tages. Comparison of the �34S of AVS(aq) species to the

2S(gas) (Fig. 7) shows similar values (except for the 340°Cxperiment), which suggests an isotopic equilibrium betweenhese sulfide species has been reached. In contrast, the metalulfides represented by the AVS(rock) show no correlation be-ween their �34S values and those of the other sulfides (Table). This lack of correlation suggests that the AVS(rock) withinhe bitumen-impregnated rock may not be in equilibrium withhe other sulfides at the collection conditions. Laboratory ex-eriments with 35S-labeled sulfur species showed rapid (lesshan 1 h) isotope exchange between H2S, S, Sx

�2 and FeS at aH 7.6 and 20°C (Fossing and Jorgensen, 1990). They suggesthat this isotope exchange occurs via polysulfide formation.

hen no elemental sulfur is present in the system no polysul-des are formed (under reducing conditions). Hence, HS� andeS reach only partial isotope exchange, at a relatively slowate (24 h). In our experiments, only negligible amounts oflemental sulfur were detected. Moreover, at pH values of 6.6

0.1 that prevail in our experimental system at ambient

Fig. 7. �34S (‰) values of H2S(gas) and AVS(aq) as a function ofydrous pyrolysis temperature (°C).

onditions during product collection, polysulfides are not stable

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5325Stable sulfur isotope partitioning during simulated petroleum formation

Schwarzenbach and Fisher, 1960; Boulegue, 1978; Bouleguend Michard, 1978). This can explain the �34S differencesetween metal sulfides and other sulfide species in our system.Significant amounts of H2S(gas) that we collected under re-

uced pressure are actually dissolved in the water phase andrganic phases at the experimental temperatures. At these ex-erimental temperatures, two main processes occur simulta-eously: (1) Sulfur cleavage from the organic matter as radicalsxtract hydrogen to form stable bisulfide or hydrogen sulfide;nd (2) Sulfur introduction into organic matter mainly viaadical attack (Aizenshtat et al., 1995; Krein and Aizenshtat,995). The rate of the first process must be higher than the latterecause there is a decrease in the sulfur content of the organicatter with increasing temperature (Fig. 1a,b). These processes

ield stabilized organic sulfur mainly as thiophene functional-ty (Nelson et al., 1995). Seewald (2001) suggested aqueousrganic compounds and iron-sulfide minerals (pyrite, pyrro-otite, magnetite) attained metastable thermodynamic equilib-ium under hydrous pyrolysis conditions (300 to 350°C). Dur-ng the stabilization and rereaction of sulfur with the organicatter (Krein and Aizenshtat, 1995), isotopic exchange with

he large sulfur pool involving HS�, H2S, and FeS could occurnder equilibration conditions. In a closed system, this wouldinimize the larger possible isotope kinetic effect during the

leavage of C-S and S-S bonds (Aizenshtat and Amrani, 2004b;mrani et al., 2005b).In contrast, in an open-system the generated sulfur would be

mmediately removed from the system and a maximum isotoperactionation would occur. Previous experiments conductednder open-system conditions using rocks and isolated kerogenith high-sulfur contents, showed a much higher isotopic dis-

rimination between the released H2S and the residual organiculfur fractions of kerogen and bitumen (Stoler et al., 2003).imilar results were obtained for a synthetic polysulfide cross-

inked polymer in open-system pyrolysis (Amrani et al.,005b). The gradual increase in temperature (1 to 10°C/m.y.),he limited void space in fine-grained rocks (5 to 10% porosity),nd high lithostatic pressures (70 to 150 MPa) in the subsurfaceake it difficult to categorically assign a closed or open system

o natural thermal maturation and petroleum generation. Notinghe amount of isotopic fractionation in natural subsurface sulfurpecies may help determine the level of openness in naturaleological systems.

.2. Pyrite Stability

.2.1. Secondary Pyrite Formation and Decomposition

It has been shown that the secondary pyrite formed at tem-eratures of 300°C or less is 34S enriched compared with thenheated pyrite (Fig. 5a). Formation of the secondary pyriteequires a source of additional iron in the rock. This is sup-orted by the observation that no secondary pyrite appears toave formed in the isolated-kerogen experiment at 340°C. Themount of pyrite in this experiment is �40 wt% less than thatn the whole-rock experiment at the same temperature (Table). In addition, the �34S of the pyrite in the isolated-kerogenxperiment (-29.3‰) is similar to that of the original unheatederogen (-29.2‰).

