Sr evolution in the Upper Permian and Lower Triassic carbonates, northeast Sichuan basin, China: Constraints from chemistry, isotope and fluid inclusions

Applied Geochemistry 27 (2012) 2409–2424

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Sr evolution in the Upper Permian and Lower Triassic carbonates, northeast Sichuan basin, China: Constraints from chemistry, isotope and fluid inclusions Kaikai Li a, Chunfang Cai a,⇑, Lei Jiang a, Liulu Cai a,b, Lianqi Jia a, Bing Zhang c, Lei Xiang a, Yuyang Yuan a a

Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China National Engineering Lab. of Biohydrometallurgy, General Research Institute for Nonferrous Metals, Beijing 100088, China c State Key Lab Oil Gas Reservoir Geology and Exploitation, Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China b

a r t i c l e

i n f o

Article history: Received 24 November 2011 Accepted 17 July 2012 Available online 25 July 2012 Editorial handling by S. Bottrell

a b s t r a c t Petrographic features, C, O and Sr isotopes, rare earth and trace elements were determined, and fluid inclusions were analyzed on various stages of interparticle cements and vug-fillings from the Upper Permian and Lower Triassic sour reservoirs in northeastern Sichuan basin. The aim was to assess the origin and evolution of palaeo-waters in the carbonates. The original water was contemporary seawater, from which marine cements precipitated with slightly high Sr contents (mean 1911 ppm), 87Sr/86Sr ratios from 0.7067 to 0.7082 and nonluminescent CL. The palaeo-seawater was diluted by meteoric water, as indicated by bright cathodoluminescence (CL) and Sr-depletion (0–516 ppm) in low-temperature calcite. When buried to temperatures of about 60–90 °C during Middle to Late Triassic, the palaeo-water was enriched in Sr released from the transformation of precursor aragonite and calcite to dolomite, resulting in precipitation of substantial pre-bitumen Sr-rich minerals (SrSO4 and SrCO3). For un-dolomitized limestone sections, aragonite neomorphism may have contributed Sr to the precipitation of small amounts of Sr-bearing minerals and calcite crystals with elevated homogenization temperatures (HTs, mainly from 90 to 130 °C) and wide Sr contents (from 34 to 3825 ppm), as recorded in stage III calcite. Since the Middle Jurassic, almost all of the early stage celestite and significant amounts of solid CaSO4 have been consumed by reactions with hydrocarbons (i.e., TSR), resulting in water enriched in isotopically light CO2 and 2þ 2þ and Eu2+, as recorded in calcite with low d13C values (down to 18.9‰), 87Sr/86Sr ratios HCO 3 ; Sr ; Ba from 0.7072 to 0.7076, high HTs (mainly 110–198 °C), positive Eu anomalies and high Sr and Ba contents. Subsequently, the water was uplifted and cooled down to about 115 °C, celestite and strontianite were precipitated with the occurrence of natural elemental S immiscible inclusions. TSR may have produced significant amounts of freshwater, which brought down Sr concentrations and salinities of the palaeowaters to not more than about 6.0 wt.% NaCl equivalent. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Thermochemical sulfate reduction (TSR) results from the reaction between SO2 4 and hydrocarbons and is responsible for ‘‘souring’’ of natural gas and petroleum by the H2S that is produced (Toland, 1960; Orr, 1977; Krouse et al., 1988; Heydari and Moore, 1989; Machel et al., 1995). These reactions are kinetically inhibited but take place in deep burial diagenetic settings (or high-temperature environments, Worden and Smalley, 1996; Cross et al., 2004). The Puguang, Maoba, Luojiazhai, Dukouhe, Yuanba and Longgang gasfields (Fig. 1) of the Upper Permian Changxing Fm and Lower Triassic Feixianguan Fm (P3ch–T1f) in the northeastern (NE) Sichuan basin, west China, are known to contain H2S of TSR origin (Cai et al., 2004; Li et al., 2005; Zhang et al., 2007; Ma et al., 2008; Hao et al., 2008). Interestingly, in these areas, formation waters ⇑ Corresponding author. Tel.: +86 10 82998127; fax: +86 10 62010846. E-mail address: [email protected] (C. Cai). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.07.013

associated with the sour gas show elevated Sr concentrations (up to 695 mg/L), the values being close to those reported from the Triassic brines in Xuanda (Xuanhan-Daxian) and other salt basins in east Sichuan (Lin et al., 2004; Huang et al., 2007a), where superlarge Sr deposits (i.e., Huaying Mountain area) developed in carbonate layers of the Lower Triassic Jialingjiang Fm (Xu and Liao, 1994; Zhu and Wu, 1999). Despite the enrichment of both Sr and SO2 4 in the formation waters (Zhu et al., 2006; Jiang et al., 2009), only minor to negligible amounts of Sr-bearing minerals have been found in P3ch–T1f strata in the study area, in contrast to the development of Sr deposits in similar carbonates. The effects of mineral dissolution and precipitation, fluid mixing, and organic maturation reactions are reflected in the present-day chemical compositions of reservoir fluids (Stoessell and Moore, 1983). As indicated in previous studies (Banner, 1995; deVilliers et al., 1994, 1995; Stoll and Schrag, 2000; Huang et al., 2007b), Sr enrichment in carbonate diagenetic fluid is usually considered to have been derived from aragonite–calcite conversion,

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Fig. 1. Map showing the geology, tectonics and locations of gas pools.

dissolution and precipitation of Sr-rich minerals, dolomitization and mixing with Sr-rich fluid. However, this mechanism alone does not necessarily produce significant amounts of Sr in brines as indicated by a host of previous case studies (Korsch et al., 1991; Banner et al., 1994; Minissale et al., 1997). Another case in point is the Upper Sinian formation waters in the Weiyuan gasfield in SW Sichuan Basin, which have no elevated Sr concentrations (147 mg/L) in the salt basins (Lin and Pan, 2001) in spite of the massive dolomitization (Korsch et al., 1991; Wei et al., 2008). On top of that, TSR has been shown to generate freshwater, and thus decrease salinity and O isotopes of the fluid (Worden et al., 1996; Simpson et al., 1996; Yang et al., 2001; Li et al., 2011) and thus Sr concentration. The P3ch–T1f carbonates in NE Sichuan basin have undergone aragonite neomorphism, dolomitization and TSR (Zheng et al., 2007; Ma et al., 2007a,b,c), thus it is expected that the Sr has a complicated evolution history. To account for the Sr evolution, this study presents petrography, fluid-inclusion data, trace element compositions and C, O and Sr isotopes from the interparticle cements and fracture/vug-fillings in the P3ch–T1f carbonates. Integration of these types of data is expected to supply reliable information to constrain the origin and evolution of palaeo-waters. 2. Geological setting Large H2S-rich gas accumulations have been discovered recently in the Upper Permian Changxing Fm (P3ch) and Lower Triassic Feixianguan Fm (T1f) in NE Sichuan basin, located in the Puguang, Luojiazhai, Dukouhe, Tieshanpo, Maobachang, Longgang and Yuanba structures (Fig. 1). Detailed descriptions of the geolog-

