Geochemical signatures of thermochemical sulfate reduction in controlled hydrous pyrolysis...

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Organic Geochemistry 39 (2008) 308–328

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OrganicGeochemistry

Geochemical signatures of thermochemical sulfate reductionin controlled hydrous pyrolysis experiments

Tongwei Zhang a, Geoffrey S. Ellis b, Clifford C. Walters c, Simon R. Kelemen c,Kang-shi Wang a, Yongchun Tang a,*

a Power, Environmental, and Energy Research Center, California Institute of Technology, 738 Arrow Grand Circle, Covina, CA 91722, USAb US Geological Survey, Box 25046, MS 939, Denver Federal Center, Denver, CO 80225, USA

c ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, NJ 08801-0998, USA

Received 16 May 2007; received in revised form 13 December 2007; accepted 18 December 2007Available online 25 December 2007

Abstract

A series of gold tube hydrous pyrolysis experiments was conducted in order to investigate the effect of thermochemicalsulfate reduction (TSR) on gas generation, residual saturated hydrocarbon compositional alteration, and solid pyrobitu-men formation. The intensity of TSR significantly depends on the H2O/MgSO4 mole ratio, the smaller the ratio, the stron-ger the oxidizing conditions. Under highly oxidizing conditions (MgSO4/hydrocarbon wt/wt 20/1 and hydrocarbon/H2Owt/wt 1/1), large amounts of H2S and CO2 are generated indicating that hydrocarbon oxidation coupled with sulfatereduction is the dominant reaction. Starting with a mixture of C21–C35 n-alkanes, these hydrocarbons are consumed totallyat temperatures below the onset of hydrocarbon thermal cracking in the absence of TSR (400 �C). Moreover, once thelonger chain length hydrocarbons are oxidized, secondarily formed hydrocarbons, even methane, are oxidized to CO2.Using whole crude oils as the starting reactants, the TSR reaction dramatically lowers the stability of hydrocarbons leadingto increases in gas dryness and gas/oil ratio. While their concentrations decrease, the relative distributions of n-alkanes donot change appreciably from the original composition, and consequently, are non-diagnostic for TSR. However, distinctmolecular changes related to TSR are observed, Pr/n-C17 and Ph/n-C18 ratios decrease at a faster rate under TSR com-pared to thermal chemical alteration (TCA) alone. TSR promotes aromatization and the incorporation of sulfur and oxy-gen into hydrocarbons leading to a decrease in the saturate to aromatic ratio in the residual oil and in the generation ofsulfur and oxygen rich pyrobitumen. These experimental findings could provide useful geochemical signatures to identifyTSR in settings where TSR has occurred in natural systems.� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Throughout the generation, expulsion, migration,and accumulation processes, petroleum is potentially

0146-6380/$ - see front matter � 2008 Elsevier Ltd. All rights reserveddoi:10.1016/j.orggeochem.2007.12.007

* Corresponding author. Tel.: +1 626 858 5077; fax: +1 626 8589250.

E-mail address: [email protected] (Y. Tang).

exposed to large quantities of brine. These aqueoussolutions can serve as a solvent that facilitates reac-tions that are sluggish or not favored in oil or gasphases (Seewald, 2003). The most prominent abioticalteration process in hot carbonate reservoir rocks isthe reduction of sulfate to sulfide coupled with theoxidation of hydrocarbons to carbon dioxide,which collectively is termed thermochemical sulfate

.

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T. Zhang et al. / Organic Geochemistry 39 (2008) 308–328 309

reduction (TSR). TSR is well documented in numer-ous petroleum systems and metal sulfide deposits (seereferences in Zhang et al., 2007). The occurrence ofTSR is most commonly recognized by the presenceof high concentrations of H2S (>10%) and CO2

(Worden et al., 1995; Machel, 2001). The presenceof alkali earth elements and base or transition metalsoften results in the precipitation of carbonate minerals,particularly calcite and dolomite, and metal sulfides;thereby obscuring the characteristic geochemical sig-nature of TSR by sequestering large quantities ofCO2 and H2S (Machel, 2001). Calcite that precipi-tates during TSR typically occurs as milky white,coarse crystalline cements and/or as replaced massesafter anhydrite (Machel and Anderson, 1989; Machelet al., 1995; Worden et al., 1996). Common metal sul-fide by products of TSR are galena and sphalerite,often as Mississippi Valley Type deposits (Sangsteret al., 1998). If iron is available during TSR, it tendsto precipitate as pyrite or marcasite that replacesanhydrite (Dixon et al., 1989; Machel, 2001). Ironmay be limited in sour gas reservoirs as most of thesedimentary iron may have already been fixed dur-ing early diagenesis by bacterial sulfate reduction(BSR) (Morse et al., 1987). Worden and Smalley(2001) suggested that TSR occurred in a clasticreservoir that resulted in substantial amounts ofpyrite precipitation and very low residual H2Sconcentration.

Field observations suggest that TSR can signifi-cantly affect the composition of petroleum hydro-carbons through oxidation and thermal crackingprocesses. In particular, the extent of souring withinreservoirs has been correlated with increases in thegas to oil ratio (GOR) (Claypool and Mancini,1989) and gases generated in association with TSRare found to be relatively dry (i.e. low C2+ content)(Krouse et al., 1988; Noth, 1997). TSR altered oilsare enriched in thiophenic and sulfidic hydrocar-bons, which are formed in the reservoirs via reac-tions between H2S and hydrocarbons (Ho et al.,1974; Cai et al., 2003). Light hydrocarbon distribu-tions and stable carbon isotope composition alsocan be very diagnostic of TSR alteration (Peterset al., 2005).

Efforts to simulate TSR in the laboratory havefocused generally on the determination of the reac-tion kinetics and attempts to understand the detailsof the reaction mechanisms involved (Toland, 1960;Dhannoun and Fyfe, 1972; Kiyosu, 1980; Trudingeret al., 1985; Kiyosu et al., 1990; Kiyosu and Krouse,1993; Goldhaber and Orr, 1995; Cross et al., 2004).

Recently, the effects of TSR alteration on gas chem-ical and isotopic compositions were investigated byPan et al. (2006) and the influence of petroleum com-position on TSR reaction rate was investigated byZhang et al. (2007). Geologic observations suggestthat the most easily oxidized hydrocarbons are thegasoline range branched and normal alkanes fol-lowed by the cyclic and monoaromatic compounds(Krouse et al., 1988; Manzano, 1995; Rooney,1995; Manzano et al., 1997; Cross et al., 2004) andthat TSR lowers the saturate/aromatic hydrocarbonratio, API gravity, and C15+ hydrocarbon content ofpetroleum while increasing the GOR (Orr, 1974,1977; Claypool and Mancini, 1989; Manzano et al.,1997). These relationships have not been validatedadequately in the laboratory. Prior simulations ofthe TSR process were conducted under unusualchemical conditions or under very high temperatureswhere it is difficult to differentiate hydrocarbon ther-mal cracking from sulfate reduction.

The objectives of this study are to:

(1) quantify the gas chemical composition changewith the increase of the extent of TSR,

(2) characterize the differences in the chemistry ofresidual liquid hydrocarbons following ther-mal cracking with and without TSR,

(3) examine the effect of TSR on the thermal sta-bility of oil during thermal maturation, and

(4) characterize pyrobitumen formation resultingfrom TSR.

In order to address these objectives, we designeda series of gold tube hydrous pyrolysis experimentsthat allow the reaction of hydrocarbons withMgSO4 to occur under a thermal regime wherehydrocarbon oxidation by sulfate is the dominantreaction while hydrocarbon thermal cracking isminimized. Tang et al. (2005) reported that MgSO4

is an effective oxidant of hydrocarbons without theaddition of initial hydrogen sulfide, which was usedas an initiator in other experiments (Hoffmann andSteinfatt, 1993; Goldhaber and Orr, 1995). Weintentionally employed extremely oxidizing condi-tions in order to amplify the effects of hydrocarbonoxidation associated with sulfate reduction and tominimize the relative effect of hydrocarbon crack-ing. These experiments provide an opportunity toinvestigate how TSR destabilizes hydrocarbons,the characteristics of the products (gases, liquids,and solids), and a means of quantifying the extentof TSR alteration in natural systems.

