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Journal of Petroleum Science and Engineering

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Origin and evolution of formation water from the Ordovician carbonatereservoir in the Tazhong area, Tarim Basin, NW China

Hongxia Lia, Chunfang Caia,b,⁎

a Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, CAS, Beijing 100029, Chinab Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, China

A R T I C L E I N F O

Keywords:Formation waterFluid migrationWater-rock interactionThermochemical sulfate reductionEnvironmental isotopesTarim Basin

A B S T R A C T

Chemistry, H, O and Sr isotopes of formation waters were determined from the Ordovician carbonate reservoirsin the Tazhong area, Tarim basin. The aim is to elucidate their origin and migration and their alteration bythermochemical sulfate reduction (TSR) and water-rock interactions. The waters were originated fromevaporated seawater, and are enriched in Ca and Sr, and depleted in Mg and SO4 compared with the seawaterevaporation trajectory (SET), thus are considered to result from dolomitization, anhydrite dissolution and insitu TSR. δ18O values show positive shift due to water-rock interactions, and δD values show negative shift withincreasing TSR extents or total thiaadamantanes concentrations of the associated oils, likely resulting fromhydrogen isotope exchange with TSR-H2S. However, the δD-δ18O relationship was mainly controlled by themixing with paleo-meteoric water with δD of −50‰ and δ18O of −7.5‰. Paleo-meteoric water may haveleached the Silurian siliciclastic rocks and thus has 87Sr/86Sr up to 0.715. Upper Ordovician (O3l) formationwater shows more negative δD and δ18O values, higher 87Sr /86Sr ratios and lower total dissolved solids (TDS)than the O1–2y water, which may have resulted from mixing of paleo-meteoric water. Lower Ordovician (O1–2y)formation water shows 87Sr /86Sr ratio of 0.7095–0.7105, TDS from 160 to 240 g/l, and Sr concentrations from300 to 1000 mg/L, which may have resulted from mixing with hydrothermal fluid up-migrated from Ediacaranand Lower Cambrian siliciclastic rocks. Significant mixings with paleo-meteoric water and hydrothermal fluidoccurred at paleo-highland at the east of No.1 Fault-Slope Zone and active faulting zone near No.10 StructuralBelt, respectively. Hydrothermal fluid and oils probably shared similar migration pathways in No.10 StructuralBelt. Formation water geochemistry may thus provide clues to constraint on petroleum migration.

1. Introduction

Although the importance of subsurface formation water for hydro-carbon migration, mineralization and hydrogeology has been widelyrecognized (Worden et al., 1999; Moldvanyi et al., 1993; Cowie et al.,2014; Bagheri et al., 2014), in most instances, the origin and evolutionhistory of formation water remain controversial. Chemical and isotopiccompositions are widely employed in sedimentary basins to investigatethe origins, migration pathways and water-rock interactions overgeological time. Origins of formation water have previously beenattributed to halite dissolution, evaporation concentrated seawaterand meteoric water (Clayton et al., 1966; Holser et al., 1986; Walteret al., 1990; Birkle and Maruri, 2003; Elias et al., 2007; Lowensteinet al., 2003; Panno et al., 2013).

Cl/Br and Na/Br ratios are used as indicators for seawaterevaporation and halite dissolution-precipitation, because Br is aconservative element for most water-rock interactions except halite

(Carpenter, 1978; Walter et al., 1990; Stueber and Walter, 1991;Warner et al., 2012). 87Sr/86Sr ratios of formation water are onlyinfluenced by the input of radiogenic 87Sr rather than temperature,pressure or phase that can impact the element and/or stable isotopecomposition. Therefore, 87Sr/86Sr ratios are used to constrain theorigin of formation water and interpret regional scale hydrogeochem-ical processes, such as fluid flow issues (Barnaby et al., 2004; Chackoand Deines, 2008; Frost and Toner, 2004; Luz et al., 2009; Pennisi andLeeman, 2000).

Tazhong area is the most complex zone for petroleum exploration inthe Tarim Basin. It has been modified by the Caledonian, Hercynian,Indosinian, Yanshanian and Himalayan tectonic movements since theOrdovician, increasing difficulties to illustrate the origin and evolutionhistory of formation water. Cai et al. (2001b) identified three endmember water in the basin, including high salinity Cambrian evaporiticwater, paleo-meteoric water and highly radiogenic shale-derived waterfrom the Awati Sag. More recently, more than 40 new wells have been

http://dx.doi.org/10.1016/j.petrol.2016.10.016Received 27 April 2016; Received in revised form 4 October 2016; Accepted 6 October 2016

⁎ Corresponding author at: Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, China.E-mail address: cai_cf@mail.iggcas.ac.cn (C. Cai).