ical settings of the basin have been published previously (Cai et al., 2003, 2004; Li et al., 2005; Ma et al., 2007b; Hao et al., 2008; Liu et al., 2010). In brief, it is a late Mesozoic–Cenozoic foreland which has experienced several important tectonic episodes since the establishment of the basement framework, including the Caledonian (±320 Ma), Yunnan (±270 Ma), Dongwu (±256 Ma), Indosinian (205–195 Ma), Yanshanian (180–140 Ma), and Himalayan (80– 3 Ma) movements (Hao et al., 2008). Of these orogenies, the Yanshanian–Himalayan tectonic movement, characterized by intensive lateral compression, exerted the strongest influence on P3ch–T1f lithological–structural traps (Li et al., 2005). The distribution of the Changxing–Feixianguan reservoirs in the aforementioned sour gas fields is controlled by depositional facies and the effective reservoirs are developed mainly in the platformmargin shoal and evaporate platform environments (Ma et al., 2005, 2007b). The platform-margin reef facies, mainly developing in the P3ch formation, is made up of gray limestones and the dolomites of a baffling sponge reef, and gray limestones and dolomites of a frame sponge reef (Table 1). The platform-margin bank facies, chiefly developed in the P3ch formation and T1f formation, is made up of thick-bedded to massive oolitic dolomite and dolarenite (Table 1, Ma et al., 2007b). High-frequency sea-level fluctuation occurred during the P3ch–T1f periods, leading to the occasional exposure and leaching of reefs and shoals (Zhao et al., 2006; Ma et al., 2007a). In contrast to thick, basin-wide anhydrite beds in the Jialingjiang and Leikoupo Formations, less anhydrite occurs in the Feixianguan Formation in the Platform-marginal belt, except in the top of the Feixianguan Formation (T1f4 member) with several meters of anhydrite beds associated (Table 1, Cai et al., 2004; Li et al., 2005).

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K. Li et al. / Applied Geochemistry 27 (2012) 2409–2424 Table 1 Synthetic stratigraphy for northeast Sichuan basin. System

Series

Stage

Symbol

Triassic

Lower

Jialingjiang

Permian

Thickness (m)

Depositional environment

Tectonic cycle

T1j

1010

Supratidal and open platform carbonates and evaporite

Indosinian

Feixianguan

T1f

450

Intertidal and open platform carbonates and evaporite

Upper

Changxing

P3c

180

Platform carbonates

Middle

Longtan

P2l

120

Maokou

P2m

280

Alternating marine and terrestrial marl and mudstone Open platform carbonates

QixiaLiangshan

P1(q  1)

220

Littoral and platform carbonates

Lower

Lithology

The burial and geothermal history of Well PG2 in NE Sichuan basin shows that rapid sedimentation took place during the early Triassic, Middle and late Jurassic (Fig. 2). Temperatures in the P3ch–T1f reservoirs reached 90 °C at about 200 Ma, 120 °C at 170 Ma, and 150 °C at 165 Ma. The stratigraphic section experienced maximum burial (about 8000 m) and maximum temperatures (about 210 °C) during late Jurassic. The major oil generation period of potential petroleum source rocks for P3ch and T1f reservoirs, including the Upper Permian Longtan Fm and Changxing Fm (Cai et al., 2004, 2010; Wang et al., 2010), was between 210 Ma and 190 Ma, that is, from the late Triassic to the early Jurassic (Fig. 2, Wang et al., 2010). The cracking of liquid hydrocarbon within the P3ch–T1f reservoirs to natural gas has been proposed to have taken place during the Middle Jurassic (Wang et al., 2010). Significant uplifts occurred during the Cenozoic as a result of the Hercynian Orogeny, which contributed to a constant decrease of temperatures in the P3ch–T1f reservoirs. The temperatures reached 120 °C at about 10 Ma and a minimum temperature of 100–110 °C at present.

Hercynian

3. Sampling and analytical methods Nineteen core samples from twelve wells with high H2S concentrations or abundant elemental S associated (e.g. Well JX1) were collected from the P3ch–T1f strata. The samples with vug/ fracture-filling authigenic minerals (calcite, dolomite, quartz, etc.) or different generations of cements were chosen from the petroleum reservoirs. Finely polished thin sections were studied using transmitted light microscopy, cathodoluminescence (coldCL) petrography and electron microprobe (EMP) analyses. CL analyses were performed on 34 thin sections using a Technosyn cold cathodoluminescence stage with a 15 kV beam and a current intensity of 500 lA. EMP analyses were carried out using a Cameca Camebax BX 50™ instrument equipped with three spectrometers and a backscattered electron (BSE) detector on 20 carboncoated thin sections. The operating conditions were set at an accelerating voltage of 20 kV, a probe current of 12 nA and a focused beam diameter of 10 lm. Precision of analyses was better than 0.5 mol%.

Fig. 2. Diagram showing burial and thermal history of well Puguang 2 (Ma et al., 2008).

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Fig. 3. Different generations of calcite cements at 4904.43 m in Well Ma2, T1f. (A) View taken under PL. The acicular or fibrous circumgranular stage I calcite cements are followed by bladed and granular stage II calcite cements, and the stage III calcite occurs as mosaic to poikilitic crystals in the center of pores. (B) View taken under CL. The stage I and III calcite shows dull red to non-luminescent, in contrast to stage II calcite cements with red to orange yellow colors.

Table 2 Sr, Fe and Mn contents, homogenization temperatures and salinities of fluid inclusions and CL colors of different cements.a Cements Stage I calcite cement Stage II calcite cement Stage III calcite cement Stage I dolomite cement Stage II dolomite cement Vug Cc-1 Vug/Fracture Cc-2 Vug-filling dolomite Quantz Vug Cs-1 Vug Cs-2 a b c d e

Sr contents (lg/g) 905–2581/1911 0–516/173 34–3825/617 0–68/31 0–525/93 239–372/326 198–22632/5539 237–550/361 1303–1464/1432 –b –b

Fe contents (lg/g) 544–2403/1352 0–158 996–2621/1912 132–311/222 0–488/106 0–158 171–887/431 832–918/875 233–1711/972 –b –b

Mn contents (lg/g) 0–209/58 124–356/240c 0–225/47 0–465/232 0–786/167 0–164 0–294/52 0–155/51 0–248/65 –b –b

HTs (°C) b

– –b 90–130d –b –b 0–50 106–218 111–179 90.8–210.3 –b 107.9– 130.8

Salinities (wt.% equiv. NaCl) b

– –b –b –b –b 0–3.5 0.2–21 0.2–18.4e 0.2–21.3 –b 7.7–21.5

CL colors Non-luminescent Dull red to orange red Dull red to non-luminescent Dull red Dull red Dull red to orange yellow Non-luminescent Dull red to dull orange yellow Non-luminescent Non-luminescent Non-luminescent

Data are expressed as range/average. No measurement or data available. Data from Meng (2003). Data from Wang (2004). Parts of data from Zhang (2010).