310 T. Zhang et al. / Organic Geochemistry 39 (2008) 308–328

2. Samples and experimental methods

2.1. Sample description

This study was performed on n-octane, a syn-thetic paraffin mixture, and on two crude oils ofvarying composition. The paraffin mixture is aresearch grade mix of C21 to C35 normal alkanes,with a melting point of 52–58 �C. This mixture wasanalyzed for its hydrocarbon composition by gaschromatography-flame ionization detection (GC-FID) following the experimental protocol describedin Section 2.2.3. The n-octane and paraffin mixtureare model compounds that contain no sulfur.

Two unrefined, wellhead crude oils were used inthis study. These oils are referred to in this paperas ‘‘low-sulfur oil” and ‘‘high-sulfur oil” to protectproprietary interests of the oil companies thatdonated the samples. Their chemical propertiesare listed in Table 1. The low-sulfur oil was pro-duced from a Carboniferous aged carbonate reser-voir in an onshore field near the Caspian Sea inKazakhstan. The oil has an approximate APIgravity of 45�, contains �0.5% sulfur and haslow polar content with no asphaltenes. The high-sulfur oil was produced from an onshore field inthe Oman. The oil contains 3.0 wt% (±0.1%) sul-fur (Stainforth, pers. comm.), has an API gravityof 31.0�, and contains 15.3% asphaltene and7.5% resins. The low and high-sulfur oils werecharacterized by GC-FID prior to reaction usingthe experimental protocol detailed in Section2.2.3. The gas chromatography for the paraffinmix, high-sulfur and low-sulfur oil was reportedby Zhang et al. (2007).

Table 1Some chemical properties of the hydrocarbons used in this study

Paraffinmixture

Low-sulfuroil

High-sulfuroil

Saturates (%) 100 76 47.9Aromatics (%) 0 20 29.3Resins (%) 0 4 7.5Asphaltenes

(%)0 0 15.3

Total sulfur(%)

0 0.5 3.0

Labile sulfura

(mg/g oil)0 1.1 17.3

a Total sulfur of the labile organosulfur species in oils wasestimated based on the hydrogen sulfide produced from oilthermal cracking at R0 of about 1.12% according to Zhang et al.(2007).

2.2. Experimental methods

2.2.1. Hydrous pyrolysis

Gold is the material of choice for this workbecause of its high chemical inertness and flexibilitythat allows for external control of the confiningpressure. All experiments were conducted in sealedgold tubes with a length of 60 mm and 70 mm, aninternal diameter of 3.5 mm, and a wall thicknessof 0.35 mm giving a total reactor volume of approx-imately 0.5 ml. Prior to loading the samples, theopen ended tubes were heated to 600 �C to removeany residual organic material. One end of each tubethen was crimped and sealed using an argon arcwelder. Liquid organic reactants (oils or modelcompounds) and distilled water were loaded intothe tubes by means of a 25 ll syringe. Preciseamounts of MgSO4 powder and solid hydrocarbons(paraffin mixture) were accurately weighed andtransferred to the tubes. The tubes were then flushedwith argon for 5 min to remove air. The open end ofthe gold tube was crimped and welded while theother end was submerged in an ethanol liquid nitro-gen mixture (�80 �C) in order to minimize the lossof any volatiles created during the welding process.

Individual sealed gold tubes were placed in sepa-rate stainless steel autoclaves and inserted into apyrolysis oven. Pyrolysis experiments were con-ducted under isothermal conditions at a tempera-ture of 350 �C for 24 h or non-isothermal conditionsfrom 250 to 500 �C at a 20 �C/h heating rate. Tem-perature was controlled to within 1 �C of the setvalue, and was monitored using a pair of thermo-couples secured to the outer wall of each autoclave.Constant confining pressure was maintained atapproximately 24.1 MPa (3500 psi) by a waterpump in order to prevent rupturing of the goldtubes at elevated temperatures. When the desiredreaction temperature or time was reached, the stain-less steel autoclave was withdrawn from the ovenand then rapidly cooled to room temperature byquenching in water. Once the autoclaves weredepressurized, the gold capsules were taken outand weighed to determine if leakage had occurred.The leakage verified gold tubes could be recoveredfor detailed analysis of their contents.

2.2.2. Gas analysisEach gold tube was loaded separately into a pierc-

ing unit and connected to a custom made glass vac-uum line with a residual pressure of 0.1 Pa. Afterisolation from the vacuum pump, each gold tube


was punctured with a stainless steel piercing deviceto allow the product gases (C1, C2, C3, C4, C5,CO2, H2S, H2, and N2) to volatilize into the glassvacuum line. The heavier gaseous compounds werecold trapped with liquid nitrogen (�196 �C), whilethe permanent gases were concentrated into a pre-calibrated volume using a mercury Toepler pump.Replacing the liquid nitrogen with a mixture of dryice and acetone (�80 �C) released the other volatilespecies, excluding water and organic compoundsheavier than C5. These gases were drawn into thesame calibrated volume in order to determine totalgas yields, and the total number of moles of gas werecalculated assuming ideal gas behavior (PV = nRT).

Identification and quantification of individualhydrocarbon and non-hydrocarbon gas componentswere carried out using a two channel Hewlett–Pack-ard 6890 Series Gas Chromatograph (GC) that wascustom configured by Wasson ECE Instrumenta-tion. The details of GC operation conditions aredescribed in Zhang et al. (2007).

2.2.3. Liquid hydrocarbon analysis

The hydrocarbon composition of the initial par-affin mixture and (non-pyrolyzed) oils were deter-mined by gas chromatography-flame ionizationdetector (GC-FID) analysis using a Varian 3400GC equipped with an Rtx-1 fused silica capillarycolumn (30 m length, 0.53 mm ID, 0.25 lm filmthickness). The non-reacted samples were preparedby dissolving 15 mg of sample into 480 ll of CS2

and then adding 20 ll of a 1.01% perdeuteratedinternal standard solution (C24D50). One microliterof prepared sample was then injected into the GC.The column was initially held at 50 �C for 1 min,and then heated to 325 �C at 10 �C min�1. Integra-tion of the FID signal was performed using PENelson Turbochrom 4 software. Compound identi-fications were based on relative retention times ascompared to standard sample analyses.

The liquid hydrocarbons remaining after reactionwere analyzed in gold tube experiments conductedin parallel with those conducted for gas analysisusing a procedure designed to prevent the loss oflow molecular weight volatiles during sample recov-ery. Following post-pyrolysis cooling in water, agold tube was frozen in liquid nitrogen (�196 �C)for 5 min to completely condense all volatiles. Thegold tube was removed from the liquid nitrogencup and gently scored around the middle, takingcare not to puncture the tube. The gold tube thenwas placed into a 5 ml vial filled with 20 ll internal

standard solution (1.01% C24D52) plus 480 ll of CS2

and the tube was then manually flexed it to break italong the score, and the vial capped immediatelythereafter. The vial was agitated for several minutesto allow the soluble pyrolysis products to dissolveinto the solvent. One microliter of prepared samplewas then injected into the GC and analyzed follow-ing the same procedure described above for the ini-tial reactants.

2.2.4. Residual solid analysis by X-ray photoelectron

spectroscopy (XPS)Residual solids after reaction were recovered

from gold tubes by manually cutting the gold tubesto small pieces and collecting the solids. After dry-ing the solids at room temperature, XPS analysesof the residual solids were conducted using a Kra-tos Axis Ultra system equipped with a monochro-matic Al Ka radiation source and automaticcharge neutralization. Solid residues were mountedon a metallic sample nub using Scotch double sidednon-conducting tape. An energy correction wasmade to account for sample charging based onthe carbon (1s) peak at 285.0 eV. The relativeamount of aromatic carbon was determined bythe method of Kelemen et al. (1993) from the cali-brated intensity of the p ? p* signal intensity.Curve resolution of the carbon (1s) spectra wasperformed following the method summarized inWalters et al. (2006). Inorganic carbonate carbonappears as a peak at 290.5 eV. The amount oforganic oxygen was determined by analyzing oxy-gen’s effect on the carbon (1s) signal of adjacentcarbon atoms. The C–O, C@O and O@C–O car-bon (1s) curve resolved peaks are associated with0.5, 1.0 and 2.0 oxygen atoms, respectively. Curveresolution of the sulfur (2p) spectra followed themethod in Kelemen et al. (1990). Two chemicalenvironments are found at energy positions consis-tent with aromatic (thiophenic) sulfur (164.1 eV)and sulfate (169.7–170.2 eV).

