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

Li, Hongxia; Cai, Chunfang; Jia, Lianqi; Xu, Chenlu; Zhang, Ke

doi:https://doi.org/10.1155/2017/6351382

Geofluids

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Abstract Introduction Analysis Results Discussion Conclusions Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2017 | Article ID 6351382 | https://doi.org/10.1155/2017/6351382

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

Hongxia Li,^1,2,3,4Chunfang Cai ,^3,4,5Lianqi Jia,^3,4Chenlu Xu,^3,4and Ke Zhang⁶

Academic Editor: Marco Petitta

Received16 Feb 2017

Revised15 Aug 2017

Accepted05 Sept 2017

Published15 Oct 2017

Abstract

Formation water chemistry, sulfate sulfur isotopes, and associated H₂S contents and sulfur isotopes were measured from the Ordovician in Tazhong area, Tarim Basin. The aim is to elucidate the effects of geochemical composition of formation water on thermochemical sulfate reduction (TSR) and potential usage of SO₄/Cl ratios as a new proxy for TSR extents in areas, where H₂S and thiaadamantanes (TAs) data are not available. The formation water has SO₄/Cl ratios from 0.0002 to 0.016, significantly lower than 0.04 to 0.05 from 3 to 7 times evapoconcentrated seawater. Thus, the low values are explained to result from TSR. Furthermore, the SO₄/Cl ratios show negative correlation relationships to TAs and H₂S concentrations, indicating that TSR occurred in a relatively closed system and SO₄/Cl ratio can be used to indicate TSR extents in this area. Extensive TSR in the Cambrian in the Tazhong area, represented by low SO₄/Cl ratios and high H₂S and TAs concentrations, is accompanied by formation water with high TDS and Mg concentrations, indicating the effects of water chemistry on TSR under a realistic geological background. In contrast, the low TSR extent in the Ordovician may have resulted from limited TSR reaction duration and total contribution of aqueous .

1. Introduction

Thermochemical sulfate reduction (TSR), a process whereby aqueous sulfate and petroleum compounds react at temperatures higher than 120°C (C_nH_2n+2 + + H₂S + altered petroleum), is considered to result in elevated H₂S concentrations in many carbonate reservoirs [1–8]. 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 [11–14]. 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 () and/or magnesium sulfate contact ion-pairs (MgSO₄ CIP) are most likely reactive sulfur species involved in TSR. Experiments indicated that the concentrations of MgSO₄ CIP are related to temperatures and SO₄/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.

H₂S, common in the Ordovician carbonate reservoir in the Tazhong area, is generated by TSR [20, 21]. H₂S 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 H₂S 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.

2. Geological Setting

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)). 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].

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 platform-marginal carbonates. The Ordovician strata include the Upper Ordovician Sangtanmu (O₃s) and Lianglitage (O₃l) Formations and the Lower and Middle Ordovician Yingshan () and Penglaiba (O₁p) 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.

2.2. 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(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].

(a)

(b)

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 . 7 H₂S 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 SO₄) 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 . The major cations in the diluted solutions (5000 times for all cations) were analyzed with a Varian Vista-Pro inductively coupled plasma-optical emission spectrometer (ICP-OES) with an analytical precision better than 5%.

Dissolved was quantitatively precipitated as BaSO₄ by reacting with excess BaCl₂. This reaction was performed at a pH between 3 and 4 with HCl to prevent precipitation of BaCO₃. The precipitation of BaSO₄ was then filtered using a Buchner funnel and washed with distilled water. And then precipitation of BaSO₄ was dried and used for sulfur isotopic analysis on a Thermo Finnigan Delta S mass spectrometer. H₂S was precipitated immediately in the field by the quantitative reaction with excess zinc acetate, Zn(CH₃COO)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 μm filter on site. In the laboratory, ZnS was transformed to Ag₂S by adding HCl and passing the evolved H₂S under an inert atmosphere through AgNO₃ solution at a pH of 4. Ag₂S 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 ³⁴S relative to the Vienna Canyon Diablo Troilite (VCDT) standard. The reproducibility for ³⁴S measurement is ±0.3.