Fluid inclusions were observed on double polished thick sections, using a calibrated Linkam THMSG 600 fitted a UV lamp to determine whether they were oil or aqueous inclusions. Microthermometric analyses were performed using a Linkam THM600 heating–cooling stage with 1 °C precision. The final melting temperature of ice (Tm) was used to calculate salinity expressed as wt.% NaC1 equivalent (Bodnar, 1993): Salinity = 0.00 + 1.78Tm  0.0442Tm2 + 0.000557Tm3, where Tm is the freezingpoint depression in degrees Celsius (°C). Representative fluid inclusions in Sr-bearing minerals were analyzed with a Raman microprobe using a 514 nm wavelength for radiation. Diagenetic minerals showing coarse, vug/fracture-filling spar and texturally a late diagenetic event were selected using a dentist’s drill. The powder samples were subjected to trace element analyses and C, O and Sr isotope measurements. Carbonate samples were dissolved in anhydrous H3PO4 to release CO2 gas, which was analyzed on a Finnigan MAT 251 mass spectrometer. The d13C and d18O are reported as ‰ relative to the Pee Dee Belemnite (VPDB) standard. The precision is ±0.1‰. Sample powders (50– 100 mg) were dissolved in 2.5 N HCl. Subsequent treatments were conducted as described in Diener et al. (1996) and Ebneth et al. (1997). The 87Sr/86Sr ratios were measured on a Finnigan MAT261 mass spectrometer. NBS987 was used as a standard reference and the precision for 87Sr/86Sr measurement is ±0.00003–0.00007. Fourteen calcite samples, one celestite sample and one strontianite sample were dissolved for REE and trace element analysis. The dissolutions of carbonates and strontianite were by the methods of

Nothdurft et al. (2004) and Souissi et al. (2007), respectively. The measurement was conducted on a Finnigan ICP-MS. Repeat analysis on standards and samples gave precisions better than ±8%. 4. Results 4.1. Petrology and elemental composition of fracture-fillings and cements Preliminary criteria developed to distinguish early and late stage minerals is established based on paragenetic relationships between minerals and solid bitumen inclusions. That is, an authigenetic mineral that either contains solid bitumen inclusions or grows from bitumen-coated grain surfaces is considered to precipitate later than the one without associated organic inclusions, as elucidated by Heydari (1997). Interparticle calcite cements, which commonly distribute in limestone sections, are frequently present in three stages. Almost none of the calcite crystals have organic inclusions. Submarine diagenesis led to precipitation of acicular or fibrous circumgranular calcite cements (stage I), which are nonluminescent under CL (Fig. 3A and B) with Fe 1352 ppm and Mn 58 ppm (Table 2). This calcite is commonly followed by stage II calcite in high frequency cyclothems, and the latter is bladed and granular and shows red1 1 For interpretation of color in Figs. 1–5, 7–15 and 17–19, the reader is referred to the web version of this article.

K. Li et al. / Applied Geochemistry 27 (2012) 2409–2424

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Fig. 4. The chronological relationship of stage I poikilitic celestite crystals, stages I and III calcite cements at 5011 m in Well Hb102, T1f, PL. Both the celestite crystals and stage III calcite cements grow outward from fibrous circumgranular stage I calcite cements and occupy part of the residual interparticle pore spaces.

Fig. 6. Photomicrographs showing stage I fine crystalline equant vug-filling calcite and stage II interparticle granular calcite cements at 6709.23 m in Well Yb5, P3ch, planar polarized light (PL).

to orange yellow CL colors, with low Fe contents (<158 ppm) and relatively high Mn contents (from 124 to 356 ppm, mean 240 ppm) (Fig. 3B). Stage III calcite occurs as mosaic to poikilitic crystals in the center of pores and occasionally coated directly by stage I calcite (Fig. 4), revealing that no meteoric diagenesis occurred. This stage III calcite shows dull red to non-luminescent under CL (Fig. 3B), with an Fe content of 1912 and Mn of 47 ppm. Two stages of dolomitic cements occur in dolomite. Crystal sizes and shapes of the two stages of cements in dolomite sections resemble stage I and stage II calcite cements in the limestone, respectively (Figs 3 and 5). Under cathodoluminescence (CL), both of the two types dolomite cements are dull red (Fig. 5B), which indicates a similar origin for them. Slight differences in Fe and Mn contents of the two are likely attributed to the different contents characteristics of the precursor calcite cements (Table 2). In addition, the sharp interfaces between stage II dolomite cement and solid bitumen give compelling evidence for dolomitization being prior to precipitation of solid bitumen (Fig. 5). The main fracture- and vug-fillings encountered in P3ch–T1f sour reservoirs include early stage vug-filling calcite (Vug Cc-1) and celestite crystals (Vug Cs-1), and late stage fracture- and vug-filling calcite (Vug/Fracture Cc-2), quartz, dolomite (Vug Dm) and celestite/strontianite crystals (Vug Cs-2/Vug St). Vug Cc-1 occurs commonly as equant fine crystals filling intraparticle dissolution pores (Fig. 6), as well as in monocrystal

or polycrystal forms in molds. The calcite spars show dull red to orange yellow colors under cathodoluminescence (CL) with Mn in the range 0–158 ppm and Fe 0–164 ppm (Table 2), and no associated solid bitumen inclusions. In some thin sections, no vug Cc-1 occurs while vug/fracture Cc-2does. Vug/Fracture Cc-2 crystals occur as replacements of anhydrite or cements filling in interparticle dissolution pores, enlarged dissolution vugs and fractures are milky white and exhibit coarse crystal sizes (Figs 7 and 8). The crystals contain abundant solid bitumen (pyrobitumen) inclusions and grew outward from bitumen-coated grain surfaces (Figs 7 and 8A), and show a different CL response from Vug Cc-1: nonluminescent CL (Figs. 8A and 9B) with relatively high Fe and low Mn contents, 431 and 52 ppm, respectively (Table 2). Vug Dm usually appears as a fine-medium crystalline pore-filling cement rather than a replacement mineral. These dolomite crystals are subhedral to euhedral in shape, with Vug Cc-2 occasionally associated (Fig. 10). The paragenetic relationship of the two vug-fillings is not well defined by petrographic analysis alone. However, as indicated by solid bitumen inclusions associated in the crystals, vug Dm must postdate solid bitumen precipitation, which is similar to Vug/Fracture Cc-2. Under CL, this dolomite displays nonluminescent to dull red colors with bright red rims (Fig. 9B) and generally contains high Fe (from 832 to 918 ppm, mean 875 ppm) and low Mn contents (<155 ppm, mean 51 ppm, Table 2).

Fig. 5. Different generations of dolomite cements at 4790.3 m in Well D5, T1f. (A) View taken under PL. (B) View taken under CL. Crystal sizes and shapes of the two stages of dolomite cements resemble stage I and stage II calcite cements in Fig. 11, respectively.

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Fig. 7. Stained section showing stage II vug-filling coarse calcite crystals (red spars) with associated bitumen at 3249.18 m in Well Lj2, T1f, PL.

Fig. 10. Stained section showing the chronological relationship of vug-filling dolomite crystals (white spars) and stage II vug-filling coarse calcite crystals (red spars) in Well PG5, T1f, PL.

Authigenic quartz was found in various pore spaces, e.g., interparticle/intragranular dissolution pores and pressure-solution cleavages (Figs 5 and 11). These vug-fillings commonly occur as medium-coarse crystalline and anhedral crystals, growing outward from bitumen-coated grain surfaces (Fig. 11B). In addition, fine crystalline quartz may occur along local serrated sutures in association with solid bitumen and crosscut the stage III calcite crystals

(Fig. 11A). A combination of the wide spread in crystal sizes and homogenization temperatures (as discussed below) indicate that the quartz may have experienced precipitation for a long period. There exist two generations of celestite fillings (Vug Cs-1 and Vug Cs-2). The earlier Vug Cs-1 appears as pore-filling poikilitic cement without solid bitumen inclusions (Fig. 4), indicating a pre-bitumen precipitation. The crystals usually grow outward from

Fig. 8. Stage II vug-filling coarse calcite crystals growing outward from bitumen-coated grain surfaces at 3267.4 m in Well Lj2, T1f. (A) View taken under PL. (B) View taken under CL. The calcite spars show nonluminescent CL, in contrast to the dolomite host rocks with red to purplish red colors.

Fig. 9. Photomicrographs of vug-filling dolomite crystals and stage II vug-filling coarse calcite crystals in Well PG5, T1f. (A) View taken under PL. (B) View taken under CL. The dolomite crystals displays dull red colors with bright red rims.

