geochemistry and origin of some natural gases in the ...hydrogen isotope geochemistry of some...

ABSTRACT

We sampled gases from 22 wells and 5 cores inwestern Pennsylvania and eastern Ohio to evaluatethe possible use of stable isotope geochemistry forinterpreting the origins of natural gases in thePlateau province of the central Appalachians andfor correlating these gases with their probablesources. The isotope data suggest that several ofthese gases have multiple sources or were alteredby geological or biogeochemical processes.

Gases produced from Upper Cambrian Rose Runsandstones in Portage County, Ohio, are condensate-associated hydrocarbons. These gases can be corre-lated to local Ordovician source rocks. Gases pro-duced from the Upper Cambrian BeekmantownDolomite in Coshocton County, Ohio, also are ther-mogenic, but the maturity of these hydrocarbons isgreater than that observed in the local Ordoviciansource rocks. The Beekmantown hydrocarbonsprobably migrated from deeper in the Appalachianbasin along the Knox unconformity.

Lower Silurian Medina Group gases in north-western Pennsylvania are late mature and probablyoriginated in Ordovician source rocks. MedinaGroup gases might be mixtures of thermogenic

hydrocarbons, or their geochemistry may indicatereservoir leakage due to gas diffusion through caprock. Gases produced from structural traps in theDevonian Ridgeley Sandstone and HuntersvilleChert of western Pennsylvania are isotopicallydiverse and reflect both the entrapment of matureassociated gases and probable second-order frac-tionation effects.

Autogenic gases produced from Devonian blackshales are postmature along the Allegheny Front andearly mature along the northwest basin flank. Theearly-mature gas is autochthonous and thermogenic.

Most Upper Devonian gases produced across thePlateau are oil-associated gases that were emplacedin reservoirs prior to maximum burial of thePaleozoic section during the Alleghanian orogeny.Some Upper Devonian gases, however, could beresidues of diffusive gas leakage or possible mix-tures of thermogenic gases that migrated into orfrom reservoirs located along regional fractures.

Lower Silurian Tuscarora Formation gases pro-duced from fractured sandstones near theAllegheny structural front in central Pennsylvaniaare postmature hydrocarbons that correlate withlocal source rocks. Gases produced from fracturedOrdovician Bald Eagle Formation rocks near theAllegheny Front, however, are enigmatic. Thesegases might be postmature hydrocarbons of possi-ble hydrothermal or geothermal origin, or theirchemistry could be the result of postgenetic frac-tionation due to diffusion or oxidation.

Gases produced from bituminous Pennsylvaniancoals of the Appalachian Plateau have variedmethane δ13C and methane δD contents and maybe mixtures of thermogenic gas and microbial gas.

INTRODUCTION

Studies of the stable isotope geochemistry ofcommercial natural gases have increased markedlyin recent years (Schoell, 1983, 1988; Whiticar,1994; Prinzhofer and Huc, 1995; Martini et al.,1996). The stable isotopic composition of methaneand other natural gas components can be used to

317

©Copyright 1998. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received April 23, 1996; revised manuscript receivedJanuary 21, 1997; final acceptance August 25, 1997.

2Pennsylvania Department of Conservation and Natural Resources,Topographic and Geologic Survey, Subsurface Geology Section, 400Waterfront Drive, Pittsburgh, Pennsylvania 15222-4740.

3Pennsylvania Department of Environmental Protection, EnvironmentalCleanup, 500 Waterfront Drive, Pittsburgh, Pennsylvania 15222-4740.

We thank the following individuals and companies for their assistance inobtaining gas samples: Ron Riley, Ohio Geological Survey; NGODevelopment Corporation; Belden and Blake; Michael Canich, Eastern StatesExploration Company; James Castle, Woody Lutz, and Harry Mayo; TheCabot Oil and Gas Corporation; The Consolidated Natural Gas Company;and James Ulery and William Diamond, U.S. Bureau of Mines. We alsoextend special thanks to Dennis Coleman, Isotech, for his patient advice andassistance throughout the course of this investigation. We thank JohnHarper, Samuel Berkheiser, Robert Smith, and Kathy Flaherty for theirreviews of earlier drafts of this manuscript. We also appreciate theconstructive reviews of R. C. Burruss, G. E. Claypool, and A. T. James; theirreviews helped us to refine our understanding of the data and improve themanuscript. Released by permission of the director, Pennsylvania Bureau ofTopographic and Geologic Survey.

Geochemistry and Origin of Some Natural Gases in the Plateau Province, Central Appalachian Basin,Pennsylvania and Ohio1

C. D. Laughrey2 and F. J. Baldassare3

AAPG Bulletin, V. 82, No. 2 (February 1998), P. 317–335.

determine the type and thermal maturity of theorganic matter that is the source of the hydrocarbons(Schoell, 1983; Wiese and Kvenvolden, 1993). Stableisotope geochemistry also can help to differentiateoriginal unaltered gases from a single active sourcerock from gases that are mixtures of different hydro-carbons of varying origins or from gases that werealtered through processes such as migration, oxida-tion, or overprinting with biogenic gas (Schoell,1983; Whiticar, 1994; Prinzhofer and Huc, 1995).

In this paper, we describe the stable carbon andhydrogen isotope geochemistry of some commercialnatural gases from various Paleozoic reservoirs of theAppalachian Plateau in western Pennsylvania andeastern Ohio. The purpose of our reconnaissanceinvestigation was to evaluate the potential use of sta-ble isotope geochemistry for interpreting the originof these gases and for correlating them with theirprobable sources. We were particularly interested incomparing our data on gases from deeper portions ofthe central Appalachian basin with the data on gasesfrom the basin f lanks published by Jenden et al.(1993a). We also wished to compare our data withthe small quantity of isotopic data from selectedPaleozoic gases in Pennsylvania published byZartman et al. (1961), Wasserburg et al. (1963),Claypool et al. (1978), and Barker and Pollock (1984).

We obtained stable isotope analyses from 27 gassamples (Figure 1; Table 1). Twenty-two samples arefrom producing gas wells in western Pennsylvaniaand eastern Ohio. Five samples are from sealed corecontainers stored at the U.S. Bureau of Mines inPittsburgh, Pennsylvania, for use in studies ofcoalbed desorption gases. All samples were analyzedfor methane δ13C and δD. Ethane δ13C was deter-mined for ten samples and propane δ13C was mea-sured in eight samples (Table 2). The isotopic analy-ses were performed under contract with one of twocommercial laboratories, Isotech Laboratories orKrueger Enterprises Geochem Laboratories Division.We also compiled isotopic analyses of Pennsylvaniaand Ohio natural gases from the literature (Table 3).

