Article pubs.acs.org/EF
Sulfur Species in Source Rock Bitumen before and after Hydrous Pyrolysis Determined by X‑ray Absorption Near-Edge Structure Trudy B. Bolin,*,†,‡ Justin E. Birdwell,§ Michael D. Lewan,§,∥ Ronald J. Hill,§,⊥ Michael B. Grayson,# Sudipa Mitra-Kirtley,# Kyle D. Bake,∇ Paul R. Craddock,∇ Wael Abdallah,○ and Andrew E. Pomerantz∇ †
Argonne National Laboratory, Argonne, Illinois 60439, United States Central Energy Resources Science Center, United States Geological Survey, Denver, Colorado 80225, United States # Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803, United States ∇ Schlumberger-Doll Research, Cambridge, Massachusetts 02139, United States ○ Schlumberger Dhahran Carbonate Research Center, Dhahran 31942, Saudi Arabia §
ABSTRACT: The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fraction of thiophene sulfur (sulfur bonded to aromatic carbon) to the extent that thiophene is the dominant sulfur form in all post-pyrolysis bitumen samples. This suggests that sulfur moiety distributions might be used for estimating thermal maturity in source rocks based on the character of the extractable organic matter.
1. INTRODUCTION Bitumen is a viscous petroleum-like phase present in organic-rich sedimentary rocks that is distinguished from kerogen by its solubility in organic solvents (e.g., chloroform and benzene). Bitumen is generally distinguished from oil on the basis of viscosity or mobility and is retained within the rock, while oil is an expelled liquid petroleum phase. It is important to differentiate native bitumen (the indigenous, biomarker-rich extractable organic matter present in immature source rocks) from that present in thermally mature or pyrolyzed samples, because native bitumen differs compositionally from bitumen generated during kerogen decomposition. As source rocks undergo thermal maturation, bitumen is generated from the thermal decomposition of kerogen prior to oil generation. Bitumen can represent a polar-rich intermediate product in the generation of hydrocarbon-rich oil.1,2 In semi-open pyrolysis, oil expulsion is driven by volatility and bitumen represents the non-volatile but organic-solvent-soluble fraction of organic matter in the rock.3 In hydrous pyrolysis, oil expulsion is driven by a net volume increase as a result of the conversion of water-bearing, polar-rich bitumen to form an immiscible, hydrocarbon-rich oil within the bitumensaturated rock matrix2,4 and can be facilitated by mineral dissolution, leading to increases in porosity and permeability. Once this oil is expelled from the rock into the surrounding water phase during a hydrous pyrolysis experiment, buoyancy drives it to the water surface. The retained bitumen is represented by the extractable organic matter remaining in the rock.2,4 In semi-open and hydrous pyrolysis, a combination of factors affects the distribution of petroleum products, with the highly viscous and © 2016 American Chemical Society
non-volatile bitumen remaining within the rock matrix partially sorbed to residual kerogen and mineral surfaces. Sulfur chemistry impacts the generation of petroleum. For example, weak carbon−sulfur bonds break easily during kerogen thermal maturation, accelerating petroleum generation. In an earlier study, it was confirmed that source rocks containing immature kerogens with high S content (S/C > 0.04)5 produce oil under reduced thermal stress because C−S bonds are weaker than C−C bonds.6 It was also shown that the presence of S free radicals produced