Thiadiamondoids as proxies for the extent of thermochemical sulfate reduction

Organic Geochemistry 44 (2012) 53–70

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Thiadiamondoids as proxies for the extent of thermochemical sulfate reduction Zhibin Wei a,⇑, Clifford C. Walters b, J. Michael Moldowan c, Paul J. Mankiewicz d, Robert J. Pottorf a, Yitian Xiao a, Will Maze a, Phuc T.H. Nguyen a, Marlene E. Madincea a, Ngami T. Phan a, Kenneth E. Peters c,e a

ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, TX 77252-2189, USA ExxonMobil Research and Engineering Company, Annandale, NJ 08801-0998, USA Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA d ExxonMobil Exploration Company, 233 Benmar, Houston, TX 77060, USA e Schlumberger Information Solutions, Mill Valley, CA 94941, USA b c

a r t i c l e

i n f o

Article history: Received 12 January 2011 Received in revised form 22 November 2011 Accepted 23 November 2011 Available online xxxx

a b s t r a c t Thiadiamondoids have been analyzed in a suite of Smackover-derived oils from the US Gulf Coast to determine whether their abundance and distribution reflect alteration by thermochemical sulfate reduction (TSR). The sample suite includes oils and condensates having various thermal maturities that are characterized as being unaltered by TSR, altered by TSR, or of uncertain affinities due to inconsistencies between conventional geochemical indicators of TSR. Nearly all samples contain thiadiamondoids, indicating that small amounts of these compounds can be generated from sulfur rich kerogen. TSR results in the generation of H2S, sulfides and thiophenic aromatic hydrocarbons, either by reaction with sulfate or by back reactions with the evolved H2S. Evidence shows that thiadiamondoids originate exclusively from reactions involving TSR. Once generated, their high thermal stability permits thiadiamondoids to accumulate with little further reaction and their abundance reflects not only the occurrence of TSR, but the extent of the alteration. The abundance of thiaadamantanes (1-cage structures) is particularly diagnostic of the onset of TSR. Examination of condensates from reservoirs >180 °C indicates that the thiadiamondoids can be thermally degraded. They are more thermally stable than the dibenzothiophenes, but are less stable than diamondoid hydrocarbons. Their stability appears to increase with increasing cage number, suggesting that the thiatriamantanes are the best proxy for the extent of TSR alteration in very high temperature reservoirs. Polythiadiamondoids (diamondoids with multiple sulfur substitutions) were detected in trace amounts and are also indicators of TSR. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Thermochemical sulfate reduction (TSR) is a reservoir alteration process in which petroleum hydrocarbons are oxidized by inorganic sulfate, ultimately yielding CO2 and H2S (Orr, 1977; Machel et al., 1995; Nöth, 1997). In addition to a net reduction in petroleum quality and quantity, H2S and CO2 are toxic and corrosive, leading to greater production costs, and TSR forms an insoluble, sulfur rich solid bitumen (Machel et al., 1995; Zhang et al., 2008), which can negatively impact reservoir porosity and permeability. Consequently, the ability to predict and quantify the extent of TSR is very important in the economic assessment of plays and prospects and the development of affected fields. Ideally, one or more molecular and isotopic measurement would provide a benchmark to indicate the extent of TSR of an oil or gas sample. While petroleum altered by TSR exhibits characteristic changes in molecular and isotopic compositions, these changes ⇑ Corresponding author. E-mail address: [email protected] (Z. Wei). 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2011.11.008

are not necessarily systematic or diagnostic of the extent of the reactions. The abundance of associated non-hydrocarbon gases is an unreliable indicator, as gases can migrate into and out of the reservoir, and H2S and CO2 can precipitate as minerals. Oil properties and compositions change as TSR advances, but competing processes can produce inconsistent trends. For example, low maturity oils derived from Type IIS kerogen commonly contain relatively high concentrations of organosulfur compounds that decrease with increasing source maturation or in-reservoir thermal cracking. However, if altered by TSR, the bulk sulfur content of oil may increase due to sulfur addition reactions. Similarly, the ratio of C15+ saturate to aromatic hydrocarbons typically increases with thermal maturity. However, in TSR altered oils, the saturated hydrocarbons decrease in proportion to aromatic hydrocarbons due to the preferential reactivity of saturated hydrocarbons and the formation of thiophenic compounds that elute in the aromatic fraction (Manzano et al., 1997). Stable isotopes of carbon and sulfur have proven to be particularly diagnostic tools for monitoring the extent of TSR. Genetically related oils exhibit fairly consistent d13C values for both bulk

3.5 2.6 2.3 5.8 2.3 13.1 n.a. 4.5 4.3 5.8 6.2 1.6 16.1 n.a.

CSIA of light hydrocarbons from Rooney, 1995 except for Zion Chapel, Little Escambia Creek, and Perdido which are reported in this study. Multiple samples from Northwest Gulf (wells 114-2 and 114-3) and Bon Secour Bay (wells 63-1, 78-1, and 823 A-5). See Mankiewicz et al., 2009 for individual well information. n.a.: not available. 1 Classification of sample type (oil/condensate) and associated H2S mole% from Kopaska-Merkel et al. (1993); except where noted. 2 Based on cumulative oil and gas production as recorded by the Alabama (through 2010) and Mississippi (through 2007) Oil and Gas Boards.

24.3 22.5 22.8 20.9 21.7 20.1 16.9 10.7 6.4 5.7 24.0 12.2 21.0 22.0 0.4 0.3 0.2 1.4 1.0 0.8 n.a. 1.75 0.045 0.07 25.69 16.7 29.1 5.88 1765 2167 1664 15837 12061 36367 4877102 49.7 51.1 51.2 39.4 48.3 32.4 24 Oil Oil Oil Condensate Condensate Condensate Condensate 137 127 132 138 145 177 210 12902–12938 13992–14004 14747–14766 15380–15455 16173–16286 17295–17535 20456–21034 Appleton Vocation Huxford Big Escambia Creek Chatom South State Line Bon Secour Bay TSR-Altered

Escambia, Al Monroe, Al Escambia, Al Escambia, Al Washington, Al Greene, Ms Mobile Bay

1.1 1.1 2.4 1.1 2.3 2.3 2.3 1.8 2.3 1.8 23.3 23.9 23.9 22.1 23.0 7.4 5.2 3.0 14.7 5.1 0.8 1.0 0.4 0.1 0.1 17.43 22.87 8.78 trace 0.62 1086 1973 1327 1318 5830 43.9 40.3 48.4 47.2 55.6 Oil Oil Oil Oil Condensate 109 116 140 135 157 13573–13590 14059–14078 15733–15769 16510–16540 18316–18334 Gin Creek Zion Chapel Little Escambia Creek Perdido Hatter’s Pond Uncertain

Choctaw, Al Choctaw, Al Escambia, Al Escambia, Al Mobile, Al

3.1 n.a. n.a. n.a. 2.2 n.a. n.a. 2.0 1.8 0.8 2.2 1.5 n.a. 2.8 n.a. n.a. n.a. 3.3 n.a. n.a. 0.1 0.7 1.2 2.9 1.1 n.a.

d13C CH-iso-C6 d13Coil ‰

23.9 23.9 23.9 23.9 23.9 23.4 23.3 23.9 23.9 23.4 23.4 23.7 25.7 2.0 2.2 2.6 1.8 0.9 8.0 4.2 12.5 5.6 7.7 2.4 0.6 21.0

d34Soil ‰ Sulfur wt%

1.9 1.7 1.4 0.8 0.8 0.2 0.6 0.1 0.1 0.2 0.2 0.1 n.a. 1.98 2.1 2.1 1.6 <0.001 <0.001 <0.001 0 0.77 0 0.01 0 0.01

H2S Mole %1 GOR2 (scf/b)

