Temperature-dependent Li isotope ratios in AppalachianPlateau and Gulf Coast Sedimentary Basin saline water

G. L. MACPHERSON1, R. C. CAPO2 , 3 , B . W. STEWART2 , 3 , T . T . PHAN2 , 3 , K . SCHROEDER4

AND R. W. HAMMACK4

1Department of Geology, University of Kansas, Lawrence, KS, USA; 2Department of Geology and Planetary Science,

University of Pittsburgh, Pittsburgh, PA, USA; 3National Energy Technology Laboratory-Regional University Alliance,

Pittsburgh, PA, USA; 4U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, PA, USA

ABSTRACT

Lithium (Li) concentrations of produced water from unconventional (horizontally drilled and hydraulically frac-

tured shale) and conventional gas wells in Devonian reservoirs in the Appalachian Plateau region of western

Pennsylvania range from 0.6 to 17 mmol kg�1, and Li isotope ratios, expressed as in d7Li, range from +8.2 to

+15&. Li concentrations are as high as 40 mmol kg�1 in produced waters from Plio-Pleistocene through Jurassic-

aged reservoirs in the Gulf Coast Sedimentary Basin analyzed for this study, and d7Li values range from about

+4.2 to +16.6&. Because of charge-balance constraints and rock buffering, Li concentrations in saline waters

from sedimentary basins throughout the world (including this study) are generally positively correlated with chlo-

ride (Cl), the dominant anion in these fluids. Li concentrations also vary with depth, although the extent of depth

dependence differs among sedimentary basins. In general, Li concentrations are higher than expected from sea-

water or evaporation of seawater and therefore require water–mineral reactions that remove lithium from the

minerals. Li isotope ratios in these produced waters vary inversely with temperature. However, calculations of

temperature-dependent fractionation of d7Li between average shale d7Li (�0.7&) and water result in d7Liwater

that is more positive than that of most produced waters. This suggests that aqueous d7Li may reflect transport of

water from depth and/or reaction with rocks having d7Li lighter than average shale.

Key words: Appalachian Plateau, diagenesis, fluid transport, formation water, Gulf Coast Sedimentary Basin,

lithium, produced water, saline water, stable lithium isotope ratios, water–rock reaction

Received 15 October 2013; accepted 5 May 2014

Corresponding author: G. L. Macpherson, Department of Geology, University of Kansas, 1475 Jayhawk Blvd, Rm.

120 Lindley Hall, Lawrence, KS 66045, USA. Email: glmac@ku.edu. Tel: +1 (785)864-2742. Fax: +1 (785)864-5276.

Geofluids (2014) 14, 419–429

INTRODUCTION

Lithium, a low-mass lithophile alkali element, has two sta-

ble isotopes, 6Li and 7Li; the Li isotope ratio (7Li/6Li) has

been used to understand crustal processes, including

water–rock reactions in near-surface environments (see

Tomascak 2004; Millot et al. 2010b; Tipper et al. 2012;

Romer et al. 2013; and references therein). The behavior

of Li in marine sediments and pore waters has been used

to understand processes in sedimentary basins (Kharaka

et al. 1973, 1977; Breen et al. 1985; Macpherson 1989).

Li isotopes in saline waters can aid in identifying sources of

Li, mixing of different fluids and progressive water–rock

reactions (Zhang et al. 1998; Chan et al. 1992, Chan

et al. 1994; Chan et al. 2002; Bottomley et al. 2003;

Millot et al. 2011; Eccles & Berhane 2011; see Table 1).

In marine settings, Li is removed from seawater during

weathering of ocean basalt and incorporated into authigen-

ic smectite during which isotopic fractionation enriches the

rock in 6Li and the fluid in 7Li. The fractionation is larger

at lower temperatures, and the process may be complicated

by ion-exchange reactions with ammonium (Zhang et al.

1998). Seafloor hydrothermal fluids are enriched in Li and

have lower 7Li/6Li compared with seawater (Magenheim

et al. 1995), suggesting that the relatively incompatible Li

is removed from unspecified minerals in the sediments, and

the progression is toward isotopic equilibrium with a

source rock having a lower 7Li/6Li. Removal of Li from

rock without isotopic fractionation has also been docu-

mented for metamorphic reactions (Qiu et al. 2011a,b).

Sedimentary basins are generally warm, low pH, reduc-

ing environments with decreasing porosity with depth

© 2014 John Wiley & Sons Ltd

Geofluids (2014) 14, 419–429 doi: 10.1111/gfl.12084

(Day-Stirrat et al. 2010) and fluids with salinity mostly

greater than seawater. Processes affecting water chemistry

in sedimentary basins (Morton & Land 1987; Land &

Macpherson 1992; Land 1995; Kharaka & Hanor 2003)

include the following.

(1) Mixing with other aqueous fluids, including inherited

evaporated seawater and mineral-dehydration water.

(2) Mineral dissolution, such as evaporite minerals, feld-

spars, calcite, quartz, and heavy minerals.