Journal of Petroleum Science and Engineering 148 (2017) 103–114

0920-4105/ © 2016 Elsevier B.V. All rights reserved.Available online 06 October 2016

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drilled, from which new formation water, associated petroleum andH2S gas were produced from the Ordovician Lianglitage (O3l) andYingshan (O1–2y) Formations (Cai et al., 2015b), thus providing newconstraints on the origin and migration in this area. Xiang et al. (2010)proposed that water chemistry of the Ordovician formation water inTazhong area was altered by thermochemical sulfate reduction (TSR)by showing the relationships of H2S concentrations to CO2, SO4

2−, Ca2+

and HCO3− concentrations. However, H2S concentrations were shown

to increase due to release from formation water, and thus cannot besimply used to indicate the extents of TSR (Cai et al., 2013; 2015b).Instead, thiaadamantanes (TAs) have been measured from the asso-

ciated oils and their concentrations have been used to indicate TSRextents (Cai et al., 2016), thus providing possibilities to discuss TSReffects on formation water chemical and isotopic compositions.

In this study, we integrated water chemistry, H, O and Sr isotopiccompositions of the Ordovician formation water in Tazhong area.Specifically, we try to address the following questions: (1) What ischaracteristics of the water, and any difference between O3l and O1–2ywater? (2) What is the origin of the water? (3) Whether and how didwater-rock interactions, TSR and fluid mixing change formation waterchemistry, H, O and Sr isotopic compositions? (4) Can formation watergeochemistry be used to trace fluid mixing and oil migration?

Fig. 1. (a) Map of Tarim Basin showing tectonic units and location of Tazhong Uplift; (b) Map of study area showing geological tectonics and location of wells from which formationwater samples were collected.

H. Li, C. Cai Journal of Petroleum Science and Engineering 148 (2017) 103–114

104

2. Geological setting

Tazhong area (or Central Tarim) is located in the center of TazhongUplift, Tarim Basin, northwest China. It is surrounded by the ManjiaerSag, South Depression, Bachu Uplift and Tadong Uplift (Fig. 1a). It canbe divided into the No.1 Fault-slope Zone, North Slope, No.10Structural Belt, Central Faulted Horst Belt, South Slope and EastBurial Hill Zone (Fig. 1b). Tazhong Uplift is one of the major petroleum

production areas in the Tarim Basin. Oils and natural gases have beenfound in the Cambrian-Ordovician carbonate reservoirs and theSilurian-Carboniferous clastic reservoirs (Lu et al., 2004).

Tazhong Uplift has undergone Caledonian, Hercynian, Indosinian,Yanshanian and Himalayan tectonic movements since the Ordovician.During the Cambrian to Early Ordovician, central faulted horst belt wasformed, resulting in an eastward-dipped paleo-geomorphology (Chen,2005; Lin et al., 2009; Zhang, 2006). During the Middle-Late

Fig. 2. Stratigraphic column for Tazhong Uplift.

H. Li, C. Cai Journal of Petroleum Science and Engineering 148 (2017) 103–114

105

Table

1Geo

chem

ical

andisotop

iccompositionsof

form

ationwater

inTazhon

gArea.

Well

Dep

th(m

)Age

pH

HCO3(m

g/l)

Cl(m

g/l)

SO4(m

g/l)

Ca(m

g/l)

Mg(m

g/l)

K(m

g/l)

Na(m

g/l)

Sr(m

g/L)

TDS(g/L

)H

2S（％）

87Sr/8

6Sr

TAs(μg/g)

δ18O

(‰)

δD(‰

)

