Organic Geochemistry 82 (2015) 54–68

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Organic Geochemistry

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Enrichment of nitrogen and 13C of methane in natural gasesfrom the Khuff Formation, Saudi Arabia, caused by thermochemicalsulfate reduction

http://dx.doi.org/10.1016/j.orggeochem.2015.02.0080146-6380/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: P.O. Box 12642, Dhahran 31311, Saudi Arabia. Tel.:+966 13 872 3862.

E-mail addresses: peter.jenden@aramco.com (P.D. Jenden), R.Worden@liverpool.ac.uk (R.H. Worden).

1 Tel.: +44 151 794 5184.

Peter D. Jenden a,⇑, Paul A. Titley b, Richard H. Worden c,1

a Saudi Aramco EXPEC Advanced Research Center, Room GA-221, Building 2291, Dhahran 31311, Saudi Arabiab Saudi Aramco Eastern Area Exploration Department, Room R-E-3580, Engineering Bldg (728A), Dhahran 31311, Saudi Arabiac Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, United Kingdom

a r t i c l e i n f o

Article history:Received 19 August 2014Received in revised form 2 February 2015Accepted 23 February 2015Available online 5 March 2015

Keywords:Natural gasNitrogenHydrogen sulfideSulfateMethane oxidationStable isotope

a b s t r a c t

Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gaswith highly variable nitrogen concentrations. Rough correlations between N2/CH4, CO2/CH4 and H2S/CH4

suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR).In Khuff gases judged to be unaltered by TSR, methane d13C generally falls between �40‰ and �35‰

VPDB and carbon dioxide d13C between �3‰ and 0‰ VPDB. As H2S/CH4 increases, methane d13Cincreases to as much as �3‰ and carbon dioxide d13C decreases to as little as �28‰. These changesare interpreted to reflect the oxidation of methane to carbon dioxide.

Khuff reservoir temperatures, which locally exceed 150 �C, appear high enough to drive the reductionof sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonlyrimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena.Assuming that nitrogen is inert, loss of methane by TSR should increase N2/CH4 of the residual gas andleave d15N unaltered. d15N of Paleozoic gases in Saudi Arabia varies from �7‰ to 1‰ vs. air and supportsthe TSR hypothesis. N2/CH4 in gases from stacked Khuff reservoirs varies by a factor of 19 yet the varia-tion in d15N (0.3–0.5‰) is trivial.

Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter,we have attempted to assess the extent of TSR using plots of methane d13C versus log(N2/CH4). Observedvariations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotopefractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90%of the original methane charge in the most extreme instance. The possibility that methane may becompletely destroyed by TSR has important implications for deep gas exploration and the origin of gasesrich in nitrogen as well as hydrogen sulfide.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Although most commercial natural gases contain only a fewpercent nitrogen, fields producing 20% or more are relativelycommon (Jenden et al., 1988; Krooss et al., 1995). High nitrogencontents decrease the commercial value of natural gas depositsand can increase production costs if treatment is required to meetcommercial standards (Kuo et al., 2012). Nitrogen in natural gasmay have a variety of sources including volcanic and geothermal

activity, burial alteration of organic rich sedimentary rocks,devolatilization of metasedimentary ‘‘basement’’ rocks and airdissolved in recharging surface water (Jenden et al., 1988; Kroosset al., 1995; Littke et al., 1995; Zhu et al., 2000; Ballentine andSherwood-Lollar, 2002; Mingram et al., 2005). Importantly, nitro-gen contamination can be introduced during sampling due to addi-tion of air or the use of nitrogen as a lift gas in exploration wells tostimulate flow from damaged or low permeability reservoirs.

1.1. Nitrogen from high maturity sedimentary and metasedimentaryrocks

In the absence of elevated heat flows or geothermal activity,nitrogen in commercial gases is most likely to be derived from

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 55

sedimentary or metasedimentary sources. Deep, high maturitysedimentary rocks are regarded as a major source of nitrogen inUpper Carboniferous to Triassic natural gases from the CentralEuropean Basin. Rotliegend reservoirs overlying Westphalian coalbeds with > 3% vitrinite reflectance produce gases with > 50% nitro-gen (Littke et al., 1995). Pyrolysis experiments (Krooss et al., 1995,2006; Jurisch et al., 2012) show that gases evolved at high tem-peratures from coal and shale may be enriched in nitrogen relativeto methane. High N2/CH4 may therefore indicate fractional entrap-ment of late thermogenic gas (Krooss et al., 1995; Battani et al.,2000). Liu et al. (2008) have suggested that N2/CH4 be used as amaturity indicator for gases generated by coal sourced naturalgases. Natural gases produced from Rotliegend reservoirs in north-ern Germany are characterized by d15N between �3‰ and 19‰,methane d13C up to �20‰ and crustal (radiogenic) helium (Littkeet al., 1995; Gerling et al., 1997). Likely sources of Rotliegend nitro-gen include organic nitrogen in Westphalian coal beds and ammo-nium in underlying Namurian shales (Krooss et al., 1995, 2006;Mingram et al., 2005).

Liberation of nitrogen during the metamorphism of pelitic rocksis indicated by a decrease in bulk nitrogen concentrations from1000 ppm or more in shales to less than 50 ppm in high gradegneisses (Mingram et al., 2005; Jia, 2006). However, establishinga link between nitrogen in commercial gas fields and nitrogen inmetamorphic rocks has proved to be difficult. One example maybe the giant Hugoton–Panhandle complex of the central UnitedStates, which produces gas with 5–75% nitrogen from shallowPermian reservoirs (< 1000 m). Ballentine and Sherwood-Lollar(2002) used noble gas measurements to argue that Hugoton–Panhandle nitrogen originated by mixing of low grade metamor-phic volatiles, characterized by d15N = �3‰ and traces of mantlehelium, with sedimentary nitrogen characterized by d15N = 13‰

and no resolvable helium component. Metamorphic nitrogenmay also be present in the Great Valley of California where drygas fields with up to 87% nitrogen and d15N between 1‰ and 4‰

are trapped in Cretaceous reservoirs at < 3000 m (Jenden et al.,1988; Bebout and Fogel, 1992). Nitrogen concentration increaseswith proximity to basement, methane d13C ranges up to �15‰

and the gas fields contain mantle derived helium, suggesting theinvolvement of metasedimentary rocks subducted beneath thewestern margin of North America.

1.2. Nitrogen enriched by thermochemical sulfate reduction?

Thermochemical sulfate reduction (TSR) is well known in bothoil and gas reservoirs (Orr, 1977; Machel et al., 1995). Althoughthe reaction preferentially attacks oil and gas condensate, at tem-peratures exceeding 140 �C even methane may be oxidized(Worden et al., 1995; Worden and Smalley, 1996, 2004; Cai et al.,2004, 2013). Assuming that methane is the primary hydrocarboninvolved, the net reaction would be:

CaSO4ðanhydriteÞ þ CH4 ! CaCO3ðcalciteÞ þH2SþH2O:

As hydrocarbons are depleted in 12C relative to marine carbon-ates, secondary calcite cements formed by TSR can have stronglynegative d13C (Krouse et al., 1988; Heydari and Moore, 1989;Worden and Smalley, 1996). Other products include elemental sul-fur, solid bitumen and carbon dioxide. Carbon dioxide may beformed by the reaction of sulfate with trace amounts of C2+ gases,condensate liquids or solid bitumen. Carbon dioxide can also beformed by the decomposition of carbonate minerals, for exampleby the reaction of hydrogen sulfide with siderite or ferroan calcite(Liu et al., 2013) or with dissolved base metals, releasing protonsand leaching calcite or dolomite.

Machel (1998, 2001) and others have disputed whethermethane in oil and gas fields is significantly altered by TSR.

Although reaction of sulfate with methane is thermodynamicallyfavorable (Worden and Smalley, 1996), rupture of S–O bonds insulfate has a high activation energy and methane is the most resis-tant of all hydrocarbons (Xia et al., 2014). Most TSR experimentshave been carried out with higher molecular weight compoundsand at temperatures of 300 �C and above. Yuan et al. (2013)recently demonstrated TSR of methane at temperatures as low as250 �C but speculated that reaction rates at geological time scaleswould be prohibitively slow at temperatures below 200 �C.