2.2.5. Residual solid analysis by X-ray diffraction

(XRD)Residual solids were suspended in deionized water,

and sonicated to ensure complete mixing. Suspendedmixtures were freeze dried, loaded into aluminumholders, and analyzed using a Siemens D5000 X-raydiffractometer equipped with a h–2h goniometer anda copper anode X-ray tube. Standard methods wereemployed for data acquisition and mineral identifica-tion using DiffracPlus software.


3. Results

3.1. Isothermal pyrolysis of n-octane with TSR

3.1.1. Gas yields from n-octane cracking with variable

amounts of MgSO4

In order to investigate the effect of TSR on hydro-carbon thermal cracking, a series of experimentswere conducted involving fixed amounts of n-octane(25 mg) and distilled water (5 mg) with a variableamount of MgSO4. All samples were pyrolyzed at350 �C for 24 h. Gas yields from these experiments(Table 2) show clearly that H2S and CO2 yieldsincrease with increasing amount of MgSO4

(Fig. 1A). Under these reaction conditions the extentof the TSR reaction is directly proportional to theamount of MgSO4 in the system as is the intensityof octane oxidation. This might imply that theTSR reaction occurs on the MgSO4 solid surface.

The amounts of generated methane and C2–5

hydrocarbon gases increase, while the amounts of eth-ylene and propylene decrease, with increasing extentof TSR (Table 2), suggesting that the thermal stabilityof n-octane is lowered by TSR. Another importantobservation is the increase of C1–C5 total gas yieldsand the ratio of C1/

PC1–5 gases generated (gas dry-

ness) from 0.12 to 0.39 with increasing gas sourness

Table 2Gas yields from octane cracking in the presence of MgSO4 from goldpressure

Expt. run Reactants (mmol) Gas yields (mmo

C8 MgSO4 H2O H2O/MgSO4 C1 C2 C

Series 1: Effect of variable MgSO4 amounts on octane cracking

1 0.22 0.42 0.28 0.67 32.7 20.74 02 0.23 0.21 0.28 1.33 21.16 16.68 03 0.22 0.11 0.28 2.55 8.79 9.6 04 0.22 0.05 0.28 5.60 3.00 5.52 05 0.22 0 0.28 1.02 1.93 0

Series 2: Effect of the presence of water on MgSO4 reduction by octane

6 0.22 0.42 0.00 0.00 36.39 23.08 07 0.22 0.43 0.15 0.35 24.86 15.77 08 0.22 0.42 0.61 1.46 18.90 13.52 09 0.22 0.42 0.82 1.94 8.10 8.13 010 0.21 0.42 1.14 2.71 0.86 3.06 011 0.22 0.42 1.27 3.02 1.47 5.46 012 0.22 0.42 1.71 4.05 0.66 2.48 013 0.22 0.42 1.99 4.71 0.78 2.86 014 0.22 0.42 2.14 5.06 0.87 2.91 015 0.22 0.42 2.41 5.76 1.53 4.85 016 0.21 0.42 3.39 8.08 0.68 2.49 017 0.21 0.42 4.56 10.90 1.27 3.82 018 0.22 0.42 7.11 16.93 2.51 4.70 019 0.22 0.42 7.72 18.39 4.07 5.76 0

or the extent of TSR (Fig. 1B), showing that driergas is generated during hydrocarbon cracking associ-ated with TSR than during thermal cracking alone.

3.1.2. Variable H2O/MgSO4 mole ratio on TSR

reaction rate

In order to further investigate the effect of waterpresence on MgSO4 reduction, another series ofexperiments was conducted involving fixed amountsof n-octane (�25 mg) and MgSO4 (50 mg) with a var-iable amount of distilled water. All samples werepyrolyzed under the same condition of 350 �C for 24h. Gas yields from these experiments (Table 2) showthat the rate of MgSO4 reduction to H2S initiallydecreases with increasing H2O/MgSO4 mole ratio(Fig. 2), and then ceases when H2O/MgSO4 moleratio continuously increases, but with further increas-ing H2O/MgSO4 mole ratio the reduction startsagain. This experimental observation suggests thatthe mechanism of MgSO4 reduction in the presenceof water is more complicated than the simple reactionoccurring on the surface of MgSO4. In the low H2O/MgSO4 mole ratios ranging from 0 to 5, the TSR reac-tion mainly occurs on the MgSO4 solid surface. How-ever, under extremely dilute conditions with very highH2O/MgSO4 mole ratios, MgSO4 is entirely dissolvedin water. The disassociation of MgSO4 produces

tube hydrous pyrolysis at 350 �C for 24 h and 3500 psi confining

l/mol C8)

2ene C3 C3ene i-C4 n-C4 i-C5 n-C5 CO2 H2S

14.04 0.05 0.31 9.71 0.31 6.1 41.43 168.812.95 0.15 0.26 9.97 0.31 7.62 25.05 103.7

.05 8.16 0.32 0.09 6.98 0.19 6.4 12.34 44.64

.04 5.06 0.64 0.03 4.64 0.09 4.64 5.15 11.96

.02 1.89 0.61 0.02 1.56 0.02 1.54 2.48 0

.00 15.63 0.05 0.34 10.80 0.34 6.79 45.48 187.80

.04 10.48 0.01 0.28 7.55 0.24 4.60 33.48 140.12

.07 9.73 0.02 0.23 7.85 0.21 5.63 21.59 101.35

.04 6.65 0.06 0.15 5.90 0.13 5.11 5.32 40.26

.14 2.85 1.67 0.01 2.91 0.03 3.11 0.37 0.00

.20 5.17 2.78 0.01 5.23 0.06 5.78 1.59 0.00

.12 2.34 1.40 0.01 2.36 0.03 2.49 0.40 0.00

.15 2.67 1.64 0.01 2.70 0.03 2.72 0.40 0.00

.13 2.73 1.59 0.01 2.75 0.03 2.88 1.76 0.00

.19 4.54 2.33 0.01 4.54 0.05 4.85 2.00 0.00

.12 2.32 1.35 0.01 2.34 0.03 2.44 0.61 0.00

.15 3.72 1.81 3.68 1.07 0.06 3.73 1.36 0.00

.12 4.84 1.53 0.08 4.41 0.10 4.63 1.55 1.33

.14 5.60 1.71 0.13 5.16 0.14 5.10 1.12 3.05

0

40

80

120

160

200

0.0 0.5 1.0 1.5 2.0

MgSO4/octane ratio in mole

Gas

yie

lds

(mm

ol/m

ol o

ctan

e)

H2S

CO2

C1-5

0.00

0.20

0.40

0.60

0.0 0.5 1.0 1.5 2.0

Mg SO4/octane ratio in mole

Gas

so

urn

ess

and

dryn

ess Gas sourness

Gas dryness

A

B

Fig. 1. Gas yields from pyrolysis experiments involving octane with increasing MgSO4 amount under isothermal conditions at 350 �C for24 h. (A) Gas yields; (B) gas sourness [H2S/(H2S + CO2 +

P(C1–C5)).


Mg2+ and SO2�4 ions in the solution, and it is most

likely that the presence of Mg2+ ions in the solutionmay play some role to sulfate reduction.