Figures 5a and 5b show that pyrite decomposition above d

00°C leads to 34S-depletion. This trend can be interpreted toesult from the isotopically heavier secondary pyrite beingreferentially decomposed relative to the original pyrite. It isossible that this secondary pyrite is less stable or more sus-eptible to decomposition. Previous studies on pyrite structurend its abundance in samples of bituminous rocks of the Deadea area have reported that pyrite occurs as both framboids andingle crystals (�50�), which are typically coated with organicatter (Spiro, 1980; Spiro et al., 1983a,b). This coating may

rotect the original pyrite from decomposition at the higheremperatures. Conversely, some of the secondary pyrite mayeside in the contact zone of the water-rock interface making itore susceptible to aqueous dissolution resulting in its prefer-

ntial degradation.Seewald et al. (1994) suggested pyrite recrystallizes to pyr-

hotite between 325 and 400°C in hydrous pyrolysis experi-ents according to the following equation:

FeS2 � H2 →FeS � H2S (4)

he presence of molecular hydrogen and negative Eh valuesEh � �345 to �373 mV) measured at room temperatureuring product collection suggest that reducing conditions ex-sted and therefore, could have been favorable for pyrrhotiteormation. Figure 8 shows the sharp increase in AVS(rock) ashe pyrite abundance decreases at temperatures higher than00°C. Calculation of pyrite S mass loss from the peak forma-ion at 300 to 365°C and the S mass added to the AVS(rock),eveal similar quantities (�0.15 g). AVS(rock) represents sulfideontaining minerals, which includes pyrrhotite. If pyrrhotite ishe main mineral being formed, then an additional source of Fes needed. According to Eqn. 5, 1 mol of pyrite produces 2 molf iron monosulfide:

FeS2 � Fe2� � H2 →2FeS � 2H� (5)

imilar to the needed Fe for formation of the secondary pyrite,he Fe needed to form an iron monosulfide like pyrrhotite isikely to have come from the rock. Unlike other carbonaceousocks of the Zefe Efe’ deposit, the Ghareb Limestone is rela-ively rich in Fe with values from 0.8 to 1.1 wt% (Spiro, 1980).he peak formation of the secondary pyrite requires only auarter of this Fe, with the remaining Fe being more thanufficient to source formation of pyrrhotite.

.2.2. Geochemical Implications for Secondary PyriteFormation

Pyrite is believed to be the isotopic marker for bacterialeduction of sulfate during diagenesis (Anderson and Pratt,995; Goldhaber, 2004; Aizenshtat and Amrani, 2004a). How-ver, the results of the present study provide new insights onhe applicability of pyrite as an isotopic marker in thermallyature rocks. In natural maturation, thermal decomposition of

yrite is not expected, because the catagenic temperatures areuch lower than in our experiments. However, other chemical

onditions may develop and effect pyrite stability (Seewald,001). It is suggested that during catagenetic processes, sec-ndary and isotopically heavier pyrite is formed (� 300°C) asong as available iron is present, and therefore, the isotopic

ifference between pyrite and organic sulfur decreases. This

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5326 A. Amrani, M. D. Lewan, and Z. Aizenshtat

ay erase the original diagenetic isotopic signature and quan-ity of pyrite and cause erroneous interpretations of the earlyiagenetic conditions of a rock.

.3. �34S Value of Thermally Formed Early CleavedSulfur

.3.1. Direct Measurement

The kinetic-isotope effect (e.g., Coplen, 1993) suggestseaker bonds will preferentially break, i.e., 32S-S, C-32S, with

he cleaved sulfur being depleted in 34S and the residual or-anic sulfur being enriched in 34S. To evaluate this kinetic-sotope effect during thermal maturation, we have to evaluatehe �34S of the early unequilibrated sulfur species that wereleaved from the thermally matured organic matter. Figure 9hows that H2S(gas) produced at the end of each heating exper-ment is always lighter than the kerogen by up to 1.4‰ with thexception of the 260°C experiment. At the lowest experimentalemperature (200°C), the �34S of the released H2S(gas) was

0.5‰, which is 1.1‰ lighter than the original kerogen thatroduced it. Isotope mass balance of AVS(total) shows �34Salues up to 2‰ lighter than the initial organic sulfur. Since theresent experimental system is closed, the early released andighter isotopes may equilibrate with other sulfur species (asiscussed previously) leading to a smaller apparent fraction-tion.

