AUTHORS

Barbara Tilley � Department of Earth andAtmospheric Sciences, 1-26 Earth SciencesBuilding, University of Alberta, Edmonton,Alberta, Canada T6G 2E3; btilley@ualberta.ca

Barbara Tilley obtained her Ph.D. from theUniversity of Alberta in 1988. She has workedas a research associate with Karlis Muehlenbachsat the University of Alberta since 2000, studyingthe geology and geochemistry of natural gases

Gas isotope reversals infractured gas reservoirs of thewestern Canadian Foothills:Mature shale gases in disguiseBarbara Tilley, Scott McLellan, Stephen Hiebert,BobQuartero, Byron Veilleux, and KarlisMuehlenbachs

throughout the Western Canada sedimentarybasin. She received the AAPG Matson Award forher oral presentation of this work at the annualconvention in Denver in 2009.

Scott McLellan � Talisman Energy Inc.,Suite 3400, 888-3rd St. Southwest Calgary,Alberta Canada T2P 5C5;smclellan@talisman-energy.com

Scott McLellan is a structural geologist andgeophysicist, currently working for TalismanEnergy, Inc. McLellan graduated in 1995 fromthe University of Calgary with an M.Sc. degreein sedimentology and geochemistry. Since then,he has worked in many different play typesfrom deep carbonates to tight structured sand-stones and shale gas. He has previously workedfor Shell, Remington Energy, Dominian EneryCanada Limited, and Suncor.

Stephen Hiebert � Talisman Energy Inc.,Suite 3400, 888-3rd St. Southwest Calgary,Alberta Canada T2P 5C5;shiebert@talisman-energy.com

Steve Hiebert graduated in 1992 with an M.Sc.degree in structural geology from the Universityof Calgary. He has worked on various explo-ration projects across the overthrust belt ofAlberta, British Columbia, and Montana. From2001 to 2005, he lived with his family in Oman,working on Cretaceous carbonate oil plays.Currently employed by Talisman Energy inCalgary, his interests include overthrust struc-tures and basement-controlled depositionalsettings across western Canada.

ABSTRACT

Isotopically reversed gases (d13C methane > d13C ethane >d13C propane) occur in fractured mixed clastic-carbonatereservoirs of the Permian and the Triassic in the foothills atthe western edge of the Western Canada sedimentary basin(WCSB). The d13C methane values (–42 to –24‰), gas dry-ness, and organic maturity (Ro > 2.2) are indicative of maturegases, and gas maturity generally increases with reservoir ageand from the southeast to the northwest. The d13C ethanevalues range from −44 to −25, with the less negative values inisotopically normal gases to the northeast of the gas fields westudied. To explain the gas isotope reversals observed in theWCSB foothills, we adopt the concept of a closed-system shale,in which simultaneous cooking of kerogen, oil, and gas yields gaswith light d13C ethane and heavy d13C methane. This gas wasreleased from shales and trapped in fractured folds of brittleclastic-carbonate rocks during deformation and thrust faultingof the Laramide orogeny, creating some of themost prolific gaspools. These gases are actually mature shale gases. Local highabundances of H2S and CO2 are most likely the products ofthermochemical sulfate reduction (TSR) reactions in anhydrite-rich interbeds and underbeds that admixed to the released shalegas during the tectonic event. No evidence exists that TSR isresponsible for the isotope reversals. Variations in d13C ethaneare likely caused by local differences in thermal history, thetiming of gas release from shale, and the timing of the fault andfold development. Less negative d13C ethane values (resultingin isotopically normal gases) to the northeast of the fields and in

Bob Quartero � Talisman Energy Inc., Suite3400, 888-3rd St. Southwest Calgary, Alberta Can-ada T2P 5C5; bquartero@talisman-energy.com

Bob Quartero received his master’s degree ingeology from the State University of Leiden,Netherlands, in 1976. He has more than 30 yr

Copyright ©2011. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received June 15, 2010; provisional acceptance August 16, 2010; revised manuscript receivedSeptember 28, 2010; revised provisional acceptance November 11, 2010; 2nd revised manuscriptreceived December 8, 2010; final acceptance January 3, 2011.DOI:10.1306/01031110103

AAPG Bulletin, v. 95, no. 8 (August 2011), pp. 1399– 1422 1399

of experience in hydrocarbon exploration, thelast 26 yr (1984–2010) with Talisman Energy,with 17 yr in foothills exploration throughoutthe North American deformed belt. His interestsremain with exploration in thin-skinned thrustbelts and global tectonics.

Byron Veilleux � Talisman Energy Inc.,Suite 3400, 888-3rd St. Southwest Calgary,Alberta Canada T2P 5C5;bveilleux@talisman-energy.com

Byron Veilleux received his master’s degree instructural geology from the University of Alberta,Canada, in 1993. He has 15 yr of experiencein exploration for oil and gas with Petro Canada,Anadarko, and Talisman Energy. His experienceincludes exploration in thin-skinned deformedbelts in Alberta, British Columbia, and Peru,as well as extensional environments, offshoreeastern Canada.

Karlis Muehlenbachs � Department ofEarth and Atmospheric Sciences, 1-26 EarthSciences Building, University of Alberta, Edmonton,Alberta, Canada T6G 2E3; kmuehlen@ualberta.ca

Karlis Muehlenbachs is a stable isotope geo-chemist who obtained his Ph.D. at the Universityof Chicago in 1971. He joined the faculty ofthe University of Alberta in 1974, where he ini-tiated a broad stable isotope research program.Since 1994, he has been working on oil and gasisotope geochemistry of the Western Canadasedimentary basin.

ACKNOWLEDGEMENTS

We thank Talisman Energy Inc. and the NationalScience and Engineering Research Council ofCanada for financial support. We thank AAPGreviewers Robert C. Burruss, Tricia F. Allwardt,and John E. Zumberge for their thorough andconstructive reviews that helped strengthenthe manuscript. We thank Heather Plint fromHusky Oil for her constructive comments.The AAPG Editor thanks the following reviewersfor their work on this paper: Tricia F. Allwardt,Robert C. Burruss, and John E. Zumberge.

1400 Gas Isotope Reversals in Fractured Gas R

the underlying Devonian carbonates likely reflect a more openshale system where the earliest generated gas was lost. Wesuggest that isotopic reversals are restricted to closed-systemmaturation, and that their magnitude may be related to therelative volume of gas retained in shales.

INTRODUCTION

Gases from the Devonian to the Triassic reservoirs in the foot-hills of the Western Canada sedimentary basin (WCSB) are ma-ture (13C-rich methane) and dry, but along part of the extremewestern edge of the WCSB (shown on the map in Figure 1),some of Canada’s most prolific gas fields have very unusuald13C values of ethane that are more negative than the d13C ofmethane. Such carbon isotope reversals (d13C methane > d13Cethane > d13C propane; Figure 2C, D) are rare in natural gasesfrom conventional reservoirs, and their origin is poorly under-stood. Typical thermogenic gases (James, 1983; Schoell, 1983;Chung et al., 1988) have carbon isotope ratios where d13Cmethane < d13C ethane < d13C propane (Figure 2A). As thematurity of a thermogenic gas increases, the isotopic ratios formethane, ethane, and propane generally become less negativein a predictable proportional manner. Alteration processessuch as biodegration and oxidation also tend to result in 13C-enriched ethane and propane (less negative d13C values), so aprocess that would create d13C values of ethane and propanemore negative than that of methane in a single thermogenic gasis not obvious. Consequently, there has been general agreementin the literature (Jenden et al., 1993; Dai et al., 2004; SeewaldandWhelan, 2005) that isotopically reversed thermogenic gasesare the result of mixing of different gases. However, the originof themixed components is problematic. Suggested origins varyfrom amixture of low- and high-maturity gases (Appalachians,Jenden et al., 1993; Potato Hills gas field, southeastern Okla-homa, Seewald andWhelan, 2005), gas from a sapropelic sourcemixed with gas from a humic source (southern Sichuan Basin,China; Dai et al., 2004) andmature gas mixedwith gas sourcedfrom oil cracking (Appalachians, Y. Tang andM. Schoell, 2005,personal communication). We propose that isotopically re-versed gases can also be created in a closed-system organic-rich shale as a result of the accumulation of gases generated atvarious stages during ongoing maturation of organic matter ina single source without migration.

