The Effect of Water Chemistry on Thermochemical Sulfate Reduction: A Case Study from the Ordovician in the Tazhong Area, Northwest China

1Research Institute of Unconventional Oil & Gas, Northeast Petroleum University, Daqing, Heilongjiang 163318, China 2Accumulation and Development of Unconventional Oil and Gas, State Key Laboratory Cultivation Base Jointly-Constructed by Heilongjiang Province and the Ministry of Science and Technology, Northeast Petroleum University, Daqing, Heilongjiang 163318, China 3Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 4University of Chinese Academy of Sciences, Beijing 100049, China 5Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, China 6Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjiang 841000, China


Introduction
Thermochemical sulfate reduction (TSR), a process whereby aqueous sulfate and petroleum compounds react at temperatures higher than 120 ∘ C (C  H 2+2 + SO 4 2− → CO 2 + H 2 S + altered petroleum), is considered to result in elevated H 2 S concentrations in many carbonate reservoirs [1][2][3][4][5][6][7][8].Significant advance has occurred on mechanisms of TSR.A great number of organic sulfides such as thiols and thiolanes [7,9,10] and 1-to 3-cage thiadiamondoids with 1 to 4 sulfur atoms were detected from TSR areas [11][12][13][14].The presence of these organic sulfur compounds, especially labile sulfur compounds such as 1-pentanethiol or diethyl disulfide, has been experimentally showed to significantly increase the rate of TSR [15].However, hydrocarbons cannot directly react with solid sulfate in temperature from 180 ∘ C to 350 ∘ C in the laboratory [16].Reactions between solid sulfate and gaseous hydrocarbon are quite slow even under temperatures of several hundred degrees Celsius (Kiyosu et al., 1990).Water is the solvent for chemical species and provides the aqueous matrix for all chemical reactions.Theoretical calculations by Ma et al. [17] showed that bisulfate ions (HSO 4 − ) and/or magnesium sulfate contact ion-pairs (MgSO 4 CIP) are most likely reactive sulfur species involved in TSR.Experiments indicated that the concentrations of MgSO 4 CIP are related to temperatures and SO 4 /Mg ratios in the solutions [18,19].
Consequently, water chemistry and geologic environment can strongly influence the TSR process [8,17].However, the effects of water chemistry on TSR are limited to theoretical and experimental studies.More researches in the real geological setting should be done.
H 2 S, common in the Ordovician carbonate reservoir in the Tazhong area, is generated by TSR [20,21].H 2 S concentration from the Ordovician in the Tazhong area is less than 10%, which is lower than that in the Khuff formation in Abu Dhabi (up to 50% [6]), the Nisku Formation in western Canada (up to 31% [22]), and the Feixianguan Formation in the northeastern Sichuan basin (up to 17% [23,24]).The low concentrations of H 2 S in the Tazhong area are considered to result from TSR process which is limited by the burial temperature [25,26].Besides, TSR process can be limited by water chemistry [8].The Tazhong area is chosen as a research target and compared with the northeastern Sichuan Basin because relatively abundant information about TSR and formation water was published by previous studies.The work presented here seeks to address the following research questions: (1) what is the effect of water chemistry on TSR process and/or extent?(2) Why TSR extent in the Ordovician Yingshan Formation in the Tazhong area is low?A better understanding about the TSR mechanism will be provided through this work. .Tazhong Uplift is one of the major petroleum production areas in the Tarim Basin.Oils and natural gases have been found in the Cambrian-Ordovician carbonate reservoirs and the Silurian-Carboniferous clastic reservoirs [27].

Geological Setting
The general stratigraphic columns of the Tazhong area were described previously [20,25,28,29].Briefly, the Cambrian strata are composed of tidal, platform, and platformmarginal carbonates.The Ordovician strata include the Upper Ordovician Sangtanmu (O 3 s) and Lianglitage (O 3 l) Formations and the Lower and Middle Ordovician Yingshan (O 1-2 y) and Penglaiba (O 1 p) Formations (Figure 2).The Lower Ordovician is predominantly composed of thick, platform facies dolomite in the lower part and limestone in the upper part.The Upper Ordovician is represented by reef and shoal facies packstone and bioclastic limestone and slope facies limestone and marlstone [9] The Silurian to the Carboniferous sequence consists of marine sandstones and mudstones.The Permian strata are composed of lacustrine sediments and volcanic rocks.The Mesozoic and the Cenozoic are nonmarine sandstones and mudstones [20,30,31].
Anhydritic dolomites and anhydrite were observed in supratidal facies of the Middle Cambrian.Bedded anhydrites of 44 m∼98 m thick are present in the eastern ZS1 and ZS5 wells [21].No anhydritic carbonate was observed in the Ordovician, which makes the geological background of TSR in the Tazhong area differentiate from the northeast Sichuan Basin.