In the 340°C experiment, a new silver coated gasket wassed to seal the reactor. Upon opening the reactor at the end ofhe experiment, Ag2S crystals were observed on the reactorasket. The �34S of these crystals was �2.2‰, which is moreepleted in 34S than the H2S(gas) by 3.4‰, the unheated kero-en by 2.8‰, and the heated residual kerogen by 4.8‰. Themount of the sulfur deposited on the silver-coated gasket ismall and cannot balance the isotopic fractionation of the otherulfur species (e.g., H2S and organic). However, assuming itepresents early formed H2S(gas), it provides evidence that thearly released sulfur is isotopically lighter than the later-formedulfur in the system. The magnitude of this early fractionationor synthetic polysulfide cross-linked polymers and isolatederogen has been shown to be in the range of 6 to13.9‰ as

Fig. 8. Amounts of pyrite and acid-volatile monosulfides in the rockAVS(rock)) (g S/200 g rock) as a function of hydrous pyrolysis tem-erature (°C).

etermined by pyrolysis experiments under closed and open a

essel conditions (130 to 300°C) for short time periods (Am-ani et al., 2005a; b).

.3.2. Isotope Mass Balance Calculations of SecondaryPyrite �34S

It is evident from the experimental results that virtually allulfur species (except sulfate) become isotopically heavier rel-tive to the unheated rock with increasing thermal maturationTable 3). However, a good isotope mass balance is obtainedor all the experiments and indicates that lighter isotopes muste incorporated into one or more of the different sulfur phases.s discussed in the pyrite section, secondary pyrite formationas been observed. The sulfur source for this pyrite is probablyulfur species that are cleaved from the kerogen. To determinehe isotopic value of this additional sulfur, isotopic mass-alance calculations were made on pyrite. These calculationsre based on the assumption that the original pyrite did noteact and therefore has the initial unheated pyrite �34S value.he following isotope mass balance equation was used:

34Ssecondary pyrite���34Spyrite �T�i� � �1 � x��34Spyrite �T�� ⁄ �x� (6)

Fig. 9. �34S (‰) values of residual kerogen sulfur (KS) and H2S(gas)

s a function of hydrous pyrolysis temperature (°C).

Fig. 10. �34S (‰) values of the incremental and cumulative sulfurdded to form secondary pyrite as a function of hydrous pyrolysisemperature to and including 300°C. Above 300°C, the incremental andumulative �34S (‰) values represent the released sulfur formed byecondary pyrite decomposition. These values were calculated accord-ng to mass balance Eqns. 6 and 7 based on data presented Tables 2

nd 3.

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5327Stable sulfur isotope partitioning during simulated petroleum formation

Where T � previous temperature, i � increment tempera-ure, and x � the secondary pyrite S mass fraction added to theyrite at the previous pyrolysis temperature. The �34S values ofhe additional sulfur according to these calculations are shownn Figure 10. The early production of secondary pyrite at 200°Cave a calculated �34S value of �20.3‰ and the largest quan-ity of added sulfur. The incremental secondary pyrite between00 and 260°C has a much heavier �34S of �0.6‰ thatecreases at higher temperatures (260 to 300°C) to �12.4‰.he cumulative secondary pyrite formed from unheated to00°C has a calculated �34S of �16.1‰. These calculationsuggest that the early cleavage S from the kerogen is isotopi-ally much lighter than what was measured in any of the otherhases. These values can balance the other isotopically heavierulfur species in the organic and gas phases.

The decomposition of pyrite above 300°C releases isotopi-ally heavier sulfur in comparison to the unheated pyrite asetermined by the following mass balance equation:

�34Sreleased pyrite���34Spyrite �T� � �1 � y��34Spyrite �T�i�� ⁄ y (7)

here y is the pyrite S mass fraction, released from the pyritet a previous temperature greater than 300°C. According tohese calculations, the released sulfur value is between �1.4‰o �12.4‰ (Fig. 10), which suggests secondary pyrite prefer-ntially decomposes at higher temperatures.