With the development of gas shale plays, isotopicallyreversed gases are commonly being found in mature shalegases such as the Barnett, Haynesville, Fayetteville,Woodford,

eservoirs

Marcellus, Horn River, and Utica shales. Isoto-pically reversed gases are associated with the mostproductive wells in many of these shales (Ferwornet al., 2008; Zumberge et al., 2009) and in theUtica Shale (Talisman Energy Inc., 2010, personalcommunication). It follows that understanding theorigin of the isotopic reversals in thermal gases couldhave important implications for the exploration anddevelopment ofmature shale gas plays.We believethat the isotopically reversed gas that occurs in frac-tured mixed clastic-carbonate reservoirs at the west-

ern edge of the WCSB is analogous to deep shalegas that has been created by a combination of ker-ogen, petroleum, and gas cracking and stored inclosed-system shales.

We present carbon isotope data for gas from asignificant number of wells, several different gasfields, and reservoirs of theDevonian to the Triassicalong the westernmost edge of the WCSB. We alsoconsider the geographic, stratigraphic, and structurallocation of the gases and their production his-tory. Using all these data, combined with the burial

Figure 1. Locations of ourgas samples from the Devonianto the Triassic reservoirs. Iso-topically reversed gases areshown as red circles or stars,normal gases as black crossesor green stars, shale gases asstars. Locations of four structuralcross sections in Figure 10 areshown here as yellow linesacross the red circles. Note thatthe reversed gases are linedup along the extreme westernedge of the WCSB where thetopographic relief begins to thewest. The Bullmoose-Sukunkastructural trend occurs in theinner foothills in northeast Brit-ish Columbia. The Narrawayand Minnow fields occur in theouter foothills in west-centralAlberta. The gray shaded area isPaleozoic and Proterozoic out-crop. The green circle denotesthe location of the mud gas logshown in Figure 6.

Tilley et al. 1401

history and organic maturity data for the areafrom the literature, we deduce a mechanism forthe origin of the isotopic reversal, a process thatlikely occurs in many closed-systemmature shales,where the limits for preservation of oil have beenexceeded.

1402 Gas Isotope Reversals in Fractured Gas Reservoirs

GEOLOGIC SETTING

The WCSB is an extensively studied westward-dipping foreland basin (Mossop and Shetsen, 1994)with a long history of oil and gas exploration anddevelopment. The Bullmoose-Sukunka structural

Figure 2. Natural gas plots(Chung et al., 1988) for gasesfrom the study area, illustratingthe relationship of methane,ethane, and propane in (A) typicalthermogenic gases comparedwith (C) and (D) isotopically re-versed gases and (E) and (F)partially reversed gases. Gases inpanel B have d13C methaneand d13C ethane values that aremore similar than expected fortypical thermogenic gases. Gasesare further subdivided on thebasis of CO2 + H2S content:(C and E) less than 20%; (D and F)more than 20%.

trend (Figure 1) is a prolific gas-producing area thatoccurs in northeastern British Columbia at thewestern edge of the Canadian Foothills fold andthrust belt and to the west of the largely unde-formed WCSB. Newson (2001) and Cooper et al.(2004) have described the tectonic structure ofthe gas reservoirs along this trend, and others havecharacterized potential source rocks, burial history,and thermal maturity in the general Monkman area(a larger area that includes Bullmoose-Sukunka)(Kalkreuth and McMechan, 1988; Ibrahimbas andRiediger, 2004; Riediger et al., 2004). However,there has been little geochemical study of the gasesthemselves.

The evolution of the WCSB includes subsi-dence, deformation, uplift, and erosion (Mossopand Shetsen, 1994). The first stage of develop-ment of the WCSB occurred from the upper Pre-cambrian through the Middle Jurassic and con-sisted of westward deepening stable continentalshelf and shoreline sedimentation, in the form ofmainly fine-grained clastics overlain by thick inter-vals of carbonates and local evaporates. TheMiddleJurassic Columbian orogeny initiated the secondstage of development, a Late Jurassic throughTertiary foreland phase. The strata of this phaseconsist of fine- to coarse-grained clastic sediments,sourced from the emerging Cordillera to the westand deposited primarily in inland seas. During theLaramide orogeny of the early Campanian to thelate Eocene, the western margin of theWCSB wasuplifted and deformed as an eastward-migrating foldand thrust belt. Within the foothills, sedimentationwas terminated by uplift caused by the deformation.

Reservoirs

Gas is produced from the Permian and the Triassicreservoirs located at the crests of fault-propagationfolds (Newson, 2001; Cooper et al., 2004) orfaulted detachment folds (Nemcok et al., 2005).Fracturing is pervasive in the hinges and forelimbsof the folds and enhances the reservoir perme-ability to yield typical flow rates of 40 mmcf gas/day (Cooper et al., 2004). Detachments occur inCretaceous shales, shales of the JurassicNikanassinFormation and FernieGroup (Figure 3), anhydrites

of the Triassic Charlie Lake Formation, shales ofthe Lower Triassic Montney Formation, and theDevonian Besa River shales (Barss and Montandon,1981; McMechan, 1985; Cooper et al., 2004).These detachments subdivide the sedimentary se-quence into a series of tectonostratigraphic pack-ages that deform, to some extent, independently,each with its own characteristic deformation style.The summed regional shortening is likely similaramong the different tectonostratigraphic packagesbut may be distributed with varying amounts ofshortening across different detachments.

The major gas reservoirs along the Bullmoose-Sukunka structural trend are in the Permian Belloy-Belcourt formations and the Triassic Pardonet-Baldonnel formations (Figure 3). The PermianBelcourt-Belloy formations are a mixed siliciclasticcarbonate unit, deposited in a tectonically stableshallow-marine shelf setting, characterized by lim-ited clastic input and an active chemical environ-ment. Lithologies include sandstone, dolostone,chert, and subordinate limestone and shale (Cana-dian Discovery Ltd., 2007). Sour gas is producedfrom the platform carbonates of the Upper TriassicPardonet-Baldonnel formations. The Bullmoose-Sukunka area was a significant Triassic depocenter,with as much as 1200m (3937 ft) of Triassic stratadeposited (Gibson and Barclay, 1989). The Par-donet Formation consists of partially dolomitizedlimestone, dolomites, dolomitic silts and shales, andcalcareous black shaly siltstone. The BaldonnelFormation consists of bioclastic and pelloidal dolo-mites (Cooper et al., 2004). Production is optimizedin both the Belcourt-Belloy formations and thePardonet-Baldonnel formations by drilling dolomiticfacies in the highly fractured axial planar regionsof folds.

Farther south in the Alberta extension of thestructural trend (Narraway and Minnow areas),gas production is predominantly from the TriassicCharlie Lake Formation (Figure 3). The CharlieLake Formation consists of a complex interbeddedsuccession of silty dolomite, dolomitic sandstone,limestone, and anhydrite. Although overall thick-ness of the formation decreases to the southeast,the more porous units are pervasive in the upper20–30m (65.6–98.4 ft). These upper zones account

Tilley et al. 1403

for most of the gas production. When folded, thedolomitic units exhibit highly fractured zones thatgreatly enhance the matrix permeability. To opti-

1404 Gas Isotope Reversals in Fractured Gas Reservoirs

mize gas production, most horizontal drilling pro-grams have targeted the upper part of the CharlieLake Formation along the crest of tight folds.

Figure 3. Stratigraphic columnshowing reservoir and probablesource rocks (modified fromBritish Columbia Ministry ofEnergy, Mines and PetroleumResources).

Source Rocks, Burial History, and SourceRock Maturity

Possible source rocks for the gas in the Permian andTriassic reservoirs are theMississippian Banff shale,the Triassic Montney shale, and the phosphatezone at the base of the Doig Formation; the Tri-assic Pardonet Formation; and the Jurassic Gor-dondale Member of the Fernie Group (Figure 3;Table 1). The Gordondale Member of the FernieGroup has type IIS kerogen, Sorg = 11.3 wt.%(where Sorg is organic sulfur within types I, II, andIIS kerogen; Higley et al., 2009), whereas the otherpossible source rocks all have type II kerogen, Sorg =4.7 to 5.2 wt.% (Higley et al., 2009). Total organiccarbon (TOC) ranges from 4.2 to 18% (Table 1)(Richards et al., 1993; Caplan, 1997; Ibrahimbasand Riediger, 2004; Higley et al., 2009).