564 561 448 651 50 992 385 653 867 1282 6775 1072 3346185 33.4 34.3 37.5 39.9 48.0 37.7 31.5 38.0 39.4 42.4 56.0 55.5 30

API° Sample Type1

Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Condensate Oil Condensate 99 107 100 102 102 110 121 126 138 141 163 160 215

Temp °C Depth feet

11594–11650 11748–11780 12031–12049 12333–12340 12372–12389 13027–13039 12454–12473 14512–14523 15684–15695 16906–16952 18407–18434 18449–18476 21508–21878 Choctaw, Al Choctaw, Al Choctaw, Al Choctaw, Al Choctaw, Al Monroe, Al Clarke, Al Monroe, Al Baldwin, Al Mobile, Al Mobile, Al Mobile, Al Mobile Bay

County State Field

A suite of oil and condensate samples from the northern Gulf of Mexico was analyzed. The location of samples is shown in Fig. 1. All samples were generated by carbonate source rocks from the Jurassic (Oxfordian) lower Smackover Formation. Onshore samples are produced from carbonate reservoir rocks in the upper Smackover Formation. Offshore samples are produced from Jurassic (Callovian–Oxfordian) Norphlet sandstones. The oils have experienced different thermal histories after emplacement, as suggested by their present day reservoir temperatures that range from 99– 215 °C (Table 1). An effort was made to verify that the depth and reservoir temperatures are correct and consequently some of the reported values differ from prior publications. The sample suite is divided into three groups. Oils were classified as altered by TSR if they possess higher than expected sulfur

Sugar Ridge Choctaw Ridge North Choctaw Ridge Mill Creek Turkey Creek Lovetts Creek Stave Creek Wallers Creek Blacksher Movico Chunchula Cold Creek Northwest Gulf

2. Samples

Unaltered by TSR

fractions and individual hydrocarbons. Increasing thermal maturity of the source rock or thermal cracking in the reservoir impose a shift of no more than +2–3‰. In contrast, TSR preferentially cleaves 12C–12C bonds compared to 12C–13C bonds, resulting in varying d13C shifts that can exceed 15‰ in C4–C8 normal and branched alkanes, which are preferentially oxidized compared to naphthenic and aromatic hydrocarbons, where the isotopic shifts occur to a lesser degree (Rooney, 1995; Connan et al., 1996). Large carbon isotopic shifts occur in C2–C5 gases, although methane may not be greatly altered (Mankiewicz et al., 2009). H2S produced by TSR reflects the d34S value of the reacting sulfate. Consequently, back reactions of hydrocarbons with the H2S result in organic sulfur compounds that are enriched in 34S compared to those from sedimentary organic matter (Orr, 1974; Krouse et al., 1988; Orr and Sinninghe Damsté, 1990; Machel et al., 1995; Worden et al., 1995). Recently, d34S of individual benzo- and dibenzothiophenes were measured in unaltered and altered oils (Amrani et al., 2011). The authors observed that in TSR altered oils, the benzothiophenes formed during TSR were highly enriched in 34S compared to the dibenzothiophenes, which mostly reflected the d34S value of their source. The extent of TSR alteration inferred from the d34S CSIA was not in total agreement with d13C shifts. Hanin et al. (2002) reported that alkylated 2-thiaadamantanes are present only in TSR altered oils and proposed these as molecular markers for the occurrence of TSR in petroleum reservoirs. Dessort et al. (2004) further suggested that the thiaadamantane series can be used to indicate mild TSR and predict the occurrence of sour (H2S risk) production. However, the sources of these compounds are not known with certainty under geologic conditions (Hanin et al., 2002; Dessort et al., 2004; Galimberti et al., 2005). Wei et al. (2007) and Wei and Mankiewicz (2011) proposed that large quantities of thiadiamondoids originate through the sulfurization of diamondoids. As mercaptans are generated during TSR by reaction between sulfur and n-alkanes (Cai et al., 2003), thiols having diamondoid structures (diamondoidthiols) may be generated similarly that give rise to the alkylated 2-thiaadamantanes. Organic sulfur compounds appear to offer the best molecular signatures for TSR; however, it is not clear from prior studies exactly how they could be used to quantify the extent of the reaction. Wei et al. (2007) reported that thiadiamondoids and diamondoidthiols are orders of magnitude more abundant in oil altered by TSR than they are in unaltered oil, suggesting that their abundance could be a direct indicator of the extent of alteration. The objectives of this study are to investigate whether distributions and concentrations of thiadiamondoids can be used as diagnostic molecular indicators for TSR alteration of hydrocarbons and to determine whether they can provide a reliable measure of the extent of TSR alteration.

d13C MCH-n-C7

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

Table 1 Oils from onshore Alabama and Mississippi derived from Jurassic Smackover Formation source rocks.

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Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

55

Fig. 1. Sketch map showing location of samples. Base map modified from Kopaska-Merkel et al., (1994). Samples numbers are indexed in Table 1.

content (compared to unaltered oils of equivalent maturity as indicated by reservoir depth) and light hydrocarbons with anomalous d13C values. Oils were classified as unaltered by TSR if they have total sulfur content consistent with their level of thermal maturity and do not exhibit anomalous d13C values. Several oils have only one of the characteristics of TSR alteration or are associated with high H2S content. These are classified as uncertain with respect to TSR alteration. d34S values were not considered in this classification. If they were included, the Movico and Chunchula oils would move to the uncertain class.

3. Analytical methods and results

with D6-dimethylsulfide and D10-biphenylsulfide standards, followed by the addition of a few drops of dichloromethane to the sample. The oven was held at 35 °C for 2 min and then ramped at 10 °C/min to 300 °C where it was held for 17 min. The inlet temperature was 275 °C at a pressure of 9.47 psi. About 1 ll of sample was injected in split mode at 1:10. Helium was used as carrier gas at a constant flow rate of 2.0 ml/min. Detector conditions were 200 °C with a hydrogen flow rate of 4 ml/min and air flow rate of 6 ml/min. The burner temperature was 800 °C with pressure of 342 Torr. The ozone reaction cell pressure was at 8 Torr and ozone oxidant flow was set at 6.0 psi. Example chromatograms are shown in Fig. 2. Dibenzothiophenes were quantified relative to both internal standards and summed (Table 2).

3.1. GC–sulfur chemiluminescence detection (GC–SCD) Sulfur compound analysis was performed on an Agilent 6850A GC equipped with a Supelo SPB-1 sulfur column (30 m  0.32 mm i.d.  4.0 lm film thickness) and a Sievers 355 sulfur chemiluminescence detector. About 100 mg of oil was weighed and spiked

3.2. Liquid column chromatography on silver nitrate impregnated silica gel Liquid chromatography on silver nitrate impregnated silica gel was used to fractionate oils and condensates into chemical classes

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Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

1000

20

10

30

South State Line, Smackover

2000

40

DMDBTs MDBTs TMDBTs

(b)

Strongly TSR-altered %S=0.81% 34S=+21.0‰ H 2 S: 31.4% Thiadiamondoids: 685 ppm

1000

DBT

(uV)

Not altered by TSR %S=0.80% 34S=-0.9‰ H2S: 0.001% Thiadiamondoids: 9.68 ppm

0

(uV)

(a)

Turkey Creek, Smackover

2000

Sulfides BTs Mercaptans

0

20

10

30

40

Retention time (min) Fig. 2. Comparison between the sulfur-GC traces of (a) smackover oil unaltered by TSR and (b) smackover oil that is severely altered by TSR. BTs: benzothiophenes; DBT: dibenzothiophene; MDBTs: methyldibenzothiophenes; DMDBTs: dimethyldibenzothiophenes; TMDBTs: trimethyldibenzothiophenes.