(3) Mineral alteration, including progressive transforma-

tion of smectite and smectite-illite to more crystalline

illite (generally complete by approximately 100°C;Eberl & Hower 1976); albitization of plagioclase;

dolomitization of limestone; and transformation of

gypsum to anhydrite.

(4) Mineral precipitation, including formation of authigen-

ic clays, quartz, calcite, siderite, and ankerite.

(5) Maturation of organic matter.

Of these processes, the clay transformations are the most

likely to affect Li concentration and isotope ratios, because

clays are the largest reservoir of Li in this environment

(Macpherson 1989). Shale (mudrocks) contains the highest

amounts of clay minerals and also is the most abundant

lithology in most sedimentary basins (Sharp et al. 1988;

Hanor & McIntosh 2007), and so shale will be considered

the most important source of Li here. This study examines

the relationships between temperature and lithium concen-

trations and isotope ratios in waters produced from oil and

gas wells (produced waters) in two tectonically different

sedimentary basins in the United States, the Appalachian

Plateau region of the Appalachian Basin and the Gulf of

Mexico Coastal Sedimentary Basin (Gulf Basin).

In the first target of this study, the Appalachian Plateau,

use of horizontal drilling combined with hydraulic fractur-

ing to enhance porosity and permeability in shale (uncon-

ventional production) has led to greatly increased

production of methane and other hydrocarbons. Currently,

the primary target of unconventional production is the

organic matter-rich (2–15 wt.%) Middle Devonian Marcel-

lus Formation (Zagorski et al. 2011). The Appalachian

Basin is an extensively folded and faulted foreland basin

that was affected by several tectonic events in the Paleo-

zoic, culminating in the Permian Alleghanian orogeny.

Most of the large-displacement faulting and high-ampli-

tude folding are confined to the southeastern basin margin,

while the Marcellus Shale gas play, which extends from

New York and Ohio to West Virginia (Karbo et al. 2010),

is located in the less deformed Appalachian Plateau to the

west. Nonetheless, tectonically driven fluids are thought to

have affected even undeformed portions of the basin (Oli-

ver 1986; Evans 1995; Osborn et al. 2012).

In contrast, the second target of this study, the well

characterized Gulf of Mexico Coastal Sedimentary Basin

(Gulf Basin), has had a comparatively simple tectonic

Table 1 Data sources

Basin or location abbreviation Basin or location name Age of host rocks Reference Graph symbol

Lithium isotope ratios and dissolved species concentrationsIL Israel Cretaceous Chan et al. (2002) +

PB Paris Basin, France Triassic Millot et al. (2011) ΔGB Gulf Coast Sedimentary Basin,

AL to TX and offshore LA,USA

Jurassic to Pleistocene This study

AP-UD Appalachian Plateau, USA Upper Devonian This studyAP-MD Appalachian Plateau, USA Middle Devonian This study

WB Williston Basin, Canada Middle Devonian Bottomley et al. (2003)

AB Alberta Basin, Canada Devonian Eccles & Berhane (2011)

Dissolved species concentrationsKN Kettleman North Dome field,

CAMiocene Kharaka et al. (1973); Kharaka &

Berry (1974)GB Gulf Coast Sedimentary Basin,

AL to TX and offshore LA,USA

Jurassic to Pleistocene GB database, includingunpublished data; Kharaka et al.(1977); Carothers et al. (1978); Land& Prezbindowski (1981); Morton &

Land (1987); Land et al. (1988);Macpherson (1992); Land &Macpherson (1989, 1992)

GB-MSB Mississippi Salt Basin of GB,USA

Jurassic–Lower Cretaceous Kharaka et al. (1987)

AB Alberta Basin, Canada Cambrian–Triassic Eccles & Jean (2010)

AP Appalachian Plateau, PA, USA Lower Silurian–Devonian Dresel & Rose (2010)AP-Sil Appalachian Plateau, eastern

Ohio, USASilurian Albion Group(‘Clinton’)

Breen et al. (1985)

MB Michigan Basin Devonian Wilson & Long (1993)

AP, Appalachian Plateau; UD, Upper Devonian; MD, Middle Devonian; GB, Gulf Coast Sedimentary Basin; P, Plio-Pleistocene, undifferentiated; M, Miocene,undifferentiated; V, Oligocene Vicksburg Fm; W, Paleocene-Eocene Wilxoc Fm; E, Cretaceous Edwards Fm; S, Jurassic Smackover Rm.

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

420 G. L. MACPHERSON et al.

history: the basin subsided as it filled, with development of

growth faults and penetration by salt domes in some areas

but little other deformation (Sharp et al. 1988). In addi-

tion, the Gulf Basin continues to receive sediment, while

the entire Appalachian Basin is undergoing erosion without

new deposition.