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Fig. 11. Photomicrographs showing various types of authigenic quartz crystals in pore spaces. (A) Fine crystalline quartz and calcite cement 3 in local serrated sutures, with associated solid bitumen at 6998.9 m in Well Yb3, T1f, PL. (B) Vug-filling coarse quartz crystals growing outward from bitumen-coated grain surfaces at 3235.66 m in Well Lj2, T1f, PL.

(B)

(A)

CaO/SrO 10-3 5.0 3.1 2.9 2.3

calcite calcite vug vug celestite

200 m

celestite

50 m

Fig. 12. Back-scattered electron images showing (A) decrease in CaO/SrO ratio from the edge to the interior of celestite, indicating the proceeding towards the center of the celestite and (B) the replacement celestite residue (white spots in the calcite).

fibrous circumgranular stage I calcite cements and occupy all or part of the residual interparticle pore spaces, occasionally coexisting with stage III calcite cement (Fig. 4). This pre-bitumen celestite (Vug Cs-1) was replaced by Vug Cc-2 as demonstrated by electron probe imaging with the replacement celestite residue

(Fig. 12B). The CaO/SrO ratio shows a decrease from 629.02 to 0.0023 from the edge to the interior of the celestite (Fig. 12A), which is similar to previous findings on the replacement sequence (Worden et al., 2000; Cai et al., 2008). Vug Cs-2 found in P3ch–T1f reservoirs is volumetrically minor and commonly coexists with

Fig. 13. Photomicrographs of stage II vug-filling celestite crystals and the double-spheroid fluid inclusion captured. (A) Stage II vug-filling celestite crystals with associated solid bitumen at 6904.36 m in Well Yb102, P3ch, PL. (B) Double-spheroid fluid inclusions in stage II vug-filling celestite crystals at 6904.36 m in Well Yb102, P3ch, PL.

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Vug St in dissolution vugs. The two Sr-bearing minerals postdate solid bitumen formation as indicated by the occurrence of abundant solid bitumen inclusions in the crystals (Fig. 13A). 4.2. Fluid inclusion petrography, microthermometry and compositions Doubly polished thick sections were prepared and examined for fluid inclusions. Abundant fluid inclusions were observed in fracture/vug-fillings and interparticle cements. The occurrence of most fluid inclusions in these minerals confined by growth-zone boundaries indicates their primary origin. Some of the inclusions appear to cut across CL-defined growth zones of the crystals, and thus may have been trapped after growth of the crystals, so are classified as secondary fluid inclusions. Multiple types of the inclusions in the same fluid inclusion assemblage (FIA) were observed in the aforementioned diagenetic minerals: single phase inclusions, two-phase gas–liquid inclusions as well as three-phase inclusions. In the following sections, only aqueous 2-phase primary inclusions with small size, normal shape and low vapor to liquid ratios (<15%) were measured for homogenization temperatures as representative of trapping temperatures (Lacazette, 1990; Barker, 1991; Goldstein and Reynolds, 1994). No measurable fluid inclusions were observed in stages I and II interparticle calcite and dolomite cements. Fluid inclusions in stage III calcite cements have two phases with small bubbles and

consistent vapor to liquid ratios. The HTs of 106 fluid inclusions mainly range from about 90 to 130 °C, with a distinct population at around 100 °C (Wang, 2004). Vug Cc-1 contains small numbers of fluid inclusions, including all-liquid and two-phase inclusions with highly variable ratios of gas to liquid. The occurrence of monophase liquid fluid inclusions indicates low precipitation temperatures (<50 °C) for Vug Cc-1 while the distinctive variability in the vapor–liquid ratios may be caused by re-equilibration. As for Vug/Fracture Cc-2, significant amounts of vapor–liquid 2-phase, pure gas inclusions and solid bitumen inclusions were observed. A few oil inclusions have been preserved, showing brownish yellow colors under polarized light microscopy and tan UV fluorescence emission colors. Primary aqueous fluid inclusions from TSR-calcite possess a wide range of homogenization temperatures (HTs) from 106 to 218 °C (n = 181), with three peaks at 115, 155 and 185 °C, respectively (Fig. 5). These inclusions also contain water with a wide range of salinities, varying from 0.2 to 21 wt.% NaCl equivalent (Fig. 14). The two histogram modal values are about 6 and 20 wt.% NaCl equivalent. There is a weak inverse relationship between salinity and temperature (Fig. 15), generally showing that salinity decreases as the temperature of calcite growth increases. Vug Dm and quartz show similar distribution of fluid inclusions to that of Vug/Fracture Cc-2, excepting for smaller population and sizes. The HTs of aqueous 2-phase fluid inclusions in Vug Dm range

Fig. 14. Histograms showing homogenization temperatures and salinities measured from fluid inclusions of postbitumen minerals including Vug/Fracture Cc-2, Vug Dm (parts of data from Zhang, 2010), quartz and Vug Cs-2 from P3ch–T1f strata.

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Fig. 15. Relationship between homogenization temperatures and salinities of the fluid inclusions from postbitumen minerals including Vug/Fracture Cc-2, Vug Dm, quartz and Vug Cs-2 from P3ch–T1f strata.

from 111 to 179 °C, with two peaks at 135 and 155 °C. The fluid inclusions gave wide salinities that range between 0.2 and 18.4 wt.% NaCl equivalent (Fig. 14). Vug-filling quartz crystals show a wide spread HT range from about 91 to 210 °C (n = 57). The three histogram modal values are about 135, 175 and 200 °C. Salinities of mineralizing fluids vary between 0.2 and 21.3 wt.% NaCl equivalent (Fig. 14). Similar to Vug/Fracture Cc-2, the HTs of Vug Dm and Vug-filling quartz samples show weak inverse relationships to salinities (Fig. 15). Pre-bitumen Vug Cs-1 crystals contain few fluid inclusions, which are too small to be measured for HTs and salinities. However, none of the fluid inclusions showing UV fluorescence supports a pre-bitumen precipitate. As for post-bitumen Vug Cs-2, abundant fluid inclusions were observed in the crystals. Aqueous fluid inclusions are the most common inclusions and coexist with single-phase gas inclusions and solid bitumen inclusions. As distinct from other post-bitumen minerals (e.g. Vug Cc-2), a

particular type of immiscible inclusion in the form of a doublespheroid can be found in the celestite spars (Fig. 13B). Two spheroids can be distinguished from each other in the various shapes. In addition, one of the two spheroids shows mobility and homogenizes into liquid when being heated, suggesting that it is a kind of gas bubble. Meanwhile the other one is unaffected even when being heated to a high temperature. Aqueous two-phase gas– liquid inclusions analyzed in the Sr aggregation gave homogenization temperatures that mostly range between 107.9 and 119.5 °C (n = 38), with a distinct population present around at 115 °C (Fig. 4), which are close to or slightly higher than the reported temperatures of the gypsum crystals (Liu et al., 2006). The salinities vary between 3.2 and 15.8 wt.% NaCl equivalent (n = 16), mostly ranging from about 6.7 to 11.9 wt.% NaCl equivalent. Interestingly, a weak trend of decreasing temperatures with increasing salinities is evident, if three abnormal temperature values are removed (Fig. 15).

Fig. 16. Microscopic laser Raman spectra at points A, B and C of an immiscible fluid inclusion in the celestite crystal. (A1 and A2) Strong sulfur peaks and a weak H2S peak corresponding to point A; (B) the host mineral celestite peaks corresponding to B point; (C) a conspicuous H2O peak corresponding to point C. Data from Huang et al. (2006), Zheng et al. (2007) and Li (2008).