(Fig. 3; after Wei et al., 2007). About 300 mg of silver nitrate impregnated silica gel (silver nitrate 10 wt% on silica gel + 200 mesh, activated at 105 °C for 3 h) was weighed and loaded into a 146 mm Pasteur pipette column, followed by packing the top of the column with silica gel (‘Baker’ silica gel, 40 lm, 60 Å in diameter, activated at 225 °C for 16 h). About 50 mg of crude oil or condensate was weighed and spiked with deuterated diamondoids and D3-1-methyl-2-thiaadamantane as internal standards for the quantification of diamondoids and thiadiamondoids. Synthetic diamondoid internal standards include D4-adamantane, D3-1-methyladamantane, D3-1-methyldiamantane, D4-diamantane, D5ethyldiamantane and D4-triamantane. A few drops of hexane were added to dissolve the samples, which were then loaded on the top of silver nitrate impregnated silica gel columns and sequentially eluted with hexane, dichloromethane and acetone to obtain the saturate, aromatic and organosulfur compound (OSC) fractions, respectively. The saturate fraction (3 ml) was further fractionated using ZSM5 zeolite to remove n-alkanes before diamondoid analysis. The aromatic fraction (8 ml) including thiophenic aromatic hydrocarbons was spiked with d10-phenanthrene to quantify the dibenzothiophenes (DBTs). Cyclic sulfides, including thiadiamondoids and mercaptans can complex with Ag+ and are strongly retained on the nitrate impregnated silica gel column (Hanin et al., 2002). Acetone gradually replaces the OSCs by forming very stable complexes with Ag+, allowing polar OCS compounds to elute as the OSC fraction (2 ml). Care was taken to avoid drying this fraction when concentrating it to 200 ll. 3.3. GC–MS analysis of diamondoids Diamondoid analysis was carried out on the saturate fraction of oil and condensate samples. The saturates were analyzed by GC– MS using a Hewlett Packard 5890 Series II gas chromatograph interfaced to a Micromass Autospec-Q mass spectrometer, operating in SIM mode. Ions were monitored at m/z 135, 136, 140, 149,

163, 177, 191 for adamantanes, m/z 187, 188, 201 and 215 for diamantanes, m/z 239, 240, 244, 253 and 267 for triamantanes and m/z 292 for tetramantanes. Ionization was by electron impact at 70 eV. The GC was equipped with an HP-1 MS fused silica capillary column (60 m (0.25 mm i.d. (0.25 lm thickness of methyl silicone film). Hydrogen was the carrier gas with a head pressure of 15 psi. Samples were injected at 50 °C while holding constant temperature for 1 min. The oven was subsequently programmed to 80 °C at 15 °C/min, then to 290 °C at 2.5 °C/min, and finally to 320 °C at 25 °C/min, where it was held for 25 min. Quantification of adamantanes, diamantanes and triamantanes was achieved by the integration of peak heights with respect to their corresponding standards D3-1-methyladamantane, D3-1-methyldiamantane and D4-triamantane, respectively. Quantification of tetramantanes was accomplished using the D4-triamantane standard. D5-1ethyldiamantane was used to quantify ethyldiamantanes. The measured sum of the 3- and 4-methyldiadamantanes are listed in Table 2. 3.4. GC  GC–TOF analysis of thiadiamondoids Thiadiamondoid analysis was carried out directly on the oil samples spiked with D3-1-methyl-2-thiaadamantane internal standard using GC  GC–TOFMS. The use of whole oil samples eliminates the potential loss of thiadiamondoids during isolation of the OSC fraction. The GC  GC–TOFMS consists of an Agilent 6890N gas chromatograph coupled to a LECO Pegasus III-4D time-of-flight mass spectrometer. The first dimensional separation was achieved using a non-polar BPX5 GC column (30 m  0.25 mm i.d., 0.5 lm film thickness) and the second dimensional separation was achieved using a BPX50 polar GC column (2.3 m  0.10 mm i.d., 0.10 lm film thickness). About 2 ll of sample was injected in split mode with a split ratio of 1. The inlet temperature was set at 300 °C. Helium was used as carrier gas at a flow rate of 1 ml/min. The first GC column temperature was programmed to remain isothermal at

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Crude oil or Condensate GCxGC-TOFMS Thiadiamondoidanalysis

Spiked with deuterated diamondoids, D3-1-methyl-2thiaadamantane

GC-SCD Dibenzothiophene analysis

Two-layer silver nitrate-impregnated silica gel column chromatography Hexane

Dichloromethane

Saturate fraction

Aromatic fraction

Acetone

Organic sulfur compound fraction

IsoSilicalitecolumn octane chromatography

Branched and cyclic fraction

MRM-GCMS SIM-GCMS

Spiked with d10-phenanthrene

Diamondoid analysis

MRM-GCMS SIM-GCMS

MRM-GCMS SIM-GCMS

Biomarker analysis

Aromatic analysis

Full-scan GCMS MRM-GCMS SIM-GCMS Thiadiamondoidanalysis Diamondoid thiolanalysis

Fig. 3. Flow diagram shows the separation of thiadiamondoids and diamondoidthiols from crude oil and condensate samples.

Fig. 4. Surface images of trimetyl-2-thiaadamantanes detected using GC  GC–TOFMS (2-D gas chromatography-time-of-flight mass spectrometry). The concentrations of thiadiamondoids increase dramatically from TSR unaltered Wallers Creek oil to the highly TSR-altered South State Line oil.

50 °C for 1 min and then ramped up to 360 °C at 1.5 °C/min. The temperature program for the second GC column was exactly the same as the first dimension except for a temperature offset of +10 °C. The second dimension separation time was set at 8 s and controlled by two stages of thermal modulation, creating a hot

pause of 0.40 s and cold time of 3.60 s. The MS source was operated at 200 °C in an EI mode and the transfer line temperature was set at 320 °C. The detector voltage was set at 1600 with filament bias at 70 V. The acquisition rate was 100 spectra/sec for a mass range of 35–550.

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S

R

I.S.

S

R

S

R

S

20

40

60

80

R

100

Retention time (minute) Fig. 5. TIC chromatogram of the OSC fraction shows the distribution of thiadiamondoids, including thiaadamantanes, thiadiamantanes, thiatriamantanes and thiatetramantanes. The thiadiamondoid fraction was separated from South State Line, a highly TSR-altered oil, using two-layer silver nitrate-impregnated silica gel column chromatography. I.S. = internal standard (D3-1-methyl-2-thiaadamantane).

For the analysis of thiadiamondoids, ions were monitored at m/z 154, 168, 182, 196, 210 and 224 for thiaadamantanes, m/z 206, 220, 234 and 248 for thiadiamantanes, m/z 258, 272, 286 and 300 for 2thiatriamantanes and m/z 171 for the D3-1-methyl-2-thiaadamantane standard. The thiadiamondoids were quantified by integrating their peak volumes with respect to D3-1-methyl-2-thiaadamantane internal standard. Results are listed in Table 2 and representative analyses are shown in Fig. 4.

Unlike the thiadiamondoids, these polythiadiamondoids have never been reported in petroleum. Interestingly, only alkyl substituted polythiadiamondoids were detected, while the unsubstituted polythiadiamondoids are not detected. Because of their relatively low abundance and uncertainty in recovery in the preparation of the OSC fraction, the polythiadiamondoids were not quantified.