3.1.3. Mg2+ and SO2�4 ions in aqueous solution on

TSR reaction rate

In order to clarify the effect of the presence ofMg2+ and SO2�

4 ions in aqueous solution on TSRreaction rate, three comparison experiments wereconducted involving fixed amounts of n-octane(25 mg), MgSO4 (2 mg) and distilled water(380 mg) with addition of NaCl (43 mg), Na2SO4

(34 mg) and MgCl2 (24 mg), respectively. Threeaqueous solutions give a same ionic strength ofabout 2.02 M. The molarity of MgSO4 solution isinitially about 0.04 M for all three solutions. Allsamples were pyrolyzed at 350 �C for 100 h. Gasyields from these experiments are listed in Table 3.H2S yield from aqueous MgSO4 reduction increases

from 15.1 mmol/mol C8 with NaCl to 25 mmol/molC8 with MgCl2, clearly suggesting that the presenceof Mg2+ ion can enhance the sulfate reduction toH2S. However, the TSR reaction was shut downin the presence of Na2SO4, which provides addi-tional SO2�

4 and drives the precipitation of MgSO4

or its hydrated species and consequently, reducesthe Mg2+ ion concentration in the solution. Theexperimental observation clearly suggests that thepresence of Mg2+ ion in the aqueous solution playsan important role to sulfate reduction.

3.2. Non-isothermal pyrolysis of paraffin mixture(C21–C35) with and without TSR

3.2.1. Gas yields

A series of experiments was conducted involvinga paraffin mixture and distilled water (paraf-fin = 5 mg, H2O = 5 mg) with and without MgSO4

0

50

100

150

200

0 5 10 15 20H2O/MgSO4 mole ratio

H2S

yie

ld (

mm

ol/m

ol C

8)

variable water amount with afixed MgSO4 loading

variable MgSO4 amount with afixed water loading

Reduction of MgSO4 solid

Reduction of HSO 4-

Fig. 2. H2S yield from MgSO4 reduction by octane with increasing H2O/MgSO4 mole ratio under isothermal conditions at 350 �C for24 h.


(100 mg) heated at 20 �C/h. Gas yield data for C1,C2–5, CO2 and H2S from these experiments are listedin Table 4. Pyrolysis of the hydrocarbons withoutTSR shows that the yields of methane and heavierhydrocarbon gases are very low at 375 �C, andincrease dramatically when the temperature is above425 �C (Fig. 3A). The methane yield is much smallerthan the sum of the heavier hydrocarbons (C2–C5)up to 525 �C (equivalent vitrinite reflectance value�2.7% R0), which is consistent with previousreports of pyrolysis experiments involving crudeoil that generate hydrocarbon gases with methanecontents ranging from �10% to 60% (Applebyet al., 1947; Hikita et al., 1989; Horsfield et al.,1992).

Pyrolysis experiments involving the paraffinmixture with the addition of MgSO4 (H2O/MgSO4

mole ratio 1:3, named 100 mg MgSO4 case) showthat sulfate reduction begins at �300 �C, and thatthe H2S yield increases with temperature andreaches a maximum at 375 �C. H2S yield then

Table 3Effect of the presence of SO2�

4 and Mg2+ in aqueous solution on MgSOfor 100 h and 3500 psi confining pressure

Expt. run Reactants (mmol) Na2SO4 MgCl2 Gas yields (mm

C8 MgSO4 H2O C1 C2

1 0.22 0.017 21.46 32.92 69.832 0.22 0.016 21.11 0.24 18.06 63.523 0.23 0.017 21.84 0.25 41.36 66.94

decreases at temperatures >400 �C. The yield ofCO2 continuously increases with temperature from300 to 500 �C and is much higher than that fromparaffin reacting with water in the absence ofMgSO4 (Table 4 and Fig. 3C), indicating thatthe oxidation of hydrocarbons by TSR is a domi-nant process. Furthermore, the changing yields ofC1 and C2–C5 with temperature provide additionalevidence of the strong oxidation potential of thissystem. The yield of the C2–5 hydrocarbon gasesincreases with increasing temperature from 300to 375 �C and then decreases markedly from 6.4to 0.01 ml/g paraffin mix at higher temperatures(Fig. 3B). The methane yield starts to decreaseonce the heavier hydrocarbon gases are oxidizedcompletely (427 �C) (Fig. 3B). This decrease inmethane must be due to TSR associated oxidationbecause CO2 yields continue to increase in theseexperiments and methane generation from paraffinthermal cracking alone consistently increases overthis temperature range.

4 reduction by octane from gold tube hydrous pyrolysis at 350 �C

ol/mol C8)

C2ene C3 C3ene i-C4 n-C4 i-C5 n-C5 CO2 H2S

0.60 63.96 8.05 2.15 52.47 1.99 34.94 7.40 15.091.10 52.95 10.55 0.22 47.44 0.93 33.59 4.15 0.000.75 75.77 7.62 6.84 57.57 4.33 44.35 28.93 25.17

Table 4Gas yields (ml/g oil) from paraffinic hydrocarbon cracking with and without MgSO4 from 250 to 550 �C and 3500 psi confining pressure at20 �C/h heating rate

Oil type With MgSO4 No MgSO4

Temperature (�C) C1 C2–C5 CO2 H2S Temperature (�C) C1 C2–C5 CO2 H2S

Paraffinic mixture 250 0.01 0.02 0.88 0.00275 0.01 0.02 0.37 0.00300 0.02 0.02 0.50 0.88325 0.08 0.11 2.01 4.37340 0.19 0.30 1.93 12.60350 0.68 1.23 4.28 30.97360 1.27 2.30 5.80 44.50370 2.90 5.16 11.36 86.18375 4.54 6.43 24.95 108.44 375 0.02 0.05 0.20400 8.34 1.11 79.67 62.10 400 0.12 1.40 0.70427 6.35 0.06 171.65 33.61 425 1.47 17.74 0.41475 0.17 0.02 288.59 25.30 450 29.24 123.91 3.43500 0.00 0.02 443.68 33.26 478 75.06 211.51 0.92525 0.01 0.00 504.42 48.25 504 183.79 293.88 2.32550 0.00 0.01 676.89 131.02 525 263.74 264.32 2.76575 0.00 0.04 811.82 345.53 550 412.70 218.12 3.35

Low-sulfur oil 300 0.07 0.11 6.17 0.00325 0.25 0.25 7.99 3.64340 0.99 0.60 17.00 13.68350 1.23 0.88 18.50 16.74360 2.04 1.38 28.00 20.50 350.00 0.07 0.15 1.18 0.08375 5.09 3.81 35.00 39.10 375.00 0.32 0.46 3.14 0.25400 11.21 1.52 91.18 53.84 400.00 0.43 1.13 1.17 0.39427 11.72 0.03 152.62 60.38 425.00 2.75 7.63 8.19 0.59453 5.63 0.03 235.37 41.40 450.00 11.46 42.66 0.24 0.67475 0.84 0.03 306.75 68.39 475.00 44.95 114.96 1.72 0.68500 0.00 0.02 463.78 29.00 500.00 135.44 211.97 2.00 1.41550 0.00 0.00 601.11 79.20575 0.00 0.00 787.65 69.64

High-sulfur oil 300 0.11 0.31 0.33 0.88325 0.62 0.68 2.30 8.48340 1.31 1.29 5.95 10.31350 1.87 1.67 14.10 14.51360 3.32 3.05 21.35 20.30 355 0.67 1.33 0.86 4.38370 4.20 3.56 29.00 28.01 375 1.49 2.65 1.64 5.10375 10.58 4.68 34.70 70.98 400 1.65 2.03 1.16 5.03400 16.66 1.66 125.76 64.78 425 5.74 10.34 2.06 8.06427 13.01 0.04 184.90 50.05 450 23.12 55.28 3.60 8.23475 8.30 0.02 352.67 110.09 475 60.00 139.30 6.86 10.71500 16.20 0.03 376.26 144.79 500 137.16 212.00 7.88 14.01525 0.01 0.02 539.44 55.43 525 209.31 210.48 10.84 14.84550 0.01 0.01 844.73 24.27 550 319.15 192.85 16.99 14.65575 0.00 0.00 856.45 87.35

Note: For the experiment with MgSO4, oils = 5 mg, H2O = 5 mg and MgSO4 = 100 mg; for the experiment without MgSO4, oils = 15 mgand H2O = 15 mg.