The JurassicGordondaleMember of the FernieGroup is themost probablemain source for the gasin the Pardonet-Baldonnel formations. The JurassicFernie Group (Cant and Stockmal, 1989), whichis dominated by deep-water mudrocks in this area(Cooper et al., 2004), represents the lower part ofthe oldest clastic wedge in the WCSB. It is at itsthickest in the general study area (Bullmoose-Sukunka structural trend; Cooper et al., 2004) andthins to the north.

The burial history and thermal maturity ofCretaceous strata in the study area have beenthoroughly studied by Kalkreuth and McMechan(1988). Maximum rates of maturation and a large

proportion of the total maturation occurred aboutthe time of the deepest burial (∼70 Ma) in theMaastrichtian to Eocene (Kalkreuth andMcMechan,1988). A westward decrease in maturity acrossthe foothills is caused by a westward decrease indepth and duration of sedimentary burial beneathMaastrichtian–Eocene foredeep deposits and par-tially caused by a general westward decrease ingeothermal gradient (Kalkreuth and McMechan,1988).

Kalkreuth andMcMechan (1988)made vitrinitereflectance (Ro) determinations mostly on LowerCretaceous coals and used coalification profilesto estimate maturation levels over a larger strati-graphic interval. Their estimates indicate that Ro

values for the Triassic strata in both the Bullmoose-Sukunka andNarraway areas are all more than 2.2%(Figure 4), and that at the time of maximum burial(∼70 Ma), Triassic strata were at depths of 6–7 km(3.7–4.3 mi) compared with present depths of2–3 km (1.2–1.9 mi). However, their calculationspredict that some of the Jurassic in the Bullmoose-Sukunka area has Ro values less than 2.2%, whereasin the Narraway area, all the Jurassic has Ro val-ues greater than 2.2%. Therefore, their work sug-gests that the Jurassic Fernie source rock is moremature at theNarraway area than at the Bullmoose-Sukunka areas. Regardless, Kalkreuth andMcMechan(1988) state that the distribution of Ro values in-dicates that all local source rocks were at maturitiesabove the limits of wet gas preservation before thedeformation caused by the Laramide orogeny.

Table 1. Kerogen Types and Total Organic Carbon of Possible Source Rocks for Gas in the Permian and Triassic Reservoirs of the

Western Canada Sedimentary Basin Foothills

Age

Source Rock Formation Kerogen Type TOC (as much as %)

Jurassic

Fernie (Gordondale Member) IIS* 18.1* Triassic Pardonet Fm. Overmature, Residual up to 2.8** Triassic Baldonnel Fm. 1.4** Triassic Base of Doig Fm. “phosphate zone” II 3.1* Triassic Montney II 4.2** Mississippian Basal black shale of the Banff Fm. II† 14††

*Higley et al. (2009).**Ibrahimbas and Riediger (2004).†Richards et al. (1993), turbidite deposits.††Caplan, 1997 (HI average, 251 mg HC/g organic C for Banff shale).

Tilley et al. 1405

DATA AND DISCUSSION

Carbon isotope analyses were performed followingthe methods of Rowe and Muehlenbachs (1999)and Tilley and Muehlenbachs (2006) at the Uni-versity of Alberta and are reported in per mil (‰)relative to the Peedee belemnite (PDB) standard.Chemical compositions were determined by var-ious service laboratories. Carbon isotope resultsfor production gases from the Sukunka-Bullmoosearea in the inner foothills of northeast British Co-lumbia and the Narraway and Minnow areas inthe outer foothills in west-central Alberta (Figure 1)are presented in Table 2, along with correspond-ing chemical compositions and present-day in-situtemperatures and subsea depths. Note that theseare very dry gases with more than 99.5 vol. % ofthe hydrocarbon fraction being methane, lessthan 0.33 vol. % ethane, and less than 0.01 vol. %propane. Also note that H2S contents range fromless than 0.01 to 29 vol. % and CO2 from 0.4 to15.9 vol. % of the total gas.

Isotopic and Chemical Data

Hydrocarbon GasesThe isotope data for the hydrocarbon gases is pre-sented in Figure 2 as a series of natural gas plots(Chung et al., 1988), which include all samples in

1406 Gas Isotope Reversals in Fractured Gas Reservoirs

Table 2 for which methane, ethane, and propaneisotope ratios and corresponding chemical data areavailable. For visual clarity and to illustrate thathigh CO2 and H2S contents are not uniquely re-lated to reversals, fully and partially reversed gaseswere separated into different plots based on theirCO2 and H2S abundances. The gases are dividedinto four groups based on the shapes of the naturalgas plots:

1. Normal thermogenic gases with d13C methane< d13C ethane < d13C propane (Figure 2A).These include gases with less than 0.01 to 16vol. % H2S from the Upper Devonian and theTriassic gas reservoirs in the Narraway andMinnow areas and from the Permian in theBullmoose-Sukunka area.

2. The d13C methane value is approximately thesame as the d13C ethane value (Figure 2B). TheH2S for these gases varies from less than 0.01 to14.1 vol. %. These gases occur in the Triassic atthe Narraway and Minnow areas, the Permianat Narraway area, and the Permian and theMiddle Triassic at Bullmoose-Sukunka areas.

3. Fully isotopically reversed gases with d13C meth-ane > d13C ethane > d13C propane (Figure 2C,D). This type of gas is present, independent ofwhether the H2S content is high or low, at theSukunka-Bullmoose areas in the Permian andthe Triassic reservoirs and at the Narraway area

Figure 4. Palinspastic reconstructions showing vitrinite reflectance (Ro) values in (A) the Bullmoose-Sukunka area and (B) the Narrawayarea (modified from Kalkreuth and McMechan, 1988). Gray vertical bands indicate the locations of our samples. Vitrinite reflectancevalues for the Jurassic source rock at the Sukunka area are about 2.2% and at the Narraway area are more than 2.2%.

in the Triassic reservoirs. Lack of propane dataand/or chemical compositions for gases fromthe Minnow area prevents their classification inthis scheme.

4. Partially reversed gases with d13C methane >d13C ethane < d13C propane. This type of gasis present, independent of whether the H2Scontent is high or low (Figure 2E, F), at theSukunka-Bullmoose area in the Permian andthe Triassic reservoirs, at the Narraway area inthe Middle and the Upper Triassic reservoirs,and at the Minnow area in a Permian reservoir.

No apparent correlation exists between gasgroup (Figure 2), field area, formation, or CO2

and H2S abundance.On the d13Cmethane versus d13C ethane cross

plot of Figure 5, isotopically normal gases lie abovethe d13C1 = d13C2 line, whereas reversed andpartially reversed gases (d13C1 < d13C2) lie belowthis line. Taking d13C of methane as an indicatorof overall gas maturity, more mature gases shouldlie to the upper right of this diagram. Several ob-servations can be made from the distribution of thedata across the diagram:

1. In the Sukunka andNarraway areas, the Permianand the Triassic contain independent uncon-nected gas systems, and as might be expected,the gas in the Permian reservoirs is morematurethan the gas in the Triassic reservoirs. In theMinnow field, the Permian and the Triassic gasesare similar, suggesting a connection between thetwo reservoirs at that location.

2. Methane d13C values in the Narraway and Min-now fields aremore negative than at the Sukunkaarea, although most gases in all three areas are atleast partially isotopically reversed.

3. Ethane d13C values range from −43 to −24.5‰.The broad range of ethane values in each fieldcould suggest different degrees of mixing of atleast two different gas sources, but in this ar-ticle we suggest an alternate mechanism thatmight create a range of ethane values. How-ever, the very low abundance of ethane in thesevery mature dry gases means that the additionof even a small amount of an isotopically dif-

ferent gas could cause a large change in d13Cethane while not significantly affecting the d13Cmethane.

4. Gas in theDevonian reservoir at theMinnowareais a normal very mature thermogenic gas (13C-rich methane, ethane, and propane), whereasoverlying gases from the Mississippian and theTriassic in the same and nearby wells are at leastpartially isotopically reversed.