Because the higher hydrocarbons are more reactive, we suggestthat extensive reaction of methane is unlikely in TSR altered fieldsproducing oil, condensate liquids or abundant C2+ gases, such asthose studied by Krouse et al. (1988), Cai et al. (2001) andMankiewicz et al. (2009). Evidence that sulfate and methane mayreact at temperatures below 200 �C is provided by methane d13Cmeasurements of TSR altered, dry gas fields in Abu Dhabi and theeastern Sichuan Basin, China. Field data and experiments haveestablished that thermochemical oxidation of the light hydrocar-bon gases is accompanied by 13C enrichment of the unreactedresidue (Krouse et al., 1988; Kiyosu et al., 1990; Pan et al., 2006;Hao et al., 2008; Mankiewicz et al., 2009). Using a petrographicparameter to estimate the extent of reaction, Worden andSmalley (1996) concluded that TSR has increased methane d13Cof Abu Dhabi gases by as much as 10‰. Cai et al. (2013) used areaction proxy based on H2S abundance and showed that TSRmay have increased methane d13C in Sichuan Basin gases up to6‰. Under appropriate conditions, we suspect that methane canbe almost completely destroyed. In the Mississippi Salt Basin, forexample, Heydari (1997) reported on a carbonate reservoir at over200 �C that tested gas with 78% hydrogen sulfide, 20% carbon diox-ide and 2% methane. Late stage calcite cement with d13C as low as�16‰ confirms that TSR occurred in situ (Heydari and Moore,1989).

As TSR of a dry gas yields one mole of hydrogen sulfide for everymole of methane consumed, the mole fraction of nitrogen andother inert components in an altered gas cannot increase unlesshydrogen sulfide is lost from the gas phase. This could occur bydissolution of hydrogen sulfide in an active water leg or by reactionwith dissolved sulfate to form elemental sulfur. If Fe or base metalssuch as Pb and Zn are available, hydrogen sulfide could be precipi-tated as pyrite, galena or sphalerite. Regardless of the fate of thehydrogen sulfide produced, extensive TSR must increase the ratioof nitrogen to methane. d15N of the remaining nitrogen shouldretain the signature of the natural gas that charged the Khuff priorto TSR alteration.

Enrichment of nitrogen in dry natural gases subjected toextensive TSR has not been explicitly reported in the geochemicalliterature. Khuff Formation gases from Abu Dhabi reported byWorden et al. (1995) show a weak positive correlation betweenN2/CH4 and H2S/CH4. In contrast, enrichment of nitrogen by TSRis not apparent in data for dry, sour gases in Carboniferous-Triassic reservoirs from the eastern Sichuan Basin (Cai et al., 2013).

Impetus for this study was provided by a deep delineation welltargeting the Khuff Formation on the east coast of Saudi Arabia.This well tested sour dry gas at high flow rates from two reservoirsseparated by a vertical distance of < 100 m. The lower (Khuff C) gascontained 19% nitrogen whereas analyses of samples collected ondifferent days confirmed that the upper (Khuff B) gas contained74% nitrogen. To assess the origin of these gases, we measuredcarbon isotope ratios of methane, ethane, propane and carbondioxide and nitrogen isotope ratios of nitrogen gas.

The results we present here pose several questions relevant tothe assessment of gas exploration risk and deep gas resources. Inparticular, what process or collection of processes can be used toexplain extreme chemical and stable isotope variations in gasreservoirs at similar depths in the same field? How reliable are

56 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

extent of reaction parameters based on hydrogen sulfideabundance? Can nitrogen and nitrogen isotopes help us under-stand the fate of hydrogen sulfide formed by TSR? What is therange of carbon isotope fractionation factors associated with TSRof methane? Finally, does the formation of base metal sulfidesinfluence the rate or extent of TSR?

Fig. 1. Paleozoic stratigraphic column showing the Qusaiba ‘‘hot shale’’ source rock(flag) and principal gas reservoirs of the Saudi Arabian Gulf coast (circles with teeth;simplified from Cantrell et al., 2014).

2. Paleozoic natural gas in eastern Saudi Arabia

The Paleozoic stratigraphy and petroleum geology of SaudiArabia have been reviewed by Sharland et al. (2001), Pollastro(2003) and Cantrell et al. (2014). The primary source rock lies atthe base of the Qusaiba Member of the Early Silurian QalibahFormation (Fig. 1). High gamma black marine shales of theQusaiba Member range up to 50 m in thickness and contain TypeII to Type III (oxidized marine) kerogen (Jones and Stump, 1999).Total organic carbon in the source rock averages 3–5% by weightbut can reach up to 20%. Maturity modeling suggests that oilgeneration began as early as the Late Permian, wet gas was gener-ated from the Late Jurassic to Cretaceous and dry gas during theTertiary (Pollastro, 2003; Cantrell et al., 2014).

The principal Paleozoic reservoirs along the east coast of SaudiArabia and in the offshore occur in Late Permian to Early Triassicshallow marine carbonates of the Khuff Formation (Fig. 1). Gas isalso found in transgressive sandstones of the Basal Khuff Clastics,in glacio-fluvial and aeolian sandstones of the Late Carboniferousto Early Permian Unayzah Formation and in fluvial to marginalmarine sandstones of the Devonian Jauf and Jubah formations. Inthe western part of the study area, more than 1500 m ofDevonian and Silurian rocks have been eroded over a broadnorth-trending mid-Carboniferous high known as the Al-BatinArch (Faqira et al., 2009). As Unayzah Formation rocks are absentin the same region, the arch may have remained topographicallyhigh well into the Permian.

Large volumes of Paleozoic gas are trapped over north-trendingbasement highs that developed during the mid-Carboniferous.Mid-Carboniferous uplift may have been related to collision ofGondwana and Eurasia (the Hercynian Orogeny of Europe) or toaccelerated subduction beneath the volcanic arc that had devel-oped northeast of the Arabian plate (Sharland et al., 2001;Cantrell et al., 2014). Trap development for the Khuff Formationoccurred primarily during the Late Cretaceous when rapid openingof the Atlantic Ocean induced plate-wide compressional foldingand ophiolite obduction along the NeoTethys margin (Sharlandet al., 2001). Further structuring took place in the Early Miocenewhen the NeoTethys Sea closed along the Zagros suture. At siteswhere the Infracambrian Hormuz salt was thick enough to flow,traps formed over salt pillows. The Paleozoic section is generallywell sealed by shales of the Early Triassic Sudair Formation.Bedded anhydrites represent important intraformational sealswithin the Khuff and isolate the Khuff from the underlying section.Sandstone reservoirs of the Unayzah, Jauf and Jubah formations aresealed by interbedded shales and siltstones.

3. Samples and analytical methods

The present study addresses data from just over 50 natural gassamples collected from exploration or delineation wells drilledbetween 1997 and 2013 (Fig. 2). Reservoir depths and tempera-tures range from 3000 m to > 5200 m and from 100 �C to> 150 �C. Most samples are from Khuff reservoirs but pre-Khuff(Jauf, Jubah and Unayzah; Fig. 1) reservoirs are also represented.Samples are all non-associated gas (i.e., no oil phase present inthe reservoir). Condensate/gas ratios do not exceed 10 bbl/MMscf(5.6 � 10�5 m3/m3).

Natural gases were collected primarily from test separators andsampled directly into evacuated steel or titanium vessels. Samplingpressures and temperatures ranged from 0.02–8.3 MPa (gauge)and 3–56 �C.

3.1. Gas compositions

Chemical compositions were normally analyzed within days ofcollection. C1–C10 hydrocarbons, carbon dioxide and hydrogen sul-fide were measured on an HP 5890 Series II gas chromatographequipped with a 0.5 ml sample loop maintained at 150 �C, a9.1 m � 3.18 mm (30 ft � 1/8 in) stainless steel column packedwith 30% DC200 on 80/100 mesh Chromosorb PAW and thermalconductivity and flame ionization detectors placed in series.Helium carrier gas was maintained at a flow rate of 30 ml/minand oven temperature programmed from 40–180 �C. Oxygen,nitrogen and methane were analyzed on a second HP 5890 SeriesII gas chromatograph equipped with a 0.5 ml sample loop main-tained at 150 �C, a 0.53 m � 3.18 mm (21 in � 1/8 in) stainlesssteel column packed with 45/60 mesh 13X molecular sieve and athermal conductivity detector. Helium carrier gas was supplied at20 ml/min and oven temperature was held isothermal at 40 �C.