TZ20

1-1H

5966

.5O3l

––

––

––

––

––

–0.71

178

––

–

TZ24

344

65.5

O3l

6.8

220

6250

049

438

0833

415

9036

710

232

106

0.01

0.71

145

––

–

TZ26

-4H

4560

.0O3l

6.4

195

5300

029

778

9043

275

726

000

189

890.02

0.71

164

–−1.30

−38

.9TZ26

-H9

4489

.0O3l

6.6

926

5620

071

421

500

844

548

1331

018

294

0.00

0.71

204

–−2.63

−53

.0TZ62

3-H2

5243

.5O3l

––

––

––

––

––

–0.71

166

––

–

TZ62

-5H

4899

.8O3l

7.0

–44

800

––

––

––

––

0.71

305

–1.24

−30

.3TZ62

-H9

4489

.0O3l

7.4

553

5460

051

477

7044

193

127

020

218

924.56

0.71

219

–1.27

−39

.5TZ82

1–1

5235

.5O3l

7.1

339

5390

039

930

0734

912

0932

280

200

922.09

0.71

276

––

–

TZ82

855

44.5

O3l

5.6

9879

450

263

1395

077

712

90.71

096

––

–

TZ82

-H2

5613

.5O3l

6.7

525

5822

081

715

940

322

3294

2810

024

410

70.00

0.71

058

–−1.70

−46

.6TZ82

-H3

5616

.0O3l

7.1

333

6610

043

252

2938

414

6235

120

258

109

0.38

0.71

171

––

–

ZG16

2-H2

6754

.0O3l

7.0

–59

600

––

––

––

960.01

0.71

075

–−1.75

−32

.7ZG43

1-H1

5459

.0O3l

7.0

–59

900

––

––

––

––

––

−2.40

−43

.0ZG7

5527

.0O3l

––

––

––

––

––

4.38

0.71

2019

100.75

––

TZ83

–1

5638

.4O1–2y

6.4

9997

900

156

8741

684

3615

5153

088

116

40.67

0.70

985

–7.70

−23

.1ZG10

361

90.5

O1–2y

7.0

–56

800

––

––

––

–0.37

0.70

969

–9.01

−26

.9ZG10

5H63

82.5

O1–2y

7.0

–71

650

––

––

––

–1.18

0.70

947

–8.54

−47

.3ZG43

154

12.8

O1–2y

6.9

132

1160

0015

014

660

1056

4440

5390

0–

191

5.80

0.70

965

22.32

5.89

−29

.8ZG43

253

25.9

O1–2y

6.0

138

1430

0020

348

360

1149

2934

3632

036

823

37.45

0.70

987

18.02

2.83

−34

.3ZG43

3C61

33.0

O1–2y

484

1200

0073

1350

010

7051

0062

400

402

204

4.88

0.70

965

35.85

7.10

−27

.9ZG44

5646

.8O1–2y

7.2

617

1002

0045

096

3577

036

9053

510

552

170

–0.70

993

––

–

ZG44

154

68.2

O1–2y

6.5

239

1200

0028

212

000

955

5069

6665

010

4320

60.86

0.71

031

––

–

ZG44

-H2C

6055

.5O1–2y

6.8

369

9599

034

311

150

768

3678

4634

050

915

90.37

0.70

984

–2.69

−37

.2ZG45

5643

.5O1–2y

6.4

101

1200

0035

214

000

1180

4280

6260

039

520

31.38

0.70

971

47.78

6.46

−31

.0ZG5

6405

.8O1–2y

6.3

459

8490

061

743

110

474

1107

7058

371

138

–0.70

937

104.19

––

ZG51

1–2

5237

.5O1–2y

6.5

8811

2000

3410

630

1003

3914

5277

070

218

2–

0.70

968

–3.92

−29

.8ZG6

6053

.4O1–2y

6.4

127

1260

0031

1377

091

850

1259

260

–20

6–

0.70

960

147.20

7.32

−44

.1ZG7–1

5837

.0O1–2y

––

––

––

––

528

––

0.70

963

–7.01

−43

.9

-:TAsor

thiaad

aman

tanes

arefrom

Cai

etal.(201

6);H

2Sfrom

Cai

etal.(201

5b)an

d–represents

nodataav

ailable.

H. Li, C. Cai Journal of Petroleum Science and Engineering 148 (2017) 103–114

106

Ordovician the high structural point began to move eastward. Towardthe end of the Ordovician, the Tazhong paleo uplift hinged westwardand became a westward-dipping nose as the southeastern margin of thebasin was strongly compressed and uplifted (Lin et al., 2009). Duringthe Silurian, the east of Tazhong Uplift continued uplifting. To the endof the early Permian, No. 1 Fault-Slope Zone became the highest areaafter central faulted horst belt subsided (Yang et al., 2013; Zhang,2006; Chen, 2005; Ding et al., 2008). Fig. 1b shows three types ofPaleozoic faults developed in the Tazhong area: Cambrian-OrdovicianFaults, Late Ordovician Thrust Faults and Silurian-Devonian Strike-Slip Faults (Li et al., 2013). These faults and unconformities served asconduits for oil and gas migration from the Lower Cambrian sourcerocks (Cai et al., 2001a, 2009a, 2009b, 2015a; Pang et al., 2013).

The geological record of the Tazhong Uplift can be divided intoseveral tectonic events and related stratigraphic sequences (Fig. 2). TheLower Ediacaran is composed of morainic conglomerates and neriticfacies sandstones and mudstones. The Upper Ediacaran consists ofplatform dolomite, micrite and lagoonal facies. The Lower Cambriansection comprises platform to platform-marginal thick dolomites; theMiddle Cambrian is composed of supratidal anhydritic dolomites andanhydrite. Bedded anhydrite 44–98 m thick are present in the easternZS1 and ZS5 wells (Cai et al., 2015b). The Ordovician is predominantlycomposed of thick dolomite, packstone, and bioclastic limestonedeposited in an open-platform environment with intercalated reefand shoal deposits. The Silurian and Devonian are composed of marinesandstone and mudstone. Carboniferous sandstones consist of littoraland neritic clastic deposits with intercalated micritic and bioclasticlimestone. The Permian consists of marine-terrestrial transition faciesgray or brown sandstone and mudstone with intercalated volcanics.The Mesozoic and Cenozoic are mainly composed of terrestrialsandstones and mudstones.