GEOLOGIC SETTING AND HYDROCARBONOCCURRENCES

The central and southern portions of theAppalachian basin in the eastern United States con-tain important natural gas resources. Cumulative gasproduction from the Appalachian basin is estimatedas 35 tcf (de Witt and Milici, 1991). Approximatelyone-third of the gas production, a little more than 11tcf, is from Paleozoic reservoirs in north-central andwestern Pennsylvania (Cozart and Harper, 1993).Most of the gas production in Pennsylvania is on theAppalachian Plateau, a gently folded upland thatoccupies a foreland position northwest of the

intensely deformed central Appalachian Ridge andValley (Gwinn, 1964). Gas fields in eastern Ohio alsoproduce from Paleozoic rocks of the AppalachianPlateau. The Plateau region of southwesternPennsylvania, eastern Ohio, and northern WestVirginia also contains the nation’s third largest in-place coalbed-gas resource (Rice et al., 1993).

Rocks of Cambrian–latest Pennsylvanian and, to alesser extent, earliest Permian age make up the sedi-mentary succession of the Appalachian Plateau(Figure 2). In western Pennsylvania, natural gas isproduced from about 12 stratigraphic units ofCambrian–Middle Devonian age and from variousreservoirs of Upper Devonian age. Some minor pro-duction comes from Mississippian and Pennsylvanianage reservoirs, including coalbed accumulations.Most of the gas production in Pennsylvania is fromstratigraphic traps in Upper Devonian clastic reser-voirs. Only 15% of the state’s cumulative gas produc-tion is from deeper reservoirs of Middle Devonianage or older. Most of this deeper production is fromDevonian Ridgeley Sandstone and Huntersville Chertreservoirs and from tight gas sand accumulations inthe Lower Silurian Medina Group.

The Lower Silurian Medina Group accounts formost of the well completions in Ohio (Cole et al.,1987). Reservoirs in Upper Cambrian and LowerOrdovician rocks, however, are now the principaltarget of exploratory drilling in that state and arethe object of the most active current explorationplay in the Appalachian basin.

Potential source rocks in western Pennsylvaniainclude Devonian black shales and the UpperOrdovician Utica Shale and equivalent Antes Shale(Laughrey, 1997). In addition, there are gas-generatingcoal beds in the Pennsylvanian rocks of the region(Oldham et al., 1993).

The Devonian Ohio and Olentangy shales and theOrdovician Utica and Antes shales are the onlywidespread rock units in Ohio with adequate organ-ic material to be potential source rocks. TOC (totalorganic carbon) in the Ohio and Olentangy shalesranges from 0.04 to 10.88 wt. % and averages 1.5 wt. %; TOC in the Utica and Antes shales ranges from0.00 to 4.23 wt. % and averages 1.3 wt. % (Cole et al.,1987; Wallace and Roen, 1989; Drozd and Cole,1994). These intervals correlate with the recognizedsource rocks in western Pennsylvania (Figure 2).

Source rocks in Pennsylvania and eastern Ohioare oil-and-gas prone to gas prone. Devonian sourcerocks contain type I and type II kerogens that areearly mature in northwestern Pennsylvania and east-central Ohio to postmature in west-central andnortheastern Pennsylvania along the Allegheny Front(Figure 3A). The Ordovician source rocks are imma-ture along the eastern f lanks of the Cincinnati–Findlay arch, but they rapidly increase in maturity asthe rocks dip east into the Appalachian basin

318 Gas Origins, Appalachian Basin

Laughrey and Baldassare 319

Fig

ure

1—

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(Figure 3B). A pod of mature Utica and Antes shalesource rocks extends through the subsurface of east-ern Ohio and into northwestern Pennsylvania (Coleet al., 1987; Wallace and Roen, 1989; Ryder et al.,1991; Drozd and Cole, 1994). The Antes and Uticashales are postmature, however, across most of theAppalachian Plateau in Pennsylvania. These rockscontain kerogen with no significant hydrocarbon-generating capability.

RESULTS

Methane is the principal component of thegases collected in this study, ranging from 87.2 to99.4% in our samples (Table 2). Wetness rangesfrom trace values to 13.2%. The very low wetnessvalues in samples 22–27 in Table 2 are fromcoalbed gases. A plot of the ratios C2/C3 vs. C1/C2(from Table 2) using logarithmic scales reveals asteep, almost vertical trend with positive slope(Figure 4). Prinzhofer and Huc (1995) suggestedthat this trend may indicate that most of thesegases derived from secondary cracking of oil.Nitrogen in our samples ranges from less than 1 to

22.8%. Other nonhydrocarbon gases occur only insmall quantities.

The methane δ13C of our samples ranges from–55.1 to –27.24‰ (Table 2). Gases produced fromPennsylvanian coals and Upper Devonian clasticreservoirs display the largest variations in the mea-surements. Methane δ13C values of gases producedfrom Lower Devonian–Upper Cambrian reservoirsexhibit considerably less variability than those pro-duced from younger reservoirs, but there is overlapof the measurements, such as that noted by Hunt(1996). Methane δ13C increases, for the most part,with increasing geologic age of the reservoirs.

The methane δD of our samples ranges from–303 to –150‰. The lightest δD value is frommethane produced from early-mature UpperDevonian Huron Shale along the Lake Erie shore(sample 16). The heaviest methane δD value isfrom Lower Silurian Tuscarora Formation gas pro-duced at Devil’s Elbow field near the Alleghenystructural front (sample 4).

The ten samples for which we measured ethaneδ13C have values ranging from –41.79 to –29.61‰(Table 2). Eight of these samples have propane δ13Cvalues of –38.21 to –26.71‰. Two samples, both


Table 1. Well and Stratigraphic Information

Sample Well Name County State Age Reservoir Depth (m)

1 Scruggs 1 Portage OH Cambrian Rose Run Ss 21792 Mizer 1 Coshocton OH Cambrian Beekmantown 25903 PA State Tr 289 Lycoming PA Ordovician Bald Eagle Fm 40154 Shaw 1 Centre PA Silurian Tuscarora Fm 33295 Spring Creek * Warren PA Silurian Medina Group –6 Ellen 3 Venango PA Silurian Medina Group 16757 Dailey 1 Venango PA Silurian Medina Group 16138 Seven Springs 1 Fayette PA Devonian Ridgeley Ss 25139 Svonavec 1 Somerset PA Devonian Ridgeley Ss 2809

10 CBC 2 Indiana PA Devonian Ridgeley Ss 242011 CBC 3 Indiana PA Devonian Ridgeley Ss 251812 Hudson 1 Indiana PA Devonian Ridgeley Ss 251213 Rickard 1 Indiana PA Devonian Ridgeley Ss 252414 PA State Tr 243 Centre PA Devonian Harrell Fm 220015 PA State Tr 231 Centre PA Devonian Lock Haven Fm 141716 Presque Isle 1 Erie PA Devonian Huron Sh Mbr 16817 Fleming 1 Indiana PA Devonian Bradford Grp 115818 Kahl 1 Indiana PA Devonian Bradford Grp 99019 Pettigrew 1 Armstrong PA Devonian Bradford Grp 95420 Reiter 1 Allegheny PA Devonian Bradford Grp 79221 Rinehart 1 Greene PA Devonian Venango Grp 103822 Thayer Hall Tr 1 Greene PA Pennsylvanian Allegheny Grp 43223 Core** Indiana PA Pennsylvanian Allegheny Grp 13324 Core** Cambria PA Pennsylvanian Allegheny Grp 25625 Core** Cambria PA Pennsylvanian Allegheny Grp 15026 Core** Cambria PA Pennsylvanian Allegheny Grp 17627 Core** Cambria PA Pennsylvanian Allegheny Grp 242

*Gathering system for Spring Creek field.**Coalbed desorption gas samples.


