Pt

Za

b

a

ARA

KGED

1

ddpaeZsotcHsemfm

0h

Chemie der Erde 73 (2013) 469– 479

Contents lists available at ScienceDirect

Chemie der Erde

jou rn al homepage: www.elsev ier .de /chemer

rediction of buried calcite dissolution in the Ordovician carbonate reservoir ofhe Tahe Oilfield, NW china: Evidence from formation water

huang Ruana,∗, Bingsong Yua, Lidong Wanga, Yinglu Pana, Guanghui Tanb

State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, ChinaExploration and Development Research Institute, SINOPEC Northwest Company, Urumqi 830011, China

r t i c l e i n f o

rticle history:eceived 31 July 2012ccepted 18 March 2013

eywords:ibbs free energyquilibrium constantissolving tendency

a b s t r a c t

The Lower–Middle Ordovician reservoir of the Tahe Oilfield is dominated by limestones with reser-voir spaces formed by the generation of dissolution pores, meaning that buried karst formation canbe evaluated by studying water–rock reactions between groundwater and calcite. The hydrogeologicalinformation preserved in this reservoir indicates that the Ordovician groundwater were high-salinityand high-closure, characteristics that are of significance to water–calcite reactions. Theoretical chem-ical thermodynamics combined with equilibrium calcite solution ionization allowed us to establish adissolution–precipitation evaluation model for calcite, with the theoretical activity of Ca2+ in solution(aCa2+

eq ) controlled by temperature, pressure, [�CO2] − [Ca2+] and solution pH, and with the actual activ-ity of Ca2+ in solution (aCa2+) being controlled by the concentration of various ions in solution. Ionizationreaction directions are controlled by �G values; these values can be calculated using aCa2+

eq and aCa2+.

Here, ground water data were collected from 34 wells that intercepted Ordovician sediments within theTahe Oilfield, and calcite �G values were calculated for these wells. These data indicate that the ground-water in this oilfield favours the dissolution of limestone, with limestones in the west and south of theTahe Oilfield being more susceptible to dissolution, consistent with observations within the oilfield. Themethods employed during this study can also be used to quantitatively assess susceptibility to burialdissolution and to support reservoir evaluation.

. Introduction

The reservoir capacity of carbonate rocks is controlled by theevelopment of secondary porosity, which in turn is dependent onissolution reactions between rocks and surface water that takelace close to or at the surface, including penecontemporaneousnd weathering karst formation, and dissolution in deep burialnvironments during reactions with groundwater (Scholle, 1977;enger et al., 1980). The majority of researchers believe that theurface or near-surface dissolution is the most important controln secondary porosity development in carbonates, a hypothesishat has been borne out during exploration and the development ofarbonate oil and gas fields worldwide (Scholle and Halley, 1985;eydari, 1997). However, although most researchers also agree that

econdary porosity can form during diagenesis (Jin et al., 2009), theffectiveness of generation of diagenetic porosity in reservoir for-

ation has not as yet been established, and no evaluation method

or determining the effectiveness of dissolution in deep environ-ents has been proposed.

∗ Corresponding author. Tel.: +86 13810965707.E-mail address: wofe0103@163.com (Z. Ruan).

009-2819/$ – see front matter © 2013 Elsevier GmbH. All rights reserved.ttp://dx.doi.org/10.1016/j.chemer.2013.03.004

© 2013 Elsevier GmbH. All rights reserved.

Calcite is the most common authigenic mineral in sedimentaryrocks, occurring not only as pore-filling cements in many clas-tic rocks but also as the dominant mineral within limestone. Anumber of advances have been made since the 1960s that haveimproved our knowledge of calcite stability in natural groundwa-ters or seawater at room temperature and pressure (Garries et al.,1960; Peterson, 1966; Berger, 1967). However, few studies haveinvestigated calcite–water interactions under high-temperatureand high-pressure diagenetic conditions.

Water–rock reactions during burial have been extensively stud-ied (Kafri and Sass, 1996; Genthon et al., 1997; Coudrain et al., 1998;Morse and Arvidson, 2002), with Garries and Christ (1965) sum-marizing the importance of silicate mineral- and clay-based ionexchange processes, introducing basic chemical thermodynamicprinciples to mineral systems. Burley et al. (1985) concluded thatelevated temperatures add energy to mesogenetic-stage systemsinvolving water–rock reaction, and Kaiser (1984) predicted reser-voir qualities and estimated diagenetic histories using groundwatersolution-mineral equilibria, suggesting that diagenesis of the Frio

Formation in Texas, USA, was controlled by temperature, pH, activ-ity, and pressure. Furthermore, Lai et al. (2005), Liu et al. (2005),and Yu and Lai (2006) investigated chemical equilibrium relation-ships between fluids, plagioclase, and K-feldspar during sediment

4 der Er

dpaeppuesZbecwo

ccdirudoa

FODa

70 Z. Ruan et al. / Chemie

iagenesis, focussing on the impact of temperature and fluid com-ositions (pH, activity of K+, Na+, Ca2+, etc.) on feldspar precipitationnd dissolution equilibria, and suggested that feldspar is extremelyasily dissolved in acidic, low-salinity pore waters at low tem-eratures. Yu and Lai (2006), and Yu et al. (2008) studied therecipitation and dissolution of carbonate cements in clastic rockssing theoretical water–rock interaction thermodynamic phasequilibria and the mass conservation law, concluding that calciteolubility is dependent on groundwater [�CO2] − [Ca2+] values.hang et al. (2009) also experimentally examined interactionsetween two fluids with different salinities and a composite min-ral system, and suggested that acidity was the most importantontrol on the dissolution of minerals in the system, as differencesere observed in the dissolving abilities of different acids on vari-

us mineral components at identical pH values.Carbonate rocks are composed of carbonate minerals such as

alcite and dolomite, and the dissolution of these minerals directlyontributes to the properties of a reservoir. Thus, the study of theissolution trends of carbonate minerals in buried environments

s a useful tool to predict the accumulated spaces in carbonateocks in the diagenetic stage. Here, we extend the previous research

ndertaken on calcite in groundwater and establish a quantitativeissolution–precipitation model for calcite in groundwater basedn basic chemical thermodynamic principles. This model is thenpplied to the Ordovician carbonate reservoir of the Tahe Oilfield of

ig. 1. Geology, tectonics, and locations of wells in the Tahe Oilfield sampled during this srdovician. U1, Tabei Uplift; U2, Central Uplift; U3, Southeast Uplift; D1, Kuqu Depressioepth line indicating the top of the Lower Ordovician; 2, Fault; 3, Sampled wells in Table

bsent line.