4. Results

4.1. Water Chemistry

Chemical compositions of formation water are shown in Table 1. The Yingshan Formation () 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 0.54~0.89 with an average of 0.74. The range of concentrations is from 31 mg/L to 891 mg/L. The SO₄/Cl ratios (expressed in weight units) range from 0.0002 to 0.016 with a mean value of 0.005 (SO₄/Cl ratio of seawater is 0.144). The range of Mg²⁺ concentrations of 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). SO₄/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 concentration of 182 mg/L, Cl⁻ concentration of 164000 mg/L, SO₄/Cl ratio of 0.001 which is lower than most of the formation water. The Cambrian formation water has Mg²⁺ concentration of 25500 mg/L and Mg/Cl ratios of 0.155 which is higher than that of the formation water.

4.2. Sulfur Isotopic Composition

values in the carbonate reservoir range from 11.9 to 16.3 with an average of 14.2 (Table 1). ³⁴S value of H₂S from the Cambrian is 33 which is significantly higher than that from the carbonate reservoir. values of the 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). ³⁴S values of the Cambrian bedded anhydrite lie between 26.2 and 33.7, which is similar to the Cambrian seawater. ³⁴S values of H₂S 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 SO₄ and H₂S in the Ordovician lies between 8.7 and 12.3 with an average of 10.6.

5. Discussion

5.1. Sulfur Isotope Composition and Fractionation

H₂S from the Ordovician carbonate reservoir in this and previous studies have ³⁴S values from 12 to 16 which are 15~20 lighter than the counterpart in the Cambrian () [10, 21, 34]. The large differences indicate that the H₂S 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 values and values (Figure 5(a)). values tend to increase with a decrease of concentrations in the formation water (Figure 5(b)). This indicates that ³⁴S values of H₂S are related to those of the remaining . There is a positive relationship between and SO₄/Cl ratios (Figure 5(c)), likely indicating that the more dissolved is converted to H₂S, the smaller sulfur isotope fractionation occurs. This may imply that values are controlled by both values and TSR extent.

(a)

(b)

(c)

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]. H₂S in the Cambrian carbonate reservoir has values close to coeval bedded anhydrite (Figure 4), whereas H₂S in the Ordovician carbonate reservoir has values lighter than coeval seawater (22~26, [36]) and formation water (Figure 4). 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 ³⁴S values between H₂S and 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 are converted into H₂S and thus H₂S shows similar ³⁴S values to the parent anhydrite [6]. In contrast, TSR in the Tazhong area may have happened at the oil-water transition zone [13, 14, 21]. TSR around the oil-water transition zone may have not consumed all the dissolved ; may have been reduced preferentially as the result of kinetic isotopic fractionation; thus, significant 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 H₂S concentration and higher differences in the Formation than those in the Cambrian may indicate that only part of the dissolved 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 is excessive for in situ TSR in the Ordovician of the Tazhong area. SO₄/Cl ratios, relating to the remaining dissolved amounts in formation water, probably can be used as a proxy of in situ TSR extent under some circumstances, where H₂S and TAs concentrations are unavailable.

5.2. SO₄/Cl Ratio: A Potential Proxy of TSR Extent

5.2.1. Effects of Water Evolution on SO₄/Cl Ratio

When initial seawater is evaporated and concentrated to 10 times, SO₄/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). SO₄/Cl ratios of 3 times and 7 times concentrated seawater are 0.05 and 0.04, respectively. Whereas the formation water has SO₄/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 SO₄/Cl ratio, formation waters from both the Cambrian and evolved from evaporated seawater alone are expected to have SO₄/Cl ratios higher than 0.04; thus, it is unlikely for the mixing of evaporated formation water between the Cambrian and the Formation to have the low SO₄/Cl ratios (<0.016).