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 57

Nitrogen was corrected for the presence of traces of oxygenusing the area ratio of nitrogen to oxygen peaks measured on air.One or more standards were used to convert peak areas to molarquantities. Where samples were collected in titanium vessels,molar abundances of each species were normalized to 100%.Where samples were collected in steel vessels, hydrogen sulfidewas set equal to the value measured in the field (using Tutwilertitration, gas adsorption tubes, or at ppm concentrations, gas moni-tors) and the remaining components normalized to 100 – %H2S.

Fig. 2. Location of gas wells sampled in this study (solid circles). Most wells were tesreservoirs were also collected. Mesozoic oil fields are shown for reference.

3.2. Carbon isotope ratios

Individual gas components were separated on an Agilent 6890gas chromatograph equipped with a 100 ll sample loop (operatedat room temperature and pressures from 0 to 200 kPa absolute), asplit injector maintained at 250 �C, a 30 m � 0.32 mm Agilent GSQPLOT column, temperature programming from 35–240 �C and Hecarrier gas at a constant flow of 2.6 ml/min. The effluent waspassed to a Finnigan GCC III combustion interface (CuO–Ni–Pt

ted from Permian Khuff reservoirs but gases from older Unayzah, Jubah and Jauf

Table 1Chemical and stable isotopic compositions of selected gases.

Sample Field Well Reservoird N2 CO2 H2S C1 C2 C3 C4+ d13C (‰, VPDB) d15N (‰, air)

Mole% CH4 C2H6 C3H8 CO2 N2

1a A 1b KHFB 74.13 4.15 6.22 15.50 0.00 0.00 0.00 �3.0 �14.4 0.42 A 1b KHFB 73.81 4.15 6.48 15.56 0.00 0.00 0.00 �2.8 �14.3 0.53a A 1 KHFC 18.98 3.17 12.23 65.32 0.30 0.00 0.00 �33.9 �29.0 �17.9 0.34 A 2 KHFB 28.98 27.35 21.41 21.44 0.11 0.06 0.65 �26.1 �14.25 A 2 KHFC 25.14 20.88 41.98 12.00 0.00 0.00 0.00 �17.6 �2.16 B 1c KFAB 25.37 8.68 13.60 52.35 0.00 0.00 0.00 �21.6 �22.2 �2.27 B 1c KFAB 24.93 8.83 14.00 52.24 0.00 0.00 0.00 �22.0 �21.8 �2.18 B 1c KFAB 24.89 8.94 14.00 52.17 0.00 0.00 0.00 �21.9 �22.29 C 1 KHFB 33.79 5.74 2.80 57.58 0.09 0.00 0.00 �30.9 �29.0 �11.2 0.310 D 1 PKFF 26.98 6.90 0.00 65.76 0.34 0.02 0.00 �31.8 �36.2 �19.111 D 1 KHFB 42.86 2.05 10.69 42.85 0.94 0.25 0.36 �33.5 �30.0 �26.7 �12.012 E 1 KHFB 10.48 7.02 28.22 49.99 2.51 0.61 1.17 �39.2 �32.9 �30.7 �21.0 �0.9

a Splits of samples 1 and 3 were charged into isotubes and analyzed by Weatherford Laboratories, Dammam, on 14 September 2013. Sample 1 was reported to havemethane d13C = �2.8 ‰ (VPDB) and methane dD = �97‰ (VSMOW). Sample 3 was reported to have methane d13C = �33.9 ‰ (VPDB), ethane d13C = �29.0‰ (VPDB) andmethane dD = �126‰ (VSMOW).

b Collected from the same test on different days.c Collected from different tests of the same interval over the space of two weeks.d KHFB and KHFC refer to the Khuff B and C reservoirs, KFAB to the combined Khuff A and Khuff B reservoirs and PKFF to a pre-Khuff reservoir.

Fig. 3. Methane d13C plotted against the ratio of C2+ hydrocarbons to methane.Letter labels distinguish samples collected from different fields. Samples collectedfrom different wells in the same field are distinguished numerically. Regions formicrobial and thermogenic gases are adapted from Clayton (1991) assuming a d13Cvalue of �28‰ for the source rock kerogen. The stippled area illustrates thedistribution of typical Paleozoic gases from northwestern, central and eastern SaudiArabia. BKFC = Basal Khuff Clastics.

58 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

furnace at 1000 �C; Cu furnace at 650 �C) coupled to a FinniganMAT Delta Plus or Thermo Scientific Delta V Advantage isotoperatio mass spectrometer.

13C/12C measurements were scaled by analyzing pulses ofreference carbon dioxide calibrated against the NBS-19 andLSVEC carbonate standards and are reported in delta notation rela-tive to the VPDB international reporting standard. To minimizeaccuracy problems, data were compiled only for componentswithin 3 V of the reference gas peak height measured on the m/z44 cup. Any slight dependence of d13C on signal intensity wasremoved by applying a linearity correction determined for eachcompound using data for replicate analyses compiled over severalmonths. Short term reproducibilities of ± 0.2‰ for the hydrocarbongases and ± 0.2–0.4‰ for carbon dioxide have been estimated bypooling the standard deviations of multiple analyses. Analyses ofa natural gas working standard in use since 1998 suggest thatthe long term reproducibility for the hydrocarbon gases is ± 0.5‰

(1 standard deviation).

3.3. Nitrogen isotope ratios

Nitrogen isotope ratios of nitrogen gas were obtained using theequipment described above for carbon isotopes. To increaseseparation between nitrogen and methane, liquid nitrogen wasused to drop the initial oven temperature to �30 �C. The combus-tion and reduction ovens were maintained at 1000 �C and 650 �Cand a liquid nitrogen trap was placed between the GCC III andthe mass spectrometer to remove traces of carbon dioxide thatwould give an interfering peak at mass 28.

15N/14N measurements are reported in delta notation relative toatmospheric nitrogen and were calibrated using a standard ofDhahran air diluted to 7% in ultrapure He. Data were compiled onlyfor runs with nitrogen peak heights within 3 V of the reference gaspeak height as measured on the m/z 28 cup. Corrections wereapplied to remove a slight dependence of d15N on peak heightand for a small air blank. Pooled standard deviations of replicateruns indicate a short-term reproducibility of ± 0.12‰ (> 200degrees of freedom). Analyses of check standards in use for the lastfive years suggest a long term reproducibility of ± 0.25‰ (1standard deviation).

3.4. Petrology

Core samples for petrographic examination were impregnatedwith blue resin and then made into polished thin section.

Mineralogy and primary and diagenetic fabrics were determinedfor each sample using a Meiji 9000 microscope fitted with anInfinity 1.5 camera. SEM examination was undertaken on carboncoated polished sections using a Philips XL 30 SEM withtungsten filament at an accelerating voltage of 20 kV, and 8 nAbeam current for backscattered electron microscopy (BSEM).Energy dispersive secondary X-ray analysis (EDAX) providedquantitative compositional analysis of carbonate, sulfate andsulfide minerals.

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 59

4. Results

Gas fields in this study are distinguished by capital letters and,where necessary, samples collected from different wells in thesame field are indicated with a numerical suffix. Chemical andstable isotopic compositions for selected gases from fields A to Eare listed in Table 1. Data for gases from fields F to M are onlyillustrated in figures.

4.1. Hydrocarbon gases

Fig. 3, a plot of methane d13C against the molar ratio of C2+hydrocarbons to methane, shows that the geochemistry of thehydrocarbons is consistent with a non-associated thermogenic ori-gin. Methane d13C ranges from �40‰ to �29‰, with a few highermeasurements from Khuff reservoirs at fields A and B. C2+/CH4 is6 0.1. As shown by the stippled area, Paleozoic gases elsewherein Saudi Arabia can have methane d13C as low as �48‰ and C2+/CH4 as high as 0.4. The relatively high methane d13C and low C2+hydrocarbon abundance observed for coastal and offshorePaleozoic gases is attributed to their advanced maturities.