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Laser Raman spectrometric analysis was carried out on the double-spheroid fluid inclusions in celestite crystals. For the unaffected spheroid (point A, Fig. 16), strong peaks at 150.2, 218.0, 472.4 cm1 and a weak peak at 2570 cm1, which are characteristic of S and H2S, respectively, are shown in Fig. 16A1 and A2. The mobile gas bubble (point B) shows no S peaks at point A, showing peaks at 998.9, 809.2 and 456.6 cm1 representative of the host mineral celestite (Fig. 16B). Point C shows a remarkable water peak at 3404.0 cm1 in addition to the host mineral celestite peaks (Fig. 16C). The double-spheroid fluid inclusions in celestite crystals are similar to the elemental S-bearing immiscible inclusions, as reported previously in authigenic gypsum (Tritlla et al., 2000; Liu et al., 2006). 4.3.

87

Sr/86Sr, d13C and d18O of fracture-fillings

The isotopic compositions of 19 post-bitumen mineral samples filling in fractures and vugs are listed in Table 3. All of the postbitumen calcite, dolomite and strontianite crystals have 87Sr/86Sr ratios from 0.7072 to 0.7076 (Fig. 17), being close to contemporary seawater Sr isotope ratios (0.7067–0.7082, Burke et al., 1982; Korte et al., 2003, 2006). In addition, the massive dolomites in the platform-marginal belt, where sour reservoirs are locatated, also show a similar range of 87Sr/86Sr ratios from 0.7066 to 0.7082 (Huang et al., 2006; Zheng et al., 2007). Vug/Fracture Cc-2 samples have C isotope ratios ranging from 18.9‰ to 2.7‰ PDB (n = 14), most of which (n = 9) are lighter than bulk carbonates and contemporary seawater (0.3‰ to +4.9‰ PDB, Huang, 1994; Veizer et al., 1999; Korte et al., 2005). The Vug Dm sample from well DW102 also has a slightly depleted d13C value at 0.6‰. Interestingly, the post-bitumen strontianite sample from Well YB101 shows negative shift in C isotope (3.5‰) as well. All of the post-bitumen carbonate minerals show narrow d18O values from 5.4‰ to 9.0‰ (mean 7.6‰), which were slightly more depleted than bulk carbonates in the Upper Permian and Lower Triassic in the basin (Huang, 1994). 4.4. Trace and rare earth elements of fracture-fillings Various Sr contents were exhibited in different types of diagenetic minerals (Table 2). For interparticle calcite cements, stage I calcite has high Sr contents from 905 to 2581 ppm (n = 4), mean 1911 ppm, being close to or slightly higher than bulk carbonates.

Fig. 17. 87Sr/86Sr ratios of post-bitumen calcite, dolomite and celestite filling in vugs and massive dolomite in comparison with contemporary seawater of P3ch–T1f age (Burke et al., 1982; Korte et al., 2003, 2006).

By contrast, stage II calcite cements show a conspicuous decrease in Sr contents with an average of only 173 ppm. For stage III calcite, there is a wide spread in Sr contents, between 34 and 3825 ppm (mean 617 ppm). Similar to stage II calcite cement, pre-bitumen Vug Cc-1 has low Sr contents from 239 to 372 ppm, with an average of 326 ppm. By contrast, post-bitumen vug- and fracture-fillings are commonly Sr-rich minerals. Vug/Fracture Cc-2 crystals show Sr contents from 198 to 22632 ppm, mean 5339 ppm (n = 14), most of which (n = 10) are significantly higher than that of P3ch–T1f bulk carbonates (889–994 ppm, Huang et al., 2007c; Zheng et al., 2007). Authigenic quartz crystals filling vugs show narrow elevated Sr contents from 1303 to 1464 ppm. Vug Dm, as an exception, has relatively low Sr contents ranging from 237 to 550 ppm (mean 361 ppm), which are, however, higher than the values of pre-bitumen dolomite cements (with average an value of only 93 ppm). In addition to Sr enrichment, the post-bitumen Vug/Fracture Cc2 samples also exhibit high Ba contents, mainly from 93 to 1279 ppm with a maximum of 10,578 ppm (n = 14). Similarly, the post-bitumen Sr-bearing minerals (celestite and strontianite) are Ba-rich, with an average Ba content of 2951 ppm (n = 2). dEu values (dEu = Eu/Eu = EuN/(SmN  GdN)1/2) of 11 post-bitumen calcite samples range from 1.06 to 10.0. The values are higher than that of unfractionated chondrite (1.05), indicating a positive

Table 3 Trace element contents, and Sr, C and O isotopic compositions of post-bitumen fracture- and vug-fillings. Sample number

Formation

Depth (m)

Sample occurrence

Sr (lg/g)

Ba (lg/g)

Eu (lg/g)

d13C (‰)

d18O (‰)

87

Sr/86Sr

D5-a D5-b DX3 DW102 JX1-a JX1-b LJ2 M2 MB2-a P2 LG82-a LG82-b LG82-c MB2-b MB3-a MB3-b YB101-a YB101-b YB101-c

T1f T1f T1f T1f T1f T1f T1f T1f T1f T1f P3ch P3ch P3ch P3ch P3ch P3ch P3ch P3ch P3ch

4793 –a 4735.1 4818.22 –a –a 3267.4 4904.43 –a –a 4238.29 4238.95 4235.02 –a 4340.6 4414.4 6904.5 6904.36 6904.36

Vug Cc Vug Cc Frac. Cc Vug Dm Frac. Cc Frac. Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Vug Cc Frac. Cc Vug St Vug Cs

927.82 2563.8 3773.14 –a 19025.00 22632.00 835.53 6636.16 630.57 2829.70 1783.17 1677.69 197.79 7767.30 –a –a 3463.60 334916.80 106791.67

116.12 120.086 92.73 –a 9047.93 10578.00 286.72 1048.72 42.094 158.05 70.69 220.53 4.49 1279.31 –a –a 144.80 761.50 5139.64

0.037 0.028 0.09 –a 1.33 1.45 0.09 0.27 0.036 0.05 0.10 0.16 0 0.11 –a –a 0.08 0.21 0.45

16.48 –a 1.55 0.64 0.21 0.26 13.52 0.89 2.662 18.87 0.19 1.01 9.68 –a 14.08 10.25 1.49 3.54 –a

8.59 –a 6.75 8.73 8.53 8.43 8.22 6.73 6.525 8.27 7.62 8.3 9.02 –a 5.96 5.42 6.60 8.39 –a

0.70753 –a 0.70754 0.707617 0.70745 0.70745 0.70759 0.70753 0.707421 0.70750 0.70748 0.707526 0.70755 0.70724 –a –a 0.70754 0.70738 –a

Vug Cc represents vug-filling calcite; Frac. Cc represents fracture-filling calcite; Vug Dm represents vug-filling dolomite; Vug St represents vug-filling strontianite; Vug Cs represents vug-filling celestite. a No measurement or data available.

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K. Li et al. / Applied Geochemistry 27 (2012) 2409–2424 D5-a D5-b Dw3 Jx1-a Jx1-b Lj2 M2 Mb2-a P2 Lg82-a Lg82-b Lg82-c Mb2-b Yb101-a Yb101-b

2

1

0

-1

-2

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Fig. 18. Rare earth element pattern curves of postbitumen calcite and strontianite samples in the study area.

