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Impact of formation water on the generation of H2S in condensate reservoirs: a case study from the deep Ordovician in the Tazhong Uplift of the Tarim Basin, NW China

Impact of formation water on the generation of H2S in condensate reservoirs: a case study from... Pet. Sci. (2017) 14:507–519 DOI 10.1007/s12182-017-0176-z OR IGINAL PAPER Impact of formation water on the generation of H S in condensate reservoirs: a case study from the deep Ordovician in the Tazhong Uplift of the Tarim Basin, NW China 1,2 1,2 1,2 1,2 3 • • • • • Jin Su Yu Wang Xiao-Mei Wang Kun He Hai-Jun Yang 1,2 1,2 1,2 1,2 • • • • Hui-Tong Wang Hua-Jian Wang Bin Zhang Ling Huang 1,2 1,2 4 • • Na Weng Li-Na Bi Zhi-Hua Xiao Received: 25 September 2016 / Published online: 27 July 2017 The Author(s) 2017. This article is an open access publication Abstract A number of condensate reservoirs with high occurred in the Cambrian. High H S-bearing condensates concentrations of H S have been discovered in the deep are mainly located near the No. 1 Fault and NE-SW strike- dolomite reservoirs of the lower Ordovician Yingshan slip faults, which are the major migration pathway of deep Formation (O y) in the Tazhong Uplift, where the forma- fluids in the Tazhong Uplift. The redox between sulfate- tion water has a high pH value. In the O y reservoir, the CIPs and hydrocarbons is the generation mechanism of 2? 2- concentrations of Mg and SO in the formation water H S in the deep dolomite condensate reservoirs of the 4 2 are higher than those in the upper Ordovician formation. Tazhong Uplift. This finding should be helpful to predict The concentration of H S in the condensate reservoirs and the fluid properties of deep dolomite reservoirs. 2? the concentration of Mg in the formation water correlate well in the O y reservoirs of the Tazhong Uplift, which Keywords Formation water  Sulfate-CIPs  TSR indicates a presumed thermochemical sulfate reduction Condensates  Dolomite reservoir  Tarim basin (TSR) origin of H S according to the oxidation theory of contact ion-pairs (CIPs). Besides, the pH values of the formation water are positively correlated with the con- 1 Introduction centration of H S in the condensate reservoirs, which may indicate that high pH might be another factor to promote Formation water has been conventionally taken as a kind of and maintain TSR. Oil–source correlation of biomarkers in undesirable by-product in petroleum exploration and the sulfuretted condensates indicates the Cambrian source development, while it would be one of the most crucial rocks could be the origin of condensates. The formation factors in hydrocarbon generation, migration and accu- water in the condensate reservoirs of O y is similar to that mulation as well as the interaction between organic and in the Cambrian; therefore, the TSR of sulfate-CIPs likely inorganic geo-fluids. Detailed analysis of the geochemical properties of formation water could indicate the hydrody- namic conditions for hydrocarbon accumulation and the & Jin Su geochemical alteration of reservoirs (Jiang and Zhang susujinjin@126.com 1999; Cai et al. 2001; Zha et a1. 2003; Chen and Zha Key Laboratory of Petroleum Geochemistry, CNPC, 2005, 2006a, b, 2008), which are of great geological sig- Beijing 100083, China nificance for interpreting the reaction mechanisms between State Key Laboratory for Enhancing Oil Recovery, Research ions and hydrocarbons. And then, the origin of multi-phase Institute of Petroleum Exploration and Development, reservoirs could be deduced from the secondary geo- PetroChina, Beijing 100083, China chemical alteration (Land 1995; Davisson and Criss 1996; Research Institute of Tarim Oilfield Company, PetroChina, Varsanyi and Kovacs 1997; Su et al. 2011). Palmer (1984) Korla 841000, Xinjiang, China published research on the hydrocarbon composition varia- PetroChina Coalbed Methane Company Limited, tion in carbonate reservoirs as a result of the formation Beijing 100028, China water flowing across the reservoirs. In particular, diben- zothiophene would be mostly altered in its composition Edited by Jie Hao 123 508 Pet. Sci. (2017) 14:507–519 (Kuo 1994; Lafargue and Tluez 1996). In addition, high 2 Geological background H S concentration from microbial activities around the oil– water contact would increase the acidity and density of The Tarim Basin experienced three important stages of crude oil, which negatively impacts on the commercial hydrocarbon accumulation, i.e., the later Eopaleozoic, the value of petroleum and as well as the safe exploitation and Neopaleozoic to the early Mesozoic, and the late Cenozoic development of reservoirs. (Zhang et al. 2011), each of which had great impacts on the With the extension of hydrocarbon exploration to deep accumulation and distribution of hydrocarbons in the dolomite rocks in recent decades in China, a large marine reservoirs (Zhang and Huang 2005). The Tarim number of gas reservoirs with high H S have been dis- Basin has become a petroliferous basin which has under- covered. In domestic and overseas investigations, high ¨ gone multistage secondary alterations (Sun and Puttmann concentrations of H S have generally been attributed to 1996; Su et al. 2010; Sun et al. 2009a, b). The Tazhong the thermochemical sulfate reduction (TSR) between Uplift, as an inherited paleo-uplift, is located in the central anhydrite and hydrocarbons (Worden et al. 2000; Jiang of the Tarim Basin and neighbors the Bachu Uplift in the et al. 2015), during which liquid hydrocarbons are enri- west, the Tadong Low Uplift in the east, the Tangguzibasi ched with sulfur and the drying coefficient of gas reser- depression in the south and the North depression in the voirs may exceed 99%. In actual geological conditions, north (Fig. 1). After the successful exploration in the reef the formation water could act as the reaction agent of and beach reservoirs of the upper Ordovician Lianglitage TSR. In the Tazhong Uplift of the Tarim Basin, H S- Formation (O l), a large-scale petroleum resource has been bearing condensates have been discovered on a large discovered in dolomite reservoirs of the Yingshan Forma- scale in deep dolomite reservoirs, which differs greatly tion (O y), in the lower Ordovician in the North Slope of from the geochemical properties of normal condensate the Tazhong Uplift. It has proved that the lower Ordovician reservoirs around the world and also differs from gas dolomite reservoirs are mainly distributed and developed reservoirs with high H S. Previous study of sulfur iso- below an unconformity surface, about 120 m deep. The topes and concentration of individual sulfur compounds distribution of dolomite reservoirs is not controlled by in the condensate reservoirs has shown that the conden- structural highs/lows. sate reservoirs with high H S may have experienced TSR The dolomite reservoirs of the Yingshan Formation (Jiang et al. 2008; Zhang et al. 2015). Various soluble (O y) are characterized by various hydrocarbon and fluid sulfate species act as the initial oxidant of TSR, mainly phases, as well as large discrepancies in gas/oil ratio 2- including free sulfate ions (SO ) and bisulfate ions (GOR). Most high-yield wells produce large volumes of (HSO ), solvent-shared ion-pairs (SIPs) and contact ion- gas and condensate oil; in contrast, low-yield wells mainly pairs (CIPs) (Eigen and Tamm 1962; Atkinson and Pet- produce crude oil with low GOR. Based on the PVT curve, rucci 1966). Therefore, the salts of formation water may the types of reservoirs in the Tazhong Uplift include be critical to activate and affect TSR. Recently, a series unsaturated and saturated condensates, and unsaturated and of gold-tube hydrous pyrolysis experiments have been saturated oil (Fig. 1), which are heterogeneously dis- conducted to address the reaction mechanism and the tributed across the Tazhong Uplift and are not controlled factors affecting TSR, which include compositions of by structure and burial depth. The difference in reservoir hydrocarbons, labile organosulfur compounds (LSC) and phases indicates that the distribution of condensates in the the early generated H S (Tang et al. 2005; Ellis et al. whole Tazhong Uplift roughly correlates with the fluid 2007; Amrani et al. 2008). Because the relative concen- properties and burial depth and the dolomite reservoirs are tration of H S is not a reliable reaction parameter to correlated with NE-SW faults. access the reaction intensity of TSR (Ma et al. 2008; Jendu et al. 2015), a systematic study on the genetic relationship between water and mineral composition of 3 Samples and methods the Ordovician formation and high sulfur condensates would avail to clarify the formation mechanism of high 3.1 Collection of samples sulfur condensate in the actual geological conditions. These new insights could indicate the effect of ions in In