3. Sample collection and analysis

Water samples were obtained through drill-stem testing (DST) orrepeat formation testing (RFT) from oil or water intervals and filteredthrough a 0.45 µm filter. Samples were not collected until Cl concen-trations remained constant (10%) over 3 measurements per 8 h.Samples from TZ201-1H and TZ623-H2 with producing gas/waterratios higher than 500000 m3/m3 were only measured on 87Sr/86Sr, incase condensed water changes chemical and hydrogen and oxygenisotopic compositions of the formation water. pH was measured usingan electrode method within 2 h after sampling in the field. Major ionchemistry was measured by titration with Cl and SO4 by AgNO3 andBaCl2 potentiometric titration and Ca and Mg by ethylenediaminetetraacetic acid (EDTA) titration. The K+Na concentration was calculatedfrom charge balances. After being acidified to pH less than 6, formationwater was analyzed for dissolved metals K and Sr using a Jarrel-Ash110 ± 1155V inductively coupled plasma atomic emission spectrometry(ICP-AES) instrument in the Institute of Ecology, the Chinese Academyof Sciences. Precision of the analyses is 5% for Sr and 8% for K, basedupon replicate analyses. The Na concentrations were calculated by K+Na minus K. Based upon replicates, the precision for Cl measurementis better than 1.5%, for SO4 is 3%, for Mg 3% when Mg concentrationsare higher than 500 mg/L and better than 6% in the Mg range from 300to 500 mg/L. The precision for Ca is better than 3% when Caconcentrations are higher than 1000 mg/L, and is better than 5% inthe Ca range from 500 to 1000 mg/L. All the above measurement isaccording to the China National Petroleum Corporation (CNPC)standard SY 5523-92 for oilfield water analysis. Total dissolved solids(TDS) were measured by the gravimetric method according to Clescerlet al. (1999). After filtration with a 0.45-μm filter, 0.5 ml of the waterwas dried at 180 °C until a constant weight was reached.

δD, δ18O and 87Sr/86Sr analyses were performed in the StableIsotope Laboratory, Institute of Geology and Geophysics, ChineseAcademy of Sciences. δD and δ18O were determined using methods

of reduction on highly purified metallic Zn at 390 ℃ and CO2

equilibration, respectively. D/H and 18O/16O ratios were measuredon a MAT251 mass spectrometer, and the results were reported relativeto SMOW, with standard deviation of ± 1 to 2‰ for δD and ± 0.20 to0.30‰ for δ18O, respectively. For determination of 87Sr/86Sr ratios,sufficient formation water or solution was acidified to pH < 2 using100% HCl. The 87Sr/86Sr ratios were measured on a Finnigan MAT-262mass spectrometer and corrected relative to the NBS-987 standardwith a measured average value of 0.710255 ± 0.000010. The precisionfor 87Sr/86Sr measurement is 0.00003–0.00007.

4. Results

Chemical and isotopic compositions of formation water from thestudy area are shown in Table 1. The water shows a variable chemicalcomposition, pH values from 5.4 to 7.4 with an average of 6.7, and totaldissolved solids (TDS) from 75 to 233 g/L with an average of 144 g/L,being 2–7 times more saline than seawater. O1–2y formation water hasTDS from 123 to 233 g/L and a mean value of 182 g/L, and are higherthan the O3l water which range from 75 to 129 g/L with a mean valueof 99 g/L.

Na concentrations range from 7 to 67 g/L with an average of 42 g/L. Cl concentrations range from 45 to 143 g/L with an average of 83 g/L. Concentrations of other anions, such as SO4 and HCO3, are muchlower than Cl (Table 1). Ca, the second most predominant cation, hasconcentrations from 3 to 48 g/L with an average of 14 g/L.

The O1–2y and O3l samples have δD values between −47.3 and−23.1‰ and −53.0 and −30.3‰, respectively, and δ18O values rangingfrom 2.69 to 9.01‰ and −2.63 to 1.27‰, respectively. δ18O valuesfrom the O1–2y formation water are higher than the O3l (Fig. 8). δ

18Ovalues of O1–2y samples are more positive than seawater, while mostδ18O values of O3l samples are more negative than seawater.

87Sr/86Sr ratios from the O1–2y samples range from 0.70937 to0.71031 with an average of 0.70973 and are lower than O3l sampleswhich range from 0.71058 to 0.71305 with an average of 0.71141. Srconcentrations of O1–2y formation water range from 264 to 1043 mg/L,with an average value of 582 mg/L. Sr concentrations of O3l formationwater range from 173 to 258 mg/L with an average of 212 mg/L(Table 1).

5. Discussion

5.1. Relationship between formation water and evaporated seawater

The origin of salinity in formation water from sedimentary basinshas historically attributed to subaerial evaporation of seawater(Carpenter, 1978) and/or dissolution of evaporite (Land andPrezbindowksi, 1981; Birkle et al., 2009). Modern seawater has aTDS of about 35.2 g/L and is dominated by Na+(10.7 g/L) and Cl-

(19.3 g/L). Brines that evaporated beyond halite saturation level showa Na/Cl molar ratio lower than seawater (0.86) since the haliteprecipitation absorbs Na and Cl in 1:1 M ratio and consequentlydecreases the Na/Cl ratio (Bagheri et al., 2014). In contrast, thedissolution of halite will provide solute with Na/Cl molar ratio closeto 1, which is slightly higher than that of seawater (Li and Jiang, 2013).The formation water in this study has Na/Cl molar ratios from 0.37 to0.92 with an average of 0.79 (Table 1), most of which are close toseawater (Fig. 3), indicating an origin dominated by evaporated sea-water.