Eu anomaly (Fig. 9). In addition, a more obvious positive Eu anomaly is also associated with the late vug-filling strontianite sample, with a dEu value at 58.8 (Fig. 18). Interestingly, the Sr contents of all the post-bitumen calcite crystals show positive correlations with Ba (R2 = 0.95) and Eu (R2 = 0.93), as well as for Ba with Eu (R2 = 0.99) (Fig. 19). Even if the two data points with abnormally high Sr and Ba contents, which are most likely responsible for the strong relationships, are removed, the positive correlations between Sr, Ba and Eu would still be present (Fig. 19).

5. Discussion 5.1. Eu positive anomalies and Ba enrichment in high HTs and 12C-rich minerals Most of the post-bitumen calcites as well as vug-filling dolomite are characterized by high HTs (mostly >110 °C) and a negative shift of C isotope values (Fig. 14, Table 3), which are consistent with those reported by others (Krouse et al., 1988; Heydari and

Fig. 19. Cross plots showing relationships of Sr contents to Ba contents (A1) Sr contents to Eu contents (B1), and Ba contents to Eu contents (C1) for 16 postbitumen calcite crystals in this study; and (A2, B2 and C2) the relationships between Sr and Ba, Sr and Eu, and Ba and Eu corresponding to (A1, B1 and C1), respectively, if the two data points with abnormally high Sr and Ba contents are removed.

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Moore, 1989; Machel et al., 1995; Yang et al., 2001). The depletion of d13C values are interpreted to result from a substantial contribution of organic C from the oxidation of hydrocarbons, whereas the less negative values are attributed to incorporation of more inorganic C released from dissolution of dolomite. That is, these postbitumen calcite and vug-filling dolomite crystals are products and by-products of TSR, respectively. Almost all the TSR-calcite samples show Eu enrichment or Eu positive anomalies. It is generally considered that positive Eu anomalies in Ca-minerals have been derived from hydrothermal fluid with elevated temperatures (>250 °C, Lüders et al., 1993; Hecht et al., 1999). However, the available HTs of fluid inclusions associated with the TSR-calcite crystals reveal that the post-bitumen calcite crystals were precipitated at temperatures below 220 °C (Figs 14 and 15). Thus, it is less likely that the Eu anomaly results from hydrothermal fluid in the study area. However, it has been suggested in previous studies that Caminerals with Eu anomalies also precipitate in reducing Eu-rich brines (Lee et al., 2003; Cai et al., 2008). In the case of NE Sichuan basin, the reducing fluid associated in P3ch–T1f sour reservoirs is indicated by the accumulation of high H2S concentrations. Then Eu mainly occurs as Eu2+ and may have been incorporated into the lattice of Ca-minerals in a similar way to Sr2+, due to their similar charge, sizes and geochemical characteristics (Brookins, 1989). The linear positive correlation relationships of Sr and Eu in postbitumen calcites provide compelling evidences for the co-incorporation of these elements into CaCO3 lattice (Fig. 10). In addition, it is very likely that Sr and Eu are released into the diagenetic fluids by dissolution of solid CaSO4 (mainly anhydrite and gypsum) during TSR. High Sr contents in the Ca-sulfates has been indicated in numerous case studies worldwide (Butler, 1973; Bein and Land, 1983; Das et al., 1990; Kasprzyk, 2003), as well as Triassic laminated/nodular anhydrite in East Sichuan (Xu and Liao, 1994; Qin, 1998). In particular, pre-bitumen celestite is considered to be another sulfate reactant for TSR in the study area, as is illustrated from the replacement of SrSO4 by CaCO3 from the edge towards the center (Fig. 12). Dissolution of these Sr-bearing minerals would release not only volumetrically significant Sr2+ but also abundant Ba2+ and Eu2+, due to their high contents in these minerals (Butler, 1973; Schiebel, 1978; Das et al., 1990; Tekin et al., 2002; Kasprzyk, 2003). As a result, these divalent metal ions substituted for Ca2+ under reducing conditions, resulting in Ba enrichment and the positive relationships of Ba to Sr and Eu in TSR-calcites (Fig. 19). Interestingly, the 12C-rich post-bitumen strontianite sample also shows a striking positive Eu anomaly and Ba enrichment, giving particularly instructive clues regarding pre-bitumen celestite as an important supplier of the characteristic elements.

5.2. Origin of pre-bitumen celestite It can be speculated from the pre-bitumen celestite that there should be a Sr2+ concentration peak in the diagenetic fluids during the early diagenetic period (Miiller, 1962; Braitsch, 1971; Snorre, 1981). Because almost all the 87Sr/86Sr measurements from the massive dolomites, as well as post-bitumen diagenetic calcite, strontianite and dolomite samples (Fig. 17), are close to bulk carbonates and contemporary seawater (Huang, 1994; Veizer et al., 1999; McArthur et al., 2001; Korte et al., 2005), the significant amounts of Sr2+ in diagenetic fluids should be from carbonates in situ rather than continental aluminosilicates. It is further considered that the precipitation of the pre-bitumen celestite or early accumulation of Sr could result from two possibilities.

(1) Aragonite–calcite conversion: Permian and Triassic seawater are considered to be favorable for precipitation of aragonite as ooids and reef (Sandberg, 1983; Lowenstein et al., 2005). Aragonite generally contains abundant Sr owing to the geochemical behavior of Sr (Milliman, 1974). The difference in the Sr distribution coefficient between aragonite and calcite is responsible for Sr enrichment in the pore waters during aragonite neomorphism (Stoll and Schrag, 2000; Rickabym et al., 2002). However, during the early diagenetic stage, as a result of frequent exposure and meteoric water flushing of the platforms, the connate water was diluted by freshwater, as indicated by the combined occurrence of all-liquid fluid inclusions and bright CL (red to orange yellow) in Vug Cc-1 and stage II calcite cements (Goldstein and Reynolds, 1994; Muchez et al., 1998). This will lead to a significant loss of Sr in palaeo-waters, which is reflected by the low Sr contents (<516 ppm, Table 2) in the meteoric calcite. There is only one possible scenario where Sr2+ released would have been well preserved in pore fluid, if the shoal and reef sediments were influenced by meteoric water, as implied by the occurrence of a minor pore-filling pre-bitumen celestite or strontianite crystals coated by stage I calcite cements (Fig. 4). The formation of Sr-bearing minerals consumed abundant Sr and so lowered the local Sr2+ concentration in the diagenetic fluid, from which stage III calcite cements with a wide range Sr contents from 34 to 3825 ppm precipitated. (2) Dolomitization: The remarkable differences in Sr contents between massive dolomite/dolomite cements (mean 93 ppm) and limestone (mean 994 ppm, Huang et al., 2007c) suggest the possibility of release of abundant Sr2+ during dolomitization. However, the variation in Sr abundance in diagenetic fluids is not only determined by its generation, but also by the preservation conditions, being likely correlated with the diagenetic system of dolomitization. As for meteoric mixing dolomitization, the open system was obviously not favorable for the released Sr2+ to be preserved in the pore waters. By contrast, the relatively closed hydrodynamic setting during dolomitization under burial conditions facilitated Sr2+ preservation. As for northeastern Sichuan Basin, the origin of P3ch–T1f massive dolomite in the platform-marginal belt, where sour reservoirs are located, has generated many arguments, including a proposed meteoric mixing (dorag) or the burial dolomitization model (i.e., Wei et al., 2005; Zhao et al., 2005; Zheng et al., 2007). However, a re-evaluation of the mixing-zone model for dolomitization, in conjunction with convincing petrographic and geochemical evidence, has ruled out the dorag model as the process responsible for pervasive dolomitization: (i) The 87Sr/86Sr ratios of the massive dolomite are close to contemporary seawater, leading to a paradox with the discernible 87Sr-enrichment effect originated from a significant contribution of meteoric water in the mixing zone (Veizer, 1989). (ii) The crystalline dolomites commonly exhibit dull red luminescence (Figs. 5B and 8B), which is distinctive, to bright orange-yellow luminescence of meteoric mixing dolomite, with a zonation commonly associated (Ward and Halley, 1985; Gaswirth and Budd, 2007). (iii) The dolomite samples have d13C values close to contemporary seawater and negative d18O values, giving convincing evidence for burial dolomitization rather than the mixingzone model for dolomitization (Warren, 2000; Machel, 2004).