order to investigate the origin of H S, hydrocarbon the formation water on the alteration of hydrocarbons in samples of 13 wells in the Tazhong Uplift were collected deep dolomite reservoirs, which would be useful to pre- (Table 1). The saturated and aromatic hydrocarbons were dict the fluid properties of deep stratum and promote the quantified by GC–MS, including steranes and hopanes, as exploration of dolomite reservoirs. well as thiophenes. Diamantanes were detected by 123 3000 Pet. Sci. (2017) 14:507–519 509 60 75 80 R(148.0 °C, Stratum Lithology 0 10 20 km R(151.86 °C, R(124.87 °C, 66.11 MPa) Pm=56.10 MPa Pm=45.61 MPa 66.11 MPa) 63.33 MPa) 48 60 64 Pm=58.89 MPa 45 48 O s 24 30 32 ZG19 ZG17 TZ86 12 15 16 ZG162 TZ45 0 0 0 ZG16 -150 -40 70 180 290 400 -150 -40 70 180 290 400 -150 -40 70 180 290 400 Temperature, °C Temperature, °C Temperature, °C ZG20 ZG14 O3l ZG13 ZG8 ZG10 ZG5 ZG15 R(126.7 °C, ZG12 ZG22 63.58 MPa) Pm=65.84MPa No. I Fault ZG21 TZ35 ZG3 ZG111 ZG11 48 O1-2y ZG102 ZG23 ZG104 32 ZG10 ZG2 ZG8 ZG501 ZG47 ZG5 ZG44 ZG461 ZG9 -150 -40 70 180 290 400 ZG45 ZG431 TZ826 ZG48 ZG46 Temperature, °C O1p ZG43 TZ18 ZG6 ZG432 ZG7 TZ82 ZG433c 80 TZ823 TZ23c ZG7 R(127.53 °C, Pm=64.69 MPa 63.94 MPa) TZ12 TZ821 TZ83 48 TZ622 TZ72 TZ721 TZ2 TZ726 TZ621 TZ44 Є3tr TZ724 TZ61 TZ69 TZ62 TZ4 -150 -40 70 180 290 400 TZ162 Legend TZ166 Temperature, °C TZ16 TZ83 TZ168 TZ242 TZ75 TZ241 Fault Contour Є a TZ43 TZ243 2 TC1 lines TZ24-2 TZ24 Є2s TZ8 TZ102 Water Commercial well oil flow Є w TZ5 TZ18 Є1x TZ52 Formation Well Є1y depth name Fig. 1 Tectonic map of the Yingshan Formation and distribution of condensate reservoirs in the Tazhong Uplift Table 1 Features of high sulfur condensate reservoir of Yingshan Formation in the Tazhong area Well Depth, H S content, Bottom-hole Critical condensate Condensate oil Gas output, 3 3 3 m mg/m content, g/m m /d Temperature, C Pressure, Pressure, Temperature, C MPa MPa ZG14 6300 5103 141.80 72.51 62.40 361.1 305.6 130,921 ZG22 5736 2310 129.75 67.31 71.46 366.6 319.2 70,200 ZG12 6279 640 140.60 72.93 67.42 348.9 66.0 169,537 ZG11 6475 5300 141.39 73.88 60.73 332.6 376.1 95,540 ZG111 6250 3400 137.00 71.81 61.88 325.1 354.8 114,362 ZG8 6145 46,267 148.00 66.11 45.61 345.6 748.1 144,844 ZG441 5522 10,100 126.76 63.24 69.19 369.8 176.3 134,935 TZ201c 5779 20,700 126.68 63.58 65.84 362.4 195.0 74,270 ZG43 5334 36,537 124.87 63.33 50.89 346.9 451.6 83,679 ZG10 6309 36,540 151.86 68.11 56.10 337.9 386.9 218,703 ZG102 6287 77800 145.78 63.53 40.48 314.1 764.2 57,658 ZG103 6233 3867 140.30 71.58 50.29 327.0 378.5 74,366 ZG5 6460 49,480 150.60 75.09 51.02 297.3 312.2 130,518 3 3 3 3 H S content (mg/m ) = the H S weight per m condensate gas; condensate oil content (g/m ) = the condensate oil weight per m condensate gas 2 2 GC 9 GC to access the oil-cracking extent of selective detector (MSD) for both saturated and aromatic hydrocarbons. hydrocarbon fractions. The HP-5 column is 30 m long 9 0.25 mm ID 9 0.25 lm thickness. Helium main- 3.2 GC–MS analysis tained at a constant flow rate of 1.0 cm /min was used as the carrier gas. The GC oven was programmed from 50 to GC–MS analysis was performed using a Trace GC Ultra 220 Cat4 C/min with an initial hold time of 5 min and gas chromatograph interfaced to a Thermo DSQII mass then from 220 to 320 Cat2 C/min with a final hold time Pressure, MPa Tm=345.6 °C Pressure, MPa Tm=337.9 °C Pressure, MPa Pressure, MPa Pressure, MPa Tm=346.9 °C Tm=362.4 °C Tm=369.8 °C Cambrian Ordovician Lower Middle Upper Lower Middle Upper Reservoirs 5500 510 Pet. Sci. (2017) 14:507–519 of 25 min. Samples were injected in the splitless mode at a was helium with flow rate of 1.8 mL/min. The modulation constant temperature of 300 C. For quantitative determi- period was 10 s with a 3.0-s hot pulse duration. The TOF– nation of saturated and aromatic hydrocarbons, known MS instrument was operated in the electron impact mode concentrations of standard compounds (d4-cholestane and (70 eV) with a range of 40–520 Da. The ion source tem- d10-anthracene) were added to the condensate and oil prior perature was 240 C, the detector voltage was set at to the fractionation. 1475 V, and the acquisition rate was 100 spectra/s. Instrument control and data processing were done using 3.3 Diamondoid hydrocarbon analysis ChromaToF (Leco) software and Microsoft Excel. The deconvoluted spectra were compared with the National For GC 9 GC-TOF–MS analysis, a whole condensate Institute of Standards and Technology (NIST) software sample was dissolved in a solution of 5% dichloromethane library for compound identification. The quantification of (DCM) in n-hexane. Two Leco Pegasus 4D GC 9 GC adamantanes was calculated by comparison with an inter- systems were used in this study coupled with a TOF–MS nal standard of d16-adamantane, while the quantification of and a FID, respectively. They were equipped with an diamantanes required a linear correlation among the ratios Agilent 6890 N GC (TOF–MS) and an Agilent 7890A GC of various diamantane standard concentrations to the con- (FID system) and configured with a split/splitless auto-in- stant d16-adamantane concentration. jector and a dual-stage cryogenic modulator. Two capillary GC columns were fitted in the GC. The first dimension 3.4 Constituent analysis of formation water chromatographic separation was performed by a nonpolar Petro-column (50 m 9 0.2 mm I.D., 0.5 lm film thick- We have reviewed more than 200 compositions of forma- ness). The second column was connected to the TOF–MS tion water in Tazhong Uplift, which were analyzed in the instrument via a DB-17HT column (3 m 9 0.1 mm I.D., chemical laboratory of the Tarim Oilfield Company, in 0.1 lm film thickness). Temperature was programmed for order to determine the origin of formation water in the 35 C (10 min) in the primary GC oven. Then, the tem- condensate reservoirs. The analysis method for oil and gas perature was increased to 60 Cat0.5 C/min and held for field water is conducted to industry standard SY/T 0.2 min, from 60 to 220 Cat2 C/min and held for 5523-2000. As well, 14 formation water samples were 0.2 min, followed by an increase at 4 C/min to the final collected to measure the concentration of ions at ambient temperature of 300 C and kept constant for 5.0 min. The temperature. Since the temperature of the reservoirs is secondary oven was programmed 20 C above the primary mostly from 125 to 150 C, the measured ionic concen- GC oven gradient. The sample injection temperature was trations had been converted to in situ conditions using thermodynamic modeling (Table 2). 300 C with an injection volume of 0.5 lL. The carrier gas - 2- 2? 2? Table 2 Concentrations of Wells Formation Depth, pH Cl , SO , Ca , Mg , Salinity, H S, 4 2 main ions in the condensate m mg/L mg/L mg/L mg/L mg/L mg/m reservoirs converted to in situ geological condition by ZG9 O 6049.1 7.24 120,200 78 12,900 1420 197,700 616,000 thermodynamic modeling ZG7 O 5865 7.81 102300 53 8890 532 168,100 47,500 ZG48 O 5498.1 6.85 88,030 400 7694 714 150,000 1000 ZG461 O 5479.64 6.24 56,990 891 19,800 810 6800 1300 ZG45 O 5637.2 7.33 42,100 894 4327 416 70,930 15,867 ZG44 O 5603.96 7.17 100,200 450 9635 770 169,500 10,600 ZG43 O 5380 8.21 6097 550 1576 1050 11,330 36,537 ZG26 O 6085.5 6.16 70,180 381 29,690 529 116,600 100 ZG164 O 6122.13 7.42 41,900 221 2014 399 72,020 7200 ZG163 O 6140 7.11 51,750 114 3364 461 85,210 6900 ZG162 O 6123 6.17 4624 1047 155 74 9607 21 ZG15- O 5918.5 7.11 63,510 209 4256 418 107,200 4900 ZG11 O 6165 6.83 64,570 97 3791 283 109,400 5300 ZG10 O 6198 6.91 107,000 31 11,900 772 179,200 36,540 123 Pet. Sci. (2017) 14:507–519 511 concentration of H SinO y condensate reservoirs show 4 Results and discussion 2 1 that the high concentration of H S is correlated well with 4.1 Geochemical features of high sulfur condensate active formation water (Fig. 2). So, it is proposed that the origin of H S may be related to the geochemical properties reservoirs of O y formation water. Therefore, a knowledge of the geochemical properties of formation water and the accu- For condensates in the O y reservoir of Tazhong Uplift, the burial depth is more than 5500 m. The reservoir tempera- mulation process of condensates will be essential to clarify the origin of H S in the condensates of deep dolomite ture may rise beyond 130 C with a gradient of 2.03 C/ 100 m, and the pressure coefficient is 1.18. The reservoirs reservoirs. Previous studies confirmed that two sets of source rocks are weakly over-pressured (Table 1). The concentration of mainly developed in the Tarim Basin, which, respectively, condensate oil in the reservoir is generally higher than 3 3 developed in the Ordovician platform margin slope facies 100 g/m with a peak of 764.2 g/m in well ZG102. and Cambrian basin facies. The planktonic algae organic According to the difference between formation pressure and dew point pressure, the condensates