The results of these mass-balance calculations demonstratehe important role of sulfide minerals in trapping the earlyeleased and light sulfur isotopes that were produced by ther-al decompositions of organic sulfur. The removal of light

ulfur isotopes from the system gradually increases the 34Snrichment of the residual sulfur fractions, and especially therganic sulfur. The magnitude of this fractionation is dependentn the amount of available iron or other metals in the system.

.4. Reaction Mechanisms of Organic Phases

The present work identifies trends that are based on theransformation of kerogen to bitumen and bitumen to oil (Le-an, 1983; Baskin and Peters, 1992) as well as the coinciding

ulfur-functionality transformations. The initial steep part of

2S(gas) generation below 300°C (Fig. 2a) mainly representshe cleavage of weak poly sulfide (S-S) bonds in the kerogen ast decomposes to bitumen (Nelson et al., 1995). Initially, theoly 32S-32S bonds will be preferentially broken to give 34S-epleted H2S(gas) at relatively low temperatures (Fig. 2b). Thisarly-cleaved sulfur reacts with available iron to produce sec-ndary pyrite (Fig. 5b) with the excess being released to theater and gas phases. As the reaction proceeds with increasing

emperature, more of the 32S-34S bonds will be broken and the

2S(gas) will become more 34S enriched. During this stage,ome of the kerogen sulfur is also transferred to the bitumen,robably as C-S bonds in the polar fraction are cleaved. Ashown in Table 1, the amount of bitumen is still increasing at00°C, but some of it is breaking down to a hydrocarbon-nriched oil. This represents the breaking of more C-S bonds inhe bitumen, which results in free radicals that initiate theow-thermal stress cleavage of C-C bonds in bitumen to formil (Lewan, 1998). This C-C cleavage may also be enhanced by

ulfide minerals (Seewald, 2001) that formed and decomposed w

Fig. 8). The cleavage of C-S bonds in bitumen to oil wouldenerate less H2S than the cleavage of S-S bonds from kerogeno bitumen (200 to 300°C) as shown in Figure 2a by the gentlerlope. The �34S plot in Figure 2b suggests this new source of

2S is initially very 34S depleted, which when added to thearly poly S-S source results in a lowering of the �34S value. Asore C-S bonds are broken in the bitumen with increasing

hermal maturation, more of the stable C-34S bonds break,hich results in an increase in the �34S value as shown inigure 2b. As shown in Figure 6, the sulfur content in theitumen first increases to 260°C and the �34S of the generateditumen remains constant (Fig. 1b). As thermal maturity in-reases, the sulfur in the bitumen decreases as the bitumenecomposes to oil. During this process, the bitumen �34Secomes lighter compared to the oil due to the more stable-34S bonds that are broken. The amount of S in the oil that is

orming from the decomposition of the bitumen increases ashown in Figure 6 to 340°C and remains essentially constant to65°C. The �34S values of the oil during this stage becomeeavier, and are heavier than coexisting bitumen with increas-ng thermal maturation. When the oil sulfur yield becomesssentially constant from 340 to 365°C, so do the �34S values.he similar quantities of H2S generated in the dry pyrolysis

gas � 6.26 wt% S) and in the hydrous experiment (aqueous �as � 6.28 wt% S) indicate that the added water in hydrousyrolysis experiments is not a critical source of hydrogen forhe formation of H2S (Table 2).

.5. Implications for Petroleum Correlations

.5.1. Hydrogen Sulfide–Oil Correlation

It is evident from our experiments that the thermally formed

2S(gas) is 34S depleted in comparison to the coexisting oil,lthough the difference is relatively small. Therefore, we wouldxpect that if the sole source of H2S(gas) is the thermal desul-urization of organic matter in a source rock, �34S of theenerated H2S(gas) will be the same or slightly depleted in 34Sompared to generated oils under closed-system conditions.here are only a few studies that report �34S of oil and asso-iated hydrogen sulfide in natural reservoirs. These studiesnvolved well-developed oil fields (10–30 yr), which couldffect the original isotopic characteristic of the gases (Vreden-urgh and Cheney, 1971). Therefore, caution should be used innterpreting these data. In most of these �34S measurements,