A general overview of the isotope ratios of gasesfrom the Cretaceous down through to the Triassic(i.e., above the Triassic and the Permian reservoirs)is provided by the only mud gas isotope log that wehave from the area, the Ojay field (Figure 6)(Tilley and Muehlenbachs, 2006), which lies be-tween the Sukunka and Narraway areas (Figure 1).This profile shows d13C values for methane andethane gradually becomingmore positivewith depththrough the Cretaceous and into the Jurassic. Thisis the general trend expected for thermogenic gasesas their maturity increases. At the top of the Tri-assic, an inversion occurs as isotope ratios of themethane and ethane gases unexpectedly becomemore negative. As will be seen in the discussionsection of this article, the most negative d13C valuefor ethane all through the Cretaceous, the Jurassic,and down into the top of the Triassic is −33‰. It islikely that gases overlying the Triassic reservoirs inour study areas to the northwest and southeast ofOjay have similarly heavy d13C ethane. This issupported by a surface gas seep in the Sukunkaarea with a d13C ethane value of –25.9‰, similarto ethane from the Cretaceous Gething Formationat Ojay.

Figure 7 shows how the gas dryness (rep-resented here by ln[C1/C2]) andmethane, ethane,and propane isotope ratios vary with the presentsubsea depth. Note that because of uplift and ero-sion, maximum burial depths (−4700 to −8100 m[−15,420 to −26,575 ft]) were about 4000 m(13,123 ft) greater than the present subsea depths.The Sukunka Triassic, Sukunka Permian, and Al-berta Triassic gases are differentiated by coloredsymbols, and gases that are not isotopically reversedare indicated by the circled symbols. The variationsand trends within and among four depth ranges

Tilley et al. 1407

Table 2. Isotopic and Chemical Composition Data for Gases from the Western Canada Sedimentary Basin Foothills*

Sample Code Age Location** FormationT

(°C)Depth

Subsea (m)d13C1(%)

d13C2(%)

d13C3(%)

d13CO2

(%)He(%)

CO2

(%)H2S(%)

C1(%)

C2(%)

C3(%)

Sukunka-Bullmoose Inner FoothillsSU-UT11 Upper Triassic 93-P-3 Pard/Bald 73 –1293.1 –31.9 –41.1 –41.8 –4.3 0.01 11.3 22.8 65.8 0.15 0.01SU-UT12 UpperTriassic 93-P-3 Pard/Bald 72 –1232.5 –32.6 –42.0 –30.2 0.01 14.8 27.7 57.4 0.09 0.01SU-UT13 Upper Triassic 93-P-3 Pard/Bald 72 –1173.1 –31.6 –42.7 –43.0 –3.9 0.01 13.9 28.4 57.5 0.09 0.01SU-UT14 Upper Triassic 93-P-3 Pard/Bald 77 –1025.2 –31.9 –38.1 –39.1 0.01 14.5 26.9 58.5 0.08 0.01SU-UT15 Upper Triassic 93-P-4 Pard/Bald 66 –805.1 –31.8 –38.1 –7.7 0.01 8.6 13.3 77.9 0.17 <0.01SU-UT16 Upper Triassic 93-P-3 Pard/Bald 70 –1094.7 –31.8 –42.1 –42.6 –4.3 0.01 14.0 29.5 56.4 0.09 0.01SU-UT17 Upper Triassic 93-P-3 Pard/Bald 81 –1375.0 –32.3 –43.5 –31.0 –4.2 0.01 14.5 29.0 56.4 0.08 0.01SU-UT18 Upper Triassic 93-P-4 Pard/Bald 77 –1354.7 –31.4 –34.3 –33.4 0.01 12.9 21.9 65.1 0.10 <0.01SU-UT19 Upper Triassic 93-P-4 Pard/Bald 70 –1248.6 –32.2 –37.8 –7.1 0.01 7.4 12.4 80.0 0.21 <0.01SU-UT20 Upper Triassic 93-P-4 Pard/Bald 70 –1080.1 –32.0 –36.0 –38.8 –6.2 0.01 7.2 8.8 83.8 0.20 <0.01SU-UT21 Upper Triassic 93-P-4 Pard/Bald 70 –1166.2 –31.5 –36.4 –32.1 –7.1 0.01 7.3 9.3 83.2 0.23 0.01SU-UT22 Upper Triassic 93-P-4 Pard/Bald 74 –1528.7 –32.8 –37.3 –39.8 –4.6 0.01 7.5 7.5 84.7 0.24 0.01SU-UT23 Upper Triassic 93-P-4 Pard/Bald 69 –1214.6 –31.3 –35.4 –35.2 –7.8 0.01 7.2 10.0 82.6 0.20 <0.01SU-UT24 Upper Triassic 93-P-5 Pard/Bald 93 –2068.9 –31.2 –35.8 –36.7 –3.5 0.01 15.5 25.6 58.9 0.06 <0.01SU-UT25 Upper Triassic 93-P-5 Pard/Bald 97 –2074.7 –30.4 –34.2 0.01 15.7 25.4 58.9 0.06 <0.01SU-UT26 Upper Triassic 93-P-5 Pard/Bald 111 –2168.2 –31.3 –39.6 0.01 15.6 28.9 55.4 0.06 <0.01SU-UT27 Upper Triassic 93-P-5 Pard/Bald 79 –1411.3 –32.8 –38.8 –35.2 –6.2 0.01 6.8 8.3 84.5 0.28 0.01SU-UT28 Upper Triassic 93-P-5 Pard/Bald 100 –2351.7 –30.0 –33.8 –36.4 0.01 10.7 9.1 80.1 0.15 <0.01SU-UT29 Upper Triassic 93-P-5 Pard/Bald 97 –2306.6 –31.1 –35.1 –27.0 –2.2 0.01 15.9 24.3 59.7 0.08 <0.01SU-UT30 Upper Triassic 93-P-5 Pard/Bald 96 –2213.7 –30.9 –35.5 –37.4 –3.5 0.01 14.0 13.2 72.6 0.12 <0.01

SU-MT40a-isot Middle Triassic 93-O-8 Halfway nd –2156.3 –32.7 –38.4 –28.0SU-MT40b Middle Triassic 93-O-8 Halfway nd –2156.3 –31.8 –36.0 –40.8SU-MT41-isot Middle Triassic 93-P-5 Halfway nd –2971.4 –32.0 –31.4

SU-P100 Permian 93-P-4 Belcourt 106 –2882.0 –27.5 –31.2 –32.0 –8.4 0.04 4.2 3.1 92.5 0.20 <0.01SU-P101 Permian 93-P-4 Belcourt 111 –2882.0 –25.6 –30.8 –34.9 0.7SU-P102 Permian 93-P-5 Belcourt 122 –3464.6 –27.9 –26.3 –0.8 0.07 12.3 6.4 80.9 0.04SU-P103a Permian 93-P-5 Belcourt 114 –3090.4 –27.1 –32.7 –30.7 3.8 0.03 5.9 3.1 90.7 0.18 <0.01SU-P104a Permian 93-P-5 Belcourt 125 –3440.5 –27.6 –34.7 –34.6 –2.3 0.04 11.0 6.8 82.0 0.11 <0.01SU-P104b Permian 93-P-5 Belcourt 125 –3440.5 –25.7 –33.8 –34.8 0.04 10.8 6.6 82.5 0.12 <0.01SU-P104c Permian 93-P-5 Belcourt 125 –3440.5 –25.4 –32.7 –1.8SU-P106 Permian 93-P-5 Belcourt 132 –3807.4 –24.4 –29.5 –35.6 0.5 0.03 11.6 6.0 82.3 0.09 <0.01

1408Gas

IsotopeReversals

inFractured

GasReservoirs

SU-P107 Permian 93-P-5 Belcourt nd –26.8 –33.3 –1.9 0.04 10.7 6.2 82.9 0.11 <0.01SU-P108a Permian 93-P-5 U. Belcourt 115 –3030.0 –27.3 –36.1 –36.3 0.03 9.0 5.5 85.4 0.14 <0.01SU-P108b Permian 93-P-5 L. Belcourt nd –3298.7 –27.0 –34.9 –35.7SU-P109 Permian 93-P-5 Belcourt 112 –2950.8 –27.2 –34.8 –24.4 –3.2 0.03 9.0 5.7 85.2 0.14 <0.01SU-P110 Permian 93-I-14 Belcourt 98 –2265.0 –30.0 –29.0 –26.9