were classified as phase of Cambrian source rock is distributed in the eastern Manjia’er sag. A series of biomarkers (triaromatic dinos- unsaturated (pressure difference [ 0) and saturated reser- voirs (pressure difference = 0). Most of the unsaturated terane and 4-methyl-24-ethyl cholestane) indicating com- plex acritarchs, planktonic algae, such as diatoms and condensates are distributed near strike-slip faults, while the saturated condensates are distributed far away from faults. dinoflagellates, have been found in a large number of Cambrian source rocks. The mud mound organic facies of Another distinct feature of O y condensates is their high Ordovician source rock are mainly distributed in the plat- concentration of H S (Fig. 2), which is different from the form edges and transition slopes. Some biomarkers of condensate in the O l reservoirs around the Tazhong No. 1 sponge and gloeocapsa, such as 24-norcholestane and structure. For the reservoirs around the Tazhong No. 1 4-methyl-24-propyl cholestane have been detected in the fault, the concentration of H S correlates well with gas lime-mud mounds (Zhang and Huang 2005). These production. According to the previous literature (Zhang et al. 2015), the H S concentration in the condensate molecular fossils have become an important marker of oil– source correlation in the Tarim Basin. Through the distri- reservoirs of the Lianglitage Formation (O l) is presumed to originate from the charge of high-maturity gases in the bution of these special biomarkers in the condensates and normal oils in the Ordovician Yingshan Formation of late Himalayan (Zhao et al. 2009). In contrast, the H Sin the condensates of the Yingshan Formation (O y) may Tazhong Uplift, the concentration of 4, 23, 24-trimethyl triaromatic dinosterane in the condensates was higher than have not dissolved and migrated in the gas phase. The production volume of formation water and the that in the normal oils, while the concentration of 90000 1000000 60000 10000 30000 100 R=0.7302 R=0.7035 0 1 0 200 400 600 800 1000 0.001 0.01 0.1 1 10 3 3 Condensate oil content, g/m Cumulative water production, 10 t Fig. 2 Correlation between H S content and condensate oil content as well as cumulative water output of the Yingshan condensate gas reservoir in the Tazhong area H S content, mg/m H S content, mg/m 2 512 Pet. Sci. (2017) 14:507–519 3-methyl-24-methyl triaromatic dinosterane in normal oils Formation (O y) and also one of the reservoirs with the increases significantly (Fig. 3). This indicates that the highest H S concentration in the Tazhong Uplift. Well condensates in the lower Ordovician Yingshan Formation ZG43, for example, produced natural gas of 83,679 m per (O y) of Tazhong Uplift mainly migrated from the deep day with a condensate concentration of 451.6 g/m and an Cambrian source rocks. H S concentration of 36,537 mg/m . The condensate The exploration area around well ZG43 is one of the reservoirs distributed along strike-slip faults have high most hydrocarbon-rich tectonic zones in the Yingshan oil/gas yields and high GOR, while oil reservoirs located ZG9, 6370– 6374 m, O y 100 100 m/z 245 m/z 231 80 80 60 60 40 40 20 20 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min ZG10, 6309– 6334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min ZG43, 4980–5334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 20 20 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min TZ201c, 4980–5334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 20 20 0 0 76 78 80 82 74 78 80 82 84 Time, min Time, min Fig. 3 Biomarker characteristics of multi-phase reservoirs in the lower Ordovician Yingshan Formation Oil reservoir Condensate reservoirs Abundance Abundance Abundance Abundance Abundance Abundance Abundance Abundance Pet. Sci. (2017) 14:507–519 513 80 80 65 25 ZG441 ZG46 ZG43 ZG432 ZG434 64 64 52 20 48 48 48 39 15 32 32 32 26 10 16 16 16 13 5 0 0 0 0 0 - 200 90 20 130 240 350 - - - - - 150 - 40 70 180 290 400 150 40 70 180 290 400 100 0 100 200 300 400 100 30 160 290 420 550 Temperature, °C Temperature, °C Temperature, °C Temperature, °C Temperature, °C C regular/diasteranes 10×4DA/(4DA+3DA+1DA) 175 Ts/Tm 10 DNR-1 4-/1-methyldibenzothiophene C TT/C Hop 23 30 Fig. 4 Comparison of phase and maturity between condensate and normal oils in the ZG-43 block of the Yingshan Formation in the Tazhong Uplift far away from faults have low oil/gas yields and low GOR. Further detailed analyses on the composition of the This indicates that the strike-slip faults might act as path- Ordovician formation water would be needed to diagnose ways for gas charging. The intensity of gas charging has whether the geochemical conditions in the Tazhong Uplift been proved to be main mechanism by which the early oil were suitable to trigger the TSR and accumulate H Sof reservoir transferred to the coexisting condensate and oil TSR-origin. reservoirs (Zhang et al. 2011). The analysis on the relative concentrations of individual compound in condensates 4.2 Relationships between H S-origin shows that the maturity of condensates is evidently higher and formation water in the condensate than that of the oils (Fig. 4). In particular, the concentration reservoirs of diamantanes reflects the extent of oil cracking (Dahl et al. 1999; Zhang et al. 2011). The concentration of dia- According to the statistics, the formation water of the lower mantanes is 150 lg/g in the condensates of well ZG43 with Ordovician—Cambrian differs from that of the upper an H S concentration of 36.537 mg/m , which is much Ordovician in the concentration of principal ions (Fig. 5). 2- higher than that in conventional oil. So, it is indicated that In particular, the concentrations of SO in the O l for- 4 3 high concentrations of H S may originate from oil cracking mation water are higher than those in the lower Ordovi- or thermal chemical alteration, which have also led to the cian—Cambrian formation water. On the contrary, the 2? higher maturity of condensates compared with conven- Mg concentration is higher in the formation water of 2- 2? tional oil. O y-Cambrian. Both of SO and Mg are significant for 1 4 The oil cracking commonly includes two main pro- thermochemical sulfate reduction (Zhang et al. 2011). It 2- cesses, i.e., thermal cracking of oil and thermochemical has been noted that the SO commonly exists in the form sulfate reduction (TSR). It is revealed that the well bottom of contact ion-pairs, which are the actual oxidant of TSR at 2- temperature of O y condensate reservoirs does not exceed geological temperatures. The decrease in SO concen- 1 4 160C. Previous research considered that thermal cracking tration in the lower Ordovician to Cambrian formation 2- would not occur unless the temperature exceeds at least water may be due to the consumption of SO in the TSR 2? 2? 190C (Price 1980; Zhang et al. 2008). So, it could be (Su et al. 2016). The concentration of Mg and Ca is concluded that the high maturity of condensates and high lower than 1.1 and 20 g/L in the upper Ordovician for- concentration of H S in the O y reservoirs were not due to mation water and much higher in the lower Ordovician to 2 1 liquid hydrocarbon thermal cracking. So, it can be inferred Cambrian formation water. In particular, the concentration 2? that the high concentration of diamantanes and H S in the of Mg goes up beyond 1.5 g/L, which is proposed to condensate reservoirs is likely to derive from the TSR. activate TSR in actual geochemical conditions. Therefore, Maturity geochemical parameters Pressure, MPa 4-+3-methyldiamantanes, μg/g 514 Pet. Sci. (2017) 14:507–519 4000 4000 Upper Ordovician O3l formation water Upper Ordovician O3l formation water Lower Ordovician O p-Cambrian Lower Ordovician O p-Cambrian 1 1 3500 3500 formation water formation water O y formation water O y formation water 1 1 3000 3000 2500 2500 2000 2000 1500 1500 1000 1000 500 500 0 0 0 10000 20000 30000 40000 50000 60000 70000 0 1000 2000 3000 4000 5000 2- 2+ SO , mg/L Ca , mg/L 4 Upper Ordovician O l formation water Upper Ordovician O3l formation water Lower Ordovician O p-Cambrian Lower Ordovician O1p-Cambrian 3500 3500 formation water formation water O y formation water O1y formation water 3000 3000 2500 2500 2000 2000 1500 1500 1000 1000 500 500 0 0 0 500 1000 1500 2000 2500 3000 0 50000 100000 150000 2- - HCO , mg/L Cl , mg/L 5000 5000 Upper Ordovician O l formation water Upper Ordovician O l formation water 4500 4500 Lower Ordovician O1p-Cambrian Lower Ordovician O1p-Cambrian formation water formation water 4000 O y formation water 4000 O y formation water 1 1 3500 3500 3000 3000 2500 2500 2000 2000 0 50000 100000 150000 56 7 8 9 10 11 12 Cl , mg/L pH Fig. 5 Comparison of formation water ion concentration between the upper Ordovician Lianglitage Formation and the lower Ordovician Penglaiba Formation—Cambrian in the Tazhong Uplift 2- 2+ 2+ SO , mg/L Mg , mg/L Mg , mg/L 2- 2+ 2+ SO , mg/L Mg , mg/L Mg , mg/L 4 3000 Pet. Sci. (2017) 14:507–519 515 0 10 20 30 km ZG17 No. I Fault TZ451 ZG20 TZ18, 4312.5m, O TZ2, 4793.3m, O ZG9, 6263.4m, O1y TZ85 The intrusion Strong Fine crystal pore of diabase dolomitization ZG-11 1.0×10 ZG-8 ZG-10 ZG2 8×10 TZ54 TZ-10 ZG-46 ZG-431 6×10 ZG-43 ZG-432 ZG9 TZ82 ZG-433c 4×10 ZG7 2×10 TZ12 TZ18 TZ83 TZ50 H2S mL/m TZ-2 TZ62 Legend TZ70 TZ16 TZ168 Fault Magnesium TZ241 TZ401 Magnesium riched area TZ75 rich fluid TZ24 TZ261 Water Commercial well oil flow TZ26 TZ1 TZ18 Formation Well depth name 2? Fig. 6 Relationship between the content of H S in the condensate reservoirs and Mg -enriching fluid in the Ordovician of Tazhong Uplift the deep dolomite reservoirs of lower Ordovician could This indicates a very close relationship between the for- 2? provide a favorable setting for TSR. mation of H S and the activity of Mg -enriching fluid in The Tazhong Uplift has experienced multi-period the lower Ordovician Yingshan Formation. 