de 73 (2013) 469– 479

NW China. It should be noted that this modelling is still preliminary,and has a number of issues that will be addressed during futurerefinement. In addition, this study can only reflect the current sta-tus of carbonate reservoirs in buried environments, as groundwaterdata are collected from present-day formations, rather than reflect-ing the processes that occurred in the past.

2. Geological setting

The Tahe Oilfield is located in the southern portion of the TabeiUplift in the Tarim Basin of NW China, and covers a total surfacearea of 3200 km2. The Tabei or Northern Tarim uplift is surroundedto the north, south, west, and east by the Kuqu, Manjiaer, andAwati depressions, and the Kuluketage horst, respectively (Fig. 1).Ordovician sediments in this area were deposited in a platformenvironment and are now deeply buried (>5000 m; Fig. 1), withlimestone, dolomite, and other evaporites dominating the geologyof this area (Kang and Kang, 1996; Wei et al., 2000). The Ordoviciancarbonate reservoir in the study area can be subdivided into sixformations: the Penglaiba (O1p), Yingshan (O1y), Yijianfang (O2yj),

Qiaerbake (O3q), Lianglitage (O3l) and Sangtamu (O3s). The Ying-shan (O1y) and Yijianfang (O2yj) formations, which are the focusof this study, consist of wackestones and packstones (Zhang et al.,2000; Ruan et al., 2012). Many faults are present in the study area,

tudy, with a SW–NE cross-section; contours indicate depth to the top of the Lowern; D2, North Depression; D3, Southwest Depression; D4, Southeast Depression; 1,3; 4, Sampled wells in Table 2; 5, O2 + 3 absent line; 6, S + D (Silurian and Devonian)

Z. Ruan et al. / Chemie der Er

bc

d2Nhigmieaear

lmdmglsOd(cd

3

dcgtdsfighemsnorits

Fig. 2. Burial history of the Tahe Oilfield, after Chen et al. (2003).

ut most faults are small and were only active during the Ordovi-ian.

Waters within the Ordovician sediments of the Tahe Oilfield areominantly CaCl2-type, with rare Na2SO4-type waters (Liu & Zhang,005). The concentrations of major cations and anions, such asa+ + K+ + Ca2+ and Cl−, can be several times to several dozen timesigher than in the modern ocean in around 90% of these waters,

ndicative of the presence of brines. The salinity of Ordovicianroundwater is about 200 × 103 mg/L, several times higher thanodern seawater salinity. These data indicate that the groundwater

s significantly depleted and highly mixed. In addition, huge differ-nces in trace element and ion concentrations between Ordovician,nd Carboniferous and Triassic groundwater (Cai et al., 2002; Qiant al., 2003) suggest that the Ordovician groundwater is in a rel-tively closed environment, with reactions between surroundingocks and waters leading to increases in salinity.

The Ordovician reservoir of the Tahe Oilfield is characterised byong-term oil isolation, multiphase accumulation, and late transfor-

ation, with Lower Ordovician sediments having been at shallowepths until the end of the Oligocene and associated with for-ation temperatures of 50–65 ◦C, based on current geothermal

radients (Chen et al., 2003; Wang et al., 2003). These relativelyow-temperature conditions led to only minor burial diagene-is. Subsequent, deeper and rapid post-Oligocene burial of therdovician sediments led to a rapid increase in organic and burialiagenesis caused by increasing ambient formation temperaturesFig. 2). This suggests that the effect of burial diagenesis on Ordovi-ian carbonates had a more significant role than near-surfaceiagenesis on the formation of secondary porosity.

. Methods

Chemical reactions between water and rocks are continuousuring diagenesis, leading to constantly changing groundwater iononcentrations. Technical restrictions mean that only present-dayroundwaters can be analysed; however, these modern groundwa-ers are also highly significant, as if they are not conducive to theissolution of calcite in Ordovician carbonate reservoirs, it is rea-onable to assume that early formed pore spaces may have beenlled by calcite cement. The converse is also true, in that if modernroundwaters are conducive to calcite dissolution, poorly formedoles that developed prior to burial could have been extended andnlarged, thereby increasing the oil and gas reservoir space. Thiseans that the relationship between calcite and groundwater-type

olutions, and the main controls on dissolution and precipitationeed to be identified in order to predict the dissolution abilityf groundwaters reacting with Ordovician limestones. Previous

esearch has shown that a thermodynamic equilibrium model thatncorporates formation water geochemistry, and calcite dissolu-ion and precipitation characteristics needs to be used to effectivelyimulate this situation.

de 73 (2013) 469– 479 471

The dissolution–precipitation equilibrium reaction of calcite(Cc) is:

CaCO3(Cc) = Ca2+ + CO32−

The direction of this reaction is dependent on the Gibbs freeenergy (�G) of this reaction. Using the Gibbs–Duhem equation,�G is the difference between the free energy of the product(s)and the reactant(s) (�G = G0 + RT ln K, where K is an equilibriumconstant), meaning that when �G < 0, calcite dissolves; otherwise,calcite is precipitated. This means that �G is a key parameter forany study that focuses on the chemical thermodynamic modellingof a water–rock system.