Because of their low abundances, comparatively few d13Cmeasurements were made on the C2+ hydrocarbons. Ethane andpropane d13C range from �38.9‰ to �23.4‰ and �35.2‰ to�23.5‰, respectively, and increase with increasing C1/C2 and C1/C3 (Fig. 4). At similar C1/C2 and C1/C3 abundance ratios, gases fromBasal Khuff Clastics and pre-Khuff reservoirs have more negatived13C values than gases from Khuff reservoirs. This is unexpectedbecause Khuff gases are widely assumed to have migrated from

Fig. 4. Plots showing relationships between the carbon isotopic compositions andrelative abundances of ethane and propane. BKFC = Basal Khuff Clastics.

the pre-Khuff. The difference in Fig. 4 suggests that the composi-tions of the Khuff and pre-Khuff gases may have been alteredfollowing charging of the Khuff. TSR has affected the chemistry ofmany of the Khuff gases, as discussed below, and pre-Khuff reser-voirs could have received a late charge of high maturity Qusaibagas.

4.2. Non-hydrocarbon gases

Non-hydrocarbon components are abundant. Carbon dioxideconcentrations range from below detection to 27% and may beelevated in both Khuff Formation and pre-Khuff reservoirs.Hydrogen sulfide concentrations do not exceed 0.7% in BasalKhuff Clastics and older reservoirs but are typically much higherin the Khuff. Hydrogen sulfide concentrations as high as 42% havebeen measured in the Khuff C at Field A-Well 2 (Table 1, sample 5).Elevated nitrogen concentrations have been measured not only insour Khuff gases (e.g., 74% at Field A; Table 1, samples 1 and 2)but also in sweet pre-Khuff gases such as the Jauf at Field D(27%; Table 1, sample 10).

The concentrations of the non-hydrocarbon gases are positivelycorrelated. In Fig. 5, non-hydrocarbon gas concentrations havebeen normalized to methane and plotted on a logarithmic scaleso as to emphasize the relationships over a wide range of concen-trations. For reference, gases with no detectable H2S have been

Fig. 5. Positive correlations among the non-hydrocarbon components of Khuffgases suggest that their abundance is controlled by thermochemical sulfatereduction. Samples with no detectable H2S are plotted along the vertical axis(H2S/CH4 = 0.001). BKFC = Basal Khuff Clastics.

60 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

plotted along the vertical axis (H2S/CH4 = 0.001). On average, Khuffreservoirs (solid points) contain higher abundances of non-hydrocarbon gases than Basal Khuff Clastics and pre-Khuffreservoirs (half-filled and open points).

As shown in Fig. 6, increasing log(H2S/CH4) in Khuff reservoirs isaccompanied by a decrease in carbon dioxide d13C from 0‰ to�28‰. Khuff carbonate rocks typically have d13C values between�3‰ and +5‰ (unpublished Saudi Aramco data) so d13C as lowas �28‰ provides strong evidence for addition of carbon dioxidederived from TSR of gas and liquid hydrocarbons. Whole oil d13Cvalues of Paleozoic oils and gas condensates average �29.2‰ andgenerally fall between �31‰ and �27‰ (Abu Ali et al., 1991;Carrigan et al., 1998; unpublished Saudi Aramco data). Khuff Cgas at Field A-Well 2 (solid circle labeled ‘‘A-2 KHFC’’) andUnayzah gas from Field J (open circle labeled ‘‘J’’) are conspicuousoutliers discussed in more detail below.

In addition to carbon dioxide d13C, log(H2S/CH4) shows aninteresting relationship with methane d13C. Fig. 7 indicates that,for samples with H2S/CH4 > 0.1, methane d13C increases withincreasing H2S/CH4. Khuff B gas at Field A-Well 1 (solid circlelabeled ‘‘A-1 KHFB’’), has by far the most 13C enriched methanebut comparatively modest H2S/CH4. As the reservoir containsabundant secondary sphalerite and galena, H2S/CH4 may once havebeen significantly higher.

4.3. Nitrogen isotope ratios

Fig. 8 shows that d15N of gases in this study varies from �6‰ to1‰. Data fall within the range measured for Paleozoic gasesthroughout northwestern, central and eastern Saudi Arabiaalthough the latter show a maximum between �1.5‰ and�1.0‰ that is absent in the coastal and offshore dataset. Gaseswith high nitrogen concentration in Saudi Arabia tend to beenriched in 15N but correlations between nitrogen isotope ratiosand nitrogen concentration or N2/CH4 are poor.

Fig. 6. d13C of carbon dioxide becomes more negative as H2S/CH4 increases, aswould be expected for addition of carbon dioxide from the oxidation of hydrocar-bons by sulfate. Samples with no detectable H2S are plotted along the vertical axis(H2S/CH4 = 0.001). In contrast to Paleozoic hydrocarbons, for which d13C 6 �27‰,Khuff carbonates typically have �3‰ 6 d13C 6 5‰. The anomalous A-2 Khuff Csample was taken very close to the gas–water contact and the test recovered largevolumes of formation water. BKFC = Basal Khuff Clastics.

4.4. Petrological evidence for TSR in the Khuff Formation of SaudiArabia

In the area of study, the Khuff Formation is strongly dolomitizedand characterized by an abundance of anhydrite in a variety offorms including discrete beds, fracture-fills and coarsely and finelycrystalline, mm to cm sized nodules (Fig. 9A). The nodules,especially, are partially replaced by calcite which grows as a rimon the outer edges of anhydrite nodules (Fig. 9B). The replaciveTSR calcite grows as a reaction front into the body of the nodule,leaving partially corroded fragments of anhydrite within the calcite(Fig. 9C). The replacive TSR calcite can, in some circumstances,serve to isolate the remaining anhydrite from the reactive petro-leum fluid (Bildstein et al., 2001) and thus limit the extent of thehydrogen sulfide generating reaction. In some wells, sphalerite

Fig. 7. d13C of Khuff methane increases with increasing H2S/CH4. Samples with nodetectable H2S are plotted along the vertical axis (H2S/CH4 = 0.001). BKFC = BasalKhuff Clastics.

Fig. 8. Histogram of d15N for gases in this study (heavy stipple) and other Paleozoicnatural gases from northwest, central and eastern Saudi Arabia (light stipple).

Saudi Aramco: Public

1 cm5 mm

500 µm 500 µm

Dolomite matrix

Par�ally-replaced anhydrite nodules

Sulfur

Dolomite matrix

Remaining anhydrite

TSR calcite

Dolomite and calcite matrix

Remaining anhydrite

TSR calcite and par�ally replaced anhydrite

A B

C D

Dolomite matrix

TSR calcite and par�ally replaced anhydrite

Sphalerite(ZnS)

Fig. 9. Petrology of TSR and related fabrics, Khuff Formation, Saudi Arabia. (A) Image of slabbed core showing the abundance of tiny anhydrite nodules, in this case partiallyreplaced by TSR calcite, and elemental sulfur. (B) Image of polished rock sample showing TSR calcite replacement rim surrounding remaining anhydrite nodule. (C)Backscattered electron microscope image of details of TSR calcite reaction front. TSR calcite contains numerous corroded relics of the original anhydrite. (D) Backscatteredelectron microscope image of details of base metal mineralization that post-dated the onset of TSR with sphalerite (zinc sulfide) present within a corroded anhydrite nodule.Galena (lead sulfide) and pyrite are also commonly associated with mineralisation, as well as fluorite and quartz.

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 61

(ZnS; Fig. 9D), galena (PbS) and pyrite (FeS2) are present within thecorroded anhydrite nodules. These minerals grew after the start ofanhydrite replacement by calcite presumably with the sulfide sup-plied by TSR. Fluorite and quartz are also associated with basemetal sulfide mineralization in fields E, G and K which thus overallrepresent a Mississippi Valley Type deposit (Sverjensky, 1986) pre-sumably localized by the presence of TSR related hydrogen sulfide.