2? hydrothermal invasion and dolomitization. So that Mg is rich in the formation water of the tectonic area experi- 4.3 The formation mechanism of high H S encing strong dolomitization and hydrothermal activity, in the condensate reservoirs which provides the geochemical conditions for the TSR of sulfate-CIPs in the Tazhong Uplift. Intrusive diabases are The production data have shown that the concentration of seen in well TZ-18, strong dolomitization happened in well H S is not clearly related to the gas volume of condensate Tazhong-2, and a number of fine crystalline porous dolo- reservoirs in O y reservoirs, but correlated to the volume of mites have been found in the lower Ordovician Yingshan formation water and the content of condensates. In the Formation of well ZG-9 (Fig. 6). The most active region of condensate reservoirs of Well-ZG7 and ZG9 blocks with magnesium-rich fluid was mainly located in the Tazhong the highest production volume of formation water, the H S Uplift and the middle section of the No. 1 fault zone concentration is up to 600 g/m (Table 3). Through com- (Fig. 6). On the other hand, the highest concentration of paring the properties and distribution of formation water in H S in the Tazhong area is also mainly distributed in the the Tazhong Uplift, it is found that the formation water in same zone as the magnesium-rich fluid. The concentration the O y dolomite reservoirs connects with the fluids in the 3 3 of H S was more than 5% (m /m ), coinciding with the Cambrian formations. The Tazhong No. 1 fault and strike- region of reservoirs with strong dolomitization (Fig. 6). slip faults of Tazhong Uplift may act as the major 4500 516 Pet. Sci. (2017) 14:507–519 Table 3 Statistics of fluid production and H S content in the Yingshan condensate reservoir of the Tazhong Uplift 3 3 3 Well Formation GOR, m /m Output H S content, mg/m 3 3 3 Oil, 10 t Water, 10 t Gas, 10 t ZG9 O y 0 0.000 3.497 0.256 616,000 ZG501 O y 237 8.392 0.000 0.283 32,200 ZG51 O y 1500 1.791 0.337 0.269 55,100 ZG6 O y 245 1.521 3.310 0.037 593,000 ZG7 O y 2024 3.503 2.332 0.594 47,500 ZG10 O y 2372 28.984 0.931 6.616 36,500 ZG102 O y 1688 1.403 0.000 0.195 77,800 ZG103 O y 2049 8.448 0.000 1.723 3870 ZG11 O y 3300 8.771 2.398 2.586 5300 ZG111 O y 2815 18.089 0.193 5.126 3400 ZG12 O y 13,500 0.029 0.045 0.040 640 ZG13 O y 185 10.596 1.921 0.594 6080 ZG14 O y 1800 8.536 0.512 1.526 5100 ZG14-1 O y 3064 4.849 0.002 1.955 3600 ZG21 O y 886 0.057 0.031 0.005 19 ZG22 O y 2300 3.385 0.123 0.671 23,100 ZG23CH O y 0 1.282 0.009 0.047 42 ZG8 O y 1168 18.97 0.030 2.409 46,300 TZ201C O y 3900 2.881 0.010 1.144 20,700 ZG43 O y 1205 26.556 1.545 3.689 36,500 ZG431 O y 589 7.627 0.286 0.458 82,100 ZG432 O y 176 6.054 0.003 0.102 89,400 ZG433C O y 307 16.197 0.07 0.368 116,000 ZG441 O y 3008 2.939 0.032 0.962 10,100 ZG44C O y 3260 0.073 0.011 0.024 10,600 ZG45 O y 778 7.079 0.817 0.583 15,900 ZG46 O y 10,572 0.609 0.451 0.641 18,400 ZG462 O y 1924 7.274 0.014 1.329 120 ZG48 O y 4294 0.860 0.767 0.373 1000 2- 2? migration pathway for the formation water in gas con- aqueous systems, so SO could not be bound with Mg densate reservoirs, and contribute high H S in the con- directly and would form contact ion-pairs (CIPs) to densate reservoirs around the faults. accelerate TSR reactions (Azimi et al. 2007; Leusbrock Much previous research has proved TSR between et al. 2008). Based on the theory of CIPs, good correlation 2? hydrocarbons and sulfate could induce oil-cracking pro- between the concentration of H S and Mg in formation cesses in actual geological conditions (Worden and Smal- water (Fig. 7) has indicated the TSR-origin of H S in the ley 1996; Wei et al. 2012). It is generally agreed that the condensate reservoirs. In addition, the concentration of - - sulfate contact ion-pair would be the dominant mechanism principal negative ions, i.e., Cl and HCO , is separately to trigger thermochemical sulfate reduction (Rudolph et al. lower than 100 and 1.0 g/L in the upper Ordovician for- 2003; Amrani et al. 2008). So, samples of formation water mation water and higher than 100 g/L and 1.0 g/L in the were collected to test the concentration of ions at ambient lower Ordovician—Cambrian formation water. It has temperature. Then, the measured ionic concentrations had shown that the concentration of H S in the O y gas con- 2 1 been converted to in situ conditions by thermodynamic densates has increased with the alkalinity of formation modeling, in order to discuss the TSR reaction mechanism water (Fig. 7), which demonstrated that the pH of forma- with the oxidant of sulfate-CIPs in the highly sulfuretted tion water may drop down in the process of forming CIP 2? condensate reservoirs (Table 3). Mg with high ionic from magnesium sulfate (He et al. 2014). Alkaline for- strength is encircled completely by water molecules in mation water would guarantee the rightward reaction 123 Pet. Sci. (2017) 14:507–519 517 to form H S in large amounts. Therefore, it could be inferred that TSR with sulfate-CIPs might also occur in the R=0.6064 100000 deep Cambrian dolomite reservoirs. 2þ 2 Mg ðÞ aq þ SO ðÞ aq½ free hydrated ions 2þ 2 ! Mg ðÞ OH SO ðÞ aq½ 2SIP 2 4 2þ 2 ! Mg ðÞ OH SO ðÞ aq½ SIP 2þ 2 ! Mg SO ðÞ aq½ CIP ð1Þ With the initiation of TSR, the concentration of H S 2- increases and concentration of SO decreases. Therefore, it could be demonstrated that H S in the condensates of lower Ordovician dolomite reservoirs does originate from redox reaction between hydrocarbons and sulfate-CIPs. 0 200 400 600 800 1000 1200 2+ Mg , mg/L 5 Conclusions The condensates in the dolomite reservoirs of the lower Ordovician in the Tazhong Uplift are generally charac- terized by various properties and phases of hydrocarbons. The concentration of H S in the condensate reservoirs increases with the production volume of formation water. 2? Both the Mg concentration and pH in the O y forma- tion water are all higher than those in the upper 100 Ordovician reservoirs and correlate well with the H S concentration in the gas condensate reservoirs. The 2- decrease in SO concentration in the O y condensates is 4 1 R=0.5662 2- due to the consumption of SO during TSR, which is the formation mechanism of H S in the O y condensates. 2 1 0 200 400 600 800 1000 1200 The pH values of the formation water are positively 2- SO , mg/L correlated with the H S concentration in the condensate of the lower Ordovician dolomite reservoirs, which shows that high alkalinity of the formation water is another important factor to initiate and promote the TSR of sul- R=0.7599 fate-CIPs. It is thus inferred the deep dolomite reservoirs have favorable geological conditions for TSR. The prop- erties of the formation water in the Cambrian are similar to those of the high H S-bearing condensate in the O y 2 1 reservoirs. This indicated that the sulfur condensates of the O y reservoirs originated from the Cambrian source rocks based on triaromatic dinosterane and 4-methyl-24- ethyl cholestane. Therefore, it can be inferred that the 10 H S concentration of Cambrian dolomite reservoirs might be higher than that of the O y reservoirs. Acknowledgements The study is funded by the Natural Science Foundation of China (NSFC, Project No. 41473020) and the CNPC pH International Cooperation Project (Grant No. 2011A-0203-01). The extraction, separation, GC and GC–MS analyses were performed in Fig. 7 Scatter point correlation between H S content in condensate the Key Laboratory of Petroleum Geology (KLPG), PetroChina. The reservoirs and ion concentration of underground water Tarim Oilfield Company is thanked for providing the background geological information and data on formation water. The anonymous expressed in Eq. (1) to generate sufficient ion-pairs and reviewers are gratefully acknowledged for their constructive com- activate reaction in Eq. 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Impact of formation water on the generation of H2S in condensate reservoirs: a case study from the deep Ordovician in the Tazhong Uplift of the Tarim Basin, NW China