2S(gas) was found to have values close to those of its coex-sting oil (Thode et al., 1958) or 34S enriched (Vredenburgh andheney, 1971; Orr, 1974). Thode et al. (1958) explain this by

mall isotopic fractionation between oil and hydrogen sulfide.redenburgh and Cheney (1971) explained the 34S enrichmentf hydrogen sulfide by preferential removal of the 32S fractionf the H2S(gas) by differential migration of the oil and gas or byhemical reaction with metals in the carrier or reservoir bedsuring migration. However, it is known that only negligibleractionation of the sulfur occurs in the formation of metalulfides (up to 1‰, e.g., Price and Shieh, 1979; Wilkin andarnes, 1996; Bottcher et al., 1998) and therefore, even ifortions of the gas reacted with metals, �34S of the residual gas

ould not change significantly. Moreover, additional thermally

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5328 A. Amrani, M. D. Lewan, and Z. Aizenshtat

leaved H2S will be 34S depleted in comparison the organiculfur as discussed previously.

Additional problems in using �34S in correlating H2S(gas) toil may arise from reservoir processes that introduce microbialources of H2S or TSR. Microbial production of H2S requiresower temperatures and in most cases generates a distinctlyight �34S value. In deep high-temperature reservoirs, TSR ishe most reasonable way to produce H2S that is isotopicallyeavier than the coexisting oils (Orr, 1974; Worden et al.,003). As a result of these potential additional sources of H2Sn reservoirs, �34S correlations of hydrogen sulfide to oil are notikely to be reliable.

.5.2. Source Rock–Oil Correlation

Two main hypotheses on the �34S relationship betweenource rock and oil can be considered. The first is that onlyegligible �34S fractionation occurs during thermal cleavage ofrganic sulfur, and therefore, all sulfur fractions (except foryrite and sulfate) in a given maturation system have similar34S values (Orr, 1974, 1986). Orr (1986) concluded thatenerated oils have up to 2‰ enrichment relative to theirource-rock kerogen. This suggests that �34S is a good param-ter for source rock-to-oil correlations. Laboratory pyrolysesurther supported this assumption (Idiz et al., 1990). Similarly,he results of the present study show fractionation within 1‰etween kerogen, bitumen and expelled-oil. Despite the agree-ent of these results with this hypothesis, some cautionary

oints need to be made. The present study shows that �34Sractionation during early thermal cleavage of organic sulfur isarge, with values as high as 21‰. Therefore, under someatural conditions, greater differences in �34S values betweenenetically related oils and H2S gas might be possible, partic-larly in the early stages of oil and gas generation. It is alsomportant to realize that the �34S of oil is typically the collec-ive product of different lateral and stratigraphic facies of aaturing source rock. Therefore, the certainty of an oil corre-

ation will always be dependent on how representative thenalyzed rock samples are of the maturing source rock and itsrganic facies. This caution is a concern in all oil-to-sourceock correlations (Waples and Curiale, 1999) and not restrictedo the use of �34S values.

The other hypothesis is that there is a large negative frac-ionation between H2S and organic-sulfur fractions, and as aesult, the organic sulfur fractions become 34S enriched (Aizen-htat and Amrani, 2004b). This hypothesis is based on theesults of open-system pyrolyses on isolated kerogen and bitu-inous rocks of the Dead Sea area (Stoler et al., 2003) and on

losed- and open-system pyrolysis experiments on a syntheticolymer and an isolated kerogen (Amrani et al., 2005a). Resultsrom these experiments show large isotopic discrimination be-ween H2S(gas) and the residual organic sulfur. Additional sup-ort can be given by the finding that oils found in the Dead Searea are generally significantly 34S enriched in comparison toheir source kerogens (e.g., Aizenshtat and Amrani, 2004b). A

ore open system is suggested to be responsible for this greaterractionation.