5. Discussion

5.1. Source rock maturities

Gases from Field E are among the lowest maturity samples inthis study (Figs. 3 and 4; Table 1, sample 12). The Khuff B inField E produces sour gas with small amounts of condensate.C2+/CH4 ranges up to 0.1, methane d13C values cluster between�40‰ and �39‰ and d13C values of ethane (�35‰ to �33‰)and propane (�32‰ to �30‰) are the most negative measured.The highest maturity gases are more difficult to identify as theabundance of C2+ hydrocarbons and d13C of methane, ethane andpropane may have been altered by TSR. Sour Khuff gases withmethane d13C >�30‰ and extremely low abundances of C2+hydrocarbons are particularly suspect. If it is assumed that pre-Khuff reservoirs have been less significantly affected by TSR, thenthe most mature thermogenic gases may be from the JaufFormation at Field D. These have C2+/CH4 6 0.007, methane d13Cbetween �32.0‰ and �31.0‰ and ethane d13C between �36.2‰

and �36.1‰ (Figs. 3 and 4; Table 1, sample 10).According to Cantrell et al. (2014; their Figs. 7 and 8), depths to

the base of the Qusaiba Member in the area of study locally exceed7 km and modeled present day maturities range from a minimumof 1.7% to more than 3.0% vitrinite reflectance. To place our datainto a maturity context, we have applied the empirical model ofFaber (1987; cf. Whiticar, 1994), which uses d13C of the C1–C3 gasesfrom oil prone kerogens to estimate source rock vitrinite reflec-tance. Fig. 10 shows that ethane and propane data pairs (plotted

as triangles) fit the Faber relationship reasonably well althoughthe inferred maturities, which range from 0.5% to just under 2% vit-rinite reflectance, are lower than expected. Ethane and methanedata pairs (plotted as circles) tend to fall above the maturity lineFaber proposed for these gases. Maturities estimated frommethane d13C range from 1.2% to more than 3% vitrinite reflec-tance, more in line with expectations.

Why maturities estimated from methane are typically muchhigher than those estimated from ethane and propane is unclear.TSR is an unlikely explanation as several gases with inconsistentmaturity estimates are from Basal Khuff Clastics or pre-Khuffreservoirs (half-filled or open symbols; Fig. 10). Pre-Khuff reser-voirs lack anhydrite and are unlikely to have been significantlyaltered by TSR. The most extreme examples are Jauf gases fromField D, noted above, which yield maturities > 3% vitrinite reflec-tance for methane and 0.7–0.8% vitrinite reflectance for ethane.Inconsistent maturity estimates could be caused by isotopic roll-over of ethane and propane as documented in unconventionalshale gases from the U.S. Midcontinent and tight conventionalreservoirs from the Rocky Mountain foothills in Canada(Zumberge et al., 2012; Tilley and Muehlenbachs, 2013).‘‘Rollover’’, which refers to the shift from 13C enrichment to 13Cdepletion as shale gases mature beyond a threshold of roughly1.5% vitrinite reflectance, might explain why d13C of ethane andpropane tends to be more negative in pre-Khuff than in Khuffreservoirs (Fig. 4). Inconsistent maturity estimates might also beexplained if many coastal and offshore Arabian Gulf reservoirswere originally charged with oil or wet gas and then, sometimelater, with a large volume of high maturity methane.

5.2. TSR altered Khuff gases

Thermochemical sulfate reduction in Khuff gas reservoirs hasbeen documented in numerous proprietary Saudi Aramco reportsin addition to the published literature (Worden et al., 1995, 2000,2004; Worden and Smalley, 1996; Ahmed et al., 2008).

62 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

Correlations between log(CO2/CH4) log(N2/CH4) and log(H2S/CH4),noted in Fig. 5, suggest that non-hydrocarbon gas abundances arecontrolled by TSR. Carbon dioxide d13C decreases as H2S/CH4

increases, consistent with hydrocarbon oxidation by TSR (Fig. 6).That methane is an important reactant is suggested by the increasein methane d13C for H2S/CH4 above 0.1 (Fig. 7). Petrographicevidence from Saudi Aramco fields includes the replacement ofanhydrite by secondary calcite (Fig. 9C), and the presence ofsecondary sulfide minerals, most commonly pyrite but, in Khuffreservoirs at fields A, E, G and K, also sphalerite (Fig. 9D) andgalena. Secondary calcite has d13C as low as �20‰ (unpublishedSaudi Aramco data), indicating oxidation of natural gas orhydrocarbon liquids. In addition, the sulfur isotope compositionof the hydrogen sulfide and secondary sulfides is commonly onlya few permil lighter than Permo-Triassic sulfate (Carrigan et al.,1998; unpublished Saudi Aramco data).

Data from fields A and B (Table 1, samples 1–8) are particularlynoteworthy. Khuff reservoirs in these fields contain abundantanhydrite. Temperatures estimated from downhole sampling toolsand drill stem tests range from 151 �C to 157 �C, well in excess ofthe 140 �C minimum reported to be required for TSR of dry gases(Worden et al., 1995). Methane d13C values range from �33.9‰

to �2.8‰. In 95% of the gases we have analyzed from Paleozoicreservoirs in northwestern, central and eastern Saudi Arabia,methane d13C is less than �35‰. Methane d13C generally increaseswith increasing source rock maturity so dry gases generated athigh maturities may slightly exceed this range. With methaned13C as high as �2.8‰, Khuff B gas from Field A-Well 1 is clearlyanomalous.

Stable carbon isotope compositions of natural gases can, insome circumstances, be altered by cylinder leakage and bacterialoxidation. Problems with cylinder leakage have been encountered

-50

-46

-42

-38

-34

-30

-26

-22

-18

-42 -38 -34 -30 -26 -22

13C

Met

hane

, Pr

opan

e (‰

, VPD

B)

13C Ethane (‰, VPDB)

0.5% Ro

1.0

1.5

2.0

3.0

2.5

0.7

1.3

0.5% Ro

1.0

1.5

2.0

3.02.5

0.7

1.3

C1-C2

C2-C3

G

D

B

H

H

E

E DA-2

KHFC

CM

L

E

L

L

K K

KK

G

F

F

KhuffBKFCPre-Khuff

Fig. 10. Plot of methane d13C (circles) and propane d13C (triangles) against ethaned13C showing the empirical source rock maturity scale of Faber (1987) for Paleozoicoil prone organic matter. Stippled areas illustrate the distribution of most SaudiArabian Paleozoic gases. BKFC = Basal Khuff Clastics. Many gases cannot be plottedbecause d13C of ethane or propane are not available.

very infrequently in our laboratory but in one instance weobserved a 20‰ increase in methane d13C when a cylinder sampledat 0.35 MPa (gauge) dropped to atmospheric pressure over thecourse of six weeks. Welhan (1988) reported d13C of �0.6‰ formethane recovered from a warm geothermal spring in the SaltonSea, California, and attributed the unusual 13C enrichment to bac-terial oxidation. Bacteria might oxidize a commercial gas sampleif it were heavily contaminated by air.

Neither of these processes can explain the Khuff B data.Significant atmospheric contamination is unlikely as the Khuff Bgas flowed at a high test rate and samples were collected from afield separator at 2.5 MPa. Leakage is unlikely as we analyzedtwo samples collected on different days and their methane d13Cvalues are within 0.2‰ (Table 1, samples 1 and 2). Most impor-tantly, the d13C values we report for Khuff B and Khuff C gases fromField A-Well 1 have been verified by an independent laboratory(Table 1, footnote ‘‘a’’).

We interpret the Khuff B gas from Field A-Well 1 to be exten-sively TSR altered. Khuff B and Khuff C gases from Field A-Well 2and Khuff A/B gases from Field B have methane d13C valuesbetween �26.1‰ and �17.6‰ (Table 1, samples 4–8) and appearless strongly altered. With d13C = �33.9‰, methane from theKhuff C reservoir at Field A-Well 1 is only marginally higher thanexpected for normal thermogenic gas. The Khuff C gas at Field A-Well 1 could be either a very high maturity thermogenic gas or anormal thermogenic gas subjected to modest levels of TSR.

5.3. Nitrogen isotope ratios

d15N of Saudi Arabian Paleozoic gases varies from �7‰ to +1‰,a relatively small range compared to that reported for naturalgases elsewhere in the world (�20‰ to 30‰; Prasolov et al.,1991; Sohns et al., 1994; Gerling et al., 1997; Zhu et al., 2000;Ballentine and Sherwood-Lollar, 2002; Liu et al., 2012). LatePermian and younger sedimentary rocks in northwestern, centraland eastern Saudi Arabia were deposited in either passive marginor foreland basins, so mantle nitrogen (ranging widely but gener-ally �5‰ ± 4‰; Marty and Zimmerman, 1999; Cartigny, 2005)and volcanic nitrogen (which may be more enriched in 15N dueto incorporation of sedimentary volatiles; e.g., Halldórsson et al.,2013) can be excluded with reasonable confidence. Althoughd15N values are more negative than nitrogen in sedimentary rocks(generally �3‰ to 12‰; Ader et al., 2006) a sedimentary ormetasedimentary origin seems likely.