The main reaction and equilibrium constants in thewater–calcite system are determined as follows:

CaCO3(Cc) = Ca2+ + CO2−3 KCc == aca2+ aco2−

3(1)

CO2(aq) + H2O = H+ + HCO−3 K1 =

aH+ aHCO−3

aCO2 aH2O(2)

HCO−3 = H+ + CO2−

3 K2 =aH+ aCO2−

3

aHCO2−3

(3)

and the total carbonate content in the solution is given by:∑CO2 = aCO2 + aHCO−

3+ aCO2−

3(4)

The law of mass conservation states that the change in theamount of total dissolved inorganic carbon resulting from theaddition of calcite to the system equals the change in calcium con-centration, as follows:[∑

CO2

]−[∑

CO2eq

]= [Ca2+] − [Ca2+

eq ] (5)

where CO2eq and Ca2+eq represent the theoretical concentrations of

CO2 and Ca2+ in the solution in a sustained thermobaric environ-ment, respectively.

Combining Eqs. (2) and (3) means that we can deduce an equa-tion from Eq. (1) as follows:

�G = �G0 + RT ln K = −RT ln Keq + RT ln K = RT ln(K/Keq)

= RT ln

(aCa2+

aCa2+eq

)(6)

This reaction is dependent on three parameters: the activity ofCa2+ in modern groundwater (aCa2+ ), the theoretical activity of Ca2+

(aCa2+ ) in an equilibrium state, and groundwater temperature.The activity of Ca2+ in modern groundwater can be calculated

using the Debye–Huckel equation:

aCa2+ = [Ca2+]r

log ri = − AZ2i

√I

1 + a0iB√

I(7)

where A and B are constant, Zi is the charge of ion ‘i’, I is the ionicstrength, ai represents the average diameter of ion ‘i’, and ri is themean ionic activity factor of ion ‘i’.

Combining Eqs. (1)–(3) means that when the reaction reachesequilibrium, the following equations apply:

kCc = aCa2+eq

aCO2−3eq

k1 =aH+

eqaHCO−

3eq

aCO2eq aH2Oeq

k2 =aH+

eqaCO2−

3eq

aHCO−3eq

(8)

472 Z. Ruan et al. / Chemie der Erde 73 (2013) 469– 479

Table 1The equilibrium constants for carbonate reactions.

P (GPa) T (◦C) K0 K1 K2 KCc Kw

10−7

10−7

w

a

˙

m

c

Cesott1c

[

a

ı

wov

c

[

m

a

E

gmtwiatr(catT

1 × 10−9 25 3.40 × 10−2 4.52 ×1 × 10−6 100 3.56 × 10−3 9.71 ×

ith Eq. (8) being reduced to:

aHCO−3eq

=aH+

eqkCc

aCa2+eq

k2aCO2−

3= kCc

aCa2+3eq

aCO2eq =(aH+

eq)2kCc

k1k2aCa2+3eq

(9)

Eq. (10) provides a simultaneous equation that combines (1)–(3)nd (5):

CO2eq = kCc

aCa2+eq

k2

[k2 + aH+

eq+ 1

k1(aH+

eq)2]

(10)

eaning that we can derive:

= kCc

k2

[k2 + aH+

eq+ 1

k1(aH+

eq)2]

(11)

where c represents a value that is unrelated to the theoreticala2+ concentration, which allows the formula to be solved moreasily. The value of c is determined from the temperature and pres-ure conditions of the solution, as well as the PH of the water, allf which can be measured directly. In Eq. (11), K1, K2 and KCc arehermodynamic constants obtained from relevant thermodynamicables (Helgeson et al., 1978; Wang and Li, 1991; Johnson et al.,992; Tanger and Helgeson, 1988), and from KCc, K1, and K2 valuesalculated using this equation in this study (Table 1).

Eq. (5) is reduced to:

˙CO2] − [Ca2+] = [˙CO2 eq] − [Ca2+eq] (12)

nd we set:

= [˙CO2] − [Ca2+] (13)

here ı represents the difference between the total concentrationsf CO2 and the concentrations of Ca2+ in actual groundwater. Thisalue can be calculated from the ion content of the water.

After substituting Eqs. (10), (11), and (13) into equation (12), wean determine the theoretical activity of Ca2+ (Ca2+

eq):

Ca2+eq ] = −ı +

√ı2 + 4c

2(14)

In addition, when Eq. (5) reaches equilibrium, aCa2+eq

is approxi-

ately equal to the concentration in a dilute solution:

Ca2+eq

= −ı +√

ı2 + 4c

2(15)

Finally, �G can be obtained by substituting Eqs. (7) and (15) intoq. (6).

As part of this study, we determined the ion concentrations inroundwater from 48 wells associated with the O1y and O2yj for-ations of the Tahe Oilfield. All of the formation water analysed in

his study was taken from test production wells or electric pumpingells, excluding samples that may be affected by acidification or

njected water to ensure the groundwater were free of drilling fluidsnd other contaminants. The sampled groundwaters were passedhrough a Waters Oasis HLB SPE column to remove organic mate-ial. The samples were also passed through a pre-treatment columnH-type) to avoid the destruction of heavy metal cations in the ion

olumn. The anion concentration in groundwater was detected byn ion chromatograph (Dionex ICS900) while the cation concentra-ion was detected by a whole spectrum ICP emission spectrometer.he accuracy of the analytical data is within 1% and the detection

4.69 × 10−11 3.31 × 10−9 10−14

2.42 × 10−1◦ 3.47 × 10−9 5.38 × 10−13

limit is 0.5 ppb. We tested two batches of groundwater samples.The first batch of 15 samples was taken from 14 wells (Fig. 1), andthe concentrations of various ions were used to study the hydro-logical background of the groundwater in the target layer (Table 2),and then assess the feasibility of employing the hydrological back-ground in further study. The second batch of 34 samples was takenfrom 34 wells spread over a larger area than the first batch (Fig. 1),and these samples were mainly used for calculation of the calcitedissolution trend (Table 3).