K. Li et al. / Applied Geochemistry 27 (2012) 2409–2424

Taking petrography, Sr isotope ratios and d13C values of the dolomite together, it is considered that closed-system dolomitization, i.e., the intermediate-deep burial model as well as the brine reflux model (in the shallow burial realm), had been prevalent in P3ch–T1f strata NE Sichuan. Then Sr2+ was excluded from the CaCO3 lattice along with Ca2+ and mixed into local basinal waters. Probably, some Sr-bearing minerals most likely precipitated, as indicated by the minor residual celestite crystals (Fig. 4), however, most of these are considered to have been consumed by abiological oxidation of hydrocarbons during the TSR process, as discussed below. 5.3. The role of TSR in Sr enrichment As elucidated in the previous section, consumption of pre-bitumen celestite and solid CaSO4 had been involved in the process of TSR, which was responsible for continuous accumulation of Sr2+ in the diagenetic fluids. However, the highly saline formation water prior to the onset of TSR, as indicated by the occurrence of abundant fluid inclusions with low temperatures and high salinities (Fig. 15), was obviously not favorable for these solid sulfates to dissolve. Interestingly, abundant low salinity fluid inclusions (down to 0.2 wt.% NaCl) and the regular decrease in aqueous fluid inclusion salinity with increasing HTs of TSR-calcite, dolomite and quartz (Figs 14 and 15) provide particularly instructive clues regarding the production of freshwater in the overall TSR process. This is further supported by the predominant occurrence of low TDS formation water in sour reservoirs rather than sweet reservoirs (Zhu et al., 2006) and generation of water during TSR as proposed by Worden et al. (1996). Dilution of the palaeo-waters, even in relatively small TSR reaction sites, most likely facilitated the dissolution of pre-bitumen celestite and solid Ca sulfates. Importantly, the freshwater production also lowered Sr2+ concentrations in the local basinal water, despite the continuous ionic increase. Consequently, Sr2+ in the pore waters tended to be constantly enriched rather than combined with SO2 4 , which was also less likely to be present in high concentrations as a result of the volumetrically significant consumption by TSR. However, there exist minor post-bitumen Sr-bearing minerals such as SrSO4 and SrCO3 in local dissolution vugs (Fig. 13A), which probably implies a change in physicochemical conditions. As distinguished from other post-bitumen diagenetic minerals by petrography, abundant natural S-bearing immiscible inclusions were found associated with Vug Cs-2 crystals (Figs. 13B and 16). Sulfur accumulation can only take place if no further reactions with hydrocarbons occur, because the association of elemental S and many organic compounds at high temperatures (>100 °C) is thermodynamically unstable (Machel et al., 1995; Noth, 1997):

4S0 þ 1:33ð—CH2 —Þ þ 2:66H2 O þ 1:33OH ! 4H2 S þ 1:33HCO3 Therefore, the accumulation of elemental S could result from the exhaustion of organic compounds, as proposed by Machel (2001), or low availability of dissolved organic matter due to the limited amount of formation water in local areas (Alonso-Azcarate et al., 2001). A similar case was reported from the Upper Permian Khuff Formation in the Arabian Basin, Permian–Triassic, Khuff Formation (Worden et al., 1996). In addition, The lower HTs (108.2–120.1 °C) of primary aqueous fluid inclusions in the post-bitumen celestite crystals compared with those of Vug Cc-2 and authigenic quartz indicate a significant temperature decrease, most likely at the reservoir uplift stage in the late Himalayan. The decrease of formation temperatures would have resulted in deceleration of the TSR reaction, which was responsible for slowing down water production and SO2 4 consumption. Then the freshwater must have been concentrated by constant dissolution of

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evaporite and carbonate minerals. This is indicated by the relatively higher salinities (mainly from 6.7 to 11.9 wt.% NaCl, Fig. 14) of primary aqueous fluid inclusions in the post-bitumen celestite crystals and the inverse relationship between salinity and HTs (Fig. 15). The water end-member with relatively higher HTs (115.8–120.1 °C) and lower salinities (3.2–7.2 wt.% NaCl equivalent) may be the mixture of TSR-water and basinal water. The other end-member is water with lower HTs (108.2–115.1 °C) and higher salinities (9.2–15.8 wt.% NaCl equivalent). The salinities are close to or slightly lower than those of the present formation water (mainly from 40000 to 70000 mg L1, Zhu et al., 2006; Jiang et al., 2009). The concomitant increase in Sr concentrations by several times compared with those in the high–Temperature TSR stage was favorable for the formation of Sr-bearing minerals. However, the present volumetrically minor amounts of these minerals might be constrained by the salt effect initiated by the increase of TDS or pH changes in formation waters (Furter, 1978), which lowered the effective concentrations of Sr2+ and SO2 4 . Further study on this aspect is needed. 5.4. Palaeo-water evolution Visually determined fluorescence colors of fluid inclusions, in combination with petrological observation, aqueous fluid inclusion data and burial and thermal history of NE Sichuan basin can be used to constrain the precipitation sequence of the diagenetic calcite, dolomite, quartz and Sr-bearing minerals, as is summarized in Fig. 20. Diagenetic water in this area may have evolved as follows. The initial water was seawater of P3ch and T1f age, from which marine cements precipitated (stage I calcite cement). Subsequently, the sediments were repeatedly exposed as a result of high-frequency sea-level fluctuation and the pore water was diluted by meteoric water, from which stage II calcite cement and Vug Cc-1 precipitated. The minerals show bright CL (dull red to orange yellow) and low precipitation temperatures (<50 °C). Abundant Sr would be released via aragonite neomorphism during the meteoric digenetic stage, most of which, however, was prone to be lost due to the open system. By contrast in low frequency cyclothems, a relatively closed hydrodynamic setting was favorable for Sr enrichment and the precipitation of small amounts of SrSO4 and SrCO3. Since the Middle Triassic, the P3ch–T1f formation had been continuously buried to the depth of over 7 km in the early Tertiary. During the Middle to late Triassic, diagenetic processes were influenced by progressive increases in temperatures (40–100 °C), a more reducing environment (non-luminescence under CL with Fe contents up to 1912 ppm in stage III calcite cement) and pressure solution. Closed-system dolomitization during this period was responsible for Sr enrichment in diagenetic fluid and the precipitation of early Sr-bearing minerals. Stage III calcite cement that precipitated in this fluid is characterized by elevated temperature (mainly from 90 to 130 °C), dull red to non-luminescent under CL and a wide range of Sr contents (34–3825 ppm). The major period of liquid hydrocarbon accumulation was between late Triassic and early Jurassic (210 Ma and 190 Ma, Wang et al., 2010). Since the Middle Jurassic, abiological oxidation of hydrocarbons (TSR) took place at depths of 4–8 km and temperatures between 120 and 220 °C, as indicated by the burial and thermal history curve (Fig. 2). In the overall TSR process, abundant solid Ca sulfates and almost all of the pre-bitumen celestite crystals were consumed by hydrocarbons oxidation to release abundant Sr2+, Ba2+ and Eu2+ into the reducing digenetic fluids, as indicated by the accumulations of high H2S concentrations in the reservoirs. The corresponding diagenetic calcite resulting from the addition of TSR-CO2 is characterized by nonluminescence under CL, positive Eu anomaly, enrichment in Sr and Ba (up to 22632 and 10578 ppm, respectively), as well as a negative shift in C isotope values (down to

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Fig. 20. Synthetic paragenetic sequence showing main stages in the diagenetic evolution of P3ch–T1f carbonates with high associated H2S concentrations and variation of Sr concentrations and salinities of paleo-waters during the diagenesis, which provide insights into the evolution of paleo-waters.