Tab

le 2

. C

hem

ical

an

d I

soto

pic

Dat

a

C1

C2

C3

C2+

*δ1

3 C1

δDC

H4

δ13 C

2δ1

3 C3

N2

CO

2H

eH

2Sa

mp

le (

mo

l%)

(mo

l%)

(mo

l%)

(%)

(‰)

(‰)

(‰)

(‰)

(mo

l%)

(mo

l%)

(mo

l%)

(mo

l%)

Co

mm

ents

187

.26.

32.

811

.2–3

9.8

–177

.5–3

3.26

–30.

531.

70.

06–

–P

rob

able

Ord

ovi

cian

sh

ale

sou

rce

291

.45

1.4

8.1

–38.

1–1

52.9

–29.

61–2

6.71

0.5

0.02

––

Pro

bab

le O

rdo

vici

an s

hal

e so

urc

e3

95.9

20.

22.

2–2

7.2

–154

.9–3

5.83

–37.

411.

50.

20.

06–

Pro

bab

le O

rdo

vici

an s

hal

e so

urc

e4

96.2

2.3

0.2

2.7

–37.

4–1

50.0

––

1.1

0.2

––

Pro

bab

le O

rdo

vici

an s

hal

e so

urc

e5

91.8

4.2

0.8

5.6

–36.

8–1

63.4

–36.

28–3

1.59

2.7

––

–P

rob

able

Ord

ovi

cian

sh

ale

sou

rce

694

.22.

50.

23.

1–3

4.4

–154

.2–3

9.57

–38.

212.

9–

––

Pro

bab

le O

rdo

vici

an s

hal

e so

urc

e7

93.1

3.3

0.5

4.2

–35.

5–1

57.0

–36.

53–3

1.01

2.8

––

–P

rob

able

Ord

ovi

cian

sh

ale

sou

rce

897

.81.

60.

051.

7–4

6.0

–241

.0–

–0.

40.

10.

020.

01P

rob

able

Dev

on

ian

sh

ale

sou

rce

998

.40.

70.

031.

7–3

1.5

–161

.1–

–0.

40.

30.

020.

01P

rob

able

Dev

on

ian

sh

ale

sou

rce

1097

.61.

70.

061.

8–3

4.5

–162

.5–3

9.3

–0.

50.

2–

–P

rob

able

Dev

on

ian

sh

ale

sou

rce

1197

.81.

70.

051.

7–3

4.8

–162

.7–3

9.44

–0.

30.

2–

–P

rob

able

Dev

on

ian

sh

ale

sou

rce

1297

.81.

50.

051.

6–3

4.7

–165

.1–

–0.

20.

30.

020.

05P

rob

able

Dev

on

ian

sh

ale

sou

rce

1397

.81.

50.

051.

6–3

4.7

–164

.3–

–0.

20.

30.

020.

005

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e14

96.8

2.4

0.14

2.6

–38.

3–1

66.1

––

0.5

0.06

0.06

0.06

Dev

on

ian

sh

ale

gas

1595

.12.

50.

33.

4–5

0.4

–210

.0–

–1.

20.

150.

1P

rob

able

Dev

on

ian

sh

ale

sou

rce

1666

.96.

12.

913

.2–5

3.0

–303

.0–4

1.79

–36.

2722

.80.

1–

–D

evo

nia

n s

hal

e ga

s17

94.4

2.5

0.24

3.1

–41.

0–1

92.9

–40.

88–3

7.53

2.2

0.4

––

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e18

96.4

2.4

0.22

2.7

–54.

0–2

64.0

––

0.6

0.08

0.2

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e19

95.9

2.9

0.36

3.5

–54.

2–2

47.0

––

0.5

0.06

0.1

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e20

90.9

5.6

1.62

8.4

–52.

7–2

19.0

––

0.6

0.07

0.1

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e21

91.7

4.8

1.2

6.7

–51.

2–2

16.0

––

1.4

0.06

0.01

Pro

bab

le D

evo

nia

n s

hal

e so

urc

e22

93.8

0.1

–0.

05–5

5.1

–219

.0–

–3.

81.

9–

–C

oal

bed

gas

; %R

o=

0.9

2380

.80.

01–

0.02

–55.

0–1

98.0

––

17.8

0.8

––

Co

alb

ed g

as**

; %R

o=

1.1

2497

.20.

10.

007

0.09

–42.

9–1

94.0

––

2.1

0.4

––

Co

alb

ed g

as**

; %R

o=

1.6

2599

.10.

03–

0.03

–39.

9–1

21.0

––

0.4

0.4

––

Co

alb

ed g

as**

; %R

o=

1.5

226

98.7

0.01

0.00

10.

007

–48.

9–2

05.0

––

1.1

0.07

––

Co

alb

ed g

as**

; %R

o=

1.5

427

99.4

0.02

–0.

016

–45.

9–1

96.0

––

0.3

0.27

––

Co

alb

ed g

as**

; %R

o=

1.5

*C2+

= (

1 –

C1/

ÂC

n) 1

00 (

%),

from

Sch

oell

(198

3).

**C

oalb

ed d

esor

ptio

n ga

s sa

mpl

es.

from Lower Devonian Ridgeley Sandstone reser-voirs (samples 10 and 11), had insufficient propanefor isotopic analyses.

Plots of methane δ13C vs. methane δD indicatea thermogenic origin for most of our gas samples(Figures 5, 6). Most samples evidently originatedas wet; i.e., associated gases (see Figures 5, 6) gen-erated from types I and II kerogens at source rockmaturities ranging from early mature to postma-ture. Linear regression of methane δ13C and δD forall the gas well samples in Table 2 yields the equa-tion δD = 3.9 δ13C – 22.5‰, which resembles thatcalculated by Schoell (1983) for thermogenicgases generated from types I and II organic matter(δD = 3.2 δ13C – 25‰). Correlation of the samplesin our data set, however, is not strong (r2 = 0.65).The regression probably does not reflect entirelythe maturation trends of the source material andthe gas.