However, the gas generation and gas characteris-tics from paraffin thermal cracking as a function oftemperature is obviously different when the H2O/MgSO4 mole ratio increases to 10:1. Zhang et al.(2007) reported the gas yields from pyrolysis exper-imental involving the 15 mg paraffin mixture withthe addition of 10 mg MgSO4 and 15 mg distilled

water from 340 to 550 �C at 2 �C/h. We gave a nameof ‘‘10 mg MgSO4” to these experimental condi-tions. The previous experimental results clearlyshow that TSR starts at a temperature of 400 �Cfor the 10 mg MgSO4 case, which is about 100 �Chigher than that of 100 mg MgSO4 case presentedin this study. There is no observed difference in C1

0

5

10

15

300 350 400 450 500 550 600

Gas

yie

lds

(ml/g

oil)

C1

C2-5

with MgSO4

0

200

400

600

800

1000

300 350 400 450 500 550 600Temperature (°C)

Gas

yie

lds

(ml/g

oil)

H2S

CO2

with MgSO4

0

100

200

300

400

300 350 400 450 500 550 600

Gas

yie

lds

(ml/g

oil)

C1

C2-5

without MgSO4

Fig. 3. Gas generation as a function of temperature from a paraffin mixture cracking with and without the presence of MgSO4 from 300 to550 �C at 20 �C/h heating rate.


and C2–5 generation as a function of temperaturewith and without TSR for the 10 mg MgSO4 case.In other words, the dominant reaction is the thermalcracking of paraffinic compounds to gases, and TSRplays an insignificant role in the alteration of gascompositions. Therefore, the 10 mg MgSO4 case isnot suitable to differentiate the effect of TSR onthe paraffinic compounds from thermal cracking.However, the oxidation of hydrocarbons by TSRis a dominant process for the 100 mg MgSO4 case,which provides an opportunity to investigate how

TSR destabilizes hydrocarbons and the characteris-tics of the products (gases, liquids, and solids) whileminimizing the relative effect of hydrocarboncracking.

3.2.2. Residual liquid hydrocarbons

The paraffin mixture (C21–C35) was pyrolyzedwithout the addition of MgSO4 under non-isother-mal hydrous pyrolysis conditions from 370 to475 �C (20 �C/h heating rate). Concentrations ofresidual C11–C35 hydrocarbons following thermal


cracking were determined using the methoddescribed in Section 2.2.3 and are shown inFig. 4A. Above 375 �C the yield of the lighter paraf-finic components with carbon number less than C21

gradually increases and accounts for a significantpercentage of the total residual hydrocarbons at450 �C. This pattern is similar to that seen in thethermal cracking of other model compounds andcrude oils (e.g., Speight, 2003; Xiong et al., 2004;Wang et al., 2006).

Similar experiments were conducted involvingthe paraffin mix with the addition of MgSO4 undernon-isothermal hydrous pyrolysis conditions from300 to 475 �C (20 �C/h heating rate); however, thereactivity and distribution of n-alkanes in the liquidresidues are significantly different in the TSR reac-tions relative to the hydrocarbon cracking experi-

0

0.2

0.4

0.6

0.8

1

1.2

1.4

11 13 15 17 19 21 23 25Carbon

wei

gh

t am

ou

nt

(mg

)

450°

without MgSO4

0

0.08

0.16

0.24

0.32

0.4

11 13 15 17 19 21 23 2Carbo

wei

gh

t am

ou

nt

(mg

)

with MgSO4

375 °C

Fig. 4. Comparison of remaining paraffinic mixture with temperatureMgSO4 at 20 �C/h heating rate.

ments. As shown in Fig. 4B, loss of the C21 to C35

n-alkanes begins at approximately 325 �C and by400 �C, the onset temperature for thermal crackingof the paraffin mixture, almost all of the originalparaffin mixture is consumed. Furthermore, thegeneration of shorter chain length hydrocarbons(<n-C21) observed during thermal cracking of theparaffin mixture does not significantly occur whenTSR reactions dominate.

3.3. Non-isothermal pyrolysis of crude oil with and

without TSR

3.3.1. Gas yields

Gas generation from the cracking of two oils wasinvestigated. In the absence of MgSO4, C1 and C2–C5 yields from thermal cracking significantly

27 29 31 33 35 37 39 number

400 °C

375 °C

425°

room temp.

475°

5 27 29 31 33 35 37 39n number

300°

325°

340 °C

350°

400 °C

room Temp.

after heating paraffin mixture with and without the presence of

Table 5Comparison of n-C9 to n-C40 normal alkane distribution for high-sulfur oil thermal decomposition with and without TSR at 400 �Cwith 20 �C/h heating rate

Component Amount of concentration (mg/g oil)

Unheatedoil

Cracked oilwithout TSR

Cracked oilwith TSR

C9 6.90 10.18 3.56C10 7.17 8.98 2.96C11 6.84 9.04 2.39C12 6.23 7.77 1.95C13 6.59 7.75 1.76C14 5.45 7.03 1.52C15 5.60 7.08 1.79C16 4.10 4.96 0.92C17 4.00 4.63 0.80C18 3.54 4.03 0.52C19 3.85 4.17 0.51C20 2.74 3.07 0.47C21 2.21 2.42 0.29C22 1.85 2.00 0.24C23 1.85 1.97 0.24C24 1.38 1.53 0.37C25 1.44 1.56 0.33C26 1.27 1.31 0.30C27 0.92 0.97 0.20C28 0.74 0.76 0.18C29 0.71 0.74 0.12C30 0.65 0.71 0.21C31 0.54 0.47 0.10


increases above 400 �C and H2S yields are low(Table 4). The H2S yield in the presence of MgSO4

is obviously higher than that from oil thermal crack-ing alone, confirming that MgSO4 was reduced bythe two oils. The onset temperature of TSR fromthe two different oils and paraffin mixture is similar,indicating that the difference in chemical composi-tions among these hydrocarbons does not play a sig-nificant role in controlling the onset temperature ofTSR under these strong oxidation conditions. In allexperiments containing MgSO4 at temperaturesfrom 300 to �400 �C H2S yield consistentlyincreases. At temperatures above �400 �C H2S yieldbecomes highly variable and there are no consistenttrends observed with increasing temperature.

The trends with increasing temperature in CO2,C1 and C2–C5 yields from the oils are similar tothose seen in the paraffin mix (Table 4). CO2 yieldcontinuously increases at temperatures >427 �C,while methane yield starts to decrease once the hea-vier gaseous hydrocarbons (C2–C5) are consumedby TSR. The gas dryness (C1/

PC1–5) of the gener-

ated hydrocarbons in the whole oil experiments issignificantly impacted by TSR with the maximumincreasing from 67% for oil cracking to 100% withTSR (Fig. 5).

C32 0.53 0.51 0.11C33 0.54 0.45 0.09C34 0.47 0.38 0.09C35 0.41 0.35 0.08C36 0.22 0.20 0.10C37 0.18 0.17 0.05C38 0.14 0.15 0.04C39 0.11 0.11 0.03C40 0.12 0.12 0.04

3.3.2. Residual liquid hydrocarbonsResidual liquid hydrocarbons were analyzed in

experiments involving the high-sulfur oil with andwithout MgSO4 at 400 �C with 20 �C/h heating rate(Table 5). The concentrations of all saturatedhydrocarbons were depleted dramatically by TSR

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

300 350 400 450 500Temperature (°C)

C1/

C1-

5, g

as d

ryn

ess

Low-sulfur oil w/TSRLow-sulfur oil w/o TSRHigh-sulfur oil w/TSRHigh-sulfur oil w/o TSR

with TSR

without TSR

Fig. 5. Gas dryness for pyrolysis experiments involving a low-sulfur and high-sulfur oil with and without MgSO4.


(Fig. 6). Even the n-alkanes in the condensate range(C6–C14), which are thermally stable at 400 �C, areconsumed in the presence of MgSO4.