4. Thiadiamondoids as proxies for TSR alteration 3.5. GC–MS analysis thiadiamondoids and polythiadiamondoids 4.1. Total abundance of thiadiamondoids Full scan and SIM GC–MS analysis was performed on the OSC fractions. The MS source was operated at 250 °C in electron impact mode at 70 eV. Samples were injected in splitless mode at a constant temperature of 320 °C. The temperature program was 50 °C for 1 min, 50–240 °C at 2 °C/min, 240–320 °C at 10 °C/min and 320 °C for 25 min. In full-scan GC–MS, the scan time was set at 1 s for a mass range of 50–550. Representative spectra of thiadiamondoids are included Tables 1 and 2. During SIM–GC–MS analysis of thiadiamondoids, ions were monitored at m/z 154, 168, 182, 196, 210 and 224 for thiaadamantanes, m/z 206, 220, 234 and 248 for thiadiamantanes, m/z 258, 272, 286 and 300 for 2-thiatriamantanes and m/z 171 for the D3-1-methyl-2-thiaadamantane standard. The OSC fractions are highly enriched in thiadiamondoids, which constitute major components in oils that have undergone extensive TSR (Fig. 5). The chromatographic separation is sufficient to allow relatively pure mass spectra of individual thiadiamondoids as shown in the Appendix. Polythiadiamondoids also were detected in oil having undergone extensive TSR. These are thia-cage compounds with more than two sulfur atoms that replace secondary carbons in the diamondoid cages. Diamondoid hydrocarbons exhibit strong molecular ions, but the introduction of heteroatoms (e.g., sulfur) promotes fragmentation. Consequently, the base peak for methyldithiaadamantane is not the molecular ion (Appendix). Diagnostic ion chromatograms of methyl-, dimethyl- and trimethyl-dithiaadamantanes are shown in Fig. 6.

Thiadiamondoids have been identified in nearly all Smackover oils. Considering some oils are believed to be unaltered by TSR, the abundance of thiadiamondoids appears to be under geologic control. Oils in the Mississippi Interior Salt Basin (Chocktaw) have total thiadiamondoids ranging from 41–120 ppm. Oils downdip of the Manilla Embayment (Baldwin, Clarke and Monroe counties) and the Movico Field lack or have only trace amounts of thiadiamondoids. Oils and condensates from Mobile Co. and Mobile Bay contain intermediate amounts, ranging from 29–49 ppm. If the presence of thiadiamondoids alone is an indication of TSR, then it would be concluded that nearly all of the analyzed oils have been altered to some degree. This conclusion is not supported by isotopic or molecular compositions that are diagnostic of TSR alteration. It is more likely that low levels of primary thiadiamondoids can form during oil generation, with the concentrations determined by the available sulfur in the source kerogen. Maturation effects alone cannot explain the variation in thiadiamondoid content, because the samples from Mobil Co. and Mobile Bay are more mature than those downdip of the Manilla Embayment. The total thiadiamondoid content of the samples identified as being TSR altered ranges from 180–1745 ppm. Clearly, higher concentrations are indicative of TSR, but it is not immediately obvious whether the thiadiamondoids are created by the process or are enriched by the selective oxidation of hydrocarbons. Regardless of the cause, a concentration limit of >150 ppm thiadiamondoids appears to be a reliable measure that TSR has occurred. For reasons

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

a

(a)

m/z = 186

b

m/z = 204

Peaks a, c, and f = alkylated dithiaadamantanes

c

m/z = 200

e

m/z = 218

d f

m/z = 214

Peaks b, d, e, and g = alkylated trithiaadamantanes

g

m/z = 232

Partial TIC

59

a

c+d

b

f+g

e 54

52

Retention time (minute)

h

m/z = 238

(b) i j

m/z = 256

k l

m/z = 252

PartialTIC

70

Peaks h, i, k, and l = alkylated dithiadiamantanes

h+k

l i

72

j Peaks j = alkylated trithiadiamantanes

Retention time (minute) Fig. 6. Ion chromatograms of (a) alkyl-substituted dithiaadamantanes (m/z 186, 200, 214) and trithiaadamantanes (m/z 204, 218, 232) and (b) alkyl-substituted diathiadiamantanes (m/z 238, 252) and trithiadiamantanes (m/z 256) in the OSC fraction separated from the South State Line oil.

described below, it is preferable to use the concentration of thiadiamondoids that excludes the more volatile compounds for benchmarking the occurrence and extent of TSR. A concentration threshold for >30 ppm of the low volatility thiadiamondoids (Table 2) differentiates the unaltered from the TSR altered oils. Using this criterion, oils of uncertain affinity would be reclassified as unaltered (Little Escambia Creek and Hatters Pond) or TSR altered (Zion Chapel, Gin Creek and Perdido). 4.2. Comparison of the abundance of thiadiamondoids, diamondoids and dibenzothiophenes The distributions of the thiadiamondoids are similar to the diamondoids. Alkylated thiaadamantanes (1-cage) are dominant, ranging from <1 ppm to >1000 ppm in the Smackover oils and con-

densates. The thiadiamantanes (2-cage), thiatriamantanes (3-cage) and larger thiadiamondoids occur at markedly lower concentrations. As with the diamondoids, the concentrations of the more volatile thiadiamondoids may be influenced by natural and production induced phase separation, sample handling and storage, and sample preparation. To compensate for potential loss of the most volatile compounds, comparisons are based on a summation of thiadiamondoids and diamondoids that do not include the most volatile species. Comparisons of the abundance of low volatility thiadiamondoids to diamondoids and dibenzothiophenes are shown in Fig. 7. For all samples except those from offshore Mobile Bay (Table 2), the concentrations of these species correlate in a linear fashion. The deviations seen in the Mobil Bay samples can be attributed to the advanced thermal cracking these fluids have experienced

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Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

(a)

800

(b) Unaltered Uncertain TSR-altered

Unaltered Uncertain TSR-altered

700

Thiadiamondoids, ppm

Thiadiamondoids, ppm

700

800

600 500

400 300 200

500 400 300

200 100

MB

100

600

0

MB

0 0

2000 4000 6000

10

8000 10000 12000 14000

Diamondoids, ppm

(d)

800

Unaltered Uncertain TSR-altered

Thiadiamondoids, ppm

700

1000

10000

100000

Diamondoids, ppm 800 700

600

Thiadiamondoids, ppm

(c)

100

500 400 300

MB 200 100

Unaltered Uncertain TSR-altered

600 500 400 300

MB

200 100

0 0

1000 2000

3000

4000

5000 6000

7000

Dibenzothiophenes, ppm

0 10

100

1000

10000

Dibenzothiophenes, ppm

Fig. 7. Plots show good correlation of thiadiamondoids and diamondoids (a = linear; b = log) and thiadiamondoids and dibenzothiophenes (c = linear; d = log) in Smackover oils from the Gulf of Mexico. Thiadiamondoids include those identified as low volatile (Table 2). Compounds used for the thiadiamondoids (low volatile), diamondoids and dibenzothiophenes are defined in Table 2. MB = Mobile Bay condensates.

in the reservoir. For these samples, the abundance of diamondoids is much higher than the thiadiamondoids, while the dibenzothiophene concentrations are lower than expected. Hence, it is concluded that the thermal stability of the thiadiamondoids is intermediate between diamondoids and dibenzothiophenes. If thiadiamondoids were formed during generation and were enriched as the TSR process oxidized less stable hydrocarbons, then a linear correlation with the diamondoids would be expected. The different behavior of the unaltered and TSR altered Mobile Bay oils suggests that the thiadiamondoids are formed by TSR. In the case of the two unaltered oils from the northwest Gulf area, thermal cracking of less stable hydrocarbons has greatly enriched the more stable diamondoids without a corresponding increase in thiadiamondoids. This is less evident for the TSR altered oils from the Bon Secour Bay area, but the values still greatly deviate from linearity. Unless the Mobile Bay oils originated from a Smackover source facies that yields low concentrations of primary thiadiamondoids unlike the nearby onshore oils from Mobile