DISCUSSION

Upper Cambrian Reservoirs, Eastern Ohio

The Rose Run Sandstone and BeekmantownDolomite (Figure 2) are part of a complex strati-graphic interval comprised of mixed carbonate andsiliciclastic rocks trending northeast-southwestwithin a long, narrow belt in the subsurface of east-ern Ohio and western Pennsylvania. The Knoxunconformity, a major erosion surface that trun-cates Upper Cambrian and Lower Ordovician strataover most of eastern North America, plays a majorrole in controlling the distribution of reservoirs andtraps within the Upper Cambrian strata of Ohio andnorthwestern Pennsylvania (Riley et al., 1993).Mature Utica Shale is a likely source rock of thehydrocarbons produced from Rose Run andBeekmantown reservoirs (Cole et al., 1987; Wallaceand Roen, 1989; Ryder et al., 1991).

The Rose Sun Sandstone produces oil and gasfrom fault-related traps in southwestern PortageCounty, Ohio (Riley et al., 1993; Moyer, 1995).Geologic and seismic mapping in this area indicatesthe presence of a major basement fault, called theAkron-Suffield fault system, trending northwest-southeast beneath the oil and gas field (Figure 1). TheRose Sun Sandstone traps in southwestern PortageCounty probably formed through faulting and frac-turing during and subsequent to the development ofthe Knox unconformity (Riley et al., 1993).

We sampled gas from the Belden and Blake 1Scruggs well in Portage County, Ohio (sample 1).The well penetrates the downthrown side of one ofseveral small faults that offset the top of the Knoxunconformity along the Akron-Suffield fault system(Figure 1). Crossplots of gas wetness vs. methaneδ13C, and methane δD vs. methane δ13C, indicate athermogenic, condensate-associated gas generatedby oil and kerogen cracking during late catagenesis(Figures 5, 6, 7).

The carbon isotope variations among the meth-ane, ethane, and propane are normal in the Scruggswell; i.e., the methane δ13C < ethane δ13C < propaneδ13C (Figure 8). Carbon isotope separations amongmethane, ethane, and propane fit a level of organicmaturity (LOM) of about 11 or a thermal alterationindex (TAI) of approximately 3– (James, 1983)(Figure 7). These values correspond to a maturationlevel equivalent to the late oil window.

Wallace and Roen (1989) published source rockgeochemical data from a well just south of the south-western Portage County producing area and the 1Scruggs well (OH-33 in Figure 1). TOC and S1 and S2pyrolysis yields indicate good to very good sourcerock generative potential for the Utica Shale.

Hydrogen indices and the S2/S3 ratio from pyroly-sis indicate that the Utica Shale is gas and oil prone.Tmax and production indices are consistent with lateoil window maturation. We believe it is reasonable toconclude that Upper Ordovician black shales several


Table 3. Selected Gas Isotope Data From the Literature

δ13C1 δ13C2 δ13C3State County Age Reservoir (‰) (‰) (‰) Reference

PA Mercer, Devonian Venango, –47 to –45 – – Wasserburg et al. (1963)Venango, Bradford,Jefferson, and Elkand Elk Groups

PA Allegheny Devonian Venango Group –40.12 – – Coleman, unpublished dataPA Indiana Devonian Bradford Group –41.9 – – Claypool et al. (1978)PA Cameron Devonian Ridgeley Ss –34.5 – – Zartman et al. (1961)PA Crawford Silurian Medina Group –39.1 –35.2 –29.8 Barker and Pollock (1984)PA Venango Silurian Medina Group –34.7 –40.4 –30.8 Barker and Pollock (1984)OH Portage Silurian Medina Group –35.2 –37 – Barker and Pollock (1984)

hundred feet above the Knox unconformity in south-western Portage County may be the likely source ofhydrocarbons in the Rose Sun Sandstone there.

Erosional remnants of the BeekmantownDolomite are one of the primary drilling objectivesin eastern Coshocton County, Ohio. Reservoirs


Figure 2—Highly generalized stratigraphic section of rocks in the subsurface of western Pennsylvania and easternOhio. Black flags indicate potential source rocks.

occur in zones of solution-enlarged vuggy porosityassociated with paleotopographic highs (Riley etal., 1993). We sampled gas from the 1 Mizer well inCoshocton County (sample 2). This well produced2.6 bcf of gas and 9 million bbl of oil through theend of 1991 (Riley et al., 1993). The 1 Mizer well ison a northwest-southeast–trending structural highassociated with the Cambridge arch (Figure 1). Thewell penetrates the northwest side of a thick (∼25m) paleoremnant of Beekmantown Dolomite.

Plots of gas wetness vs. methane δ13C, andmethane δ13C vs. methane δD for the Mizer wellhydrocarbons indicate methane generated in theprincipal zone of gas-condensate formation duringlate catagenesis (Figures 5, 6). A plot of the carbonisotope separations between methane, ethane, andpropane fit an LOM of about 12 and a TAI ofapproximately 3 (Figure 8). The estimated thermalmaturity of the gas produced from the 1 Mizer well

is greater than that of the Utica source rocks inthe area. Wallace and Roen (1989) and Ryder et al.(1991, 1995), reported that the conodont alter-ation index (CAI) of Utica samples in CoshoctonCounty ranges from 1 to 1.5, the average Tmax is440°C, and the average production index (fromRock-Eval pyrolysis) is 0.27. Their data place theUtica Shale in the beginning to middle part of theoil window.

Ryder et al. (1991) noted that the Beekmantownand Rose Run hydrocarbons produced in theCoshocton County area are highly mature, verylight oils or condensates that likely were not gener-ated in the less mature, local Utica source rocks.The maturity of their samples is compatible, how-ever, with the level of gas maturation indicated byour isotope data. Ryder et al. (1991) suggested thatthe highly mature oils were generated in eastern-most Ohio or adjoining Pennsylvania and West


Figure 3—Maturationtrends in principal sourcerocks on the AppalachianPlateau of western Pennsylvania and easternOhio. (A) Thermal alter-ation index (TAI) map for kerogens in the basal Marcellus Shale (MiddleDevonian). Solid circlesare well locations for Marcellus cores in whichTAI was measured. Shaded area depicts the oilwindow. (B) Isopleth mapof Tmax (°C) from Rock-Eval pyrolysis of Utica and Antes shales (UpperOrdovician) samples(from Wallace and Roen,1989). Shaded area represents the oil window.

Virginia and migrated along the Knox unconformi-ty into traps in eastern and central Ohio. Our stableisotope data support that interpretation.

Lower Silurian Medina Group Reservoirs

Gas fields producing from the Lower SilurianMedina Group in northwestern Pennsylvania andeastern Ohio generally are interpreted as strati-graphic accumulations where facies changes andpermeability discontinuities furnish the traps(Laughrey and Harper, 1986). Sandstones in theMedina gas fields are underpressured, tight gasreservoirs. Zagorski (1988) and Law and Spencer(1993) suggested that these underpressured gasaccumulations might be basin-centered gas accu-mulations. Dark shales in the Cabot Head forma-tion of the Medina Group were once surmised tobe the source rock for the Medina hydrocarbons(Knight, 1969), but these rocks are organicallylean (mean TOC = 0.16% in Pennsylvania) and notlikely to have generated commercial quantities ofoil or gas. Cole et al. (1987) and Drozd and Cole(1994) correlated hydrocarbons produced fromthe Medina Group in eastern Ohio with sourcerocks in the Upper Ordovician Utica and Antesshales. Our Medina gas samples from northwesternPennsylvania are very mature (samples 5–7). The

crossplots of Medina gases in Figures 5 and 6 sug-gest that these gases are associated with conden-sate. Our isotope data support previous sugges-tions that the Utica and Antes shales are the sourcerock for Medina Group gases in northwesternPennsylvania, northeastern Ohio, and western NewYork ( Martini, 1971; Jenden et al., 1993a).