by C−S bond cleavage had a controlling influence on the kinetics of oil generation. These and other results indicate that the activation energy for oil and gas generation is in part dependent upon the sulfur content of the kerogen.2,4,6,7 Bitumen composition is likely to impact the kinetics of petroleum generation in a way that is similar to kerogen because bitumen can serve as an intermediate in the formation of oil and gas.2,3,8−12 Sulfur speciation in bitumen may impact petroleum generation rates as a result of differences in bond energies for different sulfur-containing bonds. Petroleum migration may also be affected by sulfur moiety distributions because some sulfurcontaining functional groups, such as sulfoxide, are strongly polar and could potentially interact with solid surfaces and alter their wettability.13 Received: March 30, 2016 Revised: June 14, 2016 Published: July 13, 2016 6264
DOI: 10.1021/acs.energyfuels.6b00744 Energy Fuels 2016, 30, 6264−6270
Article
Energy & Fuels Table 1. Summary of Raw and Post-pyrolysis Oil Shale and Kerogen and Bitumen Properties sample
kerogen H/C ratio
kerogen S/C ratio
raw shale bitumen content (wt %)a
raw bitumen S content (wt %)
sample
1.47 (I)c 1.21 (II) 1.38 (IIS) 1.34 (IIS) 1.19 (II) 1.26 (I) 1.27 (I) 1.13 (IIS) 1.37 (I) 1.07 (II) 1.40 (I) raw shale TOC (wt %)
0.004 0.034 0.062 0.064 0.035 0.002 0.003 0.041 0.001 0.005 0.007 raw shale Tmax (°C)
3.07 0.28 0.42 3.32 2.21 1.50 1.02 1.13 0.55 0.41 0.11 raw shale HI (mg of HC/g of TOC)
Mahogany Timahdit Ghareb-Israel Ghareb-Jordan Kimmeridge Irati Glen Davis Phosphoria Pumpherston New Albany Kukersite
23.2 (22.0)d 9.6 (9.2) 15.6 (15.5) 19.9 (18.3) 44.1 (45.0) 10.7 (9.7) 59.1 (61.3) 21.0 (21.3) 17.4 (18.5) 14.9 (14.6) 41.5 (45.2)
430 (431) 415 (413) 396 (389) 407 (404) 409 (406) 423 (422) 459 (458) 412 (412) 443 (443) 421 (419) 433 (422)
747 (871) 550 (540) 661 (659) 760 (756) 748 (726) 783 (706) 777 (973) 458 (499) 769 (777) 511 (476) 970 (984)
Mahogany Timahdit Ghareb-Israel Ghareb-Jordan Kimmeridge Irati Glen Davis Phosphoria Pumpherston New Albany Kukersite
1.21 4.92 10.64 12.89 4.00 0.52 0.49 7.11 0.17 0.87 1.79 HP shale TOC (wt %) 10.0 (8.0) 6.1 (5.5) 6.5 (5.5) 8.6 (7.0) 38.2 (31.0) 5.0 (4.3) 38.8 (26.9) 14.5 (12.4) 12.1 (8.6) 10.4 (9.6) 15.9 (8.7)
HP shale bitumen content (wt %)b
HP bitumen S content (wt %)
2.08 (2.83) 0.71 (0.86) 1.34 (1.65) 1.83 (2.36) 10.28 (15.57) 0.84 (0.95) 46.78 (55.90) 2.56 (3.07) 4.31 (4.97) 0.96 (1.07) 5.49 (10.91) HP shale Tmax (°C)
0.82 3.02 4.75 7.13 3.61 0.82 0.43 6.89 0.36 1.55 1.15 HP shale HI (mg of HC/g of TOC)
437 (447) 452 (458) 454 (459) 450 (460) 331 (495) 454 (459) 442 (453) 456 (487) 442 (448) 464 (471) 444 (430)
165 (147) 81 (48) 104 (72) 113 (72) 110 (58) 101 (58) 501 (106) 98 (71) 233 (154) 72 (57) 156 (95)
a Bitumen present in the original rock (grams of bitumen per grams of original rock × 100%). bBitumen present in the recovered rock following hydrous pyrolysis (HP) on an original rock basis (grams of bitumen per grams of original rock × 100%). This was determined by multiplying the bitumen content in the recovered rock (shown in parentheses; grams of bitumen per grams of recovered rock × 100%) following HP by the ratio of recovered rock to original rock (grams of recovered rock per grams of original rock). cKerogen type indicated in parentheses based on H/C and S/C ratios. dTotal organic carbon (TOC), Tmax, and hydrogen index (HI = S2/TOC × 100) values in parentheses are for samples analyzed after bitumen extraction.