Abnormally low Na/Cl molar ratios have been observed in TZ26-H9(0.37) and ZG432 (0.39). They are located in the eastern side of No.1Fault-Slope Zone and the central part of No.10 Structural Belt (Fig. 1b).Do these abnormal Na/Cl molar ratios result from halite precipitation?Formation water after halite precipitation is expected to have lowerNa/Cl and Cl/Br molar ratios and high Cl concentrations saturatedwith halite. However, 1) Cl/Br molar ratios of Paleozoic formation

H. Li, C. Cai Journal of Petroleum Science and Engineering 148 (2017) 103–114

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water are close to seawater (Cai et al., 2001b); 2) no halite is found inthe area (Fig. 2). These two facts rules out the possibility that theabnormally low Na/Cl molar ratios resulted from halite precipitation.Other processes will be discussed in the next part.

5.2. Water-rock interactions

Seawater evaporation trajectory (SET) has been extensively used todetermine the origin of water salinity and water-rock interactions (Cai

and Mei, 1997; Birkle et al., 2009; Cai et al., 2001b; 2006). Fig. 4 showsvariations of ions to Cl relative to the SET. Ca is plot above the SET andthus rich relative to seawater evaporation while Mg and SO4 aredepleted.

5.2.1. AlbitizationCations of formation water can be changed significantly by water-

rock interactions. Most of the Ordovician formation water has Naplotted on the SET (Fig. 4a) and only ZG432 and TZ26-H9 water aredepleted in Na. In sedimentary basins, water-rock interactions invol-ving sodium are either albitization of plagioclase and K-feldspar orhalite dissolution and precipitation (Davisson and Criss, 1996).Dissolution or precipitation of halite has been excluded as the originas discussed in the last section. Thus, albitization of plagioclase or K-feldspar is most likely. The albitization with an exchange of 2 Na+ for 1Ca2+ or 1 Na+ for 1 Ca2+ can result in Na depletion and Ca enrichmentin basin fluid (Davisson and Criss, 1996). Davisson and Criss (1996)found a linear relationship between Nadeficit and Caexcess for manybasins, which was named as Basinal Fluid Line (BFL):

RCa = 0.967 Na + 140.3 ( = 0.98)excess deficit

The excess of Ca and the deficit of Na are defined as follows:

Ca =[Ca −(Ca/Cl) *Cl ]*(2/40.08)excess meas sw meas

Na = [(Na/Cl) *Cl −Na ]*(1/22.99)deficit sw meas meas

Nadeficit and Caexcess are plotted on Fig. 5. Ca excess and Na deficitcan be found both in O1–2y and O3l formation water (Fig. 5). Thesamples are plotted on the Basinal Fluid Line (BFL) showing anexchange of 2 Na+ for 1 Ca2+. Albitization of plagioclase by 2 Na+ for1 Ca2+ exchange could be responsible for the water. Plagioclase occursin clastic rocks, and formation water from the Silurian to Carboniferous

Fig. 3. Relationship between Na/Cl molar ratios and Cl concentrations of Ordovicianformation water. The solid line is the Na/Cl=0.86 line defined by seawater. Evaporationof seawater will increase the Cl concentrations coupled with unchanged Na/Cl molarratios.

Fig. 4. Relationships of formation water Cl to Na, Ca, Mg, K, Sr and SO4 relative to seawater evaporation trajectory (SET). H means halite, and SW means seawater.

H. Li, C. Cai Journal of Petroleum Science and Engineering 148 (2017) 103–114

108

clastic rocks is characterized by Na+ depletion and Ca2+ enrichmentdue to albitization (Cai et al., 2001b). Thus, the Na-deficit water in theOrdovician carbonate reservoirs may have mixed with formation waterfrom clastic rocks in the Silurian to Carboniferous, or hydrothermalfluid from the Ediacaran and Lower Cambrian as initially proposedbased on cement and vein geochemistry (Cai et al., 2008).

O1–2y samples are broadly parallel to SET and slightly enriched inK. However, some of O3l samples are slightly depleted in K relative toSET (Fig. 4d). K feldspar precipitation and illitization will decrease Kconcentration, while K feldspar dissolution and albitization willincrease K concentration (Birkle et al., 2009).

5.2.2. DolomitizationCa/Cl molar ratios of the Ordovician formation water range from

0.05 to 0.34 with an average of 0.12, which is 500% enrichment relativeto seawater (0.02). Ordovician formation water has an average Na/Clmolar ratio of 0.79, being 8% depleted comparing to seawater (0.86).Hence, only about 4% Ca enrichment is produced according to the 2Na+ for 1 Ca2+ albitization. Dolomitization of carbonates is consideredto be the main mechanism for Ca enrichment coupled with Mgdepletion (Fig. 4b, c), which is widely reported in many basins

(Bottomley et al., 1999; Stueber and Walter, 1991). Mean Mg/Cl molarratio of 0.01 yields 90% Mg depletion relative to seawater (0.1).Assuming that dolomitization is the only process accounting for thedepletion of Mg in the formation water, 90% Ca enrichment is expectedfrom dolomitization. Hence, other sources must have contributed Caleading to Ca enrichment up to 500% in these formation water, and arelikely from dissolution of anhydrite, gypsum and/or calcite.