The Medina gases plot well above the linesdescribing the relationships of cogenetic methane,ethane, and propane shown in Figure 7, suggestinga possible mixture of thermogenic gases in thereservoirs. Two of the samples in our database(samples 6 and 7, Table 2) and one from Barkeyvillefield in Venango County reported by Barker andPollock (1984) (Table 3) exhibit isotopic reversalsbetween the methane and the ethane. In additionto gas mixing, these isotopic reversals might alsosuggest oxidation of methane or leakage due to dif-fusion of gases through the reservoir cap rock(Figure 9).

Devonian Shales

Devonian black shales are major petroleumsource rocks in the Appalachian basin (Roen,1984). These rocks also produce commercialamounts of autogenic gas in southwestern Virginia,eastern Kentucky, southwestern West Virginia, and


Figure 4—Plot of theln(ethane/propane) vs.ln(methane/ethane) forthe data in Table 2.

southern Ohio (Milici, 1993). The shales also pro-duce gas along the southern shore of Lake Eriefrom Ohio to New York, but the shales are onlymarginally productive there (Milici, 1993).

Claypool et al. (1978) used chemical and stableisotope data to determine the regional thermalmaturity of organic matter in the Devonian shalesof the central Appalachian basin. Claypool et al.(1978) showed that methane δ13C ratios of thegases increased systematically from lighter valuesof –54‰ in central Ohio to heavier values of about–42‰ near the Allegheny Front in Pennsylvania.

We sampled Devonian shale gas from two wells.One of the gas samples (sample 16) is from theHuron Shale in Erie County, Pennsylvania. Therocks generating this gas are early mature. TheHuron methane sample is relatively light, with amethane δ13C of –53‰, and with light ethane and

propane also (see Table 2). The other gas sample(sample 14) is from postmature shales of theHarrell Formation near the Allegheny Front inCentre County, Pennsylvania. The sample from theHarrell Formation is heavy, with a methane δ13C of–38.3‰.

Milici (1993) noted that the Devonian blackshale gas fields developed along the south shoreof Lake Erie produce from rocks reportedly con-taining immature organic matter, implying thatthe gas may have migrated from elsewhere in theAppalachian basin or that it is locally generatedmicrobial gas. Martini et al. (1996) suggested thatgas in the Devonian Antrim Shale of the Michiganbasin (a correlative of the Huron Shale) originatedfrom the mixing of thermogenic and microbialgases having similar methane δ13C values. Our datafrom gas produced at Presque Isle in Erie County,


Figure 5—Plot of methane δ13C vs.methane δD for various Paleozoicgases from western Pennsylvaniaand eastern Ohio. Symbols andsamples are the same as thoseshown in Figure 1. Compositionalfields for various genetic types ofgases are modified from Schoell(1983). Ro scale is from Jenden etal. (1993a). Usage of the terms“associated” and “nonassociated”differs from petroleum industrypractice. Here, these terms have agenetic connotation implying thatthe gases formed or accumulatedas associated or nonassociatedgas. These gases may or may notbe so today. Associated gases, forinstance, may have formed withoil or condensate in a source rockand then migrated away, or associated gases may have migrated away from an oil accumulation to form a dry gasreservoir with the isotopic signature of an associated gas.Examples are given in the text and in Schoell (1983).

however, suggest a local, thermogenic origin forthe hydrocarbons. The crossplot of methane δ13Cvs. methane δD (Figure 5) indicates an oil-associatedgas generated within the early oil window. A plot ofthe observed carbon isotope separations in the gasvs. source rock LOM implies a TAI of approximate-ly 2 to 2+, which also would denote the early oilwindow (Figure 8).

Data obtained from Rock-Eval pyrolysis andbiomarker analyses of Huron Shale samples fromthe Erie County well (well 16 in Figure 1) supportour interpretation of the gas isotope data.Production indices (0.05–0.10), Tmax values(440–441°C), and the ratios of bitumen to totalorganic carbon (0.058–0.072) all indicate an earlyoil window. Several biomarker maturation parame-ters, including C31 homohopane isomerization(0.59–0.61), Ts/(Ts + Tm) ratios (0.63–0.65), andboth 20S/(20S + 20R) and ββ/(ββ + αα) C29 sterane

isomerization ratios (0.471–0.475 and 0.54–0.56,respectively), also indicate early oil generation(Peters and Moldowan, 1993).

The Harrell Formation gas sample (sample 14) isfrom Black Moshannon field in Centre County,Pennsylvania. This gas is produced from thin, siltyand sandy carrier beds in the upper shale memberof the Harrell Formation. The actual source rock isthe subjacent Burket Shale Member of the HarrellFormation. Measured TOC in the Burket Shaleranges from 1.34 to 2.13%. Pyrolysis S2 yields aretoo small (<0.10 mg/g) to obtain productionindices or meaningful Tmax values, but Ro in theBurket averages 2.45% and TAI averages 3.8. TheBurket Shale is postmature. The gas chemistry sug-gests a relatively mature, condensate-associated gas(Figures 5, 6). The gas isotope data provide a rea-sonable correlation between the hydrocarbons pro-duced from the upper shale member of the Harrell


Figure 6—Plot of wetness(C2–C5/C1–C5) vs. methane δ13C,with a comparison of Jenden etal. (1993a) data for western New York gases to our data forwestern Pennsylvania and eastern Ohio gases. See Figure 1for symbol explanations. Compositional fields for variousgenetic types of gases are modified from Schoell (1983).

Formation and the source rocks in the BurketMember.

Lower Devonian Ridgeley Sandstone andMiddle Devonian Huntersville Chert

The Ridgeley Sandstone and Huntersville Chert(Figure 2) comprise the most significant deep-gasreservoirs in western Pennsylvania. These inter-vals account for almost 80% of all the pre-LateDevonian gas produced there (Harper and Cozart,1992). Most of this gas is produced from fracturedHuntersville Chert and Ridgeley Sandstone onfaulted anticlines within isolated fault blocks(Flaherty, 1996).