Fractionation of nitrogen isotopes during the formation of natu-ral gas is poorly understood. During burial alteration, organic nitro-gen is liberated primarily as ammonia and this reportedly occurswith little or no shift in d15N (Boudou et al., 2008). However, nitro-gen can be stored as an ammonium ion substituting for potassiumin clays, micas and feldspars, and substantial equilibrium isotopefractionations have been established between NH4

+, NH3 and N2

(Jia, 2006). In addition, recent experiments have demonstrated thatthe decomposition of NH3 to N2 may be accompanied by a largekinetic isotope fractionation (Li et al., 2009). As a result, nitrogenliberated from sedimentary and metasedimentary rocks isexpected to be depleted in 15N relative to its organic and mineralprecursors. Consistent with this finding, studies of natural gasesand sedimentary rocks have suggested that d15N of nitrogen gasincreases with increasing thermal stress (Gerling et al., 1997;Zhu et al., 2000).

Fig. 11 plots nitrogen d15N versus methane d13C for gases in thisstudy. Notwithstanding the expectation that both parameters areinfluenced by thermal stress, no correlation is apparent. Samplescollected more than seven years ago were not analyzed for d15Nand the absence of data for Field A-Well 2 is particularly regret-table. The distribution of data for pre-Khuff gases from

Fig. 11. Cross plot of nitrogen d15N versus methane d13C. Stippled area indicates thedistribution of pre-Khuff gases from northwest, central and eastern Saudi Arabia.Removal of methane by TSR would cause methane d13C of the residual gas toincrease without affecting nitrogen d15N.

2 If we begin with n moles of methane, then after TSR has consumed fraction f(0 6 f 6 1), n*(1 � f) moles of methane remain and n*f moles of hydrogen sulfide havebeen produced. GSI = H2S/[H2S + CH4] = n*f/[n*f + n*(1 � f)] = f.

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 63

northwestern, central and eastern Saudi Arabia, shown in stipple,indicates the natural range of d15N and d13C values for unalteredSilurian Qusaiba sourced gas that may have initially charged theKhuff.

Saudi Arabian gases with unusually high methane d13C plot wellto the right of the stippled region in Fig. 11 and as indicated by thinhorizontal arrows appear to be altered by TSR. Differences in thedegree of TSR alteration seem particularly plausible for the gasesfrom fields A and C. These fields are in close proximity and,notwithstanding large variations in nitrogen content, d15N valuesare quite similar (0.3–0.5‰; Table 1, samples 1–3 and 9). The simi-larity in d15N for TSR altered gases from fields B and M is presum-ably coincidental as these wells are located 100 km apart. Despiteits occurrence in a clastic reservoir where access to sulfate is lim-ited, Unayzah gas from Field J may be slightly altered by TSR. Thisis suggested not only by unexpectedly high methane d13C(�34.0‰) but by a very low abundance of higher hydrocarbons(C2+/CH4 = 0.0004) and the presence of 0.4% hydrogen sulfide and8% carbon dioxide, the latter with d13C = �28‰ (Fig. 6).

5.4. Modeling the effects of TSR

The chemical and stable isotope evolution of a dry gas beingaltered by TSR has been modeled. Where f is the fraction of initialmethane destroyed, N refers to the molar N2/CH4 ratio and the sub-script o refers to original conditions prior to TSR,

N ¼ No=ð1� f Þ: ð1Þ

The stable carbon isotope composition of the remainingmethane, d, can be calculated from the Rayleigh distillation equa-tion for any value of f (e.g., Whiticar, 1994). Given an original stableisotope composition, do, and the stable isotope fractionation factor(a) associated with methane destruction,

d ¼ ð1000þ doÞð1� f Þa�1 � 1000: ð2Þ

Fig. 12 shows model calculations for an original gas with N2/CH4

of 0.05–0.50 (4.8–33% nitrogen on an acid-gas-free basis) andmethane d13C equal to -37‰. Reaction progress, represented by f,is plotted on the abscissa. Panel A shows that as the extent of reac-tion increases, so does d13C of the methane remaining in the

reservoir. The rate of increase in d13C is dependent upon the instan-taneous isotope fractionation factor associated with methaneremoval,

a ¼ ½1000þ d13CðCH4 removedÞ�=½1000þ d13CðCH4 residualÞ�:

Values for a here have been broadly constrained by publishedexperimental data and field measurements. Kiyosu et al. (1990)carried out open system experiments with methane and solid cal-cium sulfate or calcium sulfate-hematite mixtures at 600–900 �Cand calculated 0.983 6 a 6 0.988. Significant reaction rates couldnot be obtained below 600 �C, however, and scatter preventsextrapolation of their results to lower temperatures. Pan et al.(2006) investigated the reaction of wet natural gas and magnesiumsulfate at 350 �C in the presence of water. They could not calculatea for methane because methane was generated in their experi-ments by TSR of the C2 to C5 hydrocarbons, but they determineda = 0.988–0.989 for ethane. As the isotopic discrimination formethane is expected to be greater than ethane, a = 0.989 may bea good upper limit for TSR of methane. The lower limit is difficultto determine from the data of Pan et al. (2006) as temperaturesof commercial reservoirs are all well below 350 �C and a typicallydecreases as temperature declines. Using a completely differentapproach, Cai et al. (2013) estimated a for methane from publisheddata for dry, sour natural gases from the Sichuan Basin, China.Plotting methane d13C against an extent of reaction parameterbased on H2S abundance, they fitted a = 0.984.

To illustrate a wide range of possible results, Panel A of Fig. 12shows model calculations for 0.98 6 a 6 0.99. An increase inmethane d13C from �37‰ to values near �3‰, as observed inthe Khuff B at Field A-Well 1, would require the removal of 82–97% of the original methane. Fig. 12, Panel B shows that removalof methane by TSR is accompanied by an increase in the molarratio of nitrogen to methane. Given a starting gas with 0.05 < N2/CH4 < 0.50, TSR alteration to N2/CH4 = 4.8, similar to the Khuff Bat Field A-Well 1, would require the removal of 90–99% of theoriginal methane. Unless the reservoir is recharged with freshhydrocarbon gas, extensive TSR will produce a substantial loss ofreserves (Fig. 12, Panel C). For reference, on an acid-gas-free basis,the reserves remaining at f = 1 consist entirely of nitrogen gas.

5.5. Application to coastal and offshore gases

To apply the TSR model to Khuff gases from the east coast ofSaudi Arabia, a proxy variable is required to replace the extent ofreaction parameter f. The most obvious candidate is the ‘‘GasSouring Index’’, or GSI, defined by Worden et al. (1995) as H2S/(H2S + CH4). If the precursor gas is sweet, one mole of hydrogensulfide is produced for every mole of methane consumed, and nohydrogen sulfide is removed from the gas phase, then GSI andthe extent of reaction, f, are equal.2 Hydrogen sulfide is highly reac-tive gas, however, and GSI is unsuitable as an extent of reactionparameter in fields where extensive sulfide mineralization has beenobserved. As an alternative, we have investigated the molar ratio ofnitrogen to methane. Unlike hydrogen sulfide, nitrogen is inert.Given the ratio of the precursor gas, N2/CH4 can be used to calculatethe extent of methane reaction. In addition, our TSR model predictsthat when methane d13C is plotted against log(N2/CH4), samples thathave the same thermogenic precursor and differ only in the extent ofTSR will plot along a nearly straight line with a slope proportional to1 � a.

A plot of methane d13C against log(N2/CH4) is shown Fig. 13.Most samples fall within the stippled area defined by more than

Fig. 12. Modeled changes in the chemical and stable isotope composition of a dry natural gas undergoing TSR. Calculations assume an original gas with methane d13C = �37‰

and 0.05 6 N2/CH4 6 0.50. a is the instantaneous isotope fractionation factor (see text).