We report here lithium concentrations and isotope ratios

in produced waters from (i) unconventional natural gas

wells completed in the Appalachian Plateau Middle Devo-

nian Marcellus Group (1 vertical well, 3 horizontal wells)

in Pennsylvania, (ii) conventional natural gas wells com-

pleted in the Appalachian Plateau Upper Devonian Brad-

ford and Venango Groups (3 wells) in Pennsylvania, and

(iii) conventional oils and gas wells in the Gulf Basin (Plio-

Pleistocene to Jurassic-aged reservoirs, 6 wells located in

Texas and offshore Louisiana). In order to understand the

distribution of Li in sedimentary basins and to explore

whether there are commonalities in the lithium isotope

ratio systematics despite differences in basin tectonics and

production reservoir composition, these are described in

context of published lithium isotope ratios and/or concen-

trations reported for the Alberta Basin, Michigan Basin, an

oilfield in Israel, Kettleman North Dome field in the San

Joaquin Basin in California, the Paris Basin and a single

sample from the Williston Basin (Table 1).

METHODS AND DATA

Sample collection and treatment

Appalachian Plateau samples were collected by personnel

from University of Pittsburgh, DOE-NETL and USGS.

Samples were collected as close to the wellhead as possible,

although some are separator, and some are holding tank

samples. Middle Devonian reservoir samples were collected

from unconventional gas wells in which fresh water (not

recycled saline water) was injected during the fracturing

process. Produced water chemistry changes with time dur-

ing early production, with most cation concentrations

increasing rapidly over the first several weeks of produc-

tion, then reaching near steady-state levels thought to rep-

resent in situ water chemistry (Chapman et al. 2012).

Samples for this study were collected during this later state.

Samples from the Gulf Basin reservoirs are part of a Uni-

versity of Texas database of Gulf Basin fluids, some of

which were collected by the first author.

All samples were filtered in the field, and most were

field-acidified to pH <2 using standard protocol (Kharaka

& Hanor 2003); a comparison of the d7Li of samples

filtered and acidified in the field with samples from the

same well and sampling time (i) filtered in the field but

acidified in the laboratory and (ii) refiltered in the labora-

tory and acidified in the laboratory, show no significant

differences (2r is within � 0.5&).

Analytical methods

Lithium and other cation concentrations were determined

by ICP-OES or ICP-MS; anion concentrations were deter-

mined by ion chromatography or titration (alkalinity). For

lithium isotope determination, lithium was separated from

other cations using column chromatography, following a

protocol modified from James & Palmer (2000). For every

sample, the 5 ml aliquot before and after the aliquot used

for lithium isotope determination was analyzed for lithium

and other cation concentrations to assure complete separa-

tion of lithium from other cations. Lithium isotope ratios

were measured with a Neptune Plus multicollector ICP-

MS at the University of Pittsburg, using the bracketing

method (blank–standard–blank–unknown–blank–standard–

blank) with 7Li and 6Li ions measured simultaneously. Iso-

tope ratios are reported as d7Li using the standard del

notation in units of &: (Rsmp/Rstd – 1) 9 1000, where

‘R’ is the 7Li/6Li atomic ratio, ‘smp’ is the sample, and

‘std’ is the NIST RM 8545 (LSVEC) standard. RM 8545

has a reported atomic 6Li/7Li of 0.08215 � 0.00023. The

precision of our repeated measurement of RM 8545, pur-

chased in 2012, was � 0.52& (2r, standard error of the

mean, n = 16). Mean and precision of our measurements

of IRMM-16 (reported as 0.35 � 1.41&, Qi et al. 1997)

was 0.07 � 0.35& (n = 11) and of Li7-N (reported as

30.4 � 1.1&, Carignan et al. 2007) was 30.07 � 0.16&(n = 11). We manufactured a Na-Ca-Cl fluid with total

dissolved solids of 110 parts per thousand and 50 ppm Li

to test the chromatographic separation of Li on a fluid

with a composition similar to the produced waters. Aver-

age d7Li of the 5 full procedural replicates and 3 within-

run replicate measurements of the lithium separated from

the Na-Ca-Cl fluid was 79.1 � 0.8&. Average of two rep-

licate measurements of the Li stock solution used to make

the artificial brine was 79.2 � 0.7&. For the produced

water samples, within-run replicates, long-term replicates,

and full procedural duplicates are all within less

than � 0.7& from the average, and within-run precision

was less than � 0.2& (Table 2). Full procedural blanks,

using a 6Li-enriched solution for isotope dilution determi-

nation, contained <0.04 ng lithium, and thus contribute

insignificantly to Li isotope ratios measured in samples,

that contained approximately 5 lg Li.

Lithium concentration and isotope ratios for our samples

are given in Table 2.