Ridgeley and Huntersville gases produced atStrongstown and Living Waters fields in IndianaCounty, Pennsylvania (samples 10–13) plot in thefields of gases associated with condensate (Figures5, 6). Two of these gases for which we were able toobtain ethane δ13C measurements (samples 10 and11) exhibit methane δ13C > ethane δ13C. These iso-tope reversals suggest that the hydrocarbons pro-duced at this field are mixed thermogenic gases, orthat the gases are a residue of diffusive gas leakage(Figures 7, 9). We prefer the latter interpretation

because Strongstown field lies along the trend ofthe Home-Gallitzen lineament, a known surfaceexpression of a high-angle subsurface fracture zone(Harper, 1989).

Gas produced from the Seven Springs pool inthe Spook Hill field (sample 8) is substantially dif-ferent from these gases despite the fact that reser-voir depths, trap configurations, source rocks, andmaturations are similar at all three fields (Laughrey,1997). Although liquid hydrocarbons are not pro-duced at Spook Hill field, the gas is mature and wasgenerated within the oil window (Figure 5). Thegas is very dry (wetness = 1.7%), which is surpris-ing given its methane δ13C composition of –46‰.Perhaps this dry methane was originally cogenerat-ed with wet gases and C15+ hydrocarbons in theprincipal oil window, and then left in the reservoiras a methane-rich residue due to intense post-expulsion fractionation of C1–C4 hydrocarbongases (Price, 1995). Basilone (1984) documentedthe presence of open fractures in these rocks duringthe Alleghanian orogeny. These fractures were effec-tive conduits for fluid migration. Basilone (1984)showed that methane-saturated brines and immisci-ble methane gas were the principal f luids thatmigrated through these fractures and that ethane,and perhaps even higher homologs, migrated


Figure 7—Relationship ofstable carbon isotoperatios for cogeneticmethane vs. ethane andpropane in natural gases(from Faber, 1987) andplots of gas isotope datafrom Table 2. Most of theAppalachian Plateau gasesfall off the expected relation defined by Faber(1987) and discussed byWhiticar (1994), indicatingmixed or altered gases.Numbers next to datapoints are the samplenumbers in Table 2.

through the fractures after they had been open fora considerable period of time.

Sample 9 is from the Svonavec 1 well in theSomerset East field. The methane δ13C of this gas is–31.5‰, one of our heaviest δ13C measurements.Crossplots of methane δ13C vs. wetness and δD(Figures 5, 6) indicate a postmature gas. This inter-pretation of the gas is supported by the geochem-istry of source rock samples analyzed from theAmoco Svetz 1 well west of the Somerset East field.In the Svetz well, Devonian source rocks have anaverage TAI of 4.2, a mean Ro of 3.28%, and produc-tion indices, from Rock-Eval pyrolysis, of 0.63–0.73.

Upper Devonian Sandstone Reservoirs

All but one of the Upper Devonian sandstonegases that we sampled appear to be early-matureoil-associated gases or mixed microbial and thermo-genic gases (samples 17–21, Table 2; Figures 5, 6).Most of these gases are isotopically light. The oneexception is gas from the Kane sand of theBradford Group (sample 17), which appears to belate mature (Figures 5, 6). Other investigators havereported methane δ13C of –47 to –40.12‰ in vari-ous gases from Upper Devonian sandstones inwestern Pennsylvania (Table 3).

The most striking fact about the Upper Devoniansandstone gases is that several light, oil-associated

isotopic signatures occur all the way across theAppalachian Plateau regardless of source rockmaturity. Figure 10 shows the TAI values of organicmatter in the Marcellus Shale along with the welllocations and methane δ13C values of the UpperDevonian gas samples from Tables 2 and 3. The TAIvalues increase systematically from northwest tosoutheast across the Plateau. The methane δ13C val-ues from Devonian shale source rocks also becomemore mature from northwest to southeast, increas-ing from about –54‰ along the Lake Erie shore toabout –38‰ near the Allegheny Front. Although afew of the methane δ13C values from UpperDevonian gases correlate with the observed sourcerock maturity, most of the values do not. The gasesalong the Allegheny Front are particularly surpris-ing. Here, postmature gases in the Upper DevonianHarrell bear no isotopic resemblance to nearbyUpper Devonian sandstone gases.

One possible interpretation of these observationsis that gases generated along with oil in the Devonianshale source rocks migrated into many of the UpperDevonian sandstone traps relatively early, beforemaximum burial and subsequent uplift during theAlleghanian orogeny. Although both oil and gas accu-mulated in the sandstones, oil was progressivelydestroyed by thermal degradation during deeperburial, whereas some gases moved away from theoils. Other gases, however, remained trapped insome reservoirs, and their isotopic composition is a


Figure 8—James’s (1983)theoretical maturation diagram illustrating calculated carbon isotopicseparations between gascomponents plottedagainst source rock level oforganic maturation (LOM).Thermal alteration index(TAI) and Ro are also indicated at the bottom ofthe plots. Data are fromUpper Cambrian gases ineastern Ohio (samples 1and 2, Table 2) and HuronShale along Lake Erie,Pennsylvania (sample 16).

vestige of an earlier genesis within the oil window.Several relevant geologic observations support ourhypothesis. Bruner and Smosna (1994) documentedthe presence of “dead oil” or bitumen in the porespaces of the Upper Devonian reservoirs producinggas along the Allegheny structural front inPennsylvania, including the sandstone reservoir thatproduced the gas we sampled at Council Run field(sample 15, Table 2). Bruner and Smosna (1994)showed that solid bitumen comprises an average of3% of the bulk composition of these sandstones.They noted 5–10% of the available pore volumes inthese sandstones were filled with bitumen. Bitumencoatings in these pores are 0.01–0.02 mm thick andact like clay coatings in the rocks, inhibiting cemen-tation and preserving primary porosity (Bruner andSmosna, 1994). Thus, isotopically light thermogenicmethane produced near the Allegheny Frontmight actually reside in pores preserved by itsnow-degraded cogenetic oils.

One problem with this interpretation is the factthat isotopically heavier gases, cracked from ther-mally degraded oils in the reservoirs, should havemixed with the lighter gases generated during theearly-mature stage to yield a methane with a less neg-ative δ13C value than that observed at Council Runfield. Gas isotope data collected from both natureand laboratory experiments by Price (1995) mighthelp to explain this apparent enigma. Price (1995)demonstrated that methane carbon isotopes can belargely invariant through the oil generation window;he documented several examples of methane becom-ing slightly lighter, and hydrocarbon gases becomingwetter, with increasing maturity during the middleto late stages of oil generation. He further noted thatorganic matter type can influence gas and isotope

composition more than maturity. Finally, Price(1995) argued that methanes with δ13C of –44 to–50‰ possibly contain a significant microbial gascomponent inherited from in-situ mixing of thermo-genic and early microbial methane.