Besides, some samples of reservoir rock were sampled for litho-logical research. All of the groundwater analysed during this studyis from depths of 5500–6100 m below the surface, with rock sam-ples obtained during this study comprising wackestone, packstone,and grainstone. Abundant dissolved pores are also present in thesesamples. Scanning electron microscope (SEM) observations wereperformed using a LEO-435 VP SEM instrument at China Univer-sity of Geosciences, and were used to identify rock microstructuresand minerals. The saturation index of calcite in groundwaters wascalculated using Phreeqc Interactive 3.0 software (U.S. GeologicalSurvey).

4. Results and discussion

4.1. Petrology of the Ordovician reservoir

Reservoir rocks in the study area are dominated by O1y

and O2yj formation limestones that are subdivided into grain-stones, micrites, dolomitic and algal limestones, and biolithites.Grainstones and micrites form 32.9% and 49.9% of the reser-voir rocks in the study area, respectively, and, as such, are themost important rock types. Grainstones are generally calciru-dites and calcarenites with calcsparite cements, and contain lowabundances of narrow-salinity-range biological debris, includingcrinoids, sponges, sponge bone needles, brachiopods, and trilo-bites (Fig. 3a and b). Micrites are dominated by microcrystallinecalcite and contain minor amounts of arenes, pellets, brachiopods,crinoids, trilobites, sponges, sponge spicules, bivalves and ostra-cods (Fig. 3c). Dolomitic limestones are mainly confined to thelower part of the O1y formation, and are macroscopically por-phyritic and contain microcrystalline calcite without non-skeletalcalcarenite components (Fig. 3d). Algal limestones are alwayspresent in the O1y formation. Such limestones have been identifiedin wells S68, S72, S76 and S86, and contain a clear algal bond-ing structure (Fig. 3e) that bonds pellets and gravel debris, as wellas brachiopods, crinoids, sponge spicules, moss insects, red algae,ostracods and other insects. Biolithites are dominated by sponges;these are the main reef-building organisms in these rocks, whichcontain an oncoid layer and clear holes within reef frameworksthat are generally filled with calcsparite (Fig. 3f). Analyses of 254samples indicate that the porosity of limestones in the Ordovicianreservoir of the Tahe Oilfield varies between 0.01% and 10.80%,with an average of 0.96% and with a permeability distribution inter-val that ranges from 0.001 to 5052 × 10−3 �m2, with an arithmeticaverage permeability of 2.34 × 10−3 �m2. Some 71.52% of sampleshave <1% porosity, whereas only 6.46% samples have porosities

that are >2%. In addition, some 94.39% samples have permeabili-ties of <3 × 10−3 �m2, with 5.61% samples having permeabilities of>3 × 10−3 �m2. The main pore types in this Ordovician carbonatereservoir are structural fractures, dissolution pores, and unevenly

Z. Ruan et al. / Chemie der Er

Tab

le

2Th

e

hyd

roge

olog

ical

con

dit

ion

of

Ord

ovic

ian

grou

nd

wat

ers

in

the

Tah

e

Oil

fiel

d.

Sam

ple

s

Dep

th

(m)

Den

(g/c

m3)

T-Sa

l (m

g/l)

Ion

con

cen

trat

ion

(mg·L

−1)

100

×

rSO

4/r

Cl

rCa/

rMg

rHC

O3

+

rCO

3/r

Ca

Cl−

SO4

2−H

CO

3−

CO

32−

I−B

r−N

a++

K+

Ca2+

Mg2+

T205

-1

5713

1.18

9

306,

772.

82

186,

427.

3

400

495.

48

0

6

160

111,

361.

28

7204

.18

696.

32

0.15

83

6.27

76

0.02

26T2

05-2

5610

–564

0

1.19

280,

630.

47

170,

784.

98

300

473.

52

0

10

60

99,5

22.7

6

9102

.97

538

0.12

96

10.2

66

0.01

71TK

203

5100

1.09

8

145,

654.

59

88,4

54.1

3

500

672.

44

0

6

240

47,0

29.1

6

8352

.67

506.

41

0.41

69

10.0

08

0.02

64TK

304X

5462

.4–5

8791.02

1

35,4

50

16,5

50

6000

279.

5

0

1

0

10,8

00

1065

888

26.7

404

0.72

77

0.08

62TK

307

5520

1.14

421

2,25

3.72

129,

846.

9710

0

183.

670

12

320

69,2

23.1

5

11,7

98.1

5

759.

62

0.05

68

9.42

40

0.00

51TK

316

5535

–560

01.1

42

199,

962.

19

124,

350.

8

1500

932.

39

0

6

0

23,1

07.6

3

49,0

71.9

5

759.

62

0.88

97

39.1

97

0.00

62TK

406

5025

–502

81.1

125

8,12

1.1

157,

000

1000

438.

1

0

8

0

84,6

70

14,6

20

450.

5

0.46

98

19.6

91

0.00

98TK

428

5404

–544

01.1

133,

774.

54

82,1

28.7

9

200

75.6

6

0

16

8

43,6

90.8

3

6509

.39

1164

.7

0.17

96

3.39

11

0.00

38TK

432

5585

1.16

2

242,

508.

01

148,

432.

7

200

172.

69

0

7

175

80,2

24.5

1

12,1

88.7

3

1122

.54

0.09

94

6.58

83

0.00

47TK

436

5625

–568

71.1

5521

3,85

2.27

130,

846.

320

0

660.