5.2.2. Consumption of Aqueous 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 H₂S concentration, higher TAs concentration, and lower SO₄/Cl ratio were observed (Table 1). TSR consumes dissolved , leading to lower SO₄/Cl ratios in the formation water than original seawater. The depletion of in the formation water was not compensated by anhydrite dissolution as no anhydrite or anhydritic carbonate rocks develop in the Ordovician strata (Figure 2).

H₂S, 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 SO₄/Cl ratios versus H₂S concentrations and SO₄/Cl ratios versus TAs concentrations (Figures 6(a) and 6(b)). This indicates that was transformed to H₂S by TSR and subsequently to incorporate into TAs in a relatively closed system. Thus, SO₄/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.

(a)

(b)

5.3. Influence of Water Chemistry on TSR Initiation

TSR is commonly observed in carbonate reservoirs with high-temperature, but it is difficult to repeat the TSR process in the laboratory under conditions resembling nature. Dissolved , 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 (MgSO₄) rather than “free” dissolved sulfate ions () or solvated sulfate ion-pairs. MgSO₄ has been proved to be an effective catalysis for TSR in the laboratory [17, 42]. MgSO₄ exists as a main magnesium-bearing specie in solutions with Mg²⁺ being dominant [43]. As temperature increases, MgSO₄ solutions were separated into MgSO₄-rich phase and MgSO₄-poor phase due to the formation of the complex Mg²⁺- ion association in the fused silica capillary capsules, and the phase separation temperature decreases with increasing Mg/SO₄ ratios [19]. This indicates that formation water with high Mg concentrations and high temperature is preferable to form MgSO₄ and initiate TSR.

Figures 7(a), 7(b), and 7(c) show a negative relationship between Mg concentrations and SO₄/Cl ratios, a positive relationship between Mg concentrations and H₂S 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 prove the catalysis of MgSO₄. TDS concentrations show a negative correlation with SO₄/Cl ratios, a positive correlation with H₂S concentrations, and a positive correlation with TAs concentrations (Figures 7(d), 7(e), and 7(f)). This also indicates the catalysis of MgSO₄ as Mg concentration is in proportion to TDS in this study.

(a)

(b)

(c)

(d)

(e)

(f)

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/SO₄ 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/SO₄ ratios in the formation water from the Feixianguan Formation resulted from the high SO₄ concentration contributed by anhydrite dissolution. The limitation on TSR initiation from low Mg/SO₄ 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/SO₄ 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 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 H₂S concentrations in the Tazhong area than that in the northeast Sichuan Basin.

6. 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) MgSO₄ contact ion pair in formation water is catalyst for TSR. High Mg/SO₄ ratios and high temperatures are preferable to form MgSO₄ contact ion pair in solutions and thus will increase TSR extent. High Mg/SO₄ ratios of the 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 contribution in the reservoir.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work is supported by the Natural Science Foundation of China (Grants nos. 41672143, 41730424, and 41502148) and Special Major Project on Petroleum Study (2017ZX05008003-040).

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Copyright © 2017 Hongxia Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Geofluids

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

Abstract

1. Introduction

2. Geological Setting

2.1. Structural Units and Stratigraphy

2.2. Burial and Thermal History

3. Sample Collection and Analysis

4. Results

4.1. Water Chemistry

4.2. Sulfur Isotopic Composition

5. Discussion

5.1. Sulfur Isotope Composition and Fractionation

5.2. SO4/Cl Ratio: A Potential Proxy of TSR Extent

5.2.1. Effects of Water Evolution on SO4/Cl Ratio

5.2.2. Consumption of Aqueous by TSR

5.3. Influence of Water Chemistry on TSR Initiation

6. Conclusions

Conflicts of Interest

Acknowledgments

References

Copyright

5.2. SO₄/Cl Ratio: A Potential Proxy of TSR Extent

5.2.1. Effects of Water Evolution on SO₄/Cl Ratio