During thermal maturation of Type-IIS kerogen, highmounts of H2S(gas) are produced. We report in the present

tudy that more than 70 wt% of the original organic sulfur of r

he kerogen (Fig. 6) is converted to H2S(gas), which is dissolvednto the water, expelled oil and bitumen at elevated tempera-ures (�300°C). These results suggest that in natural systemshe huge amounts of generated H2S have to accumulate some-here or escape from the system. Accumulation could occur

hrough formation of metal sulfides (Worden et al., 2003)imilar to the secondary formation of pyrite in our experiments.his process can happen in the source rock or during migration

n carrier beds that contain available iron or other metalsSeewald, 2003). In addition, H2S is highly soluble in waterSuleimenov and Krupp, 1994) and much of it could be lost asn aqueous phase to formation waters of carrier beds or todjoining formation waters by diffusion. Preferential loss ofsotopically light H2S during maturation and migration from itsource kerogen could yield 34S-enriched oil. In most cases,etroleum reservoirs are spatially separated from their sourceock indicates that the generation, expulsion, and migration ofil are open-system processes (Seewald, 2003). This may causeignificant isotopic discrimination between the oil and itsource kerogen. The magnitude of this discrimination is sug-ested to be highly dependent on the mineral facies of eacheservoir and carrier bed. Therefore, correlation between oilnd its source rock using �34S measurements can serve as aeliable tool only when thermal maturation and petroleumeneration are in a more closed system.

.5.3. Oil-Oil Correlation

�34S values of the early- and late-generated expelled oilsave a narrow range (0.7 to 2.4 ‰), which is in generalgreement with some studies of natural crude oils. Thode andonster (1970) reported �34S values with a standard deviation

f �2.6‰ for oils from 12 fields extending over a distance of20 km in northern Iraq. Gaffney et al. (1980) also reported34S values with a small standard deviation of �2.1‰ forurassic oils of different thermal maturities from an unspecifiedasin. Oils in the Winnipeg–Red River, Bakken-Madison, andennsylvanian-Tyler systems in the Williston basin had �34Standard deviations of �1.2, �2.8, and �0.7 ‰, respectivelyThode, 1981). Vredenburgh and Cheney (1971) reported a34S standard deviation of �2.8 ‰ for Pennsylvanian-Permianrude oils representing a wide range of thermal maturities in theind River Basin. Manowitz et al. (1990) reported a �34S

tandard deviation of �1.8 ‰ for Bolivar coastal field oils thatxtend over 2500 km2 in northern Venezuela. Oils and heavyils from the Dead Sea area that are considered to be sourcedrom the Ghareb Limestone have a mean �34S value of 1.6 ‰ith a standard deviation of �1.2 ‰ (Aizenshtat and Amrani,004b). These natural oil values compare well with the 0.7 to.4 ‰ �34S values for the oils generated from the sample ofhareb Limestone used in this study (Table 3).Although this agreement between laboratory-generated oils

nd natural crude oils encourages the use of �34S in oil-oilorrelations, some caution should be taken in correlations thatnvolve oils that have been altered by water washing andiodegradation. Asphaltic tars in the Dead Sea area show auch wider range of �34S values (0.0 � 4.7‰, Aizenshtat andmrani, 2004b) than observed in the laboratory-generated oils

nd natural crude oils. This wider range of values may be a

esult of variations in the �34S of kerogen locally responsible

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5329Stable sulfur isotope partitioning during simulated petroleum formation

or the asphaltic tars or alteration of the original oils or bitumeny water washing and biodegradation. Thode (1981) showslteration of Williston-basin oils by this type of process resultsn �34S values becoming heavier by almost 7‰. Another con-ideration in using �34S values for oil-oil correlations is thermalaturity of the oil. Although changes in �34S values of the

ydrous-pyrolysis oils became heavier by only 1.7‰, greaterractionations due to thermal maturation of natural crude oilsave been reported. Orr (1974) reports mean �34S valueshanging from �3.5 to 6.3 ‰ in oils from the Big Horn Basinith increasing thermal maturity. Vredenburgh and Cheney

1971) reports mean �34S values from �5.0 to � 5.0, fromigh-sulfur, low API gravity shallow crude oils to the lowulfur higher gravity oils below present depths of 3300 m,espectively. As discussed earlier, this larger difference in someatural crude oils may be related to a more open-system duringatural oil generation than the closed-system used in hydrousyrolysis. Lastly, �34S variations in different organic facies ofmaturing source rock may also be responsible for the larger

ariations observed in some natural crude oils. Dinur et al.1980) report �34S values of organic-sulfur kerogen in Gharebimestone of the Dead Sea area from �10.7‰ in the south

Negev) to � 8.3‰ in the north (Nebi Musa).