Alberta Outer FoothillsAB-UT200 Upper Triassic 56-4W6 Charlie Lk 101 –1992.1 –36.1 –24.5 –23.0 –13.9 0.01 4.5 6.4 88.2 0.51 0.04AB-UT201 Upper Triassic 56-4W6 Charlie Lk nd –2386.0 –35.0 –26.0 –23.3 –8.8 0.01 5.3 5.4 88.8 0.42 0.03AB-UT202 Upper Triassic 56-4W6 Charlie Lk 106 –2394.2 –35.7 –28.9 –28.0 –17.7 0.02 4.1 8.2 87.5 0.19 0.01AB-UT203 Upper Triassic 56-5W6 Charlie Lk nd –1977.4 –35.3 –38.2 0.02 4.3 5.7 89.8 0.14 <0.01AB-UT204 Upper Triassic 56-5W6 Charlie Lk nd –1912.5 –36.1 –38.8 –37.2 –15.8 0.02 2.9 5.5 91.1 0.16 0.16AB-P205a Mississippian 56-5W6 Debolt nd –2146.8 –36.5 –38.6 –35.2 –12.1 0.02 2.9 6.0 90.8 0.18 <0.01AB-UD205b Upper Devonian 56-5W6 Leduc 136 –4094.8 –30.8 –25.8 –13.5 0.06 4.6 16.2 79.1 0.02 <0.01AB-UT206a Upper Triassic 56-6W6 Charlie Lk 89 –2201.5 –37.2 –38.9 –37.8 –10.7AB-UT206b Upper Triassic 56-6W6 Charlie Lk nd –2201.5 –41.7 –41.1 –26.1 –14.7AB-UT207 Upper Triassic 57-6W6 Charlie Lk 112 –2591.6 –36.9 –35.9 –39.1 0.01 6.4 4.9 88.6 0.14 <0.01

AB-MT220 Middle Triassic 60-11W6 Halfway 70 –1238.6 –38.4 –42.4 –42.7 –22.0 0.02 2.4 5.2 92.2 0.11 <0.01AB-UT221 Upper Triassic 60-11W6 Charlie Lk 76 –1030.8 –41.2 –41.5 –37.1 –18.7 0.01 4.8 1.2 93.9 0.11 <0.01AB-UT222a Upper Triassic 60-11W6 Charlie Lk 75 –1216.6 –36.2 –38.6 –10.9 0.01 5.0 1.0 93.8 0.12 <0.01AB-UT222b Upper Triassic 60-11W6 Charlie Lk 75 –216.6 –35.9 –38.7 –11.7AB-UT223 Upper Triassic 61-11W6 Charlie Lk nd –2136.0 –33.7 –35.8 –29.9 –5.2 0.01 5.7 0.5 93.7 0.11 <0.01AB-UT224 Upper Triassic 61-12W6 Charlie Lk nd –1090.6 –34.4 –39.9 –37.8 –12.4 0.03 4.7 2.3 92.9 0.12 <0.01AB-MT225 Middle Triassic 61-12W6 Halfway nd –1139.6 –35.6 –43.2 –46.5 –17.7 0.13 5.0 <0.01 94.8 0.05 <0.01AB-UT226 Upper Triassic 61-12W6 Charlie Lk 68 –827.6 –35.2 –40.0 –41.7 –12.6 0.04 4.9 3.7 91.3 0.11 <0.01AB-UT227a Upper Triassic 61-12W6 Charlie Lk ~79 –936.6 –35.6 –42.8 –44.0 –11.1 0.33 0.4 0.4 98.7 0.09 <0.01AB-MT227b Middle Triassic 61-12W6 Halfway ~80 –1139.6 –35.3 –43.2 –31.6 –14.9 0.05 3.8 4.0 92.1 0.07 <0.01AB-MT227c Middle Triassic 61-12W6 Doig ~81 –1250.6 –37.0 –44.0 –39.0 –13.8 0.04 5.6 2.8 91.6 0.06 <0.01AB-UT228 Upper Triassic 61-13W6 Charlie Lk 70 –690.5 –35.4 –41.8 –42.0 –11.9 0.04 3.4 10.1 86.4 0.10 <0.01AB-UT229 Upper Triassic 61-13W6 Charlie Lk 69 –995.2 –35.2 –39.3 –38.3 –8.6 0.03 5.4 0.8 93.7 0.11 <0.01AB-UT230 Upper Triassic 61-13W6 Charlie Lk 65 –847.4 –33.8 –42.2 –40.1 –5.2 0.02 4.5 0.5 94.8 0.15 <0.01AB-UT231 Upper Triassic 61-13W6 Charlie Lk 68 –1079.3 –34.8 –32.7 –38.5 –9.6 0.02 5.5 0.8 93.6 0.14 <0.01AB-UT232 Upper Triassic 62-11W6 Charlie Lk –2462.1 –41.6 –36.0 –33.8 –6.8 0.01 3.8 0.03 96.0 0.20 <0.01AB-UT233 Upper Triassic 62-13W6 Charlie Lk 70 –892.4 –38.2 –42.4 –15.0 0.02 4.3 0.9 94.6 0.15 <0.01AB-P234 Permian 63-11W6 Taylor Flat –3099.4 –32.7 –32.4 –28.2 –27.6 0.03 6.0 14.1 79.8 0.10 <0.01

*Temperatures (T) are present-day formation temperatures, chemical compositions are volume percent, and isotope ratios are reported in per mil with respect to the Peedee belemnite standard.**British Columbia Geographic System of Mapping.

Tilleyetal.

1409

are discussed below and illustrated in Figure 7 byred lines:

1. Zone d1 (700–1600 m [2297–5249 ft]): TheAlberta Triassic gases are drier (higher ln(C1/C2)than the Sukunka Triassic gases and show a gen-eral trend toward increasing dryness with depththat is not shown by the Sukunka Triassic gases.The d13C methane in the Triassic Sukunka andAlberta gases is constant with depth, but thed13C methane in the Alberta Triassic gases ismore negative than the d13C methane in theSukunka Triassic gases. The degree of scatter inethane and propane isotope ratios increases withdepth in this depth range, as some values becomeless negative.

2. Zone d2 (1900–2600 m [6234–8530 ft]): TheAlberta Triassic gases that are not isotopicallyreversed show a trend toward increasing drynesswith depth in this zone, random isotope ratios formethane, and interesting trends with depth to-ward more negative isotope ratios for ethaneand propane.When comparedwith the reversedgases, the nonreversed gases are generally morenegative formethane and less negative for ethaneand propane. The Sukunka Triassic reversed gasesin this depth range become wetter with depth,and d13C methane tends toward slightly less neg-ative values. The d13C ethane in the Sukunka

1410 Gas Isotope Reversals in Fractured Gas Reservoirs

Figure 6.Mud gas isotope depth profile from the Ojay field thatlies southeast of the Sukunka-Bullmoose areas and northwest ofthe Narraway area (Figure 1). The most negative d13C value forethane throughout the profile is –33‰.

Figure 5. A d13C methane ver-sus d13C ethane crossplot of WCSBfoothills gases. Different methaneversus ethane isotope ratios forgases in the Triassic and Permianreservoirs at the Sukunka area,and the Triassic reservoirs at theNarraway and Minnow areasindicate unconnected gas systemswith increasing maturity towardthe upper right of the diagram.Reversed gases are to the lowerright of the d13C1 = d13C2 line.Different colors of shadingindicate the different locationof gases.

Triassic gases is constant with depth, whereasthe d13C propane is constant in this depth rangebut more negative than many of the SukunkaTriassic gases in zone d1.

3. Zone d3 (2900–3800 m [9514–12,467 ft]): TheSukunka Permian gases occur in depth zone d3.The reversed Permian gases show good trends ofincreasing dryness and less negative d13Cmethanewith increasing depth. The d13C ethane is onlyless negative than the Triassic Sukunka gases ofzone d2 in two of the samples in this zone, thetop and bottom samples.Otherwise, d13C ethanedoes not change significantly with depth fromzone d2 to zone d3. The d13C propane is onlyvery slightly less negative in zone d3 than inzone d2, excepting a few local samples with lessnegative values. Nonisotopically reversed gasesin zone d3 are drier, have more negative d13Cmethane, and more positive d13C ethane andpropane.

4. Zone d4 (4100m [13,451 ft]): The oneDevoniansample is not isotopically reversed. It is drier thanall other gases. The d13C methane is more neg-ative and the d13C ethane is less negative thanthe Sukunka Permian gases in zone d3.