RESULTS

Li concentrations and Cl concentrations in Appalachian

Plateau produced waters analyzed in this study range from

about 0.6 to 17 and about 360 to 3000 mmol kg�1,

respectively (Table 2, Fig. 1). In Gulf Basin produced

waters analyzed in this study, Li and Cl concentrations

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

Li isotope ratios in saline water 421

range from about 0.6 to 40 and about 800 to

4500 mmol kg�1, respectively. In other sedimentary basin

data compiled for this study, all but Kettleman North

Dame samples fall within the ranges for our data; Kettle-

man North Dome samples are more dilute.

d7Li of Appalachian Plateau samples analyzed for this

study range from about +8.2 to +15.2&, while Gulf Basin

samples analyzed for this study cover a wider range, from

about +4.2 to +16.6&. Compiled values of d7Li in pro-

duced waters used in this study fall with the range of Gulf

Basin samples except for Israel oil field samples, which

range up to the d7Li of seawater (about +31&).

DISCUSSION

Li concentrations

Li concentrations generally increase with depth in most

sedimentary basins, but there is also large variation at any

one depth, even within a single basin (Fig. 1a). In the

Alberta Basin and Appalachian Plateau, the maximum

aqueous Li concentration occurs at about 2.5 km. The Li

maximum in the Appalachian Plateau fluids

(17 mmol kg�1) is slightly higher than the maximum in

the Alberta Basin (about 14 mmol kg�1). In the Gulf

Table 2 Lithium concentrations and isotope ratios, new data for this study

Database ID Analysis type State County or offshore block Depth, m* Temperature, °C*

(a) Sample identification and propertiesAppalachian Plateau samples

AP-UD002 PA Clarion Co. 335* 15.3–17.4*AP-UD003 PA Greene Co. 1400 38.6–47.5*AP-UD004 PA Greene Co. 1250 36.2–44.4*AP-MD002 PA Greene Co. 2450 60.9–77.1*AP-MD002 MSDAP-MD002 FPDAP-MD003 PA Greene Co. 2450 60.9–77.1*AP-MD004 PA Washington Co. 1446–2460* 39.8–76.1*AP-MD004 MSDAP-MD005 PA Westmoreland Co. 1906–2937* 48.5–88.2*

Gulf Coast Sedimentary Basin SamplesGB-P020 LA offshore Eugene Island, Block 313 2822 84GB-M015 LA offshore State Lease 340 2972 91GB-V007 TX Hidalgo Co. 3749 154*GB-V007 MSD

GB-W039 TX Goliad Co. 2319 80GB-W039 MSDGB-E017 TX Karnes Co. 4090 157GB-S001 AL Mobile Co. na naGB-S001 MSD

Database ID Cl, mmol kg�1 Li, mmol kg�1 d7Li,& �2 s

(b) Concentrations and isotope ratiosAppalachian Plateau samples

AP-UD002 361 0.56 15.23 0.24AP-UD003 2290 2.83 10.50 0.24AP-UD004 2390 3.23 12.49 0.19AP-MD002 2290 11.5 9.48 0.17AP-MD002 9.51 0.18AP-MD002 9.54 0.16

AP-MD003 2210 17.4 9.50 0.16AP-MD004 2970 9.67 9.32 0.15AP-MD004 9.38 0.16AP-MD005 2020 7.40 8.15 0.16

Gulf Coast Sedimentary Basin samplesGB-P020 1930 0.62 10.69 0.15GB-M015 1360 0.62 8.79 0.15

GB-V007 2920 5.47 9.32 0.15GB-V007 10.05 0.15GB-W039 816 0.7 8.11 0.14GB-W039 7.85 0.16GB-E017 4530 39.5 4.21 0.16GB-S001 4220 9.96 16.60 0.17

GB-S001 16.27 0.14

*Temperature estimated from depth, mean annual temperature and geothermal gradient; depth estimated from countywide range of depth to Marcellus Shale.MSD, Measurement same-day duplicate; FPD, Full procedural duplicate; na, data not available.

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

422 G. L. MACPHERSON et al.

Basin, the maximum Li concentration, about

40 mmol kg�1, is at approximately 4 km depth. The Mis-

sissippi Salt Dome Basin, located in the southwestern part

of the state of Mississippi (USA), is a sub-basin within the

Gulf Basin containing very high salinity, metal-rich fluids

(Kharaka et al. 1987) that are distinct from most Gulf

Basin produced water. In samples from this basin, the max-

imum Li concentration is found at a depth of about 4 km

as it is in the rest of the Gulf Basin, but it is a only about

10 mmol kg�1. There are few published data from other

basins, but Li concentrations and depths to maxima are

similar to those in the Alberta and Gulf Basins.

Aqueous cation concentrations generally increase with

increasing Cl concentration in produced waters (Hanor

1994, 1996, 2001) because of the dominance of Cl among

the anions, charge-balance constraints, and fluid chemistry

buffering by carbonate and silicate mineral phases. In gen-

eral, salinity increases in a nonuniform way with depth in

many sedimentary basins (Kharaka & Hanor 2003),

because of dissolution of evaporite minerals in bedded salt

and salt domes, water–rock reactions and fluid transport.