5.2.3. TSR effects on formation water SO4 and D valuesAll the water samples show a strong depletion in SO4 with the water

in the O1–2y being more depleted than in the O3l (Fig. 4e). In situthermochemical sulfate reduction (TSR, reaction 1), redox reactionsbetween dissolved sulfates and petroleum at temperatures > 120 °C(Cai et al., 2001a; 2003), is likely to have resulted in the SO4 depletionand H2S generation as supported by the ubiquitous distribution of H2Sin the Ordovician reservoir (Cai et al., 2001a; 2015b).

MgSO + Ca + petroleum → Mg + CaCO + H S+ altered petroleum42+ 2+

3 2

(1)

H2S in the O3l reservoir is less than 4.7%, while H2S in O1-2yreservoir can reach up to about 15% (Cai et al., 2015b). SO4 depletionand H2S enrichment probably resulted from TSR in situ (Fig. 6a). H2Shas been shown to dissolve in formation water, precipitate as pyriteand have been incorporated into oils and solid bitumens producingalkylthiolanes, alkylthiols and alkyl 2-thiaadamantanes in the area,thus H2S concentrations cannot be used to reflect TSR extents in thiscase (Cai et al., 2015b; 2016). Thiaadamantanes (TAs) concentrationsin petroleum are considered to better reflect TSR extents (Wei et al.,2012; Cai et al., 2016), and show increase with decreasing SO4

concentration in the formation water (Fig. 6b). O3l formation watershow less SO4 depletion than O1-2y water (Fig. 4e), which means thatTSR in O3l reservoir is weaker. Overall, TSR is considered to be themain reason for the SO4 depletion in the Ordovician formation water.

The water with the most negative δD value has the highest TAs andH2S concentrations, indicating that δD values shifted negatively withincreasing TSR extents (Fig. 7c and d). This probably resulted fromisotope exchanges between formation water and TSR-H2S. Liquidhydrocarbons have δD values ranging from −250 to −50‰ (Baiet al., 2014; Radke et al., 2005; Liu et al., 2006) and gaseoushydrocarbons are usually lower than −100‰ (Hu et al., 2013; Liuet al., 2016; Xiao, 2012). Therefore, the H2S is expected to have muchmore negative δDH2S values than formation water if it was derived fromTSR by petroleum as suggested by Cai et al. (2001a; 2005 and, 2016).

Much poorer correlation exists between δ18O and H2S (Fig. 7e) andespecially between δ18O and TAs (not shown), indicating that δ18Ovalues of the formation water were not altered by TSR. This observa-

Fig. 5. Formation water Nadeficit and Caexcess plot on model predictions for differentprocesses (Davisson and Criss, 1996). The 1 Ca for 2 Na exchange line is a theoreticaltrend for albitization of primary seawater. The evaporation trend of seawater initiallyfollows a vertical shift and afterward produces large Nadeficit along a horizontal trend.Reactions involving Ca only produce vertical shifts, such as precipitation or dissolution ofCaCO3 or CaSO4.

Fig. 6. (a) Relationship between H2S in gas composition and formation water SO4 concentrations; (b) Relationship between oil thiaadamantanes concentrations (TAs) and formationwater SO4.

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tion is different from Jiang et al. (2015b), in which water was proposedto have been generated during TSR and thus decreased formationwater salinities and δ18O values. The differences may be ascribed asdifferent hydrocarbons involved in TSR. Liquid hydrocarbons wereinvolved in TSR in this case (Cai et al., 2015b; 2016) whilst methane-dominated TSR occurred in NE Sichuan basin (Cai et al., 2004; 2013;Jiang et al., 2015b). Water was proposed to have been generated

Fig. 7. Crossplots showing, (a) A positive relationship between formation water δ18O and TDS; (b) No relationship between formation water δD and TDS; (c) A weak negativerelationship between formation water δD and oil TAs; (d) A weak relationship between δD and H2S when H2S concentration is higher than 2%; (e) No relationship between δ18O andH2S concentration.

Fig. 8. Relationship between δD and δ18O of Ordovician formation water. Theregression line represents the mixing relationship between paleo-meteoric water andOrdovician formation water. Arrows indicate the expected shifts in δD and δ18O duringevaporation, water-rock interactions, water-H2S hydrogen isotope exchange. GlobalMeteoric Water Line (GMWL) is from Craig (1961) and seawater evaporation trajectory(SET) is from Moldvanyi et al. (1993).

Fig. 9. 87Sr/86Sr vs 1000/Sr crossplot showing two mixing trends in the Ordovicianformation water in the Tazhong Uplift. Line 1 represents formation water mixed withpaleo-meteoric water. Line 2 represents formation water mixed with hydrothermal water.

Fig. 10. 87Sr/86Sr vs TDS crossplot showing two mixing trends. Line 1 representsformation water mixed with paleo-meteoric water. Line 2 represents formation watermixed with hydrothermal water.

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during TSR only by methane via reaction (2) not by C4+ hydrocarbonsvia reaction (3) (Pan et al., 2006).