Comparison of geochemical parameters such asPr/Ph, Pr/n-C17 and Ph/n-C18 with and withoutTSR is listed in Table 6. The Pr/Ph ratio in theresidual oils after reaction with MgSO4 generallyincreases with the extent of TSR and the Pr/n-C17

and Ph/n-C18 ratios decrease. Thermal cracking ofthe oil produces the same trend, though to a lesserdegree at equivalent temperatures.

3.4. Residual solids characterization by XPS after

TSR

Residual solids from the TSR reaction of paraf-finic mixture and MgSO4 over a range of tempera-

Table 6Comparison of geochemical parameters for remaining oils with and wi

Conditions Temperature (�C) Low-sulfur oil

n-C14�/n-C14+ Pr/Ph Pr/n-C

Unheated oil 1.69 1.04 0.46

With TSR 300 1.44 1.06 0.43325 1.56 1.06 0.44350 1.41 1.10 0.42375 3.46 1.42 0.30400 3.43 1.55 0.12

Without TSR 375 1.62 0.98 0.42400 1.59 1.06 0.38425 2.14 1.11 0.31450 6.40

0

2

4

6

8

10

12

9 11 13 15 17 19 21 23Carbon

wei

gh

t (m

g/g

oil) Cracked oil without TSR

Unheated oil

Oil with TSR

Fig. 6. Comparison of saturated hydrocarbon distributions from high-s

tures from 275 to 575 �C at a 20 �C/h heating ratewere analyzed by XPS. Quantities of sulfur, carbon,and oxygen species in the residual solids determinedby XPS are listed in Table 7. Sulfur (2p) spectra andexamples of curve resolution are shown in Fig. 7.XPS is a surface sensitive technique and the chang-ing nature of the proportion of sulfate to reducedsulfur reflects the deposition of thiophenic solidsonto the exterior portion of the sulfate minerals.At 275 �C, sulfur exists exclusively as S6+ and 93%of the curve is resolved into the two 2p3/2 and2p1/2 components characteristic of a single sulfatechemical environment (Fig. 7). At higher tempera-tures the sulfate peak broadens significantly indicat-ing that the sulfate exists in two distinct chemicalenvironments: MgSO4 and hydrated MgSO4

species. XRD analysis of the residues confirmed

thout MgSO4 after being heated from 300 to 450 �C at 20 �C/h

High-sulfur oil

17 Ph/n-C18 n-C14�/n-C14+ Pr/Ph Pr/n-C17 Ph/n-C18

0.48 1.53 0.70 0.49 0.78

0.43 1.36 0.65 0.39 0.700.45 1.38 0.59 0.35 0.690.41 1.51 0.55 0.31 0.650.26 2.33 0.62 0.21 0.430.18 3.82 0.99 0.14 0.20

0.44 1.39 0.62 0.37 0.680.38 1.76 0.64 0.34 0.610.30 2.55 0.81 0.29 0.40

5.71

25 27 39 31 33 35 33 39 number

ulfur oil thermal decomposition at 400 �C with and without TSR.

Table 7XPS analysis of carbon, sulfur, and oxygen species in the residual solids forming during TSR of a paraffin mix reacting with MgSO4 at20 �C/h heating rate

Temperature (�C) Per 100 carbons Mole percent Aromatic carbon/thiophenic sulfur ratioAromatic

carbonCarbonatecarbon

Organicoxygen

Sulfatesulfur

Thiophenicsulfur

Thiophenic SO4 (1) SO4 (2)

275 0 0 8.1 99.4 0 0 93 7300 9 0 8.6 51.1 0 0 95 5325 13 0 10.6 51.5 1.1 2 93 5 11.8350 31 0 6.5 40 3 7 81 12 10.3375 53 0 8.1 21.1 9.5 31 59 11 5.6400 46 2.2 9.3 14.5 10.9 43 43 15 4.2427 61 1 9.5 19.4 13.5 41 46 14 4.5475 76 1.8 8.7 20.6 16.1 44 42 14 4.7500 78 1.9 7.2 18.3 16.9 48 38 14 4.6550 91 1.9 7.6 16.8 15.6 48 38 14 5.8575 94 2 10 31.4 12.2 28 51 20 7.7

Reactors heated at 20 �C/h under 3500 psi constant confining pressure to the indicated maximum temperature.

275 oC

XP

S S

ulf

ur

(2p

) S

ign

al

172 168 164 160Binding Energy (eV)

SO4 S0

300 oC

325 oC

350 oC

375 oC

400 oC

425 oC

475 oC

275 oCSO4

375 oCSO4

S0

173 169 165Binding Energy (eV)

475 oCSO42P3/2

2P1/2

Fig. 7. X-ray photoelectron sulfur (2p) spectra and curve-resolved spectra of solid residues from the reaction 100 mg MgSO4 + 5 mgparaffin mix and 5 mg H2O at a 20 �C/h heating rate.


varying amounts of kieserite (MgSO4 � H2O),magnesium sulfate hydroxide hydrate (Mg1.33SO4-(OH)0.66 � 0.33H2O), magnesium sulfate hydrate(MgSO4 � 1.25H2O), hexahydrate (MgSO4 � 6H2O),and epsomite (MgSO4 � 7H2O).

Trace amounts of reduced sulfur forms appear inthe XPS sulfur 2p spectrum at 325 �C correspondingto the onset of H2S generation (Fig. 8). The amount

of the reduced sulfur is sufficient in experiments ter-minated at 350 �C and higher to curve resolve theirspectra, showing that this reduced sulfur exists in asingle chemical environment at an energy positionconsistent with thiophenic sulfur (Fig. 7). The pro-portion of reduced organic sulfur and inorganic sul-fate remains fairly constant until 575 �C where thethiophenic sulfur decreases and the H2S yield

0

20

40

60

80

100

275 325 375 425 475 525 575Temperature (°C)

Su

lfu

r sp

ecie

s in

so

lids

(mo

le %

su

lfu

r)

0

20

40

60

80

100

Car

bo

n a

rom

atic

ity

(mo

le %

car

bo

n)

SO4(1)

SO4(2)

Thiophenic S

Aromatic carbon

Fig. 8. Relative mole percent of sulfur species and aromatic carbon in the residual solids as a function of temperature from paraffinmixture with MgSO4 at 20 �C/h heating rate.


substantially increases. This coincides with anincrease in the aromatic carbon to thiophenic sulfurratio. No elemental sulfur, thiosulfate, or sulfiteforms are evident in the residual solids from theseexperiments.

4. Discussion

4.1. Possible mechanism of MgSO4 reduction under

our experimental conditions

Our experimental observations show that theH2S yield from anhydrous MgSO4 reduction byoctane is the highest and dramatically decreaseswith the increase of H2O/MgSO4 mole ratio. Thissuggests that the anhydrous MgSO4 itself is reactivetowards hydrocarbons. Under our highly oxidizingconditions (H2O/MgSO4 mole ratio of 1:3), XRDanalysis of the residues confirmed that sulfate existsin two distinct chemical environments: MgSO4 andhydrated MgSO4 species. As a result, the reactionof TSR occurs at a much lower temperature(�300 �C) under highly oxidizing condition withH2O/MgSO4 mole ratio of 1:3 than that of H2O/MgSO4 mole ratio of 10:1 (Zhang et al., 2007)because of the excess of MgSO4 solid presence.