County, the deviations cannot be explained by enrichment. Similarly, the roughly linear correlation between dibenzothiophenes and thiadiamondoids is perturbed by TSR altered oils from Northwest Gulf 823 A-5 and Bon Secour Bay 63-1, which are depleted in dibenzothiophenes. Dibenzothiophenes can arise as primary compounds from the source or via secondary back reactions with hydrocarbons and H2S. Hence, a correlation between the concentrations of dibenzothiophenes and thiadiamondoids is expected and both will increase with increasing TSR or reaction with TSR generated H2S. The deviation from linearity seen in the two samples above suggests that thermal cracking preferentially removed some of the dibenzothiophenes compared to the thiadiamondoids. This effect is not obvious in the unaltered oils from the north central Gulf area because of the low concentrations of both species. There is laboratory evidence for the formation of thiadiamondoids by TSR reactions. Wei et al. (2007) reported that alkylated 2thiaadamantanes dominated by 5,7-dimethyl-2-thiaadamantane

61

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70 Table 2 Concentration of dibenzothiophenes, diamondoids and thiadiamondoids in Smackover oils and condensates. Field

DBTs ppm

Diamondoids ppm

Thiadiamondoids, ppm Low-volatile

1-Cage

2-Cage

3-Cage

Sum

Unaltered by TSR

Sugar Ridge Choctaw Ridge North Choctaw Ridge Mill Creek Turkey Creek Lovetts Creek Stave Creek Wallers Creek Blacksher Movico Chunchula Cold Creek Northwest Gulf 114-2 Northwest Gulf 114-3

93 67 62 431 431 45 231 192 270 57 78 118 74 113

70 259 112 224 169 22 42 100 102 39 357 309 8606 6612

16 14 7 22 16 0 0 2 2 0 14 17 14 27

87 62 38 104 73 0 0 2 2 2 18 42 14 13

10 9 3 13 8 0 0 2 2 0 17 7 15 27

1.06 0.00 0.44 2.52 0.62 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0 4

98 71 41 120 82 0 0 4 4 2 35 49 29 44

Uncertain

Gin Creek Zion Chapel Little Escambia Creek Perdido Hatters Pond

925 893 53 892 81

753 482 70 648 324

61 41 0 83 20

303 169 5 204 47

27 22 0 34 10

3.69 4.29 0.00 2.83 0.50

334 195 5 241 58

Altered by TSR

Appleton Vocation Huxford Big Escambia Creek Chatom South State Line Bon Secour Bay 63-1 Bon Secour Bay 78-1 Northwest Gulf 823 A-5

691 803 983 2345 1564 6313 950 1765 96

532 669 512 3144 1736 7981 8668 8301 12050

60 58 43 296 149 685 203 166 199

200 216 161 1418 676 1282 106 54 158

36 23 18 102 57 407 120 73 72

0.84 1.52 1.22 6.59 6.25 56.35 74 95 82

237 241 180 1527 739 1745 300 222 312

DBTs = Dibenzothiophene and its alkylated groups including methyl-, dimethyl- and trimethyl-dibenzothiophenes. Diamondoids = 3- and 4-methyldiamantanes. Low volatility thiadiamondoids: C3-C4-alkylated thiaadamantanes, C0-C3 thiadiamantanes, and C0-C3 thiatriamantanes. These thiadiamondoid species are relatively less volatile that C0-C2 alkylated thiaadamantanes and may not be as greatly influenced by phase separation processes and sample handling. 1-Cage: thiaadamantanes. 2-Cage: thiadiamantanes. 3-Cage: thiatriamantanes for all detected C0 and Cn alkylated species. Sum: 1- + 2- + 3-cage thiadiamondoids for all detected C0 and alkylated species.

and 3,5,7-trimethyl-2-thiaadamantane were created from 1,3-dimethyladamantane during TSR simulation experiments, indicating that thiadiamondoids may originate from their precursor diamondoids via sulfurization under TSR conditions. Thiocholesterol also yielded trace amounts of dimethyl-2-thiaadamantane upon heating with montmorillonite at lower temperatures, suggesting that low levels of thiadiamondoids may originate by molecular rearrangement of polycyclic sulfides and thiols in the presence of acidic clay minerals since these sulfur compounds are present in crude oil that has not been altered by TSR (Wei et al., 2007). It appears that other geologic processes can make trace amounts of thiadiamondoids. However, the formation of large quantities of thiadiamondoids requires sulfurization of diamondoids during TSR in high temperature petroleum reservoirs (Wei et al., 2007; Wei and Mankiewicz, 2011). 4.3. Comparison of the abundance of thiadiamondoids with other indicator of TSR As noted above, abundant H2S, the d34S of the organosulfur compounds, together with the d13C of the oil, oil fractions and light hydrocarbons can be diagnostic of TSR. The classification of the samples as unaltered or TSR altered is based on these conventional criteria. These measurements may not always agree, resulting in some oils being assigned to the uncertain class. Furthermore, even when these criteria are met, they do not always provide an accurate assessment of the extent of TSR. The d34Soil values for the unaltered samples span from 12.5‰ to +7.7‰, which is consistent with the isotopic range for primary organic sulfur derived from Smackover source facies. The lightest values are common to oils from downdip of the Manilla

Embayment, while heaviest values are found further downdip in Mobile County. d34Soil values of TSR altered samples range from +5.7‰ to +21.0‰ and approach the values of Jurassic anhydrite (+16‰ to +24‰; Claypool et al., 1980; Heydari and Moore, 1989). For these oils, the thiadiamondoid content increases in a logarithmic fashion with respect to the whole oil d34S (Fig. 8a). The correlation between these measurements is well established for oils with low volatility thiadiamondoid content (>30 ppm), the empirical threshold for indicating TSR alteration. Samples with lower concentrations of thiadiamondoids exhibit no correlation with d34Soil values. Carbon isotope ratios of whole oils and individual light hydrocarbons become heavier with increasing thermal maturation and TSR alteration. The shift imposed by thermal maturation on whole oil d13C values is limited to about +2‰, while shift imposed by TSR can be greater. Thiadiamondoid content exhibits a logarithmic correlation with the d13Coil values (Fig. 8b) for oils identified as altered by TSR. Appleton is an outlier from this trend for reasons that are unclear. Samples with <30 ppm low volatility thiadiamondoid content show no dependency with d13Coil and both measurements are assumed to reflect their primary values inherited from the source rock. Thermal maturation and in-reservoir cracking imparts at most a +2‰ to +3‰ shift of the d13C values of individual light hydrocarbons with only minor variations across species. In contrast, selective oxidation by TSR of saturated and aromatic hydrocarbons can impose >+15‰ shifts on the more reactive normal and branched alkanes while leaving the less reactive naphthenic and aromatic species close to their unaltered state (Rooney, 1995). The difference between light hydrocarbons apparently reflects the extent of TSR reactions. The differences in d13C between two

62

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

(a)

30

(b)

-20

25

Unaltered Uncertain TSR-altered

20

Unaltered Uncertain TSR-altered

-21

δ13Coil, ‰

δ34Soil, ‰

15 10 5

-22

-23

0 -5

-24

-10 -15 0

1

10

100

-25

1000

0

5

0

-5

-10

-15

(d)

4 2

δ13Cmethylcylcohexane

δ13Ccylcohexane

δ13Cisohexane, ‰

(c)

1

10

100

1000

Thiadiamondoids, ppm

δ13Cn-heptane, ‰

Thiadiamondoids, ppm

Unaltered Uncertain TSR-altered

-20

0 -2 -4 -6 -8 -10

Unaltered Uncertain TSR-altered

-12 -14

0

200

400

600

800

Thiadiamondoids, ppm

0

200

400

600

800

Thiadiamondoids, ppm

Fig. 8. Comparison of the abundance of thiadiamondoids (low volatility) to oil properties known to reflect TSR alteration (a) d34Soil, (b) d13Coil, (c) d13Ccyclochexane and (d) d13Cmethylcyclochexane d13Cn-heptane.