CO2 and H2S Gases and the Effect of ThermochemicalSulfate ReductionVariations in H2S and CO2 contents of the gasesare listed in Table 2 and shown as a function of

depth in Figure 7. The H2S values range up to 29vol. % of the total gas, which raises the question ofthe origin of the high H2S. Possible sources of H2Sin deeply buried reservoirs are thermal decom-position of organosulfur compounds in kerogenand oil (Tissot and Welte, 1984; Hunt, 1996) andthermochemical sulfate reduction (TSR) (Orr,1974, 1977; Machel, 1987, 2001; Worden andSmalley, 1996; Cai et al., 2003, 2004). As thermaldecomposition of organosulfur compounds nor-mally only contributes less than 5% H2S to the gas(Orr, 1974, 1977; Worden and Smalley, 1996),most of the H2S in these deeply buried gas res-ervoirs must be a result of TSR (Orr, 1974, 1977;Worden and Smalley, 1996; Mougin et al., 2007).Thermochemical sulfate reduction requires tem-peratures greater than 100°C (212°F), the presenceof reactive organic material and sulfate in the hostrocks (Noth, 1997), and results in the creation ofH2S + CO2 + pyrobitumen. In our study area, thetemperature requirement was met at least at thetime of maximum burial, reactive organic materialobviously exists, and sulfate is present in the formof anhydrite in the Triassic Charlie Lake Forma-tion (Figure 3), which underlies the main Triassicreservoir in the Sukunka area. It is likely that an-hydrite was also present within carbonate layers ofthe reservoirs. Therefore, assuming that TSR is thedominant source of H2S, we will examine our datafor effects of TSR and, of particular interest here,

Figure 7. Plots ofln(C1/C2), d

13C1, d13C2,

d13C3, d13C CO2, vol. %

CO2, and vol. % H2Sversus subsea depth forWCSB foothills gases.Depths in brackets aremaximum burial subseadepths. Zones d1, d2, d3,and d4 are depth zonesdescribed in the text.Red solid and dashed lineshighlight approximatedepth trends in the data.

T

illey et al. 1411

the possible effect of TSR on the isotopic ratios ofmethane, ethane, and propane that could con-tribute to the isotopic reversals.

One might expect to see an increasing abun-dance of H2S as the depth, temperature, and corre-sponding rate of TSR reaction increase (Orr, 1974,1977; Machel et al., 1995). However, Figure 7shows that the very H2S and CO2 rich gases areonly present in the upper depth zones d1 and d2.In the Sukunka Triassic gases, where H2S and CO2

are most abundant (blues crosses in zones d1 andd2 of Figure 7), no trend of increasing H2S andCO2 with depth exists, and gases with both lowand high contents of H2S and CO2 occur at thesame depths. For example, sample SU-UT16 has asubsea depth of –1094.7 m (–3892 ft) and H2S =29.5 vol. %, CO2 = 14 vol. %, whereas sample SU-UT20 has a subsea depth of –1080.1 m (–3544 ft)and H2S = 8.8 vol. %, CO2 = 7.2 vol. % (Table 2).This suggests that the supply of H2S and CO2 islocally controlled in the Triassic. Various other re-searchers have also concluded that regional varia-tions in H2S were related to the sedimentary faciesof the reservoir rocks (Jiang et al., 2002) and thepresence and thickness of anhydrite-bearing evap-oritic rocks interbedded or intercalated with thereservoirs (Cai et al., 2003, 2004; Li et al., 2005).

Carbon dioxide derived from decompositionof organic matter during TSR reactions shouldhave a light d13C CO2 signature (<–10‰), whereasCO2 derived by dissolution of marine carbonatesshould be 0 ± 4‰ (Arthur et al., 1983). In zone d3,where H2S contents are relatively low, CO2 abun-dance is relatively high and increases with depth.The d13CCO2 values are within the range expectedfor dissolution of carbonates, suggesting that theincrease in CO2 abundance with depth may re-flect increasing decarbonation of carbonates withincreasing temperature.

Following Worden et al. (1995) and Hao et al.(2008), in Figure 8, we use a gas souring index,GSI = H2S/( H2S + C1 + C2)%, as an indicator ofthe extent of TSR and plot it against various chem-ical and isotopic parameters: ln(C1/C2), vol. %CO2, and the isotope ratios of carbon dioxide,methane, ethane, and propane. If the hydrocarbonpart of the gases in our study has been involved in

1412 Gas Isotope Reversals in Fractured Gas Reservoirs

TSR reactions, we should see certain trends inthese diagrams.

1. As TSR progresses, the chemical composition ofthe gases should change (Worden et al., 1995),with the wetter gases being attacked first(Mankiewicz et al., 2009), such that ln(C1/C2)increases. The ln(C1/C2) plot (Figure 8A)shows increasing ln(C1/C2) at low levels of GSI(as much as about GSI = 8 for the SukunkaPermian reversed gases and GSI = 15 to 17 forthe Sukunka Triassic reversed gases). AboveGSI = 17, ln(C1/C2) is essentially constant asGSI increases. These relationships suggest thatit is possible that TSR may have influenced thechemical composition of the hydrocarbon gases atrelatively low levels of TSR. However, at higherlevels of TSR, TSR reactions had no influenceon the chemical composition of the hydrocarbongases.

2. As TSR increases, the abundance of CO2 shouldalso increase. Figure 8B shows that the abun-dance of CO2 increases with increasing GSI forboth the Sukunka Permian and Triassic gases,but their specific trends are different. Figure 7shows a general increase of CO2, with depth forthe gases with lower abundances of CO2. How-ever, the higher CO2 contents in the SukunkaPermian gases of zone d3 (Figure 7) are asso-ciated with lower H2S contents than the lowCO2 Sukunka Triassic gases in zone d1. Thissuggests that the greater abundance of CO2 inthe Sukunka Permian gases may be more a resultof thermal decomposition of carbonates than ofTSR. The d13C CO2 values for the SukunkaPermian gases (Figures 7, 8C) are also consistentwith buffering by carbonates.

3. If the source of carbon in CO2 is the oxidationof organic material, this should be reflected inthe d13CCO2 values trending toward that of theorganic material (i.e., approaching d13C < –20)as TSR increases. Figure 8C shows a general trendof decreasing d13C CO2 values (–7 to –28‰)with increasing GSI for both reversed and non-reversed Alberta gases, as might be expected forCO2 created by TSR reactions. In contrast, d13CCO2 values that are less negative (–8 to +4‰)

for the Sukunka Triassic and Permian reversedgases, and which show no correlation to GSI,appear to be carbonate buffered.

4. As a given gas component is destroyed by TSR,the residue becomes enriched in 13C becauseTSR favors the removal of 12C (Rooney, 1995).The result is that as TSR progresses, the d13Cvalues of gases become increasingly less nega-tive. Mankiewicz et al. (2009) report d13C val-ues for ethane affected by TSR that range from

–26 to –10‰, but found no TSR effect on theisotope ratios of methane (d13C = –39 to –36‰).On the d13C1 versus GSI plot in Figure 8D, thegases from Alberta that are not isotopically re-versed show the most obvious trend of in-creasing d13C1 with increasing GSI, suggestingthat the methane in these gases has been in-volved in TSR reactions. In contrast, the con-stant d13C1 for Sukunka Triassic gases, even asthe GSI increases to high levels, suggests that

Figure 8. Crossplots ofgas souring index (GSI)versus (A) ln(C1/C2),(B) CO2 vol. %, (C) d

13CCO2, (D) d

13C methane,(E) d13ethane, and (F)d13 propane for theWCSB foothills gases.See Figure 7 for a legendof the symbols. Linesindicate approximatetrends of data.

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this methane was not involved in TSR reac-tions. Similar relationships of d13C2 with GSIin Figure 8E suggest that although ethane mayhave been involved in TSR reactions in the Al-berta nonreversed gases, ethane in the reversedand very H2S rich Sukunka Triassic gases wasnot involved in TSR reactions. The very negativenature of the d13C2 values in the isotopicallyreversed gases also suggests that ethane was notinvolved in TSR reactions. The random variationof 13C3 at high levels of GSI (Figure 8F) suggeststhat propane was also not involved in TSR re-actions at high levels of H2S.