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Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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Pet. Sci. (2017) 14:507–519 DOI 10.1007/s12182-017-0176-z OR IGINAL PAPER Impact of formation water on the generation of H S in condensate reservoirs: a case study from the deep Ordovician in the Tazhong Uplift of the Tarim Basin, NW China 1,2 1,2 1,2 1,2 3 • • • • • Jin Su Yu Wang Xiao-Mei Wang Kun He Hai-Jun Yang 1,2 1,2 1,2 1,2 • • • • Hui-Tong Wang Hua-Jian Wang Bin Zhang Ling Huang 1,2 1,2 4 • • Na Weng Li-Na Bi Zhi-Hua Xiao Received: 25 September 2016 / Published online: 27 July 2017 The Author(s) 2017. This article is an open access publication Abstract A number of condensate reservoirs with high occurred in the Cambrian. High H S-bearing condensates concentrations of H S have been discovered in the deep are mainly located near the No. 1 Fault and NE-SW strike- dolomite reservoirs of the lower Ordovician Yingshan slip faults, which are the major migration pathway of deep Formation (O y) in the Tazhong Uplift, where the forma- fluids in the Tazhong Uplift. The redox between sulfate- tion water has a high pH value. In the O y reservoir, the CIPs and hydrocarbons is the generation mechanism of 2? 2- concentrations of Mg and SO in the formation water H S in the deep dolomite condensate reservoirs of the 4 2 are higher than those in the upper Ordovician formation. Tazhong Uplift. This finding should be helpful to predict The concentration of H S in the condensate reservoirs and the fluid properties of deep dolomite reservoirs. 2? the concentration of Mg in the formation water correlate well in the O y reservoirs of the Tazhong Uplift, which Keywords Formation water  Sulfate-CIPs  TSR indicates a presumed thermochemical sulfate reduction Condensates  Dolomite reservoir  Tarim basin (TSR) origin of H S according to the oxidation theory of contact ion-pairs (CIPs). Besides, the pH values of the formation water are positively correlated with the con- 1 Introduction centration of H S in the condensate reservoirs, which may indicate that high pH might be another factor to promote Formation water has been conventionally taken as a kind of and maintain TSR. Oil–source correlation of biomarkers in undesirable by-product in petroleum exploration and the sulfuretted condensates indicates the Cambrian source development, while it would be one of the most crucial rocks could be the origin of condensates. The formation factors in hydrocarbon generation, migration and accu- water in the condensate reservoirs of O y is similar to that mulation as well as the interaction between organic and in the Cambrian; therefore, the TSR of sulfate-CIPs likely inorganic geo-fluids. Detailed analysis of the geochemical properties of formation water could indicate the hydrody- namic conditions for hydrocarbon accumulation and the & Jin Su geochemical alteration of reservoirs (Jiang and Zhang susujinjin@126.com 1999; Cai et al. 2001; Zha et a1. 2003; Chen and Zha Key Laboratory of Petroleum Geochemistry, CNPC, 2005, 2006a, b, 2008), which are of great geological sig- Beijing 100083, China nificance for interpreting the reaction mechanisms between State Key Laboratory for Enhancing Oil Recovery, Research ions and hydrocarbons. And then, the origin of multi-phase Institute of Petroleum Exploration and Development, reservoirs could be deduced from the secondary geo- PetroChina, Beijing 100083, China chemical alteration (Land 1995; Davisson and Criss 1996; Research Institute of Tarim Oilfield Company, PetroChina, Varsanyi and Kovacs 1997; Su et al. 2011). Palmer (1984) Korla 841000, Xinjiang, China published research on the hydrocarbon composition varia- PetroChina Coalbed Methane Company Limited, tion in carbonate reservoirs as a result of the formation Beijing 100028, China water flowing across the reservoirs. In particular, diben- zothiophene would be mostly altered in its composition Edited by Jie Hao 123 508 Pet. Sci. (2017) 14:507–519 (Kuo 1994; Lafargue and Tluez 1996). In addition, high 2 Geological background H S concentration from microbial activities around the oil– water contact would increase the acidity and density of The Tarim Basin experienced three important stages of crude oil, which negatively impacts on the commercial hydrocarbon accumulation, i.e., the later Eopaleozoic, the value of petroleum and as well as the safe exploitation and Neopaleozoic to the early Mesozoic, and the late Cenozoic development of reservoirs. (Zhang et al. 2011), each of which had great impacts on the With the extension of hydrocarbon exploration to deep accumulation and distribution of hydrocarbons in the dolomite rocks in recent decades in China, a large marine reservoirs (Zhang and Huang 2005). The Tarim number of gas reservoirs with high H S have been dis- Basin has become a petroliferous basin which has under- covered. In domestic and overseas investigations, high ¨ gone multistage secondary alterations (Sun and Puttmann concentrations of H S have generally been attributed to 1996; Su et al. 2010; Sun et al. 2009a, b). The Tazhong the thermochemical sulfate reduction (TSR) between Uplift, as an inherited paleo-uplift, is located in the central anhydrite and hydrocarbons (Worden et al. 2000; Jiang of the Tarim Basin and neighbors the Bachu Uplift in the et al. 2015), during which liquid hydrocarbons are enri- west, the Tadong Low Uplift in the east, the Tangguzibasi ched with sulfur and the drying coefficient of gas reser- depression in the south and the North depression in the voirs may exceed 99%. In actual geological conditions, north (Fig. 1). After the successful exploration in the reef the formation water could act as the reaction agent of and beach reservoirs of the upper Ordovician Lianglitage TSR. In the Tazhong Uplift of the Tarim Basin, H S- Formation (O l), a large-scale petroleum resource has been bearing condensates have been discovered on a large discovered in dolomite reservoirs of the Yingshan Forma- scale in deep dolomite reservoirs, which differs greatly tion (O y), in the lower Ordovician in the North Slope of from the geochemical properties of normal condensate the Tazhong Uplift. It has proved that the lower Ordovician reservoirs around the world and also differs from gas dolomite reservoirs are mainly distributed and developed reservoirs with high H S. Previous study of sulfur iso- below an unconformity surface, about 120 m deep. The topes and concentration of individual sulfur compounds distribution of dolomite reservoirs is not controlled by in the condensate reservoirs has shown that the conden- structural highs/lows. sate reservoirs with high H S may have experienced TSR The dolomite reservoirs of the Yingshan Formation (Jiang et al. 2008; Zhang et al. 2015). Various soluble (O y) are characterized by various hydrocarbon and fluid sulfate species act as the initial oxidant of TSR, mainly phases, as well as large discrepancies in gas/oil ratio 2- including free sulfate ions (SO ) and bisulfate ions (GOR). Most high-yield wells produce large volumes of (HSO ), solvent-shared ion-pairs (SIPs) and contact ion- gas and condensate oil; in contrast, low-yield wells mainly pairs (CIPs) (Eigen and Tamm 1962; Atkinson and Pet- produce crude oil with low GOR. Based on the PVT curve, rucci 1966). Therefore, the salts of formation water may the types of reservoirs in the Tazhong Uplift include be critical to activate and affect TSR. Recently, a series unsaturated and saturated condensates, and unsaturated and of gold-tube hydrous pyrolysis experiments have been saturated oil (Fig. 1), which are heterogeneously dis- conducted to address the reaction mechanism and the tributed across the Tazhong Uplift and are not controlled factors affecting TSR, which include compositions of by structure and burial depth. The difference in reservoir hydrocarbons, labile organosulfur compounds (LSC) and phases indicates that the distribution of condensates in the the early generated H S (Tang et al. 2005; Ellis et al. whole Tazhong Uplift roughly correlates with the fluid 2007; Amrani et al. 2008). Because the relative concen- properties and burial depth and the dolomite reservoirs are tration of H S is not a reliable reaction parameter to correlated with NE-SW faults. access the reaction intensity of TSR (Ma et al. 2008; Jendu et al. 2015), a systematic study on the genetic relationship between water and mineral composition of 3 Samples and methods the Ordovician formation and high sulfur condensates would avail to clarify the formation mechanism of high 3.1 Collection of samples sulfur condensate in the actual geological conditions. These new insights could indicate the effect of ions