pairs of closely eluting light hydrocarbons (cyclohexane – isohexane; methylcyclohexane – n-heptane) exhibit roughly linear correlations with thiadiamondoid content for the TSR altered samples (Fig. 8c and d). While the effect of TSR on the d13C values of the light hydrocarbons is obvious for the more strongly altered samples, it is comparatively subtle for the mildly altered samples and is not always consistent. High reservoir concentrations of H2S (>5 mol%) indicate that TSR has occurred within a basin, but do not necessarily reflect the extent of TSR alteration affecting the associated oil. This would only be true in a closed system with no external sources or sinks for H2S. Reservoirs containing unaltered oil may be charged with H2S rich gases from downdip reservoirs where TSR has occurred or TSR generated H2S may be sequestered from the affected reservoir by precipitation as sulfide minerals. Comparing the thiadiamondoid concentration in the Smackover oils and condensates to the amount of associated H2S reveals that both processes have occurred in the region (Fig. 9). As expected, all oils classified as unaltered have low concentrations of associated H2S. The abundance of thiadiamondoids exhibits a roughly logarithmic relationship with

d13Cisohexane

respect to H2S in oils that are classified as TSR altered (Fig. 9). The clear exceptions are the condensates from Mobile Bay. These condensates have certainly been altered by both TSR and thermal cracking. They are produced from Norphlet sandstones where massive pyrite beds with d34S values consistent with Jurassic anhydrite are common (Mankiewicz et al., 2009). Accepting the thiadiamondoid abundance as a proxy for TSR, H2S sequestration appears to have occurred onshore within the Conecuh Embayment (Appleton, Vocation, Huxford and Perdido) because little H2S is co-produced with these oils. Three oils, all classified as having uncertain affinity, are associated with unusually high amounts of H2S. Zion Chapel and Gin Creek (Choctaw Co.) contain thiadiamondoids, suggesting some TSR alteration. They are produced from relatively low temperature reservoirs and are near other fields that contain unaltered oils with little or modest amounts of H2S that could be attributed to primary generation. In contrast, the Little Escambia Creek sample has only trace amounts of thiadiamondoids, but is near a large reservoir of high H2S gas at Big Escambia Creek. It is possible that H2S has migrated into these fields from downdip reservoirs that have

63

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

undergone TSR. Such migration would account for the gas composition without the extensive TSR alteration seen in other oils from fields having similar H2S content. These oils provide additional insight into the generation of thiadiamondoids by TSR. High amounts of migrated H2S potentially could back react with diamondoids and other hydrocarbons forming thiadiamondoids without TSR. While this potentially could explain their abundance in the Zion Chapel and Gin Creek oils, the fact that only trace amounts of thiadiamondoids exist in the Little Escambia Creek sample suggests that these compounds must form primarily by TSR. The Smackover reservoir at Little Escambia Creek is about 25–30 °C hotter than at Zion Chapel or Gin Creek. If thiadiamondoids form via back reactions with migrated H2S, it would be expected that these reactions proceed appreciably faster at higher reservoir temperature and the Little Escambia Creek oils should have a higher concentration of thiadiamondoids than measured. This observation does not prove that reaction of hydrocarbons with H2S cannot form thiadiamondoids, but it suggests that such reactions are minimal at reservoir temperatures <140 °C.

35 Unaltered Uncertain

30

TSR-altered

BEC 25

H2S, mole %

ZC 20

GC 15

H2S Addition H2S Scrubbing

10

LEC 5

MB 0 0

1

10

100

1000

Thiadiamondoids, ppm Fig. 9. Plot of low volatility thiadiamondoid concentration versus H2S (mol%) in associated gas. Samples discussed in the text are labeled. BEC = Big Escambia Creek, GC = Gin Creek; LEC = Little Escambia Creek; MB = Mobile Bay (Wells 63-1, 78-1 and 823 A-5); ZC = Zion Chapel.

4.4. Thiadiamondoids as a molecular proxy for the extent of TSR alteration It has been shown above that concentrations of thiadiamondoids are consistent with established geochemical criteria for TSR alteration of oils and condensates. Furthermore, thiadiamondoids appear to provide a clearer threshold for the occurrence of TSR even when samples have only been mildly altered. It is proposed

90

90

Unaltered

Unaltered Uncertain

110

Uncertain

110

TSR-altered

TSR-altered

130

Reservoir Temperature, °C

Reservoir Temperature, °C

130

150

170

150

170

190

190

210

210

0

500

1000

1500

2000

Total Thiadiamondoids, ppm

0

200

400

600

800

Low-volatile Thiadiamondoids, ppm

Fig. 10. Thiadiamondoid concentrations versus reservoir temperature.

64

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

that the thiadiamondoid concentration can be used as a proxy for the extent of the TSR alteration. Low concentrations of thiadiamondoids were measured in nearly all samples that are considered unaltered by TSR. Their occurrence is consistent with our experimental results showing that trace amounts of thiadiamondoids may be created from polycyclic sulfides and thiols by molecular rearrangements in the presence of acidic clay minerals during late diagenesis and/or early catagenesis (Wei et al., 2007). From the Smackover dataset, 30– 40 ppm for the low volatility thiadiamondoids, or 150 ppm for the total thiadiamondoid content establishes a threshold for their concentration from primary sources. This threshold may vary in other basins and probably depends on the sulfur content of the kerogen in the source rock. Greater quantities of thiadiamondoids appear to require generation via TSR. Thiaadamantanes isolated from TSR altered Smackover oils have d34S values of +21‰ to +22‰ (Hanin et al., 2002), close to those of Jurassic anhydrite (+16‰ to +24‰; Claypool et al., 1980; Heydari and Moore, 1989), indicating that TSR is required to form thiadiamondoids. Higher thiadiamondoids (mainly thiatriamantanes and thiatetramantanes) isolated from TSR altered Mobile Bay condensate samples have d34S values of +14.1‰ to +14.9‰ (Wei and Mankiewicz, 2011). Sulfur isotopic analyses of H2S from several wells in the Mobile Bay field indicate an inorganic origin for the sulfur with d34S values of +14.8‰ to +17.1‰. These values are similar to Jurassic evaporites, the Pine Hill Anhydrite Member and Norphlet anhydrite cements (Claypool et al., 1980; Dixon et al., 1989; Strauss, 1997), suggesting that the sulfur in the thiadiamondoids originated from these evaporites during TSR. The use of thiadiamondoids as a proxy for the extent of TSR is based on the observation that they do not form via back reactions with H2S. Unlike thiophenes, benzothiophenes and sulfides that can easily form in the presence of elemental sulfur, polysulfides, or other reactive sulfur species, thiadiamondoids are not metasta-

90

4.5. Thiadiamondoid cage number distribution as a proxy for the extent of TSR The cage number distribution of the thiadiamondoids exhibits interesting characteristics that can be exploited (Fig. 11). 1-Cage species appear to be the most diagnostic of early onset and limited TSR alteration, but they are most susceptible to destruction by thermal cracking. In contrast, 3-cage species are less diagnostic of TSR onset, but appear to be less susceptible to destruction by thermal cracking. The abundance of 3-cage species in high temperature reservoirs is consistent with the observation of even higher homologues, up to 6-cages, occurring in the Mobile Bay condensates (Wei and Mankiewicz, 2011). 2-Cage species have intermediate behavior between the 1- and 3-cage species. The cage number distribution of the thiadiamondoids can be explained by their relative rates of formation and thermal stability. Furthermore, 1-cage thiadiamondoids may be more strongly influenced by phase separation within the reservoir or during production than the higher cage-numbered species. The fact that 3-cage structures appear to persist in samples that have experienced extensive thermal cracking suggests that the abundance of these

90

90

Unaltered

Unaltered

Unaltered

Uncertain

110

Uncertain

110

TSR-altered

Reservoir Temperature, °C

ble reaction intermediates and are difficult to decompose. Thiadiamondoids have high thermal stability, between dibenzothiophenes and diamondoids and once generated during TSR they remain relatively stable and become more abundant as TSR progresses. In contrast, dibenzothiophenes may originate from less stable compounds (e.g., thiophenes, benzothiophenes) that form via hydrocarbon reactions with H2S, which may migrate into the reservoir. The systematic increase in the concentrations of thiadiamondoids with increasing TSR suggests that they can be used as a molecular proxy for the extent of TSR. This relationship appears to hold for reservoir temperatures up to 180 °C, above which significant thermal cracking of hydrocarbons can occur (Fig. 10).