Our experimental observation also shows thatthe presence of Mg2+ ion in the aqueous solutioncan significantly enhance the rate of sulfate reduc-tion under the same H2O/MgSO4 mole ratio. Previ-

ous experimental studies by Bischoff and Seyfried(1978) revealed that seawater becomes an acid attemperatures >200 �C. Heating of seawater to300 �C at 500 bars resulted in large decreases inthe concentrations of Mg2+, SO2�

4 and pH (Janeckyand Seyfried, 1983). The precipitation of a magne-sium hydroxide sulfate mineral (MHSH) wasidentified as the cause of the change in seawatercharacteristics as temperature increased above200 �C (Janecky and Seyfried, 1983). The reactionthat caused these changes may be written as follows:

ð1þ nÞMg2þ þ SO2�4 þH2O ¼ ðnÞMgðOHÞ2

: MgSO4 : ð1� 2nÞH2Oþ ð2nÞHþ; ð1Þ

where n in reaction 1 can vary between 0 and 0.5(Janecky and Seyfried, 1983). Therefore, the pres-ence of Mg2+ ion at our experimental temperatureresults in the decrease of pH that, in turn, increasesthe concentration of HSO�4 ion, which is the domi-nant sulfate species in acidic aqueous solutions.Molecular modeling calculations have shown thatHSO�4 ions are much more reactive than SO2�

4 ionsin aqueous solution; the calculated activation en-ergy of the HSO�4 ion reacting with ethane is about55 kcal/mol, which is about 23 kcal/mol lower thanthat of SO2�

4 ion with ethane (Ma et al., unpublisheddata). Kiyosu (1980) observed that sulfuric acid andsodium bisulfate were reduced to H2S by dextrose attemperatures above 300 �C, with an initial pH of the


aqueous solution in the range of 0.9–1.35 (at�25 �C). However, the reduction of sodium sulfateto H2S did not occur when the initial pH of theaqueous solution was �7, which is consistent withour observation of the effect of Na2SO4 presenceto TSR reaction. Both experimental and theoreticalresults suggest that the reduction of the HSO�4 ionby HC is more energetically favorable than theSO2�

4 ion. Therefore, HSO�4 is thought to be thereactive sulfate species in aqueous MgSO4 solutionunder our high temperature condition.

4.2. Effect of TSR on gas compositions

4.2.1. Increase of gas sourness

Gas sourness H2S/[H2S+CO2 +P

(C1–C5)] isconsidered a direct indicator of the extent of TSR(Worden and Smalley, 1996; Cai et al., 2003). Ourexperimental observations show that gas sournessincreases with the extent of TSR in the presence ofexcess hydrocarbons. We observe that from 300 to400 �C H2S yield increases as TSR proceeds forexperiments involving the two oils and the paraffinicmixture. However, above approximately 400 �CH2S yield becomes highly variable. Experimentsconducted with the paraffin mixture show that from375 to 475 �C H2S yield decreases and then increasesagain to 575 �C. In contrast, experiments involvingthe two whole oils show complex patterns of H2Syield at temperatures above approximately 400 �Cwithout any consistent trends. We speculate thatthis is the result of a complex series of reactionsinvolving the formation and thermal destructionof organic sulfur species (e.g., thiophenes, benzothi-ophenes and thiols) or reduced sulfur (elementalsulfur), and is highly dependent on the type ofhydrocarbon presence.

This speculation can be partially substantiated byexamining the XPS data. Fig. 8 shows that thiophe-nic compounds are produced during TSR experi-ments involving the paraffin mixture, and thatthiophene formation increases dramatically whenthe temperature is above 375 �C, correspondingwith the observed decrease in H2S. Although XPSanalysis does not indicate the presence of elementalsulfur in the solid residues of our experiments, a pre-vious study of sulfur speciation in residues of TSRexperiments analyzed by GC-sulfur chemolumines-cence detection (GC-SCD) showed that elementalsulfur is produced from reactions involving paraffinand MgSO4 at 400 �C (Ellis et al., unpublisheddata). Experiments involving the complex mixtures

of organic compounds found in whole crude oilsmight be expected to exhibit complex patterns oforganic sulfur species formation and H2S yields. Itis important to note that reactions that occur atthese high temperatures (>400 �C) and highly oxi-dizing conditions are not likely to be very represen-tative of processes that take place in naturalsettings.

4.2.2. Increases in gas dryness

In our experiments, the relative contribution ofmethane to the total amount of hydrocarbon gasesgenerated by TSR increases as the extent of TSRincreases (Fig. 5). When compared to the hydrocar-bon gases generated from thermal cracking alone,the TSR associated hydrocarbon gases are signifi-cantly drier at the same thermal maturity. Seewald(2001) reported that methane rich gas is producedin the stepwise oxidation of hydrocarbons. Ourexperimental observations show that, for the twooils studied, gas dryness (C1/

PC1–5) reaches 1.0

(100% methane remains because heavier gaseoushydrocarbons were consumed by oxidation) atapproximately 425 �C when TSR is present, yetwithout TSR gas dryness does not exceed 0.67 atnearly 500 �C (Table 4). Admittedly, the major gasproducts from reactions involving TSR are H2Sand CO2 rather than methane, suggesting that ifTSR were a significant mechanism for the genera-tion of dry gas then one would expect to encounterhigh concentrations of H2S and CO2 associated withthis gas. It should also be noted that our experimen-tal conditions are extremely oxidizing because of theaddition of MgSO4 in excess and that the actualeffect of TSR on gas dryness under reservoir condi-tions may be much less pronounced.

4.3. Effect of TSR on oil stability

Our results show that the thermal stability of allhydrocarbons, even methane, can be significantlylowered under the strong oxidation conditions usedin our TSR experiments. TSR does not merelyaccelerate thermal cracking, but follows differentchemical pathways that involve hydrocarbon oxida-tion. The effect of these processes are illustrated inFig. 6, which compares the distributions of n-alkanes in the high-sulfur oil with the remainingproducts at 400 �C in the absence and presence ofMgSO4. Without TSR, the n-alkanes with carbonchain lengths <C15 increase in concentration whilethe >C15 saturated hydrocarbons are slightly


reduced in concentration relative to the unheatedhigh-sulfur oil, indicating an increase in condensaterange compounds resulting from the cracking oflonger carbon chain components. In other words,<C15 saturated hydrocarbons are more thermallystable than their longer chain length homologs orprecursors and the rate of their generation exceedsthat of their destruction. In contrast, these com-pounds are dramatically depleted in the oil thathas experienced TSR as compared to the unheatedoil and the oil cracked without MgSO4 present. Thisdepletion is due to the oxidation of all saturatedhydrocarbon species to CO2 and the generation ofhydrocarbon gases (C1–C5) that are associated withsulfate reduction.

Since the abundance of the n-alkanes decreasewhile their carbon number distribution is relativelyunaffected, the extent of TSR experienced by anoil can be measured by the fractional conversionof an individual or grouping of n-alkanes. This isdefined as

X i ¼ 1�Mi=M0

where Xi is the conversion index for component i inthe paraffinic mixture by TSR or thermal cracking,M0 and Mi are the amount of component i in theparaffinic mixture before and after pyrolysis at acertain given temperature, respectively. Xi becomeslarger as the extent of reaction proceeds. A plot ofthe conversion index of n-pentacosane (n-C25) ver-sus temperature from the experiments involvingthe paraffin mix clearly shows the difference in net

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

250 275 300 325 350 3Temper

Co

nve

rsio

n in

dex

Xi

Thermochemical sulfate reduction

Fig. 9. Consumption of n-pentacosane during TSR reactions involving awith oil thermochemical alteration (TCA) alone.

effects of thermal cracking and TSR (Fig. 9). Penta-cosane (n-C25) is almost totally consumed by sulfatereduction before 400 �C, whereas almost 90% n-C25

remains when undergoing only thermal cracking.We can assume that the consumption of C25 whenthe temperature is below 400 �C could mainly resultfrom thermochemical sulfate reduction and have aminor contribution from hydrocarbon thermalcracking under our strong oxidation experimentalconditions.

The conversion index might apply to understand-ing the extent of oil cracking with and withoutinvolving TSR reaction. As shown in Fig. 4B,C21–C35 n-alkanes in the paraffinic mixture areequally oxidized by TSR with no obvious differencein reactivity of individual homologs. In order tocompare the conversion index calculated for TSRexperiments involving the paraffin mixture withthose conducted with the whole oils, the saturatesin the two oils were grouped into two families, i.e.>C21 (C21–C40) and <C21 (C9–C20). As shown inFig. 10A, the conversion index of >C21 saturatesincreases as TSR and thermal cracking proceeds,indicating the destruction of <C21 saturates in bothprocesses. In contrast, the conversion index of <C21

saturates decreases with increasing extent of thermalcracking, indicating the generation of <C21 satu-rates from the cracking of longer carbon chainhydrocarbons. While during TSR, the destructionof <C21 saturates remains dominant and leadingto a conversion index that increases with the extentof TSR.