in In order to investigate the origin of H S, hydrocarbon the formation water on the alteration of hydrocarbons in samples of 13 wells in the Tazhong Uplift were collected deep dolomite reservoirs, which would be useful to pre- (Table 1). The saturated and aromatic hydrocarbons were dict the fluid properties of deep stratum and promote the quantified by GC–MS, including steranes and hopanes, as exploration of dolomite reservoirs. well as thiophenes. Diamantanes were detected by 123 3000 Pet. Sci. (2017) 14:507–519 509 60 75 80 R(148.0 °C, Stratum Lithology 0 10 20 km R(151.86 °C, R(124.87 °C, 66.11 MPa) Pm=56.10 MPa Pm=45.61 MPa 66.11 MPa) 63.33 MPa) 48 60 64 Pm=58.89 MPa 45 48 O s 24 30 32 ZG19 ZG17 TZ86 12 15 16 ZG162 TZ45 0 0 0 ZG16 -150 -40 70 180 290 400 -150 -40 70 180 290 400 -150 -40 70 180 290 400 Temperature, °C Temperature, °C Temperature, °C ZG20 ZG14 O3l ZG13 ZG8 ZG10 ZG5 ZG15 R(126.7 °C, ZG12 ZG22 63.58 MPa) Pm=65.84MPa No. I Fault ZG21 TZ35 ZG3 ZG111 ZG11 48 O1-2y ZG102 ZG23 ZG104 32 ZG10 ZG2 ZG8 ZG501 ZG47 ZG5 ZG44 ZG461 ZG9 -150 -40 70 180 290 400 ZG45 ZG431 TZ826 ZG48 ZG46 Temperature, °C O1p ZG43 TZ18 ZG6 ZG432 ZG7 TZ82 ZG433c 80 TZ823 TZ23c ZG7 R(127.53 °C, Pm=64.69 MPa 63.94 MPa) TZ12 TZ821 TZ83 48 TZ622 TZ72 TZ721 TZ2 TZ726 TZ621 TZ44 Є3tr TZ724 TZ61 TZ69 TZ62 TZ4 -150 -40 70 180 290 400 TZ162 Legend TZ166 Temperature, °C TZ16 TZ83 TZ168 TZ242 TZ75 TZ241 Fault Contour Є a TZ43 TZ243 2 TC1 lines TZ24-2 TZ24 Є2s TZ8 TZ102 Water Commercial well oil flow Є w TZ5 TZ18 Є1x TZ52 Formation Well Є1y depth name Fig. 1 Tectonic map of the Yingshan Formation and distribution of condensate reservoirs in the Tazhong Uplift Table 1 Features of high sulfur condensate reservoir of Yingshan Formation in the Tazhong area Well Depth, H S content, Bottom-hole Critical condensate Condensate oil Gas output, 3 3 3 m mg/m content, g/m m /d Temperature, C Pressure, Pressure, Temperature, C MPa MPa ZG14 6300 5103 141.80 72.51 62.40 361.1 305.6 130,921 ZG22 5736 2310 129.75 67.31 71.46 366.6 319.2 70,200 ZG12 6279 640 140.60 72.93 67.42 348.9 66.0 169,537 ZG11 6475 5300 141.39 73.88 60.73 332.6 376.1 95,540 ZG111 6250 3400 137.00 71.81 61.88 325.1 354.8 114,362 ZG8 6145 46,267 148.00 66.11 45.61 345.6 748.1 144,844 ZG441 5522 10,100 126.76 63.24 69.19 369.8 176.3 134,935 TZ201c 5779 20,700 126.68 63.58 65.84 362.4 195.0 74,270 ZG43 5334 36,537 124.87 63.33 50.89 346.9 451.6 83,679 ZG10 6309 36,540 151.86 68.11 56.10 337.9 386.9 218,703 ZG102 6287 77800 145.78 63.53 40.48 314.1 764.2 57,658 ZG103 6233 3867 140.30 71.58 50.29 327.0 378.5 74,366 ZG5 6460 49,480 150.60 75.09 51.02 297.3 312.2 130,518 3 3 3 3 H S content (mg/m ) = the H S weight per m condensate gas; condensate oil content (g/m ) = the condensate oil weight per m condensate gas 2 2 GC 9 GC to access the oil-cracking extent of selective detector (MSD) for both saturated and aromatic hydrocarbons. hydrocarbon fractions. The HP-5 column is 30 m long 9 0.25 mm ID 9 0.25 lm thickness. Helium main- 3.2 GC–MS analysis tained at a constant flow rate of 1.0 cm /min was used as the carrier gas. The GC oven was programmed from 50 to GC–MS analysis was performed using a Trace GC Ultra 220 Cat4 C/min with an initial hold time of 5 min and gas chromatograph interfaced to a Thermo DSQII mass then from 220 to 320 Cat2 C/min with a final hold time Pressure, MPa Tm=345.6 °C Pressure, MPa Tm=337.9 °C Pressure, MPa Pressure, MPa Pressure, MPa Tm=346.9 °C Tm=362.4 °C Tm=369.8 °C Cambrian Ordovician Lower Middle Upper Lower Middle Upper Reservoirs 5500 510 Pet. Sci. (2017) 14:507–519 of 25 min. Samples were injected in the splitless mode at a was helium with flow rate of 1.8 mL/min. The modulation constant temperature of 300 C. For quantitative determi- period was 10 s with a 3.0-s hot pulse duration. The TOF– nation of saturated and aromatic hydrocarbons, known MS instrument was operated in the electron impact mode concentrations of standard compounds (d4-cholestane and (70 eV) with a range of 40–520 Da. The ion source tem- d10-anthracene) were added to the condensate and oil prior perature was 240 C, the detector voltage was set at to the fractionation. 1475 V, and the acquisition rate was 100 spectra/s. Instrument control and data processing were done using 3.3 Diamondoid hydrocarbon analysis ChromaToF (Leco) software and Microsoft Excel. The deconvoluted spectra were compared with the National For GC 9 GC-TOF–MS analysis, a whole condensate Institute of Standards and Technology (NIST) software sample was dissolved in a solution of 5% dichloromethane library for compound identification. The quantification of (DCM) in n-hexane. Two Leco Pegasus 4D GC 9 GC adamantanes was calculated by comparison with an inter- systems were used in this study coupled with a TOF–MS nal standard of d16-adamantane, while the quantification of and a FID, respectively. They were equipped with an diamantanes required a linear correlation among the ratios Agilent 6890 N GC (TOF–MS) and an Agilent 7890A GC of various diamantane standard concentrations to the con- (FID system) and configured with a split/splitless auto-in- stant d16-adamantane concentration. jector and a dual-stage cryogenic modulator. Two capillary GC columns were fitted in the GC. The first dimension 3.4 Constituent analysis of formation water chromatographic separation was performed by a nonpolar Petro-column (50 m 9 0.2 mm I.D., 0.5 lm film thick- We have reviewed more than 200 compositions of forma- ness). The second column was connected to the TOF–MS tion water in Tazhong Uplift, which were analyzed in the instrument via a DB-17HT column (3 m 9 0.1 mm I.D., chemical laboratory of the Tarim Oilfield Company, in 0.1 lm film thickness). Temperature was programmed for order to determine the origin of formation water in the 35 C (10 min) in the primary GC oven. Then, the tem- condensate reservoirs. The analysis method for oil and gas perature was increased to 60 Cat0.5 C/min and held for field water is conducted to industry standard SY/T 0.2 min, from 60 to 220 Cat2 C/min and held for 5523-2000. As well, 14 formation water samples were 0.2 min, followed by an increase at 4 C/min to the final collected to measure the concentration of ions at ambient temperature of 300 C and kept constant for 5.0 min. The temperature. Since the temperature of the reservoirs is secondary oven was programmed 20 C above the primary mostly from 125 to 150 C, the measured ionic concen- GC oven gradient. The sample injection temperature was trations had been converted to in situ conditions using thermodynamic modeling (Table 2). 300 C with an injection volume of 0.5 lL. The carrier gas - 2- 2? 2? Table 2 Concentrations of Wells Formation Depth, pH Cl , SO , Ca , Mg , Salinity, H S, 4 2 main ions in the condensate m mg/L mg/L mg/L mg/L mg/L mg/m reservoirs converted to in situ geological condition by ZG9 O 6049.1 7.24 120,200 78 12,900 1420 197,700 616,000 thermodynamic modeling ZG7 O 5865 7.81 102300 53 8890 532 168,100 47,500 ZG48 O 5498.1 6.85 88,030 400 7694 714 150,000 1000 ZG461 O 5479.64 6.24 56,990 891 19,800 810 6800 1300 ZG45 O 5637.2 7.33 42,100 894 4327 416 70,930 15,867 ZG44 O 5603.96 7.17 100,200 450 9635 770 169,500 10,600 ZG43 O 5380 8.21 6097 550 1576 1050 11,330 36,537 ZG26 O 6085.5 6.16 70,180 381 29,690 529 116,600 100 ZG164 O 6122.13 7.42 41,900 221 2014 399 72,020 7200 ZG163 O 6140 7.11 51,750 114 3364 461 85,210 6900 ZG162 O 6123 6.17 4624 1047 155 74 9607 21 ZG15- O 5918.5 7.11 63,510 209 4256 418 107,200 4900 ZG11 O 6165 6.83 64,570 97 3791 283 109,400 5300 ZG10 O 6198 6.91 107,000 31 11,900 772 179,200 36,540 123 Pet. Sci. (2017) 14:507–519 511 concentration of H SinO y condensate reservoirs show 4 Results and discussion 2 1 that the high concentration of H S is correlated well with 4.1 Geochemical features of high sulfur condensate active formation water (Fig. 2). So, it is proposed that the origin of H S may be related to the geochemical properties reservoirs of O y formation water. Therefore, a knowledge of the geochemical properties of formation water and the accu- For condensates in the O y reservoir of Tazhong Uplift, the burial depth is more than 5500 m. The reservoir tempera- mulation process of condensates will be essential to clarify the origin of H S in the condensates of deep dolomite ture may rise beyond 130 C with a gradient of 2.03 C/ 100 m, and the pressure coefficient is 1.18. The reservoirs reservoirs. Previous studies confirmed that two sets of source rocks are weakly over-pressured (Table 1). The concentration of mainly developed in the Tarim Basin, which, respectively, condensate oil in the reservoir is generally higher than 3 3 developed in the Ordovician platform margin slope facies 100 g/m with a peak of 764.2 g/m in well ZG102. and Cambrian basin facies. The planktonic algae organic According to the difference between formation pressure and dew point pressure, the