TSR-altered

TSR-altered

130

130

130

150

150

150

170

170

170

190

190

190

210

210

210

0

500

1000

Uncertain

110

1500

1-Cage Thiadiamondoids, ppm

0

200

400

600

2-Cage Thiadiamondoids, ppm

0

50

100

3-Cage Thiadiamondoids, ppm

Fig. 11. Thiadiamondoid concentrations for 1-cage, 2-cage and 3-cage structures versus reservoir temperature.

65

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

thiadiamondoids could be used as a proxy for TSR at reservoir temperatures >180 °C. Conversely, the abundance of 1-cage thiadiamondoids may be influenced by in-reservoir cracking at temperatures >160 °C. 4.6. Polythiadiamondoids as proxies for the extent of TSR alteration Polythiadiamondoids, thia-cage compounds with more than two sulfur atoms, were detected in the most altered TSR oils. Wei et al. (2007) suggested that thiadiamondoids originated from diamondoids through TSR. Since sulfur cannot directly replace carbon atoms in the cage structure of diamondoids due to their high stability, it is likely that cage cleavage occurs in the presence of reduced sulfur species formed by TSR because sulfur radicals derived by sulfate reduction initiate the cleavage of C–C bonds. It is also possible that at least two C–C bonds at secondary carbon atoms in the cage are involved in bond cleavage during the reaction.

1

5. Conclusions TSR is associated with multiple reactions of sulfur compounds, which generate different sulfur species. The lower valance sulfur species derived from reduction of sulfate by hydrocarbons can react rapidly with other hydrocarbons to form H2S, mercaptans,

(a)

C0

m/z = 154

Subsequent addition of –SH groups to the secondary carbons forms open cage diamondoid-like thiols (Wei and Mankiewicz, 2011), which may react with reduced sulfur species, followed by cyclization to form polythiadiamondoids. These polythiadiamondoids also could be reaction intermediates from increasing sulfurization of diamondoids. The presence of polythiadiamondoids alone appears to indicate that TSR has occurred. Improved analytical methods will be needed to quantify sub-ppm concentrations of these compounds to be used as proxies for the extent of the reaction.

(b)

21

m/z = 206

C0

22 2

3

C1

C1

m/z = 168

C2 5

23

m/z = 220

24 25

m/z = 234

C2

m/z = 182 26

6

4

C3 8

m/z = 248

C3

m/z = 196 7

50

60

55

9 10

27

(c) 11

m/z = 258

C0

C4 m/z = 210

65

Retention time (minute)

13 12

29

14 16 15

31

28

C5

m/z = 272

C1 30

19

m/z = 224 17

33

C2

18 20

m/z = 286

34 32 36

m/z = 171

C3

I.S.

m/z = 300 35

25

30

35

40

70

Retention time (minute)

37 38

75

Retention time (minute) Fig. A1.

80

66

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

sulfides (e.g., cyclic sulfides), disulfides, condensed thiophenic compounds and thiadiamondoids. The latter appear to originate exclusively from TSR and not via back reaction with H2S. Once generated, the high thermal stability of thiadiamondoids allows them to remain in TSR altered oil with little further reaction if not exposed to extremely high reservoir temperatures. Consequently, the abundance of these species reflects not only the occurrence of TSR, but the extent of the alteration. Based on the analysis of a suite of oils generated from lower Smackover carbonate source rocks, the following observations and conclusions can be made. Small amounts of thiadiamondoids may be formed during oil generation, although commingling of small amounts of TSR altered with unaltered oil cannot be excluded. The abundance of these compounds appears to be controlled primarily by variations in the source facies and is not greatly influenced by thermal maturation. A threshold of

A

79

100

<30–40 ppm for the sum of low volatility thiadiamondoids or <150 ppm for all thiadiamondoids indicates that no TSR alteration has occurred. Oils and condensates having thiadiamondoids above the threshold values have been altered by TSR. The abundance of these species is consistent with other molecular and isotopic measurements that are diagnostic of TSR alteration. Thiadiamondoids do not appear to be formed by back reaction with H2S. Consequently, the extent of TSR can be determined for petroleum in reservoirs that have received a charge of migrated H2S or have lost or sequestered H2S. The abundance of these compounds in petroleum that has not undergone substantial thermal cracking (>180 °C) can be used as a proxy for the extent of TSR. Thiadiamondoids are more reliable proxies for determining the extent of the TSR reactions than conventional geochemical measurements. The thermal stability of thiadiamondoids is between that of dibenzothiophenes and diamondoid hydrocarbons and increases with increasing cage

Peak 1 154

80

123

93 107

60

40

40

77

97 111

65

20 0 50

100

150

0 50

200

168

80

Peak 2

100

200

182

79 40

111

20

20

0

0 50

100

150

200

168

100

79

100

50

Peak 3

150

200

107

100

Peak 7

80

93

80 60

150

60

111 123

79 55

100

93 Peak 6 107

80

93

60

149

20

100

Relative abundance

182

Peak 5

80

60

40

100

60

111

40 55

196

40

123

139

91

20

20

181 0 50

100

150

0 50

200

93

100

Peak 4

80

100

200

196 125

Peak 8

121 60 40

40

0 50

150

107

80

182

60

20

100

123

77

91 149

20

181

0 100

150

200

m/z Fig. A2.

50

100

150

200

67

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

number. The abundance of 3-cage and higher pseudo homologues may be used to extend the use of thiadiamondoids in high-temperature reservoirs. The presence of polythiadiamondoids is an indicator that TSR has occurred. These compounds potentially could be used as proxies for the extent of TSR.

Upstream Research Company and ExxonMobil Production Company for permission to publish these results. Appendix A A.1. Historic identification of thiaadamantanes

Acknowledgements The authors thank Professor Zachary Ball and Dr. Vincenzo Russo at Rice University for the synthesis of thiadiamondoid standards. We are also grateful to Steve Colgrove and Kuangnan Qian for their help in sulfur compound characterization during establishment of the GC–SCD method and their assistance in interpreting the mass spectra of thiadiamondoids. The paper also benefited from insightful comments by Cara Davis. We also appreciate the thoughtful and constructive comments of two anonymous reviewers and associate editor Andrew Murray. We thank ExxonMobil

B

107

100

121

100

Peak 9 196

80 60 55

Birch et al. (1952) first isolated 2-thiaadamantane from the kerosene boiling range of Middle East crude oil. This compound and its derivatives have been synthesized in the laboratory (e.g., Stetter and Held, 1961; Stetter and Schulte-Oestrich, 1962; Olsson, 1968; Janku and Landa, 1972; Moon et al., 1976; Krishnamurthy and Fort, 1981; Suginome and Yamada, 1986) and their physical and chemical properties were measured (Birch et al., 1955; Lacina et al., 1961; Janku and Mitera, 1970; Snatzke and Wolfram, 1972; Janku and Popl, 1974; Hajek et al., 1976; McCabe et al., 1977; Bishop and Lee, 1987).