Burial and Thermal History.
TZ12 is located at the central part of number 10 Structural Belt (Figure 1(b)).Based on the burial history that rebuilt on well TZ12 (Figure 3

Sample Collection and Analysis
A total of 17 water samples were collected from wells in the Tazhong area.These samples are used for analysis of water chemistry and S isotopic compositions of SO 4 2− .7 H 2 S samples were collected and analyzed for S isotope.TAs concentrations in oils are obtained from previous studies.pH was measured using an electrode method within 2 hours after sampling in the field.TDS were measured by the gravimetric method according to Clescerl et al. [32].After filtration with a 0.45 ml filter, 0.5 ml samples of the brines were dried at 180 ∘ C until a constant weight was reached.The anions were measured by ion chromatography following appropriate dilution (5000 times for Cl and 1000 times for Br and SO 4 ) with a Dionex ICS900 instrument with an AS19 ion-exchange column.The analytical precisions were better than 0.8% for Cl and 4.3% for SO 4 2− .The major cations in the diluted solutions (5000 times for all cations) were analyzed with a Varian Vista-Pro inductively coupled plasmaoptical emission spectrometer (ICP-OES) with an analytical precision better than 5%.
Dissolved SO 4 2− was quantitatively precipitated as BaSO 4 by reacting with excess BaCl 2 .This reaction was performed at a pH between 3 and 4 with HCl to prevent precipitation of BaCO 3 .The precipitation of BaSO 4 was then filtered using a Buchner funnel and washed with distilled water.And then precipitation of BaSO 4 was dried and used for sulfur isotopic analysis on a Thermo Finnigan Delta S mass spectrometer.H 2 S was precipitated immediately in the field by the quantitative reaction with excess zinc acetate, Zn(CH 3 COO) 2 , to form ZnS at a pH in the range of 10-11 (the pH was adjusted with NaOH).The solution with ZnS was put aside overnight and then filtered with a 0.45 m filter on site.In the laboratory, ZnS was transformed to Ag 2 S by adding HCl and passing the evolved H 2 S under an inert atmosphere through AgNO 3 solution at a pH of 4. Ag 2 S were used for sulfur isotopic analysis on a Thermo Finnigan Delta S mass spectrometer calibrated by a series of International Atomic Energy Agency standards.Results are presented as  34 S relative to the Vienna Canyon Diablo Troilite (VCDT) standard.The reproducibility for  34 S measurement is ±0.3‰.O 1-2 y formation waters is from 92 mg/L to 1070 mg/L with an average of 638 mg/L, and the Mg/Cl ratios lie between 0.002 and 0.014 with a mean value of 0.007 (Mg/Cl ratio of seawater is 0.067).Na/Cl molar ratios of formation water are close to that of seawater (0.86).SO 4 /Cl and Mg/Cl ratios of formation water are significantly depleted compared to seawater and vary largely compared to Na/Cl molar ratios.The Cambrian formation water has SO 4 2− concentration of 182 mg/L, Cl − concentration of 164000 mg/L, SO 4 /Cl ratio of 0.001 which is lower than most of the O 1-2 y formation water.The Cambrian formation water has Mg 2+ concentration of 25500 mg/L and Mg/Cl ratios of 0.155 which is higher than that of the O 1-2 y formation water. 34S H2S values in the O 1-2 y carbonate reservoir range from 11.9‰ to 16.3‰ with an average of 14.2‰ (Table 1). 34 S value of H 2 S from the Cambrian is 33‰ which is significantly higher than that from the O 1-2 y carbonate reservoir. 34 S SO4 values of the O 1-2 y formation water range from 22.7‰ to 29.8‰ with an average of 26‰ which is slightly heavier than that of coeval seawater (Figure 4). 34 S values of the Cambrian bedded anhydrite lie between 26.2‰ and 33.7‰, which is similar to the Cambrian seawater. 34 S values of H 2 S from the Ordovician are lighter than that of the Cambrian seawater, the Ordovician seawater, and the Cambrian bedded anhydrite (Figure 4).Sulfur isotope fractionation between SO 4 and H 2 S in the Ordovician lies between 8.7‰ and 12.3‰ with an average of 10.6‰.