Evaporation of seawater is often considered the explanation

for Cl concentrations higher than seawater, as well as for

some other dissolved species concentrations that are higher

than seawater. Many sedimentary basins contain evaporite

deposits, although the San Joaquin Basin that hosts the

Kettleman North Dome field does not (Scheirer & Ma-

goon 2007). In the Alberta Basin, Gulf Basin, and Paris

Basin, evaporites are found in the basal sediments. The

Appalachian Plateau study area contains Upper Cambrian

to Permian sediments; evaporites are well known in Silu-

rian-age sediments at intermediate depths of the approxi-

mately 5 km of sediments, and anhydrite has been

reported in Middle Ordovician (Ryder et al. 2009) and

Cambrian units (Bacon et al. 2009). The Michigan Basin

also contains evaporites at intermediate depths rather than

in basal sediments.

In the data compiled for this study (Fig. 1b), lithium

concentrations are almost all higher than seawater, with

the exception of the Kettleman North Dome dataset,

where original fluid was probably meteoric water (Kharaka

et al. 1973). All samples contain Li concentrations greater

than expected from closed-system evaporation of seawater.

The 1:1000 line shows that, in most produced water, Li

generally increases with Cl (Fig. 1b). This is similar to the

trends for major cations (see above). Only the Mississippi

0

2000

4000

6000

0 10 20 30 40 50

IL, LCre

PB, Tri

GB, this study

GB-MSB, J-UK

AP, UDev, this study

AP, MDev, this study

WB, Dev

AB, K - Tri

GB, J - Pleis

AP, Sil - Dev

MB, Dev

++

0.001

0.010

0.100

1.000

10.000

100.000

10 100 1000 10 000

Li, m

mol

kg–

1

Cl, mmol kg–1

Dep

th, m

Li, mmol kg–1

Seawater evaporation 1:1000 line KN, Mio IL, LCre GB, this study, Jur-Pleis GB, Jur-Pleis GB-MSB, Jur-UCre PB, Tri AP, UDev, this study AP, MDev, this study AP, LSil, LDev, UDev AP, Sil, E Ohio AB, MDev-UDev WB, Dev MB, Dev

(A)

(B)

Fig. 1. Relation between Li and some fundamen-

tal basin variables in a variety of produced

waters. Symbols are given in Table 1. (A) Li ver-

sus sampling depth. The distributions of large

datasets are enclosed by lines: Alberta Basin

(AB), short-dashed line; Gulf of Mexico Coastal

sedimentary basin (GB), dash-dot line; Appala-

chian Plateau (AP), gray line. Samples used in

this study from the AP and GB are shown, as

are data from an Israeli oil field (IL), the Paris

Basin (PB), the Mississippi Salt Dome Basin of

the GB (GB-MSB), and a single datum from the

Williston Basin (WB). Kettleman North Dome

(Miocene; KN) samples are not plotted, but

have Li less than 0.5 mmol kg�1 at depths

between approximately 2 to 2.6 km. (B) Li ver-

sus Cl concentrations. Modern seawater evapo-

ration trend (McCaffrey et al. 1987) and

1:1000 line with arbitrary starting point are

shown for reference.

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

Li isotope ratios in saline water 423

Salt Basin of the Gulf Basin (Kharaka et al., 1987) and the

Michigan Basin (Wilson & Long 1993) samples follow

trends similar to seawater evaporation, consistent with ear-

lier interpretations of the major dissolved species in these

fluids. Although the Li:Cl relation follows a 1:1000 line,

Cl concentration is not a good predictor of Li concentra-

tion: Li concentration varies more than two orders of mag-

nitude for a single Cl concentration. This contrasts with

major cations, where the cation concentration generally

varies over much less than one order of magnitude at any

Cl concentration (Hanor 2001). For the entire dataset

used in this study, Cl concentration ranges over about 2.5

orders of magnitude and Li concentration over nearly 4

orders of magnitude. These observations suggest that (i)

evaporation of seawater or dilution of evaporated seawater

cannot account for the anomalously high Li content of

some samples, (ii) there is contribution from diagenesis of

Li-bearing minerals, and (iii) basin-to-basin (or even strata-

to-strata) variations in the Li concentration in the source

minerals must control the Li concentrations in produced

water.

Variations in major elements in saline waters from reser-

voirs of different ages may be attributed to time-varying

seawater chemistry (Lowenstein & Timofeeff 2008). For

example, Cretaceous and Late Silurian Ca-rich brines may

reflect seawater Ca concentrations approximately 3 times

greater than present. Experimental work shows that Li/Ca

in foraminifera and inorganic calcite is controlled by salin-

ity, temperature, and species (Marriott et al. 2004). Fossil

chemistry suggests little variation in Li/Ca in foraminifera

since the end of the Cretaceous (Delaney & Boyle 1988;

Hathorne & James 2006; Lear & Rosenthal 2006), coinci-

dent with little change in major-ion chemistry of seawater

during that time. If the nearly constant Li/Ca can be

extended back in time, even a threefold increase in Ca and

Li during Ca-seawater episodes is insufficient to explain Li

concentrations in produced waters that are 10–100 times

greater than seawater and evaporated seawater. Seawater Li

is controlled by continental weathering, seafloor weather-

ing, and ocean-floor sequestration of Li (Stoffyn-Egli &

Mackenzie 1984; Hathorne & James 2006). Therefore,

differences in the chemistry of seawater deposited with the

sedimentary basin sediments cannot be the sole explanation

for present Li concentrations.