240

6.67

300

66,9

17.8

8

14,0

86.1

2

1035

.18

0.11

27

8.25

64

0.01

54TK

615

5675

–572

0.0

1.13

6

211,

422.

97

128,

472.

93

750

483.

28

0

8

250

69,8

60.9

11,0

67.2

9

522.

21

0.43

06

12.8

59

0.01

43S6

6

5612

–568

01.1

22

176,

100

107,

100

300

233.

7

0

8

200

62,1

10

5425

567.

4 0.

2066

5.80

13

0.01

41S6

7

5295

–530

0.5

1.09

2

139,

900

82,7

80

3000

624.

8

0

25

50

45,9

80

7012

642.

7 2.

6731

6.61

99

0.02

93T4

16

5468

–548

01.1

4

210,

670.

18

128,

874.

58

400

135.

46

0

11

160

68,8

25.5

6

11,1

08.7

7

1122

.54

0.22

89

6.00

46

0.00

40T4

36

900–

1000

1.15

2

229,

924.

92

140,

382

600

733.

46

0

5

125

71,9

49.7

4

15,2

43.6

3

1012

.82

0.31

52

9.13

22

0.01

58

de 73 (2013) 469– 479 473

distributed holes and fractures with diverse geometric shapes andsizes.

It is evident that abundant calcite dissolution within Ordovi-cian sediments of the Tahe Oilfield has occurred, but the originof this dissolution is complex, as it may have been caused byweathering-related karstification, penecontemporaneous dissolu-tion, or fluid-related dissolution in deep burial environments (Jinget al., 2005; Zhou et al., 2009). The focus of this study, on thecorrelation between the abundance and composition of water inburied environments and changes in the dissolution and/or pre-cipitation of calcite, means that it is important to identify the typeof calcite dissolution that these rocks have undergone. However,it is extremely difficult to distinguish burial-related calcite disso-lution from subaerial dissolution, although the following criteriaare indicative of burial-related calcite dissolution: (1) The pres-ence of hydrothermal minerals that accompany porosity increasesare often indicative of formation during burial, suggesting burial-related calcite dissolution. (2) In contrast to subaerial calcitedissolution, burial-related dissolution is more selective, with dis-solution of buried calcite developing in areas containing manyexisting pores or cracks; in addition, burial-related dissolutionoften occurs along planes that have no relationship with karst-derived structures. (3) Where no pre-existing cracks are present,less burial-related dissolution of calcite generally occurs, form-ing microscopic pores along crystal interfaces or in intracrystallinematrices (Fig. 4a and b). (4) Dissolution pores in late-formed matri-ces are indicative of late-stage dissolution, and provide strongevidence of burial-related diagenesis (Fig. 4c). (5) In the Tahe Oil-field, faults with large angles may have formed in response topost-Hercynian stage tectonic stress, and pores along these suturesmay have formed during burial-related dissolution (Fig. 4d).

4.2. Chemical composition of formation water

The chemical composition of Ordovician groundwater was ana-lysed by the first branch of 15 samples during this study. The totalsalinity values were >8 × 104 mg/L in most of these wells, indicat-ing high-salinity groundwater that are dominated by chloride ions.These waters also show a strong positive correlation between totalsalinity and chloride ions (correlation coefficient of 0.6), indicat-ing the concentrated nature of groundwater (left panel in Fig. 5).The Ordovician groundwater also have low SO4

2− and HCO3− mass

concentration values, the majority of which are <3000 mg/L (rightpanel in Fig. 5). H2S gas is more abundant in gas-productive Ordovi-cian sediments, which is related to the reduction of SO4

2− by water.In comparison, the high concentrations of calcium ions in thesewater, combined with the fact that dissolved CO2 gas producesHCO3

− ions, means that these waters have near-zero concentra-tions of CO3

2−. The majority of these waters are acidic, with pHvalues between 5.0 and 6.5. Again, this is conducive to the forma-tion of HCO3−. The Ca2+ ion is an important factor in our study,and Table 2 shows that the Ca2+ concentration in formation wateris 1065–49,072 mg/L, with the majority of values being between5000 and 15,000 mg/L. There is no clear correlation between Ca2+

concentration and salinity, because Ca2+ is involved in the precipi-tation and dissolution of authigenic minerals. In addition, the largenumber of clay minerals and the large amount of clay cement in theOrdovician carbonate strata means that Na+ and Ca2+ are readilyadsorbed, resulting in a low Ca2+ content in formation water.

Desulfurisation is common in groundwater, and not onlyremoves sulphate from solution but also adds H2S to natural gas.This indicates that desulfurisation could be a potential environ-

mental indicator, as the smaller the amount of desulfurisation,the more complete closure of the formation water. The desul-furisation coefficients (100 × rSO4/rCl) determined for all wellsbarring TK304X are 0.05–2.70, with an average of 0.4548; this low

474 Z. Ruan et al. / Chemie der Erde 73 (2013) 469– 479

Fig. 3. Characteristics of limestones from the Ordovician reservoir of the Tahe Oilfield; a, calcsparite arenitic limestone, well S76, 5592.52 m depth, plane polarized light;b, calcsparite arenstritic limestone, well S91, 5692.50 m depth, ambient light; c. calcarenite micrite, well S87, 5701.0 m depth, plane polarized light; d, dolomitic limestone,well S86, 5813.52 m depth, plane polarized light; e, algal grain limestone, well T704, 5824.01 m depth, plane polarized light; f. biohermal limestone containing sponge, wellS60, 5906.2 m depth, plane polarized light.

Fig. 4. Calcite dissolution characteristics in the Ordovician reservoir of the Tahe oilfield; a, sparry calcarenite with dissolved holes and micro-dissolved holes in arene, wellT705, 5661.55 m depth, ambient light; b, Micro-dolomitized micrite and dolomite with intergranular holes, well S86, 5812.32 m depth, ambient light; c, dissolution alonga suture, well T704, 5740.31 m depth, ambient light; d, dolomitized micrite with dissolution holes along a suture that have been filled by brown organic matter, well T702,5493.69 m depth, ambient light.