CH4 + Mg2+ + SO42- = MgCO3 + H2S + H2O (2)

4C4H12 + 5Mg2+ + 5SO42- + H2O = 5MgCO3 + 8CH4 + 5H2S + 3CO2(3)

5.3. Meteoric water mixing indicated by δD and δ18O values

Most of the hydrogen and oxygen isotope data plot below the GlobalMeteoric Water Line (GMWL: δD = 8δ18O + 10; Craig, 1961) (Fig. 8),and thus are considered as typical formation water (Kharaka et al.,1987). A least square fit to the data intersects the global meteoric waterline (GMWL) yielding an intercept at approximately −50‰ (δD) and−7.5‰ (δ18O). This may indicate that meteoric water, as suggested bythe intercept, have mixed with formation water that originated fromevaporated seawater. The regression line (dotted line) represents themixing between meteoric water and formation water. O3l water showslower δD, δ18O values and are closer to the intercept than the O1-2ywater, which is supported by lower TDS in the O3l water (Figs. 7a, band 8). Therefore, formation water of O3l mixed with the meteoricwater to greater degree than the O1-2y water.

Aksu River and Tarim River in the Tarim Basin represent modernmeteoric water, and show δD and δ18O values range from −78.0 to−54.9‰ and from −11.1 to −7.8‰, respectively (Huang et al., 2010).δD and δ18O values of the intercept (−50‰ and −7.5‰, respectively)are slightly heavier than the composition of modern meteoric water in

the Tarim Basin (Fig. 8). Tarim plate was located at lower latitudeduring the Silurian (10–20°N) than afterward with present-day loca-tion at 35°N (Fang and Shen, 2001), and paleo-meteoric water duringthe period are expected to have heavier δD and δ18O values. Thus theintercept with the heavier δD and δ18O values is likely to representisotopic composition of paleo-meteoric water during the Silurian asinitially proposed (Cai et al., 2001b). Thus the paleo-meteoric waterprobably mixed with the Ordovician formation water during theSilurian.

Water samples deviate far from the mixing line are considered tohave been affected by water-rock interactions or hydrogen isotopeexchange with TSR-H2S, or both. Oxygen and hydrogen isotopiccompositions of formation water are essentially conservative at ambi-ent temperatures. Water-rock interactions may perturb the oxygen andhydrogen isotopic composition at elevated temperatures (Gat, 1996).They could cause a positive shift in δ18O values and keep δD values ofbasin brine unchanged, depicted by right radiating arrow (Fig. 8),because oxygen is much more involved than hydrogen in carbonates(Chen et al., 2013; Clayton et al., 1966). Heavier δD and δ18O values ofO1-2y formation water may have resulted from both seawater evapora-tion and water-rock interactions. Lighter δD and δ18O values of O3lformation water may have resulted from the mixing with paleo-meteoric water to greater degree. Hydrogen isotope exchange betweenwater and TSR-H2S is represented by ZG6 with low δD value of−44.1‰ and high TAs concentration of 147.20 μg/g. This could cause anegative shift of δD values without changing δ18O values, depicted bydownward radiating arrow.

Fig. 11. Water mixing model for Ordovician formation water (See Fig. 1b for AB location).

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5.4. Water mixing indicated by Sr concentrations and 87Sr/86Sr

Strontium has three origins: (1) mantle derived strontium with87Sr/86Sr value of 0.703; (2) terrigenous strontium with 87Sr/86Sr valueof 0.720; (3) Ordovician marine carbonates derived strontium with87Sr/86Sr value of 0.708 (Veizer, 1989; Jiang et al., 2001). Strontiumshows higher contents in carbonate rocks than in siliciclastic rocks. Incontrast, carbonate rocks have 87Sr/86Sr ratios lower than siliciclasticrocks due to the higher content of Rb in siliciclastic rocks. Therelationship between 87Sr/86Sr and 1000/Sr is thus commonly utilizedto trace the origin of formation water. Both O1-2y and O3l formationwaters show higher 87Sr/86Sr ratios (0.70937-0.71031 and 0.70958-0.71305, respectively) than Ordovician limestone and coevalOrdovician seawater (0.7078-0.7090, Jiang et al., 2001; Huang et al.,2006; Chen et al., 2008), and thus they must have obtained radiogenic87Sr from siliciclastic rocks (up to more than 0.75; Cai et al., 2001b).

Line 1 in Figs. 9 and 10 indicates mixing of high 87Sr/86Sr, low TDSand Sr fluid (EM1) with Ordovician formation water. EM1 can bemeteoric water or shale-derived water. Both may have obtained 87Srfrom siliciclastic rocks, and thus are characterized by low TDS and high87Sr/86Sr ratios. Meteoric water and/or shale-derived water havemixed with formation water from the O3l as indicated by O3l watersamples lying on line 1 (Figs. 9 and 10).

Mixing with meteoric water is indicated by low temperature of fluidinclusions (50–60 °C) in calcite and quartz (Cai et al., 2008; Jia et al.,2015; Jiang et al., 2015a) and the relationship between δ18O and δDvalues (Fig. 8). O3l formation water from well TZ62-5H, located in theeast, shows 87Sr/86Sr ratio of 0.7131, Sr concentration of 173 mg/Land TDS of 75 g/L (Figs. 1b, 9 and 10). O3l formation water fromTZ82-H2, located in the west, shows 87Sr/86Sr ratio of 0.7106, Srconcentration of 244 mg/L and TDS of 107 g/L (Fig. 1b, Figs. 9 and10). This indicates that O3l water in the east of No.1 Fault-Slope Zonemixed with meteoric water to more degrees than in the west. EasternTazhong was uplifted to a greater degree than the west during the LateOrdovician to Silurian. During the Late Silurian, the O3l strata wereexposed and eroded in the southeast to well TZ26-H9 (see locations inFig. 1b) and Silurian siliciclastic rocks were eroded and weathered(Zhang, 2006; Chen, 2005). Therefore, meteoric water mixing is mostlikely to occur during the Late Silurian. This proposal is partiallysupported by the heavier δD and δ18O values of the paleometeoricwater (intersection dot in Fig. 8) than modern meteoric water.