According to previous work,14,15 sulfur speciation varies greatly in kerogens from different locations representing different depositional environments. Sulfur moiety distributions were found to mimic those of carbon, in the sense that kerogen samples with more abundant aromatic carbon also have more abundant aromatic sulfur. Pomerantz and co-workers13 showed that even kerogen and bitumen from the same location can have substantially different sulfur moiety distributions, particularly for immature shales, with the consequence that knowledge of the sulfur species present in kerogen does not mean that bitumen sulfur speciation is also known. There has been much work on how kerogen composition changes with thermal maturity.2,15−18 Fewer studies have focused on the evolution of bitumen composition during thermal maturation. Systematic studies of changes in Green River oil shale bitumen with artificial thermal maturation by semi-open pyrolysis were performed using techniques such as nuclear magnetic resonance (NMR),19 infrared,20 and X-ray absorption near-edge structure (XANES)3 spectroscopies as well as more traditional methods including elemental analysis, saturate, aromatic, resin, and asphaltene (SARA) fractionation, and gel permeation chromatography.3 These studies have demonstrated that, for Green River oil shale, the composition of bitumen varies greatly with thermal maturity. In particular, bitumen loses hydrogen, sulfur, and aliphatic carbon and shows an increase in aromatic carbon content during maturation, similar to what is observed for kerogen. Oxygenated species measured by infrared spectroscopy may increase in abundance slightly at low maturity before decreasing precipitously, with little change occurring once
peak oil generation begins. Sulfur XANES results demonstrate that bitumens are dominated by oxygenated sulfur forms, such as sulfoxide and sulfone, at low maturities but contain more thiophene with increasing maturation.3,21,22 Consistent with the infrared spectral results, most of the shift in the sulfur moiety distribution occurs prior to peak oil generation. Some of the trends parallel those observed previously in kerogen evolution, while other trends are opposite. In this study, we examine how bitumen sulfur speciation evolves with thermal maturity for a suite of 11 source rocks from around the world. Artificial thermal maturation was achieved by hydrous pyrolysis, which simulates the natural petroleum generation process at laboratory time scales.2,4 Trends in the evolution of bitumen sulfur moiety distributions are observed for two degrees of thermal maturation (immature and near the end of bitumen and oil generation) that are consistent with other hydrous pyrolysis21 and semi-open pyrolysis results.3 The potential implications of those trends are discussed.
2. MATERIALS AND METHODS 2.1. Oil Shale Samples. The oil shales included in this study represent a range of materials from sedimentary rock formations around the world.23 The samples are thermally immature and represent a variety of lithologies and kerogen types. A summary of the key rock, kerogen, and bitumen properties are presented in Table 1. The samples include Eocene Green River Formation Mahogany zone oil shale (Colorado, U.S.A.), Cretaceous Timahdit oil shale (Morocco), Cretaceous Ghareb shales from two locations (Israel and Jordan), Jurassic Kimmeridgian blackstone (England), Permian Irati Formation marinite (Brazil), Permian Glen Davis torbanite (Australia), Permian shale of the 6265
DOI: 10.1021/acs.energyfuels.6b00744 Energy Fuels 2016, 30, 6264−6270
Article
Energy & Fuels Phosphoria Formation (Montana, U.S.A.), Carboniferous Pumpherston torbanite (Scotland), Mississippian−Devonian New Albany shale (Indiana, U.S.A.), and Ordovician Narva-E mine kukersite (Estonia). Here and throughout the paper, the samples are listed in order of geologic age from youngest to oldest. 2.2. Hydrous Pyrolysis. Rock chips (0.5−2 cm) were artificially matured under hydrous pyrolysis conditions. Hydrous pyrolysis is a laboratory technique used to generate pyrolysates similar to natural petroleum from source rocks and oil shales.2,4 The method involves heating whole rock samples in the presence of liquid water to a selected temperature and holding at that temperature for a particular amount of time to achieve different degrees of thermal maturation.4 Experiments were conducted using 1 L Hastelloy C-276 non-stirred reactors (Parr Instrument Co., Moline, IL) with electric heaters (Parr model 4926) and custom-built temperature controllers. Rock samples of approximately 200 g were immersed in 400 mL of distilled water. Prior to heating, the reactors were evacuated to