Fig. 5. Cl− vs. total salinity diagram for Ordovician formation waters of the Tahe Oilfield (left), and SO42− , HCO3

− vs. total salinity plot for Ordovician formation waters of theTahe Oilfield (right).

der Erde 73 (2013) 469– 479 475

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Z. Ruan et al. / Chemie

esulfurisation coefficient is indicative of near-complete closure. Inddition, the carbonate equilibrium coefficient (rHCO3 + rCO3/rCa)an be used as an indicator of formation closure and hydrocarbonigration, with small coefficients being indicative of better closure

f formation waters or oil and gas reservoirs. The data shown inable 2 indicate that the carbonate equilibrium coefficient in allells is <0.1, suggesting stratigraphic well closure and closure of

he oil and gas reservoir. Furthermore, previous research suggestshat oil and gas within the reservoir migrated from the southeast ofhe study area (Liu & Zhang, 2005; Li et al., 2011), and that calciumnd magnesium coefficient (rCa/rMg) values reflect the degree ofetamorphism of the formation water, as longer and better closure

elates to high degrees of metamorphism, and therefore higher cal-ium and magnesium coefficient values. Calcium and magnesiumoefficient (rCa/rMg) values for 15 wells vary between 0.72 and9.19, with an average of 10.28 and most wells having coefficients5, indicating relatively intense metamorphism and better sedi-entary unit closure.

.3. Factors that control �G values

Fig. 6 provides a brief description of the methodology outlinedbove. Eq. (6) indicates that �G values are directly dependentn Ca2+ activity (aCa2+ ), the theoretical activity of Ca2+ (aCa2+

eq),

nd the formation water temperature, with the latter being cal-ulated using burial depths and the local geothermal gradient. Thisndicates that activity and theoretical activity values for Ca2+ are

undamentally important in �G value calculations. The activity ofa2+ can be calculated directly using the Debye–Huckel equation,nd Eq. (7) indicates that ionic strength calculations need molaroncentrations of various ions in solution as inputs. In addition,

able 3he components, ionic activity, ionic intensity, and the Gibbs free energy of formation wa

Well Depth (m) pH Ion concentration (mmol L−1)

Cl− HCO3− CO3

2− OH−

T914 6125 7.8 826.54 27.99 0.00 0.00

S66 5540 6.0 46.35 3.02 0.00 0.00

T901 5850 6.0 122.04 6.69 0.00 0.00

T502 7.0 411.95 25.43 0.00 0.00

TK216 5630 5.5 205.17 12.18 0.00 0.00

T701 5657 6.0 1652.40 14.12 0.00 0.00

S92 5770 6.0 1429.16 11.77 0.00 0.00

T707 5756 5.5 1330.01 1.44 0.00 0.00

TK719 5935 6.2 2865.90 35.31 0.00 0.00

T416 5800 5.9 2394.85 1.89 0.00 0.00

S70 5623.1 5.5 3316.84 3.32 0.00 0.00

S76 5570 5.0 2799.90 3.65 0.00 0.00

T749 5960 6.1 2928.06 14.53 0.00 0.00

TK622 5626 5.5 2949.75 2.65 0.00 0.00

S61 5701.6 6.5 3200.06 6.29 0.00 0.00

T313 5589.7 6.0 3453.21 3.38 0.00 0.00

T403 5633.7 6.0 3404.83 5.94 0.00 0.00

S65 5754 6.0 3832.43 3.32 0.00 0.00

T728 6085 6.5 2737.76 35.28 0.00 0.00

TK409 5670 6.0 3848.23 3.64 0.00 0.00

S47 5600 5.8 3998.45 3.30 0.00 0.00

T706 5650 6.0 3763.31 4.09 0.00 0.00

T727 5917 6.1 3433.32 11.69 0.00 0.00

TK310 5600 6.0 3637.52 3.07 0.00 0.00

T737 5813 6.0 3001.20 21.10 0.00 0.00

S98 5.5 4756.05 13.15 0.00 0.00

S105 5874 5.9 3498.66 12.63 0.00 0.00

TK729 5557 6.0 3687.23 18.20 0.00 0.00

S73 6.2 3094.98 9.51 0.00 0.00

TK736 5558 6.2 2985.65 20.48 0.00 0.00

S85 5947 6.0 2955.60 12.74 0.00 0.00

TK656 5656 5.8 3547.05 12.70 0.00 0.00

T752 5633 5.7 3492.05 27.05 0.00 0.00

S72 4.5 3581.26 24.60 0.00 0.00

Fig. 6. Flow diagram outlining the methods used in this study.

Eqs. (8)–(15) indicate that K1, K2, KCc, [�CO2] − [Ca2+], and thepH value of the solution are all used to determine the theoreti-cal activity of Ca2+. The activity of H+ can be calculated using pHvalues, with K(1, 2, Cc) values being constants that are related to

ter in the Tahe Oilfield.