75 400 425 450 475 500ature (°C)

Thermal chemical alteration

paraffinic mixture occurs at a much lower temperature compared

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500

Temperature (°C)

Co

nve

rsio

n In

dex

>C21, w/TSR High-sulfur

>C21, w/TSR Low-sulfur

>C21, w/o TSR High-sulfur

>C21, w/o TSR Low-sulfur

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1.0

250 300 350 400 450 500

Temperature (°C)

Co

nve

rsio

n In

dex

<C21, w/TSR High-sulfur

<C21,w/TSR Low-sulfur

<C21, w/o TSR high-sulfur

<C21, w/o TSR Low-sulfur

A

B

Fig. 10. Conversion index of >C21 and <C21 groups from high-sulfur and low-sulfur oils during TSR reactions and oil thermal chemicalalteration.


In natural systems, saturated hydrocarbons arepreferentially destroyed relative to aromatics bythermochemical sulfate reduction (Rooney, 1995;Manzano et al., 1997). Similar effects are observedin the laboratory simulation experiments. NormalC8–C35 alkanes are consumed as the oil is alteredby the TSR reaction and the depletion increases withthe extent of TSR (Fig. 5). The C1–5 gas yield fromhigh-sulfur oil thermal cracking without TSR at355 �C is 2 ml/g oil, which is almost equivalent tothe hydrocarbon gas yield from the paraffinic mix-ture at 400 �C (Table 4). This suggests that thedestruction of C8–C35 saturates from oils reactingwith MgSO4 at temperatures above 370 �C couldresult from both hydrocarbon oxidation by TSRand hydrocarbon thermal cracking. As a result, itis hard to accurately assess the extent of TSR onthe basis of the quantification of the remaining C8–C35 saturates from the heated oils. While the conver-sion index of >C21 saturates group in the oils mainlyrepresents the destruction of n-alkanes by TSR, itmight be used as an estimate of the minimum extentof TSR for the oils at a given temperature.

Oils reacted in the presence of MgSO4 show noobvious differences in n-alkane distributions com-pared to the original unheated oils with the increaseof TSR extent (Fig. 11). Hence, we conclude that n-alkane distributions are not diagnostic of the extentof TSR. However, the Pr/n-C17 and Ph/n-C18 ratiosobviously decrease with the increase of the extent ofTSR (Fig. 12). These ratios are significantly lowerwhen TSR occurs compared to thermal crackingalone. These observations suggest that abnormallylow Pr/n-C17 and Ph/n-C18 ratios in oils at a giventhermal maturity may be caused by TSR. Thisexperimental observation agrees with field observa-tions of oils from the Western Canada SedimentaryBasin that have experienced TSR (Manzano et al.,1997).

4.4. Effect of TSR on aromatization and sulfur and

oxygen incorporation to solid residues (pyrobitumen)

XPS analysis indicates that a significant amountof aromatic carbon appears on the surface of the325 and 350 �C reaction residues, coincident with

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Fig. 11. Gas chromatography of remaining high-sulfur oil after reacting with MgSO4 (left side) and remaining high-sulfur oil after thermalalteration without MgSO4 (right side) at different temperatures at 20 �C/h heating rate. The numbers in the bracket below the temperaturelabels are the n-C25 conversion index which indicates the reaction extent of TSR and TCA.


observed cracking of normal paraffins, indicatingthat TSR facilitates the decomposition of paraffinichydrocarbons and the production of an aromaticcarbon residue on the mineral surfaces. As a result,the saturate/aromatic ratio in remaining oilsbecomes smaller as TSR proceeds. This is consistentwith the geochemical observations of oil composi-tional changes after experiencing TSR (Manzanoet al., 1997). The thiophenic sulfur in the residue isinterpreted as reduced sulfur that is incorporatedinto the aromatic organic char as a by product ofthis modeled TSR reaction system. Table 7 showsthat the ratio of aromatic carbon to thiophenic sul-fur stays remarkably constant (4.2–5.9) between 375and 550 �C, indicating that nearly all of the aro-matic carbon is associated with thiophenic sulfurstructures. Organic oxygen species are present at rel-atively constant levels throughout the experimentaltemperature range, while CO2 yield as an indication

of hydrocarbon oxidation constantly increases withtemperature.

Obviously, TSR promotes aromatization, andthe incorporation of sulfur and oxygen into hydro-carbons. The residues from the experiments sug-gest that the chemical nature of reservoirpyrobitumens may be diagnostic for the occurrenceof TSR. This may be particularly insightful incases where the H2S from TSR was precipitatedas metal sulfides (Machel, 2001) or migrated fromthe trap that underwent TSR. The organic solidsformed in the experiments are consistent with nat-urally occurring TSR solid bitumens. Kelemenet al. (2007) report that sulfur rich organic solidsfrom TSR associated reservoir rocks from theMadison and Nisku formations are highly enrichedin aromatic carbon structures, have little or nonitrogen, and contain organic sulfur almost exclu-sively in aromatic form.

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Fig. 12. Pr/n-C17 and Ph/n-C18 ratios in oils that experience TSR obviously decrease compared with those by thermal chemical alterationalone.


5. Conclusions

Quantification of gas yields (C1–C5, H2S, andCO2), the residual saturated hydrocarbon fractionsof pyrolysates, and generated pyrobitumen from aseries of gold tube hydrous pyrolysis experimentswere carried out and provided a unique opportunityto investigate gas generation and the oxidation ofhydrocarbons by sulfate reduction before significantthermal cracking of hydrocarbons occurred. Theprincipal findings of this study are as follows:

(1) H2O/MgSO4 mole ratio determines the inten-sity of TSR, the smaller the ratio, the strongerthe oxidizing conditions. At the low H2O/MgSO4 mole ratio conditions, anhydrousMgSO4 solid is the main reactive oxidanttowards hydrocarbons.

(2) HSO�4 is thought to be the reactive sulfate spe-cies in aqueous MgSO4 solution under ourhigh temperature conditions due to the precip-itation of a magnesium hydroxide sulfate min-eral (MHSH), which lowers pH of the aqueoussolution.

(3) Thermal stability of oil is affected by thermo-chemical sulfate reduction (TSR) in geologicalsettings. The temperature required for TSR is

less than that of hydrocarbon thermal crack-ing. As a result, TSR can dramatically lowerthe thermal stability of oil.

(4) TSR preferentially generates methane and gasdryness is proportional to the extent of theTSR reaction.

(5) The distribution C8–C35 n-alkanes is not sig-nificantly altered in oil as it undergoes TSRin the laboratory experiments. Pr/n-C17 andPh/n-C18 ratios decrease in oils exposed tosimulated TSR compared to oils that undergothermal cracking.

(6) TSR promotes aromatization, and the incor-poration of sulfur and oxygen into hydrocar-bons. As a result, a decrease in the ratio ofsaturated to aromatic hydrocarbons in resid-ual oils and an increase of sulfur and oxygencontent in pyrobitumen may be observed withthe progression of TSR.

(7) Methane can be oxidized to CO2 by TSR;however, the reaction rate is relatively slowerthan heavier gaseous hydrocarbons (C2–C5).

Acknowledgements

This research was supported by the JointIndustrial Program on Thermochemical Sulfate


Reduction at the Power, Environmental and EnergyResearch Center at Caltech. Industrial sponsors in-clude BP, ChevronTexaco, ENI, ExxonMobil, Sau-diAramco, Shell Oil and Total. The technicalreview by M. Lewan, P. Lillis and D. Nichols fromUSGS Energy Resource Surveys Team were helpfuland constructive to improve the manuscript. Thiswork benefited from collaborations and discussionswith many colleagues, and we would especially liketo thank H. Wycherly, M. Haught and X. Zhangfor constructive discussion and P. Kwietak for per-forming the XPS analyses. The authors also extendtheir thanks to the reviewers Dr. C. Ostertag-Hen-ning and Dr. R. Hill.

Associate Editor—Rolando di Primio

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