The other source of EM1 is shale-derived water from the Awati Sag.Mixing with this water is supported by the three lines of evidence, 1) adecreasing trend in 87Sr/86Sr data from the west to the east is proposedto indicate that the shale-derived water from the Awati Sag wasexpelled and flowed into the strata in the northwestern Tazhong area(Cai et al., 2001b); 2) the bulk oil and its dibenzothiophenes from wellZG19 (Fig. 1b) nearby the Awati Sag shows similar δ34S values to theMiddle and Upper Ordovician kerogen, and thus the oil is considered tohave been derived from the source rock of that age in Awati Sag (Caiet al., 2015a); and 3) O3l formation water from well ZG162-H2(Fig. 1b), has high 87Sr/86Sr of 0.71075 and low TDS of 96 g/L. Thiswater sample is less likely to have mixed with paleo-meteoric waterbecause well ZG162-H2 was not located in paleo-highland but aroundsubsidence center near the Awati Sag and well ZG19 (Fig. 1a and b).Thus, the ZG162-H2 water with high 87Sr/86Sr and low TDS is mostlikely to have mixed with shale-derived water from the Awati Sag.

Line 2 in Figs. 9 and 10 represents mixing between high 87Sr/86Sr,TDS and Sr fluid (EM 2) and Ordovician formation water. EM 2 may behydrothermal fluid up-migrated from the Lower Cambrian andEdiacaran siliciclastic rocks, as suggested by Cai et al. (2008) and Jiaet al. (2015). The hydrothermal fluid may have up-migrated and mixedwith formation water along the cross points of faults near the No.10Structural Belt to more degrees. This is supported by water samplesfrom the area having higher 87Sr/86Sr ratios and higher Sr and TDSconcentrations, such as in wells ZG431, ZG432, ZG433C, ZG441, ZG45

and ZG511-2 (Figs. 1b, 9 and 10). Significantly, oils are mainlyproducing from the area, thus it may indicate that oil and hydrothermalfluid may have up-migrated along similar pathways.

5.5. Model of water flow and its relationship to oil migration

A mixing model for the Ordovician formation water is proposedbased on above discussion (Fig. 11). During the Late Ordovician toSilurian, the eastside of Tazhong was uplifted to a greater degree due totectonic compression. Sediments of the top Silurian siliciclastic rockswere removed during the Late Silurian. Paleo-meteoric water received87Sr from the Silurian siliciclastic rocks infiltrated into O3l carbonatereservoir at the eastside of No.1 Fault-Slope Zone, flowed westwardsand subsequently mixed with O3l diagenetically altered evaporatedseawater.

Hydrothermal fluid, obtained 87Sr and high TDS from the LowerCambrian and Ediacaran siliciclastic rocks, may have up-migratedduring the Permian (Cai et al., 2008; Jia et al., 2015) along faultsespecially near the No.10 Structural Belt and mixed with O1-2ydiagenetically altered evaporated seawater. Significantly, oils are foundwhere hydrothermal fluid were active near the No.10 Structural Belt,and are proposed to have up-migrated from Cambrian source rocksduring the Late Silurian to Early Devonian, and re-migrated frompreviously accumulated oil pools during the Permian (Cai et al., 2009aand b; 2016). That is, oils may have similar migration pathways to thehydrothermal water. Thus, chemical and isotopic compositions offormation water may provide clues to oil migrating direction.

6. Conclusions

(1) Formation water was originated from evaporated seawater, andenriched in Ca and depleted in Mg, SO4 and Na likely bydolomitization, albitization, anhydrite dissolution and thermoche-mical sulfate reduction (TSR).

(2) δD values of formation water were negatively shifted likely due tohydrogen isotope exchange between water and TSR-H2S.

(3) O1-2y formation water may have mixed with hydrothermal fluidduring the Permian, while O3l water mixed with paleo-meteoricwater during the Late Silurian. Thus O1-2y water show higher TDS,heavier δD and δ18O and lower 87Sr/86Sr than the O3l.

(4) Both hydrothermal fluid with high TDS and 87Sr/86Sr and oils mayhave up-migrated along faults near No.10 Structural Belt, and thusformation water geochemical composition may supply clues to oilmigration.

Acknowledgments

This work is financially supported by China National Funds forDistinguished Young Scientists (41125009), NSFC grant No. 41672143and Special Major Project on Petroleum Study (2016ZX05008-003).Thanks should be given to Dr. Peter Birkle, an anonymous reviewerand the editor for constructive and critical reviews and suggestions toimprove the manuscript.

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