aH+ I aCa2+ �G/kJ

Ca2+ Mg2+ SO42−

33.68 8.42 8.33 1.7E−08 0.53 4.71 −0.68.99 8.96 7.81 1.0E−06 0.08 3.09 −2.0

18.68 20.51 11.72 1.0E−06 0.17 4.61 −3.056.22 68.27 41.67 1.0E−07 0.55 7.68 −4.332.41 8.33 6.25 3.2E−06 0.20 7.30 −3.2

117.62 44.62 14.58 1.0E−06 1.20 10.86 −7.0235.51 38.54 3.13 1.0E−06 1.28 21.01 −7.3207.98 60.41 10.42 3.2E−06 1.22 18.98 −7.4452.02 69.90 0.52 6.3E−07 2.50 29.34 −8.2335.20 125.70 17.71 1.3E−06 2.16 23.28 −8.3515.83 76.99 2.08 3.2E−06 2.86 31.52 −8.6565.22 154.16 1.04 1.0E−05 2.85 34.58 −8.6581.63 103.86 2.08 8.1E−07 2.85 35.59 −8.6471.02 171.28 13.02 3.2E−06 2.80 29.02 −8.6621.89 113.68 2.60 3.2E−07 3.08 36.75 −8.7528.99 85.65 1.46 1.0E−06 2.96 31.81 −8.7517.13 98.08 3.26 1.0E−06 2.94 31.18 −8.7569.73 96.23 3.13 1.0E−06 3.26 32.85 −8.8941.32 51.26 2.08 3.2E−07 3.38 53.46 −8.8604.46 84.57 2.08 1.0E−06 3.31 34.62 −8.8617.92 79.37 1.35 1.6E−06 3.40 34.99 −8.9701.48 67.02 2.08 1.0E−06 3.43 39.58 −8.9850.95 74.40 3.13 8.5E−07 3.58 47.12 −8.9552.23 109.45 3.13 1.0E−06 3.15 32.31 −8.8

1025.20 97.86 5.83 1.0E−06 3.77 55.56 −9.0930.05 167.20 3.65 3.2E−06 4.59 46.48 −9.2

1547.27 105.15 1.56 1.3E−06 5.06 74.36 −9.41453.93 77.51 3.65 1.0E−06 4.92 70.65 −9.31898.71 29.47 4.17 6.0E−07 5.42 88.87 −9.52069.19 85.04 5.21 7.1E−07 5.82 94.21 −9.51077.86 115.48 3.13 1.0E−06 3.92 57.45 −9.11982.29 35.61 2.08 1.8E−06 5.82 90.27 −9.62354.78 87.99 3.13 2.0E−06 6.65 102.01 −9.73066.34 78.62 2.60 3.2E−05 8.10 123.81 −9.9

4 der Erde 73 (2013) 469– 479

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76 Z. Ruan et al. / Chemie

emperature and pressure. In addition, [�CO2] − [Ca2+] representshe difference between total carbonate content and the Ca2+ con-ent of the solution—a difference that can be calculated using theonic molar concentrations of CO3

2−, HCO3− and Ca2+. This means

hat temperature, pressure, the pH of the solution, and the dif-erence between total carbonate and calcium ion concentrationsre all controls on the theoretical activity of Ca2+. If all of theseariables can be constrained, then it should be possible to pre-ict the ability of groundwater to dissolve calcite (i.e., calculateG values).As we need to calculate the capacity of modern groundwa-

er to dissolve reservoir rocks, it is also necessary to determinehe temperature of these waters. Drilling temperature data indi-ate that the average geothermal gradient in the Tahe Oilfields 2.2 ◦C/100 m, with the majority of Ordovician sediment-osted formation water being present at depths of 5500–6000 m,orresponding to temperatures of 121–132 ◦C. The pressure con-itions of these formation waters are also crucial. Previousesearch indicated that sedimentary pressures in the Ordovi-ian carbonate reservoir ranged from 50 to 70 MPa (Li, 2007).onsidering these variables, we used a temperature of 100 ◦Cnd a pressure of 0.1 GPa for all groundwaters, with equi-ibrium constant values for 100 ◦C and 0.1 GPa being used inable 1.

In fact, the dissolution trend of dolomite in groundwater canlso be forecasted by using a quantitative model of the limestone.he main difference between calcite and dolomite is the participa-

ion of the magnesium ions in the ionization reaction. Therefore, wean calculate the �G values of dolomite in groundwater from thectivity product of calcium and magnesium ions in the actual solu-ion (aCa2+ aMg2+ ), as well as from the theoretical activity product

ig. 8. Calculated calcite dissolution tendencies for carbonate rocks of the O1y + O2yj Form, well location; 3, well location sampled during this study; 4 area of weak calcite dissolu

Fig. 7. Saturation index of CaCO3 of samples.

of calcium and magnesium ions (aCa2+ aMg2+ ). Formula (5) can berewritten as[∑

CO2

]−[∑

CO2eq

]= [Ca2+ + Mg2+] − [Ca2+

eq + Mg2+eq ] (16)

We did not calculate the �G values of dolomite becausedolomite matrix is rare in the study area.

4.4. Estimations of buried calcite dissolution

The concentrations of a number of ions in groundwater of theYijianfang and Yingshan formations (O1y and O2yj) were deter-mined for 34 wells in the Tahe Oilfield during this study. The

concentrations of CO3

2− and OH− are below the detection limitand all the samples have Ca2+ concentrations that greatly exceedthe Mg2+ concentrations, indicating relatively low dolomite abun-dances in these formations. These ionic concentrations, combined

ation of the Tahe Oilfield and the porosity of well S76, S65 and T914; 1, �G contour;tion; 5, area of strong calcite dissolution.