The isotopically heavier gas produced from theKane sand at Cushion field in Indiana County,Pennsylvania (sample 17) requires a different inter-pretation. This gas plots in the field for relativelymature oil-associated gases (Figures 5, 6). This gas,however, has an ethane δ13C of –40.88‰ and apropane δ13C of –37.53‰. These are low-maturityδ13C values. Furthermore, the methane, ethane,and propane δ13C values do not exhibit the expect-ed separation for cogenetic gases on Figure 7. TheKane hydrocarbons may represent a mixture ofthermogenic hydrocarbons, a gas altered bymethane oxidation, or the residue of diffusive gasleakage (Figure 9). We favor the possibilities ofmethane oxidation or diffusive gas leakage. TheKane sand at Cush Cushion field is a prolific frac-tured reservoir that lies along the trend of a promi-nent regional cross-structural discontinuity(Laughrey, 1982; Harper, 1989; Hussing, 1994). Anumber of geologists have suggested that f luidmigration patterns along this discontinuity wereiterative and complex throughout the geologic his-tory of the central Appalachian basin (Wheeler,1980; Rodgers and Anderson, 1984; Harper, 1989).

Ordovician and Silurian Fractured Reservoirs,Allegheny Structural Front

The Lower Silurian Tuscarora Formation andthe Upper Ordovician Bald Eagle Formation are


Figure 9—Prinzhofer andHuc’s (1995) graphicalcomparison of their thermogenic/leaking gasmodel with Jenden et al.(1993a) thermogenic gasmixing model for inter-preting isotopic reversalsamong δ13C1, δ13C2, andδ13C3 gases. The samplesplotted on the graph areidentified in Tables 1 and 2.

important fractured gas reservoirs near theAllegheny Front in central Pennsylvania. We sam-pled gases from the Tuscarora Formation atDevil’s Elbow field in Centre County (sample 4)and from the Bald Eagle Formation at Gruganfield in Lycoming and Clinton counties (sample3). Potential source rocks along the AlleghenyFront in Centre, Lycoming, and Clinton countiesare postmature (Figure 3).

The Tuscarora Formation has produced almost3 bcf of gas from a naturally fractured reservoir atDevil’s Elbow field (Harper and Cozart, 1992). Thegas is produced from open fractures within tightquartzites. The fractures are propped open by largeepigenetic quartz crystals (Wescott, 1982). Isotopedata indicate that the Tuscarora Formation gas atDevil’s Elbow field is postmature and was generatedwithin the principal zone of gas formation (Figures5, 6). Gases produced from the Tuscarora Formationat Devil’s Elbow correlate with the observed maturi-ty of local Antes Shale source rocks.

Fractured sandstones within the Upper Ordo-vician Bald Eagle Formation produce gas at onelocation in the central Appalachian basin, theGrugan field in Pennsylvania. This small field con-sists of a three-well pool that straddles the borderof Clinton and Lycoming counties in the north-central part of the state. Grugan field, its small sizenotwithstanding, has two of the most productivewells in the entire Appalachian basin. The discov-ery well, the Pennsylvania State Tract 285, pro-duced 3.6 bcf of gas between the end of 1982 andthe end of 1992. The Pennsylvania State Tract 289well, an extension located about 3.6 km northeastof the discovery well, produced 3.95 bcf of gasbetween 1985 and the end of 1992.

The gas from the Grugan field has a methane δ13Cof –27.24‰, the heaviest δ13C of any methane ever

reported in the Appalachian basin. Crossplots ofδ13C and δD for the Bald Eagle methane plot out-side of the field of most thermogenic gases(Figures 5, 6) and resemble some methanes asso-ciated with hydrothermal or geothermal gases(Laughrey and Harper, 1996). Similar to thermo-genic gases, most hydrothermal and geothermalmethanes probably have an organic source. Forexample, Des Marais et al. (1981) demonstratedthat 13C enrichment of geothermal methane withincreasing temperature at Yellowstone NationalPark was due to extensive natural pyrolysis ofsedimentary organic matter. Lacazette (1991) pro-posed that the Bald Eagle Formation in centralPennsylvania, including the reservoir at Gruganfield, was fractured by hypersaline, methane-satu-rated brines that attained superlithostatic pres-sures during the Alleghanian orogeny. On thebasis of fluid inclusion studies, he estimated thatfracturing occurred at burial temperatures of200–205°C. Deep faulting recognized at Gruganfield (Henderson and Timm 1985) may have pro-vided migration pathways for such hydrothermalfluids moving out of and through deeper layers,including the Antes Shale source rocks.

The ethane and propane in the Grugan field gasare isotopically lighter than the methane (Table 2).The observed distribution of carbon isotopes in theGrugan methane, ethane, and propane is unusualand superficially resembles that observed in someabiogenic gases (Des Marais et al., 1981; Jenden etal., 1993b); however, the isotopic reversalsobserved in the gas at Grugan field probably aredue to heterogeneities in the source rock organicmatter, mixing of gases from different sources, oxi-dation of thermogenic gas, or partial diffusive leak-age of the gas reservoir (Jenden et al., 1993b;Prinzhofer and Huc, 1995) (Figure 9).


Figure 10—Thermal alterationindex (TAI) map for the MiddleDevonian Marcellus Shale inPennsylvania and methane δ13Cvalues from Upper Devonian wellssampled for this study.

Coalbed Gases

The northern Appalachian coal-bearing basincontains an estimated 1730 billion m3 of in-placecoalbed natural gas (Rice et al., 1993). Approx-imately four-fifths of this in-place gas occurs within aconcentrated area in southwestern Pennsylvaniaand northern West Virginia (Diamond et al., 1987).Oldham et al. (1993) published a comprehensivegeological report on the coalbed gas production inthis region.

We collected six samples of coalbed gases formolecular and isotopic analyses (samples 22–27,Table 2). Sample 22 is from a multilevel completioncoalbed gas well in Greene County, Pennsylvania.The other five samples are desorption gases collect-ed from sealed cores of specific coals stored at theU.S. Bureau of Mines laboratories near Pittsburgh,Pennsylvania. Figure 1 shows the original locationsof these coal cores. The coals are high-volatile bitu-minous to low-volatile bituminous, with Ro valuesranging from 0.9 to 1.6%.

The δ13C of coalbed methanes we sampled rangefrom relatively light values of –55.1‰ to as heavy as–39.9‰. The methane δD of our coalbed gas sam-ples ranges from –219 to –194‰. On Schoell’s(1983) plot of methane δ13C vs. methane δD, fourof the samples appear to be mixtures of thermo-genic and microbial methanes, one sample appearsto be thermogenic oil-associated gas, and one sam-ple appears to be a dry, nonassociated methane(Figure 5).

The methane δ13C of coalbed gases is typicallylighter (to –60‰) at low ranks and heavier (to–40‰ and greater) at high ranks (Rice et al., 1993).Rice et al. (1993) considered coalbed gases to beboth microbial and thermogenic in origin. Theprincipal controls on the molecular and isotopiccomposition of coalbed gases are the compositionand rank of the coal and its depth and temperaturehistory. The composition of coalbed gas also isinfluenced by secondary processes such as mixingof thermogenic methane with late-stage microbialmethane that was generated by anaerobic microor-ganisms introduced into the coal by groundwater(Rice et al., 1993; Scott et al., 1994).