Sulfur Isotope Composition and
Fractionation.H 2 S from the Ordovician carbonate reservoir in this and previous studies have  34 S values from 12‰ to 16‰ which are 15‰∼ 20‰ lighter than the counterpart in the Cambrian (33‰) [10,21,34].The large differences indicate that the H 2 S in the Ordovician was probably generated from in situ TSR rather than TSR that happened in the Cambrian [13,21].A positive relationship exists between  34 S H2S values and  34 S SO4 values (Figure 5(a)). 34 S SO4 values tend to increase with a decrease of SO 4 2− concentrations in the formation water (Figure 5(b)).
This indicates that  34 S values of H 2 S are related to those of the remaining SO 4 2− .There is a positive relationship between  34 S SO4-H2S and SO 4 /Cl ratios (Figure 5(c)), likely indicating that the more dissolved SO 4 2− is converted to H 2 S, the smaller sulfur isotope fractionation occurs.This may imply that  34 S H2S values are controlled by both  34 S SO4 values and TSR extent.
Sulfur isotope fractionation is nearly negligible if complete conversion of the available sulfate during TSR [5] ( Krouse, 2001).Sulfur isotope fractionation is observed when only part of sulfate was reduced during TSR [35].H 2 S in the Cambrian carbonate reservoir has  34 S H2S values close to coeval bedded anhydrite (Figure 4), whereas H 2 S in the  Ordovician carbonate reservoir has  34 S H2S values lighter than coeval seawater (22∼26‰, [36]) and formation water (Figure 4). 34 S SO4-H2S differences of the Ordovician in the study area fall within a range of 8.7‰ to 12.3‰ with a mean value of 10.6‰ (Table 1).The differences of  34 S values between H 2 S and SO 4 2− in the Ordovician carbonate reservoir are higher than those of the Khuff Formation (2‰ to 3‰; [6]).This is probably due to the different geologic settings between the Abu Dhabi and the Tazhong area.TSR in the Khuff Formation of Abu Dhabi happened in the gas intervals with faster sulfate reduction than supply of reactive sulfates from anhydrite dissolution; consequently, almost all dissolved SO 4 2− are converted into H 2 S and thus H 2 S shows similar  34 S values to the parent anhydrite [6].In contrast, TSR in the Tazhong area may have happened at the oilwater transition zone [13,14,21].TSR around the oil-water transition zone may have not consumed all the dissolved SO 4 2− ; 34 SO 4 2− may have been reduced preferentially as the result of kinetic isotopic fractionation; thus, significant  34 S SO4-H2S differences are observed.Similar cases were reported from gas-water transitions in local areas from the western Canadian Basin [5] and the northeastern Sichuan Basin (Cai et al., 2010).The relatively lower H 2 S concentration and higher  34 S SO4-H2S differences in the O 1-2 y Formation than those in the Cambrian may indicate that only part of the dissolved SO 4 2− was reduced in situ in the Tazhong area, and TSR in the Ordovician is in the early stage.Zhang et al. [25] and Su et al. [26] also suggested that the overall TSR extent in the Ordovician of the Tazhong area is limited by the burial temperatures that reservoirs experienced.In other words, TSR extent is low and dissolved SO 4 2− is excessive for in situ TSR in the Ordovician of the Tazhong area.SO 4 /Cl ratios, relating to the remaining dissolved SO 4 2− amounts in formation water, probably can be used as a proxy of in situ TSR extent under some circumstances, where H 2 S and TAs concentrations are unavailable.