Peak 13

80

210

125

60

55 40

40

91

107

91

20

139

20

195 0

0 50

100

150

200

107

100

123

80

196

93

80

55

40

40

20

20

107 139 210 193

0

0

Relative abundance

200

Peak 14

60

55

150

123

100

Peak 10

60

100

50

100

50

150

200

121

100

100

Peak 11

100

50

Peak 16

80

80 55

60

60

150

107

200

121

210

93 153

210 40 20

40

139 55

91

195

20

195

0

0 100

50

150

200

121

100

100

Peak 12 107

80

210 139

60 40 55

195

200

121 135

Peak 17

224 55

40

20

150

80 60

91

100

50

107 91

153

193 209

20 0

0 50

100

150

200

50

m/z Fig. A2 (continued)

100

150

200

68

Z. Wei et al. / Organic Geochemistry 44 (2012) 53–70

A.2. Synthesis of D3-1-methyl-2-thiaadamantane

A.3. Identification of thiadiamondoids

The synthesis of D3-1-methyl-2-thiaadamantane is based on collaborative work with Rice University (Russo et al., 2009). It began with Meerwein’s ester, which was made in a single step condensation of dimethylmalonate and formaldehyde. The Meerwein’s ester was monomethylated to give C-methylated tetraester upon treatment with CD3I and NaOMe. Decarboxylation of the methylated Meerwein’s ester through AcOH and HCl gave methylated diketone, which upon treatment with tert-butyldimethylsiyl trifluoromethane-sulfonate in the presence of triethylamine gave the bis silylenol ether. The bis-enol ether gave diketone upon treatment with SCl2 in the presence of pyridine. The reduction of the D3-1-methyldiketone to D3-1-methyl-2-thiaadamantane was completed via the Wolf–Kishner reduction. The D3-1-methyl-2-thiaadamantane was purified to >98% by column chromatography using silver nitrate-impregnated silica gel as a stationary phase and 1:9 Me2S–Et2O as eluent.

GC and GC–MS analyses of the OSC fraction separated from Smackover oil samples indicated the presence of a series of cyclic sulfides and thiols with the cage structure of diamondoids. These compounds were identified as thiadiamondoids by co-injection of synthesized standards and comparison of their GC retention times and characteristic mass spectra reported in the literature (Hanin et al., 2002; Dessort et al., 2004; Galimberti et al., 2005). Higher ring structures were assigned based on mass spectra reported by Wei and Mankiewicz (2011) for a Bon Secour Bay condensate (Well 823 A-5) that contains relatively abundant thiadiamondoids and diamondoidthiols. Similar to diamondoids, the thiadiamondoids consist of different alkylated clusters, e.g., thiaadamantanes (C9+nH14+2nS, n P 0), thiadiamantanes (C13+nH18+2nS, n P 0) and thiatriamantanes (C17+nH22+2nS, n P 0). The homologous series of thiadiamondoids are characterized by molecular ions at [M+] 154 + 14n for

C

206

100 80

80

60

60

81 91 40 57 69 20 105 0 50

129 91 79 105 143

20

129

183

0 100

150

200

250

50

100

150

200

250

300

272

100

Peak 23

Peak 29 80

80

60

60 40

79

105 91 203

100

150

200

250

234

Peak 25

50

183

257

150

200

250

300

286 Peak 33

80

60

60

157

40

95 105

40

100

100

80

20 57

79

20

91 79 105 129

20 0

0 50 100

143

40

20

Relative abundance

Peak 27

40

220

100

258

100

Peak 21

91 105

143

197

0

0 50

100

150

200

250

248

100

50

100

150

200

250

300

300

100

Peak 26

Peak 36 80

80

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m/z Fig. A2 (continued)

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a result, the alkyl group may be stabilized since cyclization of the product ion can increase its stability. However, they are similar based on other fragments at m/z 79, 107 and 125. The molecular ion at m/z 196 and fragment ion at m/z 107 are the most abundant in the mass spectra of trimethyl-2-thiaadamantanes. The mass spectra of tetramethyl-2-thiaadamantanes are characterized by a strong molecular ion at m/z 210 and base peak at m/z 121. A molecular ion base peak occurs in the mass spectra of pentamethyl-2-thiaadamantanes. The next most abundant peak is at m/z 135. As noted by Waltman and Ling (1980), the fragmentation patterns of C3- and C4-alkylated 2-thiaadamantanes are similar to those of alkylated adamantanes. Thus, alkylated 2thiaadamantanes are thought to be sulfurized analogs of alkylated adamantanes (Hanin et al., 2002). The mass spectra of alkylated thiadiamantanes are characterized by molecular ion base peaks at m/z 206 + 14n. Likewise, the molecular ions at m/z 258 + 14n are also the base peak for alkylated thiatriamantanes. Fig. A1 shows GC–MS analysis of the OSC fraction from the highly TSR-altered South State Line oil. Diagnostic ions are as follows: (a) thiaadamantanes = m/z 154, 168, 182, 196, 210, 224; D3-1-methyl-2-thiaadamantane standard = m/z 171; (b) thiadiamantanes = m/z 206, 220, 234 and 248 and (c) 2-thiatriamantanes = m/z 258, 272, 286, 300. In general, each homolog of alkylated 2-thiaadamantanes has several structural isomers. There are two isomers for methyl-2-thiaadamantanes (m/z 168), three isomers for dimethyl-2-thiaadamantanes (m/z 182), four isomers for trimethyl-2-thiaadamantanes (m/z 196), six isomers for tetramethyl-2-thiaadamantanes (m/z 210) and four isomers for pentamethyl-2-thiaadamantanes (m/z 224). Mass spectra for the numbered peaks are in Fig. A2.

alkyl-substituted thiaadamantanes, 206 + 14n for alkyl-substituted thiadiamantanes and 258 + 14n for alkyl-substituted thiatriamantanes. The mass spectra of 1-methyl-2-thiaadamantane and 1,5-dimethyl-2-thiaadamantane were described by Hanin et al. (2002). In general, the molecular ions of cyclic sulfides undergo a number of characteristic reactions, including ring-opening followed by loss of a hydrocarbon fragment, a HS radical and a H2S molecule. One would expect that cleavage of the S–C bond, with charge retention on the sulfur-containing fragment, occurs during the fragmentation reactions. As shown in figure below, the mass spectrum of 2-thiatricyclo[3.3.1.1(3,7)]decane (2-thiaadamantane) is characterized by a base peak at m/z 79, an intense molecular ion at m/z 154 (ca. 75% relative intensity), and minor fragments at m/z 93 and 107. In addition to a base peak at m/z 93, an abundant molecular ion at m/z 168 is observed for 1-methyl-2-thiaadamantane (ca. 82%) and 5-methyl-2-thiaadamantane (ca. 68%). The spectrum of 5,7-dimethyl-2-thiaadamantane also is characterized by the base peak at m/z 93, which differs from the other two isomers, including 1,5-dimethyl-2-thiaadamantane and 1,3-dimethyl-2thiaadamantane. Interestingly, the loss of a C1- or C2-alkyl group from the C1- and C2-alkyl-substituted 2-thiaadamantanes is not observed. This differs from what is observed in C1- and C2-alkylsubstituted adamantanes that have the base peak at [M CH3+] 135 and 149, respectively, resulting from the loss of an alkyl group from either of the alkyl-substituted adamantane (Wingert, 1992). A possible explanation for the absence of an alkyl group loss from the C1- and C2-alkyl-substituted 2-thiaadamantanes is that side-chain carbon and sulfur may be involved in rearrangement upon electron ionization to form a stable larger-membered ring ion structure. As

95

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A.4. Identification of polythiadiamondoids Mass spectra of selected polythiadiamondoids are in Fig. A3. The spectra are consistent with those of synthetic polythiadiamondoids, as reported by Olsson (1966a,b, 1967, 1968). Most of these compounds show significant molecular ions, e.g., (a) methyl-dithiaadamantane at m/z 186, (b) methyl-trithiaadamantane at m/z 204 and (d) dimethyl-trithiadiamantane at m/z 256. This also is the case for (c) methyldithiadiamantane, which has a molecular ion at m/z 238, but with a base peak at m/z 57. Methyldithiadiamantane co-elutes with dimethyldithiadiamantanes as indicated by interference of the m/z 252 fragment in mass spectrum.

Associate Editor—Andrew Murray

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