All of the coals from which we collected gassamples are of relatively high rank and should havegenerated thermogenic gases. Yet, all but one ofour samples (sample 25) have methane δ13C valueslighter than anticipated for coals of the observedrank. We suspect that late-stage microbial gases,generated in the coals by anaerobic microorgan-isms, are mixed with the thermogenic gases thatformed earlier in the coals. Rice et al. (1993) statedthat groundwater flow creates a favorable environ-ment for microbial activity and the related genera-tion of late-stage gases. Groundwater movement in

the deeper (>50 m) regional aquifer of southwest-ern Pennsylvania is relatively stagnant due to lowhydraulic conductivity of the coal-bearing strata,and the lateral groundwater flow rate is very low(Stoner et al., 1987). Reducing conditions prevail atthe depths from which the coalbed gases are pro-duced (Stoner et al., 1987).

Comparisons With Previously Published Gas Data

Jenden et al. (1993a) used stable isotope geo-chemistry to investigate the origin of commercialnatural gases in western and central New York state.They recognized two genetic groups of gases inNew York (Figures 5, 6) that they interpreted as mix-tures of wet thermogenic gases generated locally inDevonian and Ordovician source rocks and dry post-mature gases that migrated up from deeper portionsof the Appalachian basin. Jenden et al. (1993a) pre-sented three principal observations in support oftheir premise. (1) The range of source rock maturi-ties inferred from stable isotopic compositions ofthe New York gases is greater than that observed inthe Paleozoic rocks of that region. (2) Geochemicalparameters, such as gas wetness, methane δ13C, andmethane δD, increase with increasing reservoir age,whereas ethane δ13C does not. (3) Several gases,especially in Silurian and Ordovician reservoirs,exhibit isotopic reversals (methane δ13C > ethaneδ13C). Jenden et al. (1993a) suggested that their mix-ing model could be applied to gases in surroundingareas, including the St. Lawrence lowlands, southernOntario, Ohio, Pennsylvania, and West Virginia.Prinzhofer and Huc (1995), however, examinedJenden et al.’s (1993a) data and noted a good positivecorrelation of δ13C1–δ13C2 vs. C1/C2, which theyinterpreted as a straightforward thermogenic trendrather than mixing. Prinzhofer and Huc (1995) inter-preted the isotopic reversals in the New York gasesto be a postgenetic leakage phenomenon due to dif-fusion of gases through the cap rock in reservoirs(Figure 9).

The plot of methane δ13C vs. methane δD inFigure 5 suggests that there also are two basicgroups of gases in western Pennsylvania and easternOhio. One group of gases consists of Pennsylvaniancoalbed methanes and Devonian methanes withδ13C values less than –40‰ and methane δD valuesless than –190‰. This group of hydrocarbonsappears to be oil-associated gases or a mixture ofmicrobial and thermogenic gases (Figures 5, 6). Asecond group consists of Cambrian, Ordovician,Silurian, and Devonian gases with methane δ13Cvalues greater than –40‰ and methane δDgreater than 180‰. In Figures 5 and 6, this sec-ond group of gases appears to consist of gases


associated with condensate. One of the coalbedgases plots within the field for nonassociated ther-mogenic methane.

When compared to the data published by Jendenet al. (1993a) (Figures 5, 6), our group of condensate-associated gases is similar to their group II gases.Our oil-associated gases, however, differ markedlyfrom their group I gases. The New York group Igases are much wetter and heavier with respect tomethane δ13C than are our oil-associated gases. Therelative shift from the position of the New Yorkgroup I gases to our group of oil-associated gases inFigure 6 might indicate that the our group of gaseshave experienced compositional changes due toshallow migration (Schoell, 1983). Alternately, boththis plot and the methane δ13C vs. methane δD plotin Figure 5 also suggest that our group of oil-associ-ated gases might be mixtures of microbial and ther-mogenic gases.

CONCLUSIONS

Stable isotope geochemistry provides someinsight into the origin of natural gases in westernPennsylvania and eastern Ohio, and provides someinformation concerning the timing and direction ofgas migration into different reservoirs. The data sug-gest that most of the gases have multiple sources orwere altered by geological or biogeochemical pro-cesses. Gas mixing, oxidation, and diffusive leakagefrom reservoirs are processes that may have affectedthe gases during their geological history.

In eastern Ohio, gases in Rose Run Sandstonereservoirs probably are derived from adjacentOrdovician source rocks, whereas gases inBeekmantown Dolomite reservoirs are probablyderived from deeper downdip source rocks. Gasesin Ordovician, Silurian, and possibly Lower–MiddleDevonian reservoirs appear to have a mixed originwith an overprint of an isotopically heavy, dry gasderived from deeper parts of the Appalachianbasin. An alternate interpretation is that these gasesbear the isotopic signature of second-order frac-tionation effects and are residues of diffusive leak-age of gases through cap rocks. We favor the latterinterpretation for gas accumulations in reservoirslocated along regional subsurface fracture zones.Several Upper Devonian reservoir gases in westernPennsylvania most likely migrated into reservoirsrelatively early in their burial history. Coalbed gasesin Pennsylvania are dry gases with a probable over-print of secondary microbial methane.

We conclude this paper by reminding readersthat this was a reconnaissance investigationdesigned to evaluate the potential utility of stableisotope geochemistry for interpreting the origin ofnatural gases in portions of the central Appalachian

basin and for correlating the gases with their proba-ble sources. We believe that a much larger databasewill be needed to support, verify, or refute ourobservations and interpretations. We are satisfied,however, that the techniques and data presentedhere do indeed sustain the information and conclu-sions of several other workers who have studiedthe origins of gases in the Appalachian basin, andwe are convinced that isotope geochemistry couldserve as an important tool for discovering newreserves in this mature basin.

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ABOUT THE AUTHORS

Christopher Laughrey

Christopher D. Laughrey is a geo-logist with the Subsurface GeologySection of the Pennsylvania Geo-logical Survey’s Geologic ResourcesDivision, where he has workedsince 1980. Laughrey worked as ageophysical analyst for the WesternGeophysical Company in Houston,Texas, before taking his presentposition in Pittsburgh, Pennsylvania.His special interests include isotopeand organic geochemistry, borehole geophysics, GISapplications in earth sciences, and reservoir petrology.

Fred Baldassare

Fred Baldassare is a hydrogeolo-gist with the Pennsylvania Depart-ment of Environmental Protection.He has 10 years experience investi-gating incidents of subsurface straygas migration. Fred currently servesas a project manager providing tech-nical reviews of chemical/hydrogeo-logical data generated at industrialsites with groundwater contamin-ation. He also serves as an advisoron the regulations and technical guidance forPennsylvania’s Land Recycling Program. His areas ofinterest include contaminant fate and transport, and iso-tope geochemistry.

geochemistry and origin of some natural gases in the ...hydrogen isotope geochemistry of some...

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