Effects of Water
Evolution on SO 4 /Cl Ratio.When initial seawater is evaporated and concentrated to 10 times, SO 4 /Cl ratio of seawater decreases from 0.144 to 0.04 as the result of the precipitation of sulfates [37,38].TDS of formation water in the study area lies between 84 g/L and 206 g/L with an average of 144 g/L, which is 3 to 7 folds of seawater (35 g/L).SO 4 /Cl ratios of 3 times and 7 times concentrated seawater are 0.05 and 0.04, respectively.Whereas the O 1-2 y formation water has SO 4 /Cl ratios from 0.0002 to 0.016, which are significantly lower than that which can be generated from the seawater evaporation.Assuming that the Cambrian to the Middle Ordovician seawater has a similar SO 4 /Cl ratio, formation waters from both the Cambrian and O 1-2 y evolved from evaporated seawater alone are expected to have SO 4 /Cl ratios higher than 0.04; thus, it is unlikely for the mixing of evaporated formation water between the Cambrian and the O 1-2 y Formation to have the low SO 4 /Cl ratios (<0.016).

Consumption of Aqueous SO 4
2− by TSR.TSR is ubiquitous in the carbonate reservoirs the Tazhong area [13,20,21,29,39,40].TSR in the Cambrian is more extensive than that in the Ordovician, as higher H 2 S concentration, higher TAs concentration, and lower SO 4 /Cl ratio were observed (Table 1).TSR consumes dissolved SO 4 2− , leading to lower SO 4 /Cl ratios in the formation water than original seawater.The depletion of SO 4 2− in the O 1-2 y formation water was not compensated by anhydrite dissolution as no anhydrite or anhydritic carbonate rocks develop in the Ordovician strata (Figure 2).
H 2 S, a direct product of TSR, can dissolve in formation water, precipitate as pyrite, and be incorporated into oils and solid bitumens producing alkylthiolanes, alkylthiols, and alkyl 2-thiaadamantanes [7,13,21].Thiaadamantanes (TAs) concentrations in petroleum are considered to better reflect TSR extents because TAs is quite stable even under high temperature [13,14,41].However, TAs concentrations are only measured in several wells.Negative relationships exist in SO 4 /Cl ratios versus H 2 S concentrations and SO 4 /Cl ratios versus TAs concentrations (Figures 6(a) and 6(b)).This indicates that SO 4 2− was transformed to H 2 S by TSR and subsequently to incorporate into TAs in a relatively closed system.Thus, SO 4 /Cl ratio is expected to be a good proxy to reflect TSR extent if they are not significantly changed by water mixing or anhydrite dissolution.

Influence of Water
Chemistry on TSR Initiation.TSR is commonly observed in carbonate reservoirs with hightemperature, but it is difficult to repeat the TSR process in the laboratory under conditions resembling nature.Dissolved SO 4 2− , with symmetrical molecular structure and spherical electronic distributions, have extremely low reactivity in the absence of catalysis [17].TSR reactions that occur in natural environments are most likely to involve magnesium sulfate (MgSO 4 ) rather than "free" dissolved sulfate ions (SO 4 2− ) or solvated sulfate ion-pairs.MgSO 4 has been proved to be an effective catalysis for TSR in the laboratory [17,42].MgSO 4 exists as a main magnesium-bearing specie in solutions with Mg 2+ being dominant [43].As temperature increases, MgSO 4 solutions were separated into MgSO 4 -rich phase and MgSO 4poor phase due to the formation of the complex Mg 2+ -SO 4 2− ion association in the fused silica capillary capsules, and the phase separation temperature decreases with increasing Mg/SO 4 ratios [19].This indicates that formation water with high Mg concentrations and high temperature is preferable to form MgSO 4 and initiate TSR.Mean Mg/Cl ratios in the formation water from the Yingshan Formation is 0.007, which is similar to that from the Feixianguan Formation (0.009 [44]).But average Mg/SO 4 ratio in the formation water from the Yingshan Formation is 6.24, which is significantly higher than that from the Feixianguan Formation (0.035 [44]).The low Mg/SO 4 ratios in the formation water from the Feixianguan Formation resulted from the high SO 4 concentration contributed by anhydrite dissolution.The limitation on TSR initiation from low Mg/SO 4 ratios in the Feixianguan Formation was probably compensated by the high temperature in the reservoir (Figure 3(b)).Similarly, the limitation on TSR initiation from low temperature in the Yingshan Formation was compensated by the high Mg/SO 4 ratios in the formation water.Duration time of TSR in the Feixianguan Formation is much longer than that in the Yingshan Formation as temperature of the Feixianguan Formation has reached 200 ∘ C since the Middle Jurassic (Figure 3(b)).And reaction rate of TSR in the Feixianguan Formation is also faster than that in the Yingshan Formation.Moreover, contribution of total aqueous SO 4 2− in the Feixianguan Formation is more than that in the Yingshan Formation as many anhydrites develop in the Feixianguan Formation.These are probably the main reasons for the lower H 2 S concentrations in the Tazhong area than that in the northeast Sichuan Basin.