Z. Ruan et al. / Chemie der Erde 73 (2013) 469– 479 477

Fig. 9. SEM images of typical burial-related dissolution of calcite in the Ordovician reservoir of the Tahe Oilfield; (a) intragranular dissolved pores and holes in sparryc and

d 8 m ded eam a

wttrdawtttcutNwuatwwtce

oovdape(

alcarenite–bioclastic limestone, well S76, 5593.8 m depth; (b) dissolved fracturesepth; (c) intergranular pores and micro-gaps in calcarenite micrite, well S86, 5338.epth; WP, intragranular dissolved pores; BC, intergranular holes; CA, Dissolution s

ith pH values, formation temperatures and pressures, the activi-ies of Ca2+ in water, and the theoretical activities of Ca2+, were usedo calculate �G. Table 3 shows that the smaller the Ca2+ to Ca2+

eqatio, the smaller the value of �G, and increasingly favourable con-itions for calcite dissolution. In addition, Ca2+ ion concentrationsre lower than Ca2+

eq values, with �G values lower than zero in allells analysed during this study, indicative of stable calcite dissolu-

ion trends in the Tahe Oilfield. In fact, a method has been developedo evaluate the nature of minerals dissolved in solution, employinghe saturation index. PHREEQC Interactive software is used to cal-ulate low-temperature hydrogeochemical reactions, and is widelysed in calculations of water geochemistry, especially to calculatehe saturation index (SI) of minerals (Parkhurst & Appelo, 1999;ath et al., 2008). In this study, six of the samples listed in Table 3ere selected for calculations of the saturation index of CaCO3sing PHREEQC Interactive software with a temperature of 100 ◦Cnd CO2 partial pressure of 1000 atm. The results show that all ofhe SI values of CaCO3 are under −1 (Fig. 7), indicating the ground-ater conditions are conducive to the dissolution of CaCO3. It isorth noting that PHREEQC Interactive software is more applicable

o solutions in low-temperature environments; consequently, thealculation results may contain errors in the case of deeply buriednvironments, and the results should only be used for reference.

The �G values for groundwater in these 34 wells are indicativef burial-related calcite dissolution trends across the stratigraphyf the O1y and O2yj formations of the Tahe Oilfield. All �G isogramalues are under zero, indicating that current groundwater con-itions are conducive to burial-related calcite dissolution (Fig. 8),

nd suggesting that the dissolution of buried calcite could haveromoted porosity and permeability development in the reservoir,specially in areas that already contained significant pore spacee.g., in areas with pore spaces between crystals or with sutures).

intergranular holes in sparry calcarenite–bioclastic limestone, well S76, 5593.8 mpth; (d) intergranular pores in fine-grained biogenic limestone, well 86, 5710.77 mnd hole.

Although the overall groundwater conditions are favourablefor calcite dissolution, the solvating capacity in different regionschanges significantly, with three weak dissolution belts present inthe study area, formed by wells T914 and T901 in the east, a centralarea near well TK219, and around wells S66 and T502 in the north.The �G values in these three regions are greater than −5 kJ, withvalues in other regions between −5 kJ and −11 kJ, indicating moresignificant potential dissolution in the latter. The result of the pre-diction could be proved by the porosity data of four wells showingin Fig. 8. The porosity in well T901 and T914 is much poorer thanthe one in well S76. In addition, a series of burial-related calcitedissolution phenomena were observed in significant potential cal-cite dissolution areas, for example in wells S76 and S86 (Fig. 9).Apparently selective calcite dissolution in bioclastic matrices, suchas algal debris, was observed in well S76, with dissolved pores dis-persed in algal debris and microcrystalline matrices accounting for70–80% of the total porosity (Fig. 9a and b). In addition, needle-likemicro-pores are uniformly distributed in the well S86 reservoir,with larger dissolved pores in biological chambers (Fig. 9c and d).The presence of these dissolution pores is not compatible with fea-tures expected to have formed during surficial karstification, andthis indicates that burial-related calcite dissolution occurred in thestudy area.

In addition, contouring of �G values allows us to speculate asto whether dissolution trends weaken or even change into pre-cipitation trends in the northern section of the study area. Recentresearch indicates significant calcite precipitation in well YQ2 inthe northern part of the study area, supporting the conclusions

presented here.

A burial history chart for the Tahe Oilfield indicates thatHimalayan tectonism was the last tectonic event to affect the studyarea (Ye et al., 2000), with the burial depth of the Ordovician strata

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78 Z. Ruan et al. / Chemie

uring this time being similar to the current depth of these sedi-ents. This indicates that the current geothermal field accurately

eflects that of the geothermal field during Himalayan tectonism,uggesting that limestone dissolution trends of modern formationater reflect the calcite-dissolving ability of formation waters dur-

ng Himalayan orogenesis.

. Conclusions

The theoretical calcite water–rock interactions androundwater–reservoir relationships presented here have pro-ided several insights. The theoretical solubility of calcite inolution is controlled by the temperature, pressure, and pH of theolution. If the temperature, pressure, [�CO2] − [Ca2+], and pHalues of a solution are determined while the calcite ionisationeaction is at equilibrium, a theoretical Ca2+ activity (aCa2+

eq ) inolution can be calculated. However, the real Ca2+ activity (aCa2+)n solution is always different from this calculated aCa2+

eq value.a2+ activities (aCa2+) are less than the theoretical activity of Ca2+

aCa2+eq ), meaning that the concentration of Ca2+ in solution has

ot reached saturation, and indicating that the solution will act toissolve calcite, yielding �G values below zero. In contrast, if thea2+ activity (aCa2+) is greater than the theoretical activity of Ca2+

aCa2+eq ), Ca2+ will be combined with carbonate components in

he solution, leading to calcite precipitation (�G > 0). Ordovicianroundwater of the Tahe Oilfield are high-salinity CaCl2-typeuids that are significantly different from formation waters inther sediments in the oilfield, with low desulfurisation andarbon balance coefficients, and higher calcium and magnesiumoefficients, indicating that these groundwater have always beenn a closed environment. This suggests that ion concentrations inhese groundwaters can be used to calculate calcite-dissolutionrends, with �G values indicating weak calcite-dissolution trendsn the eastern and northern parts of the Tahe Oilfield, and withvidence of burial-related calcite dissolution in other areas. Thispproach provides a realistic theoretical basis for the evaluation ofurial-related calcite and limestone dissolution within carbonateeservoirs.

cknowledgments

This study was financially supported by funds from the Chi-ese National 973 Program (2011CB201100-03, 2006CB202302)nd Chinese National Oil and Gas Program (2011ZX05005-004-Z06, 2011ZX05009-002-402).

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