Li isotope ratios

d7Li of produced waters varies inversely with lithium con-

centration, both in this work and in previous studies

(Fig. 2). d7Li ranges between the seawater value (d7Liapproximately 31&; for summary see Tomascak 2004) and

that of continental crust (d7Li approximately 0&; e.g.,

Teng et al. 2004). Analyses of shale sampled from cores,

to minimize the effects of recent weathering, are few,

restricted to samples from Australia (n = 10), Japan

(n = 2) and South Dakota (n = 1). The mean and median

d7Li for these 13 shales (James & Palmer 2000; Sturchio

& Chan 2003; Teng et al. 2004) are similar (�0.2& and

�0.7&, respectively), although the range is fairly wide

(8.4&).

Appalachian Plateau (Middle Devonian reservoir), Gulf

Basin (Cretaceous reservoir), and some Alberta Basin pro-

duced waters (Devonian reservoir) have both extremely

light d7Li and high Li concentration (Fig. 3). The Appala-

chian Plateau samples do not fall on simple mixing lines

between Middle Devonian and Upper Devonian Appala-

chian Plateau water (mixing line not shown) or with sea-

water (Fig. 2), even allowing the largest documented

range of past seawater d7Li (Misra & Froelich 2012). Fur-

ther, there is no discernable distinction between fluids pro-

duced from unconventional shale gas reservoirs and those

from conventional oil and gas reservoirs (Table 2). The

progression toward higher lithium concentration with

lighter d7Li, approaching continental crust and shale val-

ues, instead suggests a temperature-controlled process.

Temperature-controlled isotopic exchange between lith-

ium in solid phases and water is expressed by the fraction-

ation factor, a, which is the ratio of the lithium isotopic

ratio in the solid to that in the water at equilibrium at a

fixed temperature. There have been few determinations of

this factor at different temperatures. Chan et al. (1994)

reported results of three calculations of a over a tempera-

ture range of 2–350°C, using vent fluid and seawater

chemistry in comparison with variously altered basalt. Mil-

lot et al. (2010a) reported results of four laboratory exper-

iments over a temperature range of 25–250°C, reacting

seawater with basalt. At all temperatures, the fractionation

0

10

20

30

40

0.01 0.10 1.00 10.00 100.00

7 Li,

‰

Li, mmol kg–1

+++++++++++++++

Fig. 2. Relation between d7Li and Li concentration for this study and pub-

lished studies. Symbols same as in Figure 1 and Table 1. Two mixing lines

(Li in seawater evaporated to the Br level in the Middle Devonian samples

mixed with an extreme Middle Devonian sample, long-dashed line; modern

seawater mixed with the Gulf Basin sample with the lightest d7Li, dot-dash

line) show that mixing does not adequately explain the distribution of data.

Although estimates of Paleozoic seawater have not been published, Ceno-

zoic seawater may have had d7Li as low as 22& (Misra & Froelich 2012);

mixing with Cenozoic seawater also would not explain the data distribu-

tion.

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

424 G. L. MACPHERSON et al.

factor is less than one, showing that liquid in equilibrium

with a solid will be heavier than the corresponding solid.

Equations fit to these datasets (this work and Scholz et al.

2010) give similar but not identical trends of a depen-

dence on temperature, in that the Millot data trend pre-

dicts a slightly lower a at any temperature than the Chan

data trend. Although originally designed to investigate sea-

water–basalt interactions, the data cover a temperature

range convenient for testing equilibration of sedimentary

basin fluids with shale. Experimental work determining

fractionation factors between fluids and sediments or min-

erals (Chan et al. 1994; Williams & Hervig 2005; Vigier

et al. 2008) are significantly different from the seawater–

basalt data and reveal very different fractionation factors

among the different experiments. This is probably the

result of different initial mineralogy of the sediments as

well as secondary mineral formation. For this study, we

elected to use the more consistent fractionation factor

experiments from water–basalt reactions, recognizing that

these will only approximate the lithium fractionation when

highly saline fluids react with shale or mudrock.

Due to the difficulty in determining a reasonably accu-

rate in situ temperature (MIT 2006) as well as proprietary

issues, there are few data for temperature (and/or depth).

For the Appalachian Plateau samples reported in this study,

temperatures were estimated using estimated depth to

production interval, mean annual air temperature and

geothermal gradient. The geothermal gradient in the area

ranges from 20°C km�1 (Renner & Vaught 1979) to

26.5°C km�1 (Rowan 2006; Yeager & Meyer 2010). For

most of the Appalachian Plateau samples, exact locations

are not known, so countywide estimates of these parame-

ters were used to estimate in situ temperature.