5. CONCLUSIONS

The present study reports quantities and stable isotopes onrganic and inorganic sulfur species at different levels of ther-al maturation and petroleum formation as simulated in labo-

atory-pyrolysis experiments. The following main conclusionsave been made on the basis of these hydrous pyrolysis resultsor Type-IIS kerogen in the Ghareb Limestone.

1. The most dominant sulfur transformation is the generationof H2S(gas), which accounts for as much as 70 wt % of theoriginal organic sulfur. The H2Sgas formed during pyroly-sis is isotopically lighter than the sulfur in the kerogen andoil.

2. The early sulfides released are up to 21‰ depleted in 34Srelative to the original unheated kerogen. These early-generated sulfides react with available metals and reachequilibrium with other sulfur species in the system. Thisresults in narrowing of the isotopic differences between allactive sulfur species.

3. Secondary pyrite that is isotopically heavier than the orig-inal pyrite forms from the early sulfides released by thedecomposition of kerogen at experimental temperaturesequal to or less than 300°C. As a result, the �34S of the totalpyrite becomes 34S enriched by as much as 8.3 ‰.

4. At experimental temperatures above 300°C, secondary py-rite begins to preferentially decompose relative to the orig-inal pyrite. As a result, the �34S of the total pyrite becomes34S depleted by as much as 6.2 ‰.

5. Caution should be used in reconstructing diagenetic con-ditions based on �34S of pyrite and kerogen in thermallymature source rocks. Formation and decomposition of iso-topically heavy secondary pyrite during thermal maturationcan mask the �34S of the original diagenetic pyrite.

6. With the increase of temperature, the organically bonded

sulfur is enriched in 34S by as much as 2‰ compared with

the initial �34S values. Differences in �34S between kero-gen, bitumen and expelled-oil are within 1‰.

7. The main transformations of kerogen-to-bitumen and bitu-men-to-oil are reflected in the sulfur content and �34S ofeach phase, as well as in the amount and �34S of thegenerated H2S(gas). These changes can be attributed in partto the breakage of different C-C, C-S, and S-S bond types.

8. �34S values can be used in oil-oil correlations, but the lackof a correlation may indicate mineral facies changes in thematuring source rock or a more open-system during ther-mal maturation. More open systems are expected to havegreater differences in �34S values of oils at different ther-mal maturities.

9. If sufficient information is available on the major sulfurspecies (e.g., kerogen, oil, pyrite, H2S and sulfate), thendifferences in their �34S values may be used to evaluate theopenness of a thermally maturing system.

0. No TSR or formation of sulfate was observed under ourexperimental conditions.

1. The presence of water in the experiments at 340°C did notmake a significant difference in the amount or �34S of thegenerated H2S relative to the dry pyrolysis experiment.However, it did make a significant difference in the amountof the sulfur retained in the kerogen by more than a factorof two and a difference in �34S of 0.6‰.

cknowledgments—The authors wish to thank Eli Tannenbaum ofimron Oil & Minerals for promoting this collaborative project and forelpful suggestions. Michele Tuttle and Cyrus Berry of the USGS areppreciated for their invaluable analytical support in sulfur speciations well as fruitful discussions. Appreciation is extended to Michaelrible, Pam Gemery-Hill and Augusta Warden of the USGS for tech-ical assistance in elemental analyses, supplemental �34S determina-ions, and gas analyses, respectively. Alon Amrani thanks the Europeanssociation of Organic Geochemists (EAOG) for the Student Travelward that enabled his work at the USGS in Denver. The Riegeroundation is also gratefully acknowledged for Alon Amrani’s Ph.D.tudy Award. We also thank the Israel Academy of Sciences and theinerva Foundation for the GC-EA-IRMS instrument, which was used

or the �34S determinations in this study. Reviewers Geoffrey Ellis,avid Manning, Richard Worden, Michele Tuttle and John Curtis are

hanked for their suggestions and helpful comments.

Associate editor: J. Seewald

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