Conclusions
Formation water is the solvent for sulfates, and water chemistry has a great influence on TSR which explain the low TSR extent in the Tazhong area.
(1) MgSO 4 contact ion pair in formation water is catalyst for TSR.High Mg/SO 4 ratios and high temperatures are preferable to form MgSO 4 contact ion pair in solutions and thus will increase TSR extent.High Mg/SO 4 ratios of the O 1-2 y formation water compensated the low temperature which would limit the initiation of TSR in the Tazhong area.
(2) The lower TSR extent in the Tazhong area is limited by the shorter reaction time and less total aqueous SO 4 2− contribution in the reservoir.

2. 1 .
Structural Units and Stratigraphy.Tazhong area is located in the center of Tazhong Uplift, Tarim Basin, northwest China.It is surrounded by the Manjiaer Sag, South Depression, Bachu Uplift, and Tadong Uplift (Figure 1(a)).It can be divided into number 1 Fault-slope Zone, North Slope, number 10 Structural Belt, Central Faulted Horst Belt, South Slope, and East Burial Hill Zone (Figure 1(b)) (a)), the Lower Ordovician reached temperature of 120 ∘ C at the late Cretaceous and then reached to the maximum depth of 5000 m and temperature of 150 ∘ C at present day, whereas the Triassic Feixianguan Formation has reached temperature of 150 ∘ C since the end of the Triassic and reached temperature of 200 ∘ C at the middle Jurassic (Figure 3(b)).TSR occurred in the limestone reservoirs of the Yingshan Formation above a temperature of 120 ∘ C [20, 29].

Figure 1 :
Figure 1: (a) Map of the Tarim Basin showing tectonic units and location of Tazhong Uplift; (b) map of the study area showing geological tectonics and location of wells from where formation waters were collected.

Figure 3 :Figure 4 :
Figure 3: (a) Burial and paleotemperature history of the Ordovician Yingshan Formation (marked with grey color) based on the well TZ12 in the Tazhong area (modified from Chen et al., 2010).(b) Burial and paleotemperature history of the Triassic Feixianguan Formation (marked with grey color) based on the well LJ2 from the Northeast Sichuan Basin (modified from [23]).

Figures 7 (
a), 7(b), and 7(c) show a negative relationship between Mg concentrations and SO 4 /Cl ratios, a positive relationship between Mg concentrations and H 2 S concentrations, and a positive relationship between Mg concentrations and TAs concentrations.This indicates that the TSR extent is high in formation water with high Mg concentrations, which

Figure 7 :
Figure 7: Relationship between (a) SO 4 /Cl ratios and Mg concentrations; (b) H 2 S concentrations and Mg concentrations; (c) TAs concentrations and Mg concentrations; (d) SO 4 /Cl ratios and TDS; (e) H 2 S concentrations and TDS; (f) TAs concentrations and TDS.

Table 1
The SO 4 /Cl ratios (expressed in weight units) range from 0.0002 to 0.016 with a mean value of 0.005 (SO 4 /Cl ratio of seawater is 0.144).The range of Mg 2+ concentrations of . The Yingshan Formation (O 1-2 y) formation waters have Na + concentrations ranging from 20140 mg/L to 64000 mg/L and Cl − concentrations ranging from 50861 mg/L to 126000 mg/L, characterized by Na/Cl molar ratios of 0.54∼0.89with an average of 0.74.The range of SO 4 2− concentrations is from 31 mg/L to 891 mg/L.

Table 1 :
Geochemical and sulfur isotopic composition of formation water, concentrations and sulfur isotopic composition of H 2 S in the gas, and concentrations of TAs in the oil in Tazhong [25]e that n is from Cai et al.[10], * is from Cai et al.[21], # is Cai et al.[13], & is from Li and Cai[33], ¶ is from Zhang et al.[25], and "-" represents no data available.