64 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

300 pre-Khuff gases from central, northwestern and eastern SaudiArabia. Because of limited sulfate availability, pre-Khuff gases areunlikely to have experienced extensive thermochemical sulfatereduction. Pre-Khuff gases generally have methane d13C < �35‰,and none have N2/CH4 > 0.5. Pre-Khuff gases show a very roughpositive correlation between methane d13C and N2/CH4. This couldreflect either increasing Qusaiba source rock maturity or possiblyaddition of nitrogen-rich volatiles from metasedimentary base-ment rocks. The most extreme pre-Khuff gas compositions areobserved at Field D. Evidence for TSR is lacking in pre-Khuff sam-ples from Field D yet methane d13C falls between �32‰ and�31‰ and N2/CH4 between 0.41 and 0.44.

Khuff C gas from Field A-Well 1, which may be slightly alteredby TSR, plots within the stippled pre-Khuff area in Fig. 13. Theremaining gases from fields A and B have been interpreted asextensively TSR altered. They plot well outside the stippled areaand are anomalously enriched in both 13C (�26.1‰ 6methaned13C 6 �2.8‰) and nitrogen (0.47 6 N2/CH4 6 4.7). Khuff gasesfrom fields C, D, G and M plot outside the stippled area but are lessenriched in 13C (�33.5‰ 6methane d13C 6 �29.4‰) and nitrogen(0.07 6 N2/CH4 6 1.00). They could be altered but, if so, TSR cannothave progressed as far.

5.5.1. TSR at Field AFig. 13 presents three hypothetical TSR models constructed to

fit the average of the Khuff B points from Field A-Well 1 (‘‘A-1KHFB’’; Table 1, samples 1 and 2). The slopes of the reaction vectorsreflect the use of different fractionation factors. Possible precursorgases are indicated by open diamonds.

Model 1 is summarized schematically in Fig. 14. The Khuff B andKhuff C reservoirs in Field A-Well 1 are separated vertically by< 100 m so the model assumes that they were charged with thesame thermogenic gas precursor and differ only in the extent ofTSR. Forcing the Model 1 reaction vector to pass through the pointlabeled ‘‘A-1 KHFC’’ (Table 1, sample 3) fixes a = 0.989, at the highend of the range illustrated in Fig. 12. As abundant metal sulfidesare not recognized in the Khuff C reservoir, the extent of reactionin the Khuff C gas was set equal to its GSI of 0.158 (15.8% methanedestruction). From this, we calculated N2/CH4 = 0.245 and methaned13C = �35.8‰ for the unaltered thermogenic precursor. In order togenerate a gas with N2/CH4 = 4.76 and methane d13C = �2.9‰, theaverage of the Khuff B samples, 94.9% of the methane in the precur-sor would have to be destroyed. There is a large discrepancybetween our modeled GSI of 0.949 and the measured GSI of0.290. As noted earlier, however, the Khuff B reservoir contains

Fig. 13. Plot of methane d13C against N2/CH4. Stippled area indicates thedistribution of pre-Khuff gases from northwest, central and eastern Saudi Arabia.The numbered arrows illustrate hypothetical TSR alteration pathways passingthrough Khuff B gas from Field A-Well 1 (points labeled ‘‘A-1 KHFB’’) or Khuff A/Bgas from Field B. Thermogenic precursor gases are indicated by open diamonds. Foreach reaction pathway, a twofold increase in N2/CH4 corresponds to 50% methanedestruction and a ten-fold increase to 90% methane destruction. Fractionationfactors for pathways 1–5 are 0.989, 0.986, 0.980, 0.990 and 0.980, respectively.BKFC = Basal Khuff Clastics.

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 65

abundant sphalerite and galena. The GSI discrepancy can be recon-ciled if we assume that 98% of the Khuff B hydrogen sulfide wasprecipitated as sulfide minerals.

Model 2 assumes that the reaction vector passes through theKhuff B point from Field C (‘‘C-1’’; Table 1, sample 9). This is a

Fig. 14. Schematic of TSR Model 1. Model 1 assumes that the Khuff B and Khuff C reservoextent of subsequent TSR. The steps in the development of the model are as follows. (1) Ssulfide mineralization). (2) Use Eq. (1) to calculate N2/CH4 for the original gas charge (No

(3) Rearrange Eq. (2) and use the extents of reaction and isotopic compositions for the Khumethane carbon isotope ratio for either the Khuff B or Khuff C gas, and a, calculate thefraction of Khuff B H2S precipitated as sulfide minerals. Where G is the gas souring inde

plausible alternative to Model 1 because, as discussed in connec-tion with Fig. 11, fields A and C are in close proximity and theirgases have very similar nitrogen isotope compositions.Constraining the reaction vector to pass through point ‘‘C-1’’ yieldsa = 0.986. As the Khuff B reservoir at Field C is not known to hostabundant secondary sulfides, we assumed f = GSI = 0.046 and cal-culated an unaltered precursor with N2/CH4 = 0.560 and methaned13C = �31.5‰. According to Model 2, gas similar to the Khuff Bat Field A-Well 1 would require 88% methane destruction. Asbefore, the discrepancy between the calculated extent of reactionand the GSI of the Khuff B gas from Field A-Well 1 may be recon-ciled by assuming that a large proportion of the H2S produced byTSR was removed by precipitation of sphalerite and galena.

Model 3 uses a fractionation factor of 0.980, the low end of therange illustrated in Fig. 12. Precursor methane d13C is assumed tobe �37‰, near the average of coastal and offshore gases plottingwithin the stippled area of Fig. 13. This fixes N2/CH4 = 0.836.According to Model 3, just over 82% methane destruction wouldbe required to generate Khuff B gas at Field A-Well 1.Calculations are not particularly sensitive to the composition ofthe precursor gas. Varying the precursor methane d13C from�31‰ to �40‰, the maximum and minimum values measuredfor pre-Khuff gases in this study, shifts the estimate of methanedestruction from 76–85%.

Khuff B and Khuff C gases from Field A-Well 2 (Table 1, samples4 and 5) plot close to the reaction vector for Model 3 but theirchemical compositions may not be representative. Recovered froma flank location, Well 2 gases flowed at low rates, were sampled atpressures of 0.15–0.20 MPa and were accompanied by large vol-umes of formation water. Khuff C gas is extremely dry but KhuffB gas contained traces of C6–C7 hydrocarbons, suggesting minorcontamination. Based on their methane d13C values (�26.1‰ and�17.6‰), Model 3 predicts between 43% and 63% methanedestruction. The corresponding GSI values (0.50 and 0.78) suggestslightly higher extents of reaction but, as hydrogen sulfide is moresoluble than methane (Cai et al., 2013), GSI may have been influ-enced by gas exsolving from co-produced formation water.

irs in Field A-Well 1 were charged with the same sweet dry gas and differ only in theet the extent of reaction for the Khuff C gas equal to its gas souring index (no Khuff C= 0.245) and the extent of reaction for the Khuff B gas (fKHFB = 1 � No/NKHFB = 0.949).ff B and Khuff C gases to obtain a = 0.989. (4) From Eq. (2), the extent of reaction andisotopic composition of the original gas charge (do = �35.8‰). (5) Calculate p, thex, p = (1 � GKHFB/fKHFB)/(1 � GKHFB) = 0.978.

Fig. 15. Plot of methane d13C against N2/CH4 illustrating the effects of mixingbetween Khuff B gas from Field A-Well 1, and low nitrogen, unaltered end memberssimilar to those in pre-Khuff reservoirs from fields G and H. Dotted tie lines indicatemixing proportions. BKFC = Basal Khuff Clastics.

66 P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68

Exsolution of carbon dioxide and hydrogen sulfide from formationwater may explain why Khuff C gas from Field A-Well 2 is a con-spicuous outlier on our plot of carbon dioxide d13C vs. log(H2S/CH4) (Fig. 6).

5.5.2. TSR at Field BThe Khuff A/B reservoir at Field B flowed gas at high test rates

and was sampled three times over the space of two weeks at pres-sures of 2–5 MPa. Methane d13C and N2/CH4 for the samples fallwithin remarkably narrow ranges of �22.0‰ to �21.6‰ and0.477 to 0.485 (Table 1, samples 6–8). Models 4 and 5, fitted tothe average Field B gas composition in Fig. 13, assume a between0.990 and 0.980 and a precursor methane d13C of �37‰

(0.100 6 precursor N2/CH4 6 0.219). According to these models,generating a gas similar to that tested from the Khuff A/B reservoirat Field B would require the destruction of 54–79% of the precursormethane. The calculated extents of reaction are considerablyhigher than suggested by GSI values of the Field B gases, whichcluster tightly about an average of 0.210.