In general, the compositional and isotopic datafor the high H2S gases (GSI > 17) suggest thatmethane, ethane, and propane have not been iso-topically altered by TSR and that the H2S is notgenetically related to the associated methane,ethane, and propane. Only the 13C-enrichedmethane in the Sukunka Permian gases (d13C1 =–27.9 to –24.4‰) relative to the nonreversedDevonian gas in Alberta (d13C1 = –30.8‰, d13C2 =–25.8‰) suggests that some process caused thevery high 13C enrichment in the Sukunka Permianmethane. The available data do not allow a cleardistinction of whether this process involved de-struction of methane by TSR or methane creationat a very high level of maturity. If the SukunkaPermian methane has been involved in TSR re-actions, then this methane cannot be geneticallyassociated with the ethane and propane that areunaltered by TSR processes. Although we cannotunequivocally determine the role of TSR reactionsin the 13C-enrichment in Sukunka Permian meth-ane,muchmore negative d13Cmethane values (–38to –36‰) are unlikely the result of TSR reactions,and these values are also associated with isotopicallyreversed gases in our data set. Just as important,TSR reactions could not have created the unusuallynegative d13C ethane values. Isotopic reversals arepresent irrespective of the H2S abundance andirrespective of the d13C methane value. There-fore, although TSR reactions may have been in-volved in some gases at low levels of H2S, theyare not the fundamental factor responsible for theisotope reversals.

1414 Gas Isotope Reversals in Fractured Gas Reservoirs

Spatial Distribution of Isotope Data andRelationship of Reversals to Gas Production

Figures 9 and 10 are a series of maps that show thegeographic distribution of the data. In the SukunkaTriassic of Figure 9A, little variation exists in d13Cmethane (left map), whereas a wide range is pre-sent in d13C ethane (right map). All of these gasesare at least partially isotopically reversed with d13Cmethane > d13C ethane. The most negative d13Cethane occurs in the southeast corner of the area.Otherwise, with few exceptions, a general trendof less negative ethane values moving across theBullmoose-Sukunka area from the southwest to thenortheast occurs. The size of the symbol in Figure 9indicates the relative volume of gas produced fromthat well (based on the average monthly productionfor the first 12 months of full production). For theTriassic, the best producers are those associated withthe isotopically reversed gases with exceptionallylight d13C ethane in the southeast corner of the field.

More variation is found in d13Cmethane (–27.5to –24.4‰) for the Sukunka Permian (Figure 9B)than for the Sukunka Triassic, but no correlation ofmethane values with production performance isshown. However, in one well in the eastern part ofthe field, where the d13C ethane is heavy (–26.3‰),production performance is poor. This particularwell was no longer under production on conclu-sion of this study and, unlike the other Permianwells, had a gas-water contact. Although the poorproduction may be a structural issue (Figure 11),the correlation of poor production with the heavyd13C ethane value is interesting.

Panels A and B of Figure 10 are maps showingthe distribution of isotope ratios for methane (leftmap) and ethane (right map) for the Narraway andMinnow areas, respectively. Available productiondata were insufficient for comparison of produc-tion history and isotopic reversals in these areas. Inboth areas, the isotopically reversed gases are to thewest, and normal gases are to the east of the fields.At the Narraway area (Figure 10A), light d13Cmethane (–41.6‰) and no reversal suggest a moreimmature gas in the Charlie Lake Formation to thenortheast of the area. In the eastern part of theMinnow area (Figure 10B), d13Cmethane is similar

to the western part of the area, but d13C ethane issignificantly heavier, more within the range ex-pected for thermogenic gases. Note that the gas inthe Devonian is an isotopically normal mature gas(13C-enriched methane and ethane), whereas theMississippian and Triassic gases at the same loca-tion are apparently less mature (lighter d13Cmethane) and reversed.

Relationship between Tectonic Structure andIsotopic Reversals

Detailed structural cross sections that are based onseismic and dip meter data are presented in Figure11 fromwells across the Sukunka-Bullmoose trend(Figure 11A, B), the Narraway area (Figure 11C),and the Minnow area (Figure 11D). The d13C

Figure 9. Pool outlinesand spatial distribution ofisotope data (‰) in theSukunka field for (A) theTriassic gas pools and(B) the Permian gas pools.White triangles in panel Aare middle Triassic gaspools, blue triangles areupper Triassic pools. Thesize of the well symbolindicates the relative vol-ume of present-day pro-duction. AA′ and BB′ arelines of structural crosssections shown in Figure 11.

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methane, d13C ethane, and d13C propane values(where available) are shown above thewell in whichthey occur or as projected onto the cross section.Along the Sukunka-Bullmoose trend (Figures 11A,B), isotopically reversed gases are associated withthrust-faulted folds. In Figure 11B, in the Permian,where the gas is from the crest of a nonfaulted fold,the gas is not reversed (d13C1 = –27.9‰, d13C2 =–26.3‰). This is the same well from Figure 9Bthat was not being produced during the conclusionof the study. Therefore, at the Sukunka-Bullmoosearea, there seems to be an association of isotopereversal, thrust-faulted fold, and good productionperformance.

At the Narraway area, the magnitudes of theisotope reversals are related to the complexity of thestructure (Figure 11C). In thewest,multiply stackedthrust sheets (d13C1 minus d13C2 = 5) are found,

1416 Gas Isotope Reversals in Fractured Gas Reservoirs

whereas eastward, nomultiply stacked sheets (d13C1

minus d13C2 = 2) are present. Farther eastward(Figure 11C), where only a single low-displacementthrust fault surrounded by undisturbed layers exists,an entirely normal thermogenic gas occurs. Contraryto the Sukunka-Bullmoose area, the maturity of themethane appears to correlate with the level of thethrust sheet. For example, d13C1 = –35.2‰ is froma higher thrust sheet than themoremature d13C1 =–33.7‰. However, when palinspatically restored,this maturity difference appears to simply reflectthe duration of burial before thrusting, as sug-gested by Kalkreuth and McMechan (1988).

At theMinnow area (Figure 11D), to the west,several stacked and folded thrust sheets are pre-sent. Along this northwest-southeast structuraltrend (projecting out of the page on Figure 11D),the Triassic and Mississippian gases show isotopic

Figure 10. Spatial dis-tribution of isotope data(‰) (A) in the Narrawayfield and (B) in the Min-now field. Black numbersare isotope ratios for Tri-assic gases; blue numbers-Mississippian gases; andgreen numbers-Devoniangases. The green linesmark the location of thestructural cross sectionsin Figure 11.

Figure 11. Seismic and dip meter-constrained structural cross sections showing carbon isotope ratios (‰) (methane, ethane, propane)of production gases. Production gas depth is given above the isotope data for each well.

Tilley et al. 1417

reversals. To the east of the Minnow area, d13Cmethane is the same as to the west, but d13C ethaneis heavier, resulting in a normal gas. Whatevermechanism caused the lighter d13C ethane values tothe west, it did not occur along the structural trendto the east.

ORIGIN OF THE ISOTOPE REVERSAL

To determine the origin of the isotopically reversedgases in our study area, it is necessary to determinethemechanismbywhich a verymature drymethanegas could be accompanied by a small amount ofapparently immature ethane. The traditional solu-tion is to add deeper mature gas to shallower, moreimmature gas. In our study, the deep mature gas isthe bulk of the gas, and we would need to add asmall amount of shallower,more immature gaswith12C-enriched ethane (d13C < –40‰). However,as shown by the mud gas isotope log in Figure 6and a gas seep to the surface (Figure 9A), no ap-parent local source of shallower, more immature,12C-enriched ethane gas exists.

The large amount of H2S present in the some ofthe gases raises the obvious question as to whetherthe H2S is related to, or responsible for, the isotopicreversal. However, as shown in Figures 2, 7, and 8and the discussion in previous text, no apparentrelationship exists between the isotopic reversal andthe abundance of H2S. In addition, gases asso-ciated with TSR generally tend to have abnormallyheavy ethane and propane (Krouse et al., 1988;Mankiewicz et al., 2009), instead of the isotopicallylight ethane and propane that we see in the reversedgases. If the heavy d13C methane in the Permiangases was involved in TSR reactions, the associatedlight d13C ethane was not. This would indicatethat in the Permian gases, ethane was added to themethane-H2S-CO2 gas only after cessation of TSRreactions. In the other gases of our data set, whered13Cmethane is more negative and no reason existsto believe that methane was involved in TSR reac-tions,methane, ethane, and propaneweremost likelycogenetic, and the H2S-CO2 gas is an admixture.

The simplest hypothesis to explain the isotopicreversals is that the hydrocarbon gas was generated

1418 Gas Isotope Reversals in Fractured Gas Reservoirs

asmature shale gases (Ro > 2.2%) in closed systems,only released to and trapped in surrounding mixedclastic-carbonate rocks as a result of tectonics duringthe Laramide orogeny. The nonhydrocarbon gases(H2S and CO2) are the result of local TSR reac-tions in anhydrite-containing interlayers or under-layers and are not genetically related to the isotopi-cally reversed shale gases. These nonhydrocarbongases were admixed to the released shale gasesduring the tectonic event and stored in the reservoir.