condensates were classified as phase of Cambrian source rock is distributed in the eastern Manjia’er sag. A series of biomarkers (triaromatic dinos- unsaturated (pressure difference [ 0) and saturated reser- voirs (pressure difference = 0). Most of the unsaturated terane and 4-methyl-24-ethyl cholestane) indicating com- plex acritarchs, planktonic algae, such as diatoms and condensates are distributed near strike-slip faults, while the saturated condensates are distributed far away from faults. dinoflagellates, have been found in a large number of Cambrian source rocks. The mud mound organic facies of Another distinct feature of O y condensates is their high Ordovician source rock are mainly distributed in the plat- concentration of H S (Fig. 2), which is different from the form edges and transition slopes. Some biomarkers of condensate in the O l reservoirs around the Tazhong No. 1 sponge and gloeocapsa, such as 24-norcholestane and structure. For the reservoirs around the Tazhong No. 1 4-methyl-24-propyl cholestane have been detected in the fault, the concentration of H S correlates well with gas lime-mud mounds (Zhang and Huang 2005). These production. According to the previous literature (Zhang et al. 2015), the H S concentration in the condensate molecular fossils have become an important marker of oil– source correlation in the Tarim Basin. Through the distri- reservoirs of the Lianglitage Formation (O l) is presumed to originate from the charge of high-maturity gases in the bution of these special biomarkers in the condensates and normal oils in the Ordovician Yingshan Formation of late Himalayan (Zhao et al. 2009). In contrast, the H Sin the condensates of the Yingshan Formation (O y) may Tazhong Uplift, the concentration of 4, 23, 24-trimethyl triaromatic dinosterane in the condensates was higher than have not dissolved and migrated in the gas phase. The production volume of formation water and the that in the normal oils, while the concentration of 90000 1000000 60000 10000 30000 100 R=0.7302 R=0.7035 0 1 0 200 400 600 800 1000 0.001 0.01 0.1 1 10 3 3 Condensate oil content, g/m Cumulative water production, 10 t Fig. 2 Correlation between H S content and condensate oil content as well as cumulative water output of the Yingshan condensate gas reservoir in the Tazhong area H S content, mg/m H S content, mg/m 2 512 Pet. Sci. (2017) 14:507–519 3-methyl-24-methyl triaromatic dinosterane in normal oils Formation (O y) and also one of the reservoirs with the increases significantly (Fig. 3). This indicates that the highest H S concentration in the Tazhong Uplift. Well condensates in the lower Ordovician Yingshan Formation ZG43, for example, produced natural gas of 83,679 m per (O y) of Tazhong Uplift mainly migrated from the deep day with a condensate concentration of 451.6 g/m and an Cambrian source rocks. H S concentration of 36,537 mg/m . The condensate The exploration area around well ZG43 is one of the reservoirs distributed along strike-slip faults have high most hydrocarbon-rich tectonic zones in the Yingshan oil/gas yields and high GOR, while oil reservoirs located ZG9, 6370– 6374 m, O y 100 100 m/z 245 m/z 231 80 80 60 60 40 40 20 20 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min ZG10, 6309– 6334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min ZG43, 4980–5334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 20 20 0 0 74 76 78 80 82 78 80 82 84 Time, min Time, min TZ201c, 4980–5334 m, O y 100 100 m/z 231 m/z 245 80 80 60 60 40 40 20 20 0 0 76 78 80 82 74 78 80 82 84 Time, min Time, min Fig. 3 Biomarker characteristics of multi-phase reservoirs in the lower Ordovician Yingshan Formation Oil reservoir Condensate reservoirs Abundance Abundance Abundance Abundance Abundance Abundance Abundance Abundance Pet. Sci. (2017) 14:507–519 513 80 80 65 25 ZG441 ZG46 ZG43 ZG432 ZG434 64 64 52 20 48 48 48 39 15 32 32 32 26 10 16 16 16 13 5 0 0 0 0 0 - 200 90 20 130 240 350 - - - - - 150 - 40 70 180 290 400 150 40 70 180 290 400 100 0 100 200 300 400 100 30 160 290 420 550 Temperature, °C Temperature, °C Temperature, °C Temperature, °C Temperature, °C C regular/diasteranes 10×4DA/(4DA+3DA+1DA) 175 Ts/Tm 10 DNR-1 4-/1-methyldibenzothiophene C TT/C Hop 23 30 Fig. 4 Comparison of phase and maturity between condensate and normal oils in the ZG-43 block of the Yingshan Formation in the Tazhong Uplift far away from faults have low oil/gas yields and low GOR. Further detailed analyses on the composition of the This indicates that the strike-slip faults might act as path- Ordovician formation water would be needed to diagnose ways for gas charging. The intensity of gas charging has whether the geochemical conditions in the Tazhong Uplift been proved to be main mechanism by which the early oil were suitable to trigger the TSR and accumulate H Sof reservoir transferred to the coexisting condensate and oil TSR-origin. reservoirs (Zhang et al. 2011). The analysis on the relative concentrations of individual compound in condensates 4.2 Relationships between H S-origin shows that the maturity of condensates is evidently higher and formation water in the condensate than that of the oils (Fig. 4). In particular, the concentration reservoirs of diamantanes reflects the extent of oil cracking (Dahl et al. 1999; Zhang et al. 2011). The concentration of dia- According to the statistics, the formation water of the lower mantanes is 150 lg/g in the condensates of well ZG43 with Ordovician—Cambrian differs from that of the upper an H S concentration of 36.537 mg/m , which is much Ordovician in the concentration of principal ions (Fig. 5). 2- higher than that in conventional oil. So, it is indicated that In particular, the concentrations of SO in the O l for- 4 3 high concentrations of H S may originate from oil cracking mation water are higher than those in the lower Ordovi- or thermal chemical alteration, which have also led to the cian—Cambrian formation water. On the contrary, the 2? higher maturity of condensates compared with conven- Mg concentration is higher in the formation water of 2- 2? tional oil. O y-Cambrian. Both of SO and Mg are significant for 1 4 The oil cracking commonly includes two main pro- thermochemical sulfate reduction (Zhang et al. 2011). It 2- cesses, i.e., thermal cracking of oil and thermochemical has been noted that the SO commonly exists in the form sulfate reduction (TSR). It is revealed that the well bottom of contact ion-pairs, which are the actual oxidant of TSR at 2- temperature of O y condensate reservoirs does not exceed geological temperatures. The decrease in SO concen- 1 4 160C. Previous research considered that thermal cracking tration in the lower Ordovician to Cambrian formation 2- would not occur unless the temperature exceeds at least water may be due to the consumption of SO in the TSR 2? 2? 190C (Price 1980; Zhang et al. 2008). So, it could be (Su et al. 2016). The concentration of Mg and Ca is concluded that the high maturity of condensates and high lower than 1.1 and 20 g/L in the upper Ordovician for- concentration of H S in the O y reservoirs were not due to mation water and much higher in the lower Ordovician to 2 1 liquid hydrocarbon thermal cracking. So, it can be inferred Cambrian formation water. In particular, the concentration 2? that the high concentration of diamantanes and H S in the of Mg goes up beyond 1.5 g/L, which is proposed to condensate reservoirs is likely to derive from the TSR. activate TSR in actual geochemical conditions. Therefore, Maturity geochemical parameters Pressure, MPa 4-+3-methyldiamantanes, μg/g 514 Pet. Sci. (2017) 14:507–519 4000 4000 Upper Ordovician O3l formation water Upper Ordovician O3l formation water Lower Ordovician O p-Cambrian Lower Ordovician O p-Cambrian 1 1 3500 3500 formation water formation water O y formation water O y formation water 1 1 3000 3000 2500 2500 2000 2000 1500 1500 1000 1000 500 500 0 0 0 10000 20000 30000 40000 50000 60000 70000 0 1000 2000 3000 4000 5000 2- 2+ SO , mg/L Ca , mg/L 4 Upper Ordovician O l formation water Upper Ordovician O3l formation water Lower Ordovician O p-Cambrian Lower Ordovician O1p-Cambrian 3500 3500 formation water formation water O y formation water O1y formation water 3000 3000 2500 2500 2000 2000 1500 1500 1000 1000 500 500 0 0 0 500 1000 1500 2000 2500 3000 0 50000 100000 150000 2- - HCO , mg/L Cl , mg/L 5000 5000 Upper Ordovician O l formation water Upper Ordovician O l formation water 4500 4500 Lower Ordovician O1p-Cambrian Lower Ordovician O1p-Cambrian formation water formation water 4000 O y formation water 4000 O y formation water 1 1 3500 3500 3000 3000 2500 2500 2000 2000 0 50000 100000 150000 56 7 8 9 10 11 12 Cl , mg/L pH Fig. 5 Comparison of formation water ion concentration between the upper Ordovician Lianglitage Formation and the lower Ordovician Penglaiba Formation—Cambrian in the Tazhong Uplift 2- 2+ 2+ SO , mg/L Mg , mg/L Mg , mg/L 2- 2+ 2+ SO , mg/L Mg , mg/L Mg , mg/L 4 3000 Pet. Sci. (2017) 14:507–519 515 0 10 20 30 km ZG17 No. I Fault TZ451 ZG20 TZ18, 4312.5m, O TZ2, 4793.3m, O ZG9, 6263.4m, O1y TZ85 The intrusion Strong Fine crystal pore of diabase dolomitization ZG-11 1.0×10 ZG-8 ZG-10 ZG2 8×10 TZ54 TZ-10 ZG-46 ZG-431 6×10 ZG-43 ZG-432 ZG9 TZ82 ZG-433c 4×10 ZG7 2×10 TZ12 TZ18 TZ83 TZ50 H2S mL/m TZ-2 TZ62 Legend TZ70 TZ16 TZ168 Fault Magnesium TZ241 TZ401 Magnesium riched area TZ75 rich fluid TZ24 TZ261 Water Commercial well oil flow TZ26 TZ1 TZ18 Formation Well depth name 2? Fig. 6 Relationship between the content of H S in the condensate reservoirs and Mg -enriching fluid in the Ordovician of Tazhong Uplift the deep dolomite reservoirs of lower Ordovician could This indicates a very close relationship between the for- 2? provide a favorable setting for TSR. mation of H S and the activity of Mg -enriching fluid in The Tazhong Uplift has experienced multi-period the lower Ordovician Yingshan Formation. 