Why model calculations and measured GSI values disagree forthe Field B gases is not clear. If we assume that the GSI valuesreflect the true extent of reaction and 0.980 6 a 6 0.990, then theprecursor methane d13C would have to fall between �26‰ and�24‰. This is 5–7‰ higher than the most 13C enriched pre-Khuffgas in Saudi Arabia. An alternative explanation is that we haveunderestimated the fractionation factor. If we assume that theextent of reaction is equal to the average GSI and that the precursormethane d13C was �31.0‰ (the highest known pre-Khuff value),then a would be 0.960. This is at least twice the fractionation webelieve to be justified by field data and laboratory experiments. Athird possibility is that GSI underestimates the extent of reactionbecause hydrogen sulfide has been lost from the system.Although the Khuff A/B reservoir is not known to host abundantsecondary sulfide minerals, H2S might have dissolved in the waterleg or reacted to form species of intermediate oxidation state suchas elemental sulfur or polysulfides.

5.5.3. Origin of Khuff gas at Field DKhuff gases from Field D and the precursor gas used in TSR

Model 3 are interesting because although N2/CH4 is comparativelyhigh, methane is not particularly enriched in 13C. Khuff gases fromField D were collected from two wells that flowed gas at high ratesand have 0.8 6 N2/CH4 6 1.0 and methane d13C = -33.5‰ (e.g.,Table 1, sample 11). The fact that they plot near the origin of theModel 3 reaction vector in Fig. 13 is coincidental as fields D andA are located roughly 80 km apart and do not share a commonmigration pathway. TSR cannot have destroyed significantamounts of methane in the Khuff reservoir at Field D becausemethane d13C falls within the range of pre-Khuff gases. In addition,whereas fields A and B are extremely dry, C2+ hydrocarbons in theKhuff at Field D are comparatively abundant (0.034 < C2+/C1 < 0.037).

It is possible that the stippled pre-Khuff region in Fig. 13 under-estimates the range of N2/CH4 in gases unaltered by TSR.Alternatively, gases with elevated nitrogen abundances and littleor no 13C enrichment might be produced by mixing of TSR alteredand unaltered gases. Fig. 15 illustrates mixing lines constructed forTSR altered Khuff B gas from Field A-Well 1 and low nitrogen endmembers similar to Basal Khuff Clastics and pre-Khuff gases fromfields G and H. The mixing lines are concave upward and boundKhuff data with the most negative methane d13C values. Mixingcould be caused either by addition of pre-Khuff gas to a TSR alteredKhuff accumulation or by updip migration of TSR altered gas into ashallower, unaltered Khuff accumulation. Fig. 15 suggests thatmixing could be important not only at Field D but in other fieldssuch E and F.

5.6. Differences between Khuff B and Khuff C gases at Field A-Well 1

Why TSR appears to have altered Khuff B gas in Well 1 far moreextensively than Khuff C gas in Well 1 is puzzling. Among the mostimportant factors controlling TSR are temperature and the abun-dance and crystal size of anhydrite in the reservoir rock. As notedearlier, Khuff B and Khuff C reservoir temperatures exceed thethreshold thought necessary for TSR of methane. They differ byno more than 3–4 �C, however, and the Khuff B, which is moreextensively altered, is cooler. Anhydrite occurs as pore-fillingcements and nodules and ranges from 0–20% by volume in bothreservoirs. Petrographic data from the Khuff Formation in AbuDhabi indicate that fine, 10–30 lm, anhydrite crystals react muchmore readily than coarse, 50–200 lm crystals (Worden et al.,2000). Large, poikilotopic anhydrite crystals have been reportedin the Khuff C, but whether finely crystalline anhydrite is moreabundant in the Khuff B is unknown. Other factors known toinfluence TSR rates are hydrogen sulfide abundance, which maycatalyze the reaction by forming elemental sulfur or low molecularweight organic sulfides and the abundance of Mg in the formationwater (Zhang et al., 2008; Ma et al., 2008; Lu et al., 2011). Whethereither of these might favor the Khuff B over the Khuff C is not clear.

We offer three explanations for the differing extent of reaction.

5.6.1. Early oil charge in the Khuff BFaint hydrocarbon staining, odor and fluorescence have been

reported in the Khuff B but not in the Khuff C. Pyrolysisexperiments show that sulfate reduction is enhanced by the pres-ence of organic sulfides, such as 1-pentanethiol, which can form byreaction of light hydrocarbons with hydrogen sulfide (Amraniet al., 2008; Zhang et al., 2008). Although Khuff B gas is now extre-mely dry, the reservoir may originally have been charged with oilor gas-condensate. If so, TSR could have begun earlier than in theKhuff C and the rate of methane oxidation possibly enhanced.

5.6.2. Sulfide mineralization in the Khuff BAs noted earlier, extensive sulfide mineralization is observed in

the Khuff B but not in the Khuff C (R.F. Lindsay, personal

P.D. Jenden et al. / Organic Geochemistry 82 (2015) 54–68 67

communication, 30 April 2011). Invasion of hot mineralizing fluidsmay have produced a transient increase in reservoir temperature.Alternatively, precipitation of sulfides may have enhanced TSR byremoving an important reaction product and increasing therelative concentration of methane in the residual gas. Porosity inthe Khuff B appears to be enhanced by grain dissolution that couldbe associated with sulfide mineralization.

5.6.3. Mixing with fresh gas in the Khuff CThe apparent extent of TSR in the Khuff C may have been dimin-

ished by mixing with unaltered methane rich gas (Fig. 15). Becauseon-going TSR would destroy evidence for mixing, recharging musthave been very recent or the rate of TSR must be considerablyslower now than in the past. Basin models suggest that reservoirtemperatures in Field A reached a maximum around 35 Ma andhave cooled by approximately 5 �C since then. Such a small changeseems unlikely to have produced an appreciable effect on reactionrates. Another possibility is that TSR has been shut down by thedevelopment of calcite reaction rims around fine grained anhydritenodules (Worden et al., 2000).

6. Conclusions

Deep, dry Khuff gas reservoirs from eastern Saudi Arabia arecharacterized by 0.06 < N2/CH4 < 4.8, �39.6‰ < methane d13C <�2.8‰ and �28.4‰ < carbon dioxide d13C < �0.1‰. Broad cor-relations between each of these parameters and H2S/CH4 suggestthat methane has been destroyed by thermochemical sulfatereduction. Chemical compositions and methane carbon isotopecompositions have been fitted with simple Rayleigh fractionationmodels using initial N2/CH4 between 0.10 and 0.84, initial methaned13C between �40‰ and �31‰ and constant isotopic fractionationfactors between 0.98 and 0.99. According to these models, thehighest measured N2/CH4 and methane d13C values require thedestruction of 76–95% of the original methane charge.

Data from Field A in our study indicate that the extent of TSRmay differ markedly in Khuff reservoirs at similar depths and pre-sent day temperatures. The reason for these differences is unclear.The Khuff B reservoir at Field A apparently received an early chargeof liquids, which could have enhanced reaction rates. It has alsosuffered extensive sulfide mineralization. Mineralization may havepromoted TSR either by the invasion of hot mineralizing fluids orby the precipitation of hydrogen sulfide, which would increasethe relative abundance of methane in the residual gas. Lastly, theapparent extent of reaction in the Khuff C reservoir may have beendiminished by mixing with fresh, unaltered gas.

Acknowledgments

The authors would like to thank the Hydrocarbon PhaseBehavior Unit at the Saudi Aramco EXPEC Advanced ResearchCenter for carrying out field sampling and providing chemicalanalyses for all gas samples analyzed in this study. We areindebted to William J. Carrigan and Peter J. Jones, whose propri-etary studies contributed so much to our understanding of TSR inSaudi Arabian oil fields and to Owaidh S. Al-Harthi, who measuredthe stable isotope ratios reported here. Detailed recommendationsfrom two anonymous reviewers improved the organization, con-tent and clarity of this paper. Special thanks go to Saudi ArabianOil company (Saudi Aramco) and the Ministry of Petroleum andMineral Resources for permission to publish this study.

Associate Editor—Andrew Murray

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