Data plot along a coherent mass fractionation trend that

is subparallel to the trend calculated for d7Li (water) in

equilibrium with a solid having d7Li of �0.7&, the median

for shale (discussed above; Fig. 3). Most compiled data

with reported temperatures also plot along the same trend,

which is close to the equilibrium line for a solid with d7Liof around �5&. This is outside the range of reported d7Lifor shale from core, and the small difference between the

water–rock fractionation trends using the Chan et al.

(1994) and Millot et al. (2010a) experimentally derived

fractionation factors (Fig. 3) does not explain the offset in

published data and our data from the equilibrium fraction-

ation line. Possible explanations for this are discussed next.

Nonequilibrium isotope fractionation

During the approach to the equilibrium state such as that

which occurs in modern weathering (Millot et al. 2010b;

Tipper et al. 2012) or in seawater–seafloor reactions (Simon

et al. 2006), a fluid is not expected to be at steady state with

respect to d7Li. Instead, its d7Li reflects an end member rock

or sediment composition and fractionation associated with

formation of secondary minerals, usually taken as removal of

Li from water accompanied by an increase in d7Li in the

water. With increased weathering progress, the d7Li of theliquid approaches equilibrium with the rock or sediment

with which it is reacting. In contrast, the produced waters in

sedimentary basins have been in contact with host rocks for

many tens to hundreds of millions of years. This suggests

that equilibrium between host rock and produced water is

likely in most sedimentary basins, and attributing the offset

in the trend of d7Li with temperature from the trend

expected for average shale is not reasonable.

Equilibrium with shale of lighter d7LiIt is remarkable that samples from this study and most

samples from the Paris Basin (where temperatures are also

estimated) all fall along the equilibrium line for a �5&material, suggesting the possibility that shale in sedimen-

tary basins is lighter than most shale measured to date.

Fluid migration after irreversible fluid–rock reactions

Previous work has shown that fluids and solutes in sedi-

mentary basins can migrate, sometimes for distances of

hundreds of kilometers (Ziegler & Longstaffe 2000;

Kharaka & Hanor 2003; Eccles & Berhane 2011). If fluids

come into d7Li-shale equilibrium at depth (and higher

temperature) and migrate upward without back reaction,

then the d7Li could be indicative of the maximum burial

of the fluid. The Appalachian Basin has undergone tecto-

nism, resulting in extensive faulting and folding in the

Valley and Ridge Province and less extensive deformation

0

5

10

15

20

25

0 100 200 300

7 Li,

‰

Temperature, °C

Solid phase: –0.7‰

Solid phase: –5‰

Fig. 3. Relation between reservoir temperature and d7Li. Symbols in

Table 1. Black solid line is calculated d7Li of water in equilibrium with aver-

age shale (�0.7&) using fractionation factors summarized in Chan et al.

(1994). Black dashed line is expected d7Li of water in equilibrium with aver-

age shale (-0.7&) using Millot et al. (2010a). Gray solid line is calculated

d7Li of water in equilibrium with shale with d7Li of �5&. Error bars for d7Li

are shown, or are smaller than symbol size; errors on in situ temperatures

are not known. Positive error bar on temperature of Appalachian Plateau

samples gives range of possible temperatures based on geothermal gradient

ranging from 20 to 26.5°C km�1, and for Washington and Westmoreland

County samples, the depth estimate of the Marcellus Formation as well as

the geothermal gradient range.

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

Li isotope ratios in saline water 425

in the Appalachian Plateau Province, with the potential for

fluid movement as a result (Evans 1995; Evans et al. 2012;

Osborn et al. 2012). Rowan (2006), using vitrinite reflec-

tance, suggests that approximately 1800–3000 m (6000–

10 000 ft) of sediment has been eroded from southwestern

Pennsylvania. Before erosion, under a geothermal gradient

of 25°C km�1, the Devonian basin fill was subjected to

higher temperatures (from 45 to 75°C higher than present

in situ temperatures). In Greene County, near the western

Pennsylvania side of the Rowan (2006) cross section, cur-

rent in situ temperatures in the Middle Devonian wells are

approximately 60°C. A temperature of approximately

140°C would be required to explain the fluid-d7Li if the

shale-d7Li is approximately 0&; this is a temperature dif-

ferential of +80°C. The erosion hypothesis predicts a for-

mer temperature up to only approximately 115°C, and so

does not support, by itself, equilibrium with the median

shale d7Li-shale. In addition, Evans et al. (2012 and refer-

ences therein) suggest maximum burial temperatures of

only 80–118°C, based on fluid inclusions in late quartz

cement.

If the Gulf Basin and Paris Basin fluids, both in tectoni-

cally simpler basins than the Appalachian Plateau, indicate

fluid migration during compaction of the sediments, then

migration paths must be long: the temperature differentials

(again assuming a shale with d7Li of �0.7&) for Paris

Basin fluids are about 100°C, and the temperature differ-

entials for the Gulf Basin fluids range from about 30–

100°C, the latter requiring an upward fluid migration dis-

tance of up to approximately 4 km. Macpherson (1989),

based on mass balance constraints, proposed that Li in

Gulf Basin fluids represents the outcome of ongoing

greenschist facies metamorphism in the Gulf Basin and

upward fluid transport of Li removed from metamor-

phosed sediment.