2? hydrothermal invasion and dolomitization. So that Mg is rich in the formation water of the tectonic area experi- 4.3 The formation mechanism of high H S encing strong dolomitization and hydrothermal activity, in the condensate reservoirs which provides the geochemical conditions for the TSR of sulfate-CIPs in the Tazhong Uplift. Intrusive diabases are The production data have shown that the concentration of seen in well TZ-18, strong dolomitization happened in well H S is not clearly related to the gas volume of condensate Tazhong-2, and a number of fine crystalline porous dolo- reservoirs in O y reservoirs, but correlated to the volume of mites have been found in the lower Ordovician Yingshan formation water and the content of condensates. In the Formation of well ZG-9 (Fig. 6). The most active region of condensate reservoirs of Well-ZG7 and ZG9 blocks with magnesium-rich fluid was mainly located in the Tazhong the highest production volume of formation water, the H S Uplift and the middle section of the No. 1 fault zone concentration is up to 600 g/m (Table 3). Through com- (Fig. 6). On the other hand, the highest concentration of paring the properties and distribution of formation water in H S in the Tazhong area is also mainly distributed in the the Tazhong Uplift, it is found that the formation water in same zone as the magnesium-rich fluid. The concentration the O y dolomite reservoirs connects with the fluids in the 3 3 of H S was more than 5% (m /m ), coinciding with the Cambrian formations. The Tazhong No. 1 fault and strike- region of reservoirs with strong dolomitization (Fig. 6). slip faults of Tazhong Uplift may act as the major 4500 516 Pet. Sci. (2017) 14:507–519 Table 3 Statistics of fluid production and H S content in the Yingshan condensate reservoir of the Tazhong Uplift 3 3 3 Well Formation GOR, m /m Output H S content, mg/m 3 3 3 Oil, 10 t Water, 10 t Gas, 10 t ZG9 O y 0 0.000 3.497 0.256 616,000 ZG501 O y 237 8.392 0.000 0.283 32,200 ZG51 O y 1500 1.791 0.337 0.269 55,100 ZG6 O y 245 1.521 3.310 0.037 593,000 ZG7 O y 2024 3.503 2.332 0.594 47,500 ZG10 O y 2372 28.984 0.931 6.616 36,500 ZG102 O y 1688 1.403 0.000 0.195 77,800 ZG103 O y 2049 8.448 0.000 1.723 3870 ZG11 O y 3300 8.771 2.398 2.586 5300 ZG111 O y 2815 18.089 0.193 5.126 3400 ZG12 O y 13,500 0.029 0.045 0.040 640 ZG13 O y 185 10.596 1.921 0.594 6080 ZG14 O y 1800 8.536 0.512 1.526 5100 ZG14-1 O y 3064 4.849 0.002 1.955 3600 ZG21 O y 886 0.057 0.031 0.005 19 ZG22 O y 2300 3.385 0.123 0.671 23,100 ZG23CH O y 0 1.282 0.009 0.047 42 ZG8 O y 1168 18.97 0.030 2.409 46,300 TZ201C O y 3900 2.881 0.010 1.144 20,700 ZG43 O y 1205 26.556 1.545 3.689 36,500 ZG431 O y 589 7.627 0.286 0.458 82,100 ZG432 O y 176 6.054 0.003 0.102 89,400 ZG433C O y 307 16.197 0.07 0.368 116,000 ZG441 O y 3008 2.939 0.032 0.962 10,100 ZG44C O y 3260 0.073 0.011 0.024 10,600 ZG45 O y 778 7.079 0.817 0.583 15,900 ZG46 O y 10,572 0.609 0.451 0.641 18,400 ZG462 O y 1924 7.274 0.014 1.329 120 ZG48 O y 4294 0.860 0.767 0.373 1000 2- 2? migration pathway for the formation water in gas con- aqueous systems, so SO could not be bound with Mg densate reservoirs, and contribute high H S in the con- directly and would form contact ion-pairs (CIPs) to densate reservoirs around the faults. accelerate TSR reactions (Azimi et al. 2007; Leusbrock Much previous research has proved TSR between et al. 2008). Based on the theory of CIPs, good correlation 2? hydrocarbons and sulfate could induce oil-cracking pro- between the concentration of H S and Mg in formation cesses in actual geological conditions (Worden and Smal- water (Fig. 7) has indicated the TSR-origin of H S in the ley 1996; Wei et al. 2012). It is generally agreed that the condensate reservoirs. In addition, the concentration of - - sulfate contact ion-pair would be the dominant mechanism principal negative ions, i.e., Cl and HCO , is separately to trigger thermochemical sulfate reduction (Rudolph et al. lower than 100 and 1.0 g/L in the upper Ordovician for- 2003; Amrani et al. 2008). So, samples of formation water mation water and higher than 100 g/L and 1.0 g/L in the were collected to test the concentration of ions at ambient lower Ordovician—Cambrian formation water. It has temperature. Then, the measured ionic concentrations had shown that the concentration of H S in the O y gas con- 2 1 been converted to in situ conditions by thermodynamic densates has increased with the alkalinity of formation modeling, in order to discuss the TSR reaction mechanism water (Fig. 7), which demonstrated that the pH of forma- with the oxidant of sulfate-CIPs in the highly sulfuretted tion water may drop down in the process of forming CIP 2? condensate reservoirs (Table 3). Mg with high ionic from magnesium sulfate (He et al. 2014). Alkaline for- strength is encircled completely by water molecules in mation water would guarantee the rightward reaction 123 Pet. Sci. (2017) 14:507–519 517 to form H S in large amounts. Therefore, it could be inferred that TSR with sulfate-CIPs might also occur in the R=0.6064 100000 deep Cambrian dolomite reservoirs. 2þ 2 Mg ðÞ aq þ SO ðÞ aq½ free hydrated ions 2þ 2 ! Mg ðÞ OH SO ðÞ aq½ 2SIP 2 4 2þ 2 ! Mg ðÞ OH SO ðÞ aq½ SIP 2þ 2 ! Mg SO ðÞ aq½ CIP ð1Þ With the initiation of TSR, the concentration of H S 2- increases and concentration of SO decreases. Therefore, it could be demonstrated that H S in the condensates of lower Ordovician dolomite reservoirs does originate from redox reaction between hydrocarbons and sulfate-CIPs. 0 200 400 600 800 1000 1200 2+ Mg , mg/L 5 Conclusions The condensates in the dolomite reservoirs of the lower Ordovician in the Tazhong Uplift are generally charac- terized by various properties and phases of hydrocarbons. The concentration of H S in the condensate reservoirs increases with the production volume of formation water. 2? Both the Mg concentration and pH in the O y forma- tion water are all higher than those in the upper 100 Ordovician reservoirs and correlate well with the H S concentration in the gas condensate reservoirs. The 2- decrease in SO concentration in the O y condensates is 4 1 R=0.5662 2- due to the consumption of SO during TSR, which is the formation mechanism of H S in the O y condensates. 2 1 0 200 400 600 800 1000 1200 The pH values of the formation water are positively 2- SO , mg/L correlated with the H S concentration in the condensate of the lower Ordovician dolomite reservoirs, which shows that high alkalinity of the formation water is another important factor to initiate and promote the TSR of sul- R=0.7599 fate-CIPs. It is thus inferred the deep dolomite reservoirs have favorable geological conditions for TSR. The prop- erties of the formation water in the Cambrian are similar to those of the high H S-bearing condensate in the O y 2 1 reservoirs. This indicated that the sulfur condensates of the O y reservoirs originated from the Cambrian source rocks based on triaromatic dinosterane and 4-methyl-24- ethyl cholestane. Therefore, it can be inferred that the 10 H S concentration of Cambrian dolomite reservoirs might be higher than that of the O y reservoirs. Acknowledgements The study is funded by the Natural Science Foundation of China (NSFC, Project No. 41473020) and the CNPC pH International Cooperation Project (Grant No. 2011A-0203-01). The extraction, separation, GC and GC–MS analyses were performed in Fig. 7 Scatter point correlation between H S content in condensate the Key Laboratory of Petroleum Geology (KLPG), PetroChina. The reservoirs and ion concentration of underground water Tarim Oilfield Company is thanked for providing the background geological information and data on formation water. The anonymous expressed in Eq. (1) to generate sufficient ion-pairs and reviewers are gratefully acknowledged for their constructive com- activate reaction in Eq. 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