Thermal destruction of kerogen or wet gascomponents ultimately results in a 13C-enrichedmethane-rich gas (heavy d13C methane). Thebreakdown of oil gives rise to 13C-depleted ethane(light d13C ethane) (Hill et al., 2003; Y. Tang andM. Schoell, 2005, personal communication). In aclosed shale system, at the observed Ro values of2.2% or greater, this breakdown cascade includesnot only kerogen and gas, but also oil. Thus, 12C-enriched ethane (lighter d13C ethane) from oilcracking will be generated along with the 13C-enriched methane (heavier d13C methane). Theseideas are consistent with the closed-system pyrol-ysis experiments of Hill et al. (2003) and Jin et al.(2010). Hill et al. (2003) found that with increas-ing thermal stress in confined dry pyrolysis of oil,ethane became progressively enriched in 12C overthe Ro range of about 1 to 1.7%. Their d13C eth-ane values ranged from about –38 to –40‰ forcracking of whole oil with d13C = –29.5‰. Theselight d13C ethane values are interestingly similarto our light d13C ethane values. Isotopic reversalswere not observed in either the Hill et al. (2003)or Jin et al. (2010) experiments. However, it maybe inappropriate to directly compare the results oflaboratory pyrolysis experiments with our predic-tion of isotopic reversals in closed-system shales.The experiments may not have successfully mim-icked natural maturation in massive shale units.Regardless, isotopically reversed gases are commonin thermallymature shale gas plays (Ferworn et al.,2008; Zumberge et al., 2009; Talisman EnergyInc., 2010, personal communication). This meansthat mixing from multiple source rocks is notnecessary to create the isotopic reversals.

Our simple hypothesis becomesmore complexwhen we try to incorporate all our data. Isotopic

analyses of the organic matter from source rockswas outside the scope of this study, but the dif-ference in the isotope ratios for the Triassic and thePermian at Sukunka suggests different source rocksfor these reservoirs. Their source rocks were mostlikely the Jurassic Gordondale Formation of theFernie Group and the Triassic Pardonet Formationfor the Triassic, and maybe the Triassic MontneyandDevonian Banff and/or Exshaw for the Permian.The Gordondale with its type IIS kerogen (high-sulfur kerogen) (Table 2) may have been a minorcontributing source of H2S in the Triassic gases. Thelower maturity of the Triassic gas, relative to thePermian gas, may simply reflect the shallower depthof its source rock and implies that isotopic reversalscan occur at a range of high maturity. The similarityof isotope ratios for the Triassic, Permian, and Mis-sissippian in the Narraway and Minnow fields sug-gests that these reservoirs were connected at least atthe time of filling.

We do not yet fully understand the mecha-nisms that control the ethane d13C values of gasesin mature shales. However, in their experiments,Jin et al. (2010) found that ethane and propaneisotope ratios were very sensitive to temperaturevariation. This suggests that ethane isotope ratiosmay be a sensitive indicator of the progress oforganic maturation in shales. We suggest that ingases released at a late stage from shales, the ob-served local variations in ethane isotope ratios could(1) indicate local variations in the thermal historyof the shale before the gas was released (i.e., beforethe tectonic event) or (2) indicate local differ-ences in timing of the tectonic event and releaseof gas from the shale, such that the gas is releasedat different stages of maturity. With further re-search involving the isotopic systematics of shalegas and a better understanding of how ethane iso-tope ratios change in maturing shales, it may bepossible to interpret the observed geographic vari-ations in ethane isotope ratios in terms of localdifferences in thermal history and timing of thetectonic event.

Jin et al (2010) found that partly accumulatedgas from petroleum cracking is 13C enriched rel-ative to cumulative gas. The 13C-enriched ethaneat the Sukunka area (–26.3‰; Figure 9B) and in the

east of theMinnow area (–28.9, –26.0, and –24.5‰)may be the result of loss of the early gas from theshale source (i.e., a somewhat more open shalesystem). This might explain why the –26.3‰ eth-ane well at the Sukunka area is not a good gas pro-ducer. The normal thermogenic gas at the Narrawayarea (d13C1 = –41.6‰; d13C2 = –36‰) is less ma-ture than the reversed gases (d13C1 is more nega-tive) andmay be a conventional gas released earlierfrom an open-system shale.

Finally, the fact that the Devonian gas at theMinnow area is very mature (13C-enriched meth-ane, d13C1 = –30.8‰) but not reversed, whereasthe overlying Mississippian and Triassic gases areapparently less mature (d13C1 = –36.5‰) butreversed, supports our theory that reversed gasesare created in a closed shale system. The Devonianat this location is part of a wide-scale permeablecarbonate reef system with an extensive regionalplatform development that almost certainly wouldhave created a pathway for ongoing escape of gasesfrom its source shale (i.e., an open system).

Along the Sukunka-Bullmoose trend, the bestreservoirs are in thrust-faulted folds, in contrast todetachment folds. Our shale gas–based modelwould suggest that the mechanical breakup of therigid clastic carbonates did not necessarily lead to acomplete loss of gas but facilitated the accom-modation and trapping of additional gas. The de-formation of the shale, in addition to helping toexpel the gas into the newly fractured reservoir,also provided a seal.

IMPLICATION

Based on our interpretation of isotopic reversals infractured reservoirs in the foothills of the WCSB,we suggest that isotopic reversals are restricted toclosed-system maturation of kerogen and residualoil in shales. This would mean that isotopic re-versals only occur in mature shales where little orno gas has been lost during its maturation history.The implication is that more gas should be re-tained in a closed-system shale than in the sameshale that matured inmore open conditions, wheregas was lost, and that these closed-system shales

Tilley et al. 1419

can be identified by gas isotope reversals. Follow-ing this logic, it is then possible that the magnitudeof the reversal might be related to the relative vol-ume of gas retained in shales. To test this hypoth-esis, the production statistics of wells in shales shouldbe compared with the magnitude of isotopic re-versal as was done at the Sukunka area. If such astudy shows a consistent correlation, then the fol-lowing workflow would be possible: (1) check forisotope reversal; (2) if present, assume a closed sys-tem; (3) measure magnitude of reversal; (4) basedon existing data, use the magnitude of the reversalto anticipate the volume of gas (relative to volumescontained in the nonshale reservoirs) that could beproduced from targeting the source directly.

CONCLUSIONS

Isotopically reversed gases that are associated withprolific gas producers along the tectonically dis-turbed western edge of the WCSB are interpretedas mature (Ro > 2.2%) shale gases in disguise. Al-though they now occur at or near the crests offractured folds of mixed carbonate-clastic rocks,their chemical and isotopic compositions were es-tablished while in the source shales. The presenceof the reversed gases in the fractured reservoirsis caused by the unique combination of (1) highmaturity of the shale source rocks; (2) tight non-permeable rocks surrounding the source shale (closedsystem); (3) intense tectonic disturbance after thesource rocks reached advanced stages of matura-tion, that is, after oil and gas cracking had beeninitiated; and (4) deformation of the shale sourcerocks to both squeeze out the gas into surroundingnewly fractured brittle rock, as well as to provide aseal. The nonhydrocarbon gases (H2S and CO2)are the result of local TSR reactions in anhydrite-containing interlayers or underlayers and are notgenetically related to the isotopically reversed shalegases. These nonhydrocarbon gases were admixedto the released shale gases during the tectonic eventand stored in the reservoir.

Much remains to be understood about gas gen-eration and the resulting isotopic ratios in thermallymature shales. This work applies the concept of

1420 Gas Isotope Reversals in Fractured Gas Reservoirs

closed-system generation of gas by maturation ofkerogen combined with cracking of retained oil inmature shales to explain the observed isotopic re-versals in fractured reservoirs, in light of existingfoothills structure, source rock, tectonic history,and reservoir rock–type data. Variations in isotoperatios across the study area and with depth havebeen interpreted here in terms of the degree ofopenness of the source shale system during matu-ration. Where the system is partially open and gashas escaped from the shale at some stage during itsmaturation, d13C ratios for ethane are less negativeand the isotopic reversals are smaller in magnitudeor totally absent. An implication of this study is thatcorrelation of the production statistics of sourceshales to the magnitude of isotopic reversals couldpotentially provide an indication of the relativevolume of stored gas and the longevity of a ther-mally mature shale gas play.

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