Fractionation in sedimentary basins differing from that in

recent marine sediments

The fractionation factor–temperature trend used in this

study is based on experimental work reacting seawater and

basalt. Whether burial of mud in sedimentary basins and

the accompanying diagenetic processes fractionate lithium

isotopes differently is as yet fully unexplored. Day-Stirrat

et al. (2010) have shown that Upper Paleocene and Lower

Eocene mudstone in the Gulf Basin acts as an open system

with respect to potassium, so that potassium is imported

into the shale and is incorporated into illite transformed

from smectite. Whether this process is countered by an

export of monovalent ions and/or by fractionation of lith-

ium isotopes has not been investigated. Williams & Hervig

(2005) ran laboratory experiments at 300°C and 100 MPa

showing that larger sized clay minerals (>2 lm) may frac-

tionate Li isotopes 6& more (lighter) than smaller clay

minerals (<2 lm) during the illitization of smectite, when

exchangeable Li is excluded and when the solution con-

tains only Li and no other cations. The direct applicability

of this result to sedimentary basin diagenesis has not been

tested, but it is well established that the small size fraction

of Gulf Basin mudstones (<2 lm) is typically more than

50% clay minerals, while the larger fraction (>2 lm) is

<10% clay minerals (Hower et al. 1976).

Based on the above discussion, the d7Li–temperature

relations in the Appalachian Plateau and Gulf Basin pro-

duced waters reported here could result from fluid migra-

tion, equilibrium with a shale of lighter-than-average shale

d7Li, or a combination of these, or from isotopic fraction-

ation during the smectite-illite transition, a diagenetic reac-

tion common in sedimentary basins.

CONCLUSIONS

Values of d7Li for water produced from oil and gas wells

from the Appalachian Plateau Middle and Upper Devonian

reservoirs and Gulf Coast Sedimentary Basin (Gulf Basin)

Plio-Pleistocene to Jurassic reservoirs range from a high of

about +16.6&, which is lighter than seawater at +31&, to

a low of about +4.2&, the latter extending the published

range of d7Li in produced water.

Li concentration increases with depth more rapidly and

attains a higher concentration at shallower depth in Appa-

lachian Plateau produced water than in other sedimentary

basins. The highest Li concentration in the dataset consid-

ered is from the Gulf Basin. In all sedimentary basins con-

sidered, Li concentrations are higher than those expected

from seawater, diluted seawater or evaporated seawater.

The latter observation supports the idea that water–mineral

reactions add Li to produced water.

Concentrations of Li and Cl (the dominant anion in

produced waters) are generally positively related, but there

are wide variations in Li/Cl. Some restricted datasets, such

as Gulf Basin–Mississippi Salt Basin and Kettleman North

Dome, follow tighter Li-Cl trends, the former following a

trend parallel to but elevated in Li above seawater evapora-

tion, and the latter following a dilution trend for Li ele-

vated above seawater concentration with Cl diluted from

seawater concentration.

The d7Li of Appalachian Plateau and Gulf Basin pro-

duced waters, as well as previously published data from

produced waters in the Paris Basin, fall on a trend parallel

to the trend predicted for fractionation of lithium isotopes

in equilibrium with the median d7Li of shale. If this trend

indicates fluid migration from depth (or higher tempera-

ture), then migration paths are long. The trend can be

explained by migration in combination with reaction with

sedimentary basin rocks with a more negative d7Li than

the average shale. Because the equilibrium fractionation

factors used in these calculations were determined experi-

mentally on seawater reactions with basalt, additional work

© 2014 John Wiley & Sons Ltd, Geofluids, 14, 419–429

426 G. L. MACPHERSON et al.

is needed to resolve whether the offset from the marine–

sediment equilibrium trend is the result of a different type

of fractionation during diagenesis in sedimentary basins, or

the result of equilibration with lighter d7Li shale with or

without fluid migration. Finally, even though the Appala-

chian Plateau Middle Devonian shale producing gas is

unusually organic rich and gas production method is

unconventional, the fluids produced from this unit are sim-

ilar to other produced waters and suggest a continuum of

water–rock processes across conventional (sandstone, lime-

stone) and unconventional (tight shale) reservoirs.

ACKNOWLEDGEMENTS

This work was supported by the National Energy Technol-

ogy Laboratory’s ongoing research under the RES contract

DE-FE0004000. We also thank the Geology Foundation

of the University of Kansas, and the University of Kansas

Department of Geology. This work could not have been

completed without laboratory and field support from per-

sonnel from DOE-NETL, USGS, and University of Pitts-

burgh. The authors thank the reviewers, J.S. Hanor and P.

Tomascak, for their detailed and insightful comments that

greatly improved this manuscript. The authors declare no

conflict of interest.

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