18.9‰). Furthermore, the mineralized fluids have 87Sr/86Sr ratios ranging between 0.7072 and 0.7076 as measured from TSR-calcite and post-bitumen dolomite, being similar to contemporary seawater and the massive dolomite. One of the most important processes in the TSR-stage was freshwater production, which diluted the local basin waters from 20 wt.% NaCl equivalent to about 6 wt.% NaCl equivalent or even lower (down to 0.2 wt.% NaCl), being unusual for evaporate-related lithologies (Patterson and Kinsman, 1982; Das et al., 1990; Worden et al., 1996). Accordingly, the concomitant decrease of Sr2+ and SO2 4 concentrations most likely contributed to the free status of these divalent ions rather than in combination as SrSO4. During the advanced diagenetic stage in the Himalayan, the continuing tectonic uplift lowered the reservoir temperatures to 120 °C or lower. Diagenetic reactions in this realm were influenced by large volumes of CH4, H2S and CO2, and low temperature. As the formation water was cooled, TSR slowed with dwindling freshwater production and less SO2 4 consumption, and the constant dissolution effect led to TDS increase and Sr2+ enrichment, which was responsible for the precipitation of post-bitumen SrSO4 and SrCO3. Additionally, in this period, partial oxidation of H2S by SO2 or 4 hydrocarbons was involved in producing S0 (Orr, 1982; Machel et al., 1995; Machel, 2001) without further reaction with CH4. Then the free solid S was captured to form natural S-bearing immiscible inclusions in post-bitumen Sr-bearing minerals. Salinities of fluid inclusions in such minerals are close to the TDS of present gasfield waters, suggesting a fairly close resemblance of the two fluids and the end of Palaeo-water evolution to date in P3ch–T1f sour reservoirs in NE Sichuan Basin.

6. Conclusions (1) In the P3ch–T1f sour reservoirs in NE Sichuan basin, it is the combined effects of aragonite neomorphism, closed-system dolomitization of Sr-rich carbonates (sediments from aragonite seas) and TSR that is responsible for the accumulation of significantly high Sr2+ concentrations in present day formation waters. By contrast, a single mechanism cannot account for the volumetrically significant Sr2+. (2) Strontium enrichment in early diagenetic fluids, as indicated by the occurrence of pre-bitumen Sr-bearing minerals, is mostly attributed to aragonite–calcite conversion and dolomitization in a closed hydrodynamic setting. (3) In the subsequent TSR stage, the early bound Sr as well as additional Sr was constantly released into the palaeo-waters by dissolution of pre-bitumen celestite and Sr-rich solid CaSO4, the involvement of which also enriched the reducing diagenetic fluids with Ba2+ and Eu2+, resulting in a positive Eu anomaly and elevated Sr and Ba contents in post-bitumen calcite crystals. (4) The concomitant changes in volumes of TSR-water at different reaction rates, in response to formation temperature variations, have placed certain constraints on salinity fluctuations of the formation water and dissolution–precipitation processes of Sr-bearing minerals. The minor post-bitumen celestite and strontianite were precipitated in progressively concentrated local basinal water at the reservoir uplift stage in the late Himalayan, as is indicated by the associated double-spheroid elemental S-bearing immiscible inclusions.

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Acknowledgements This work is financially supported by united foundation of NSFC and China’s Petroleum Chemical Industry (Grant No. 40839906), State Key Lab of Petroleum Resources & Prospecting, China University of Petroleum and China National Funds for Distinguished Young Scientists (41125009). We thank Professor Simon Bottrell, an anonymous reviewer and the editor for giving us constructive suggestions, which improved the quality of the paper. References Alonso-Azcarate, J., Bottrell, S.H., Tritlla, J., 2001. Sulfur redox reactions and formation of native sulfur veins during low grade metamorphism of gypsum evaporites, Cameros Basin (NE Spain). Chem. Geol. 174, 389–402. Banner, J.L., 1995. Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis. Sedimentology 42, 805–824. Banner, J.L., Musgrove, M., Capo, R., 1994. Tracing ground-water evolution in a limestone aquifer using Sr isotopes: effects of multiple sources of dissolved ions and mineral-solution reactions. Geology 22, 687–690. Barker, C.E., 1991. A fluid inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer. Geology 18, 1003–1006. Bein, A., Land, L.S., 1983. Carbonate sedimentation and diagenesis associated with Mg-Ca-chloride brines; the Permian San Andres Formation in the Texas Panhandle. J. Sediment. Res. 53, 0243–0260. Bodnar, R.J., 1993. Revised equation and table for determining the freezing point depression of H2O–NaCl solutions. Geochim. Cosmochim. Acta 57, 683–684. Braitsch, O., 1971. Salt Deposits, Their Origin and Composition. Springer, Berlin. Brookins, D.J., 1989. Aqueous geochemistry of rare earth elements. In: Lipin, B.R., Mckay, G.A. (Eds.), Geochemistry and Mineralogy of Rare Earth Elements. Mineral. Soc. Am., pp. 201–225 (Chapter 8). Burke, W.H., Denison, R.E., Hetherington, E.A., Koepnick, R.B., Nelson, H.F., Otto, J.B., 1982. Variation of seawater 87Sr/86Sr throughout Phanerozoic time. Geology 10, 516–519. Butler, G.P., 1973. Strontium geochemistry of modern and ancient calcium sulfate minerals. In: Purser, B.H. (Ed.), The Persian Gulf. Springer-Verlag, pp. 423–452. Cai, C.F., Li, K.K., Li, H.T., Li, M., Chen, L.X., 2008. Evidence for cross formational hot brine flow from integrated 87Sr/86Sr, REE and fluid inclusions of the Ordovician veins in Central Tarim, China. Appl. Geochem. 23, 2226–2235. Cai, C.F., Li, K.K., Zhu, Y.M., Xiang, L., Jiang, L., Tenger, Cai, X.Y., 2010. TSR origin of sulfur in the Permian and Triassic reservoir bitumen in East Sichuan Basin, China. Org. Geochem. 9, 871–878. Cai, C.F., Worden, R.H., Bottrell, S.H., Wang, L.S., Yang, C.C., 2003. Thermochemical sulphate reduction and the generation of hydrogen sulphide and thiols (mercaptans) in Triassic carbonate reservoirs from the Sichuan Basin, China. Chem. Geol. 202, 39–57. Cai, C.F., Xie, Z.Y., Worden, R.H., Hu, G.Y., Wang, L.S., He, H., 2004. Methanedominated thermochemical sulphate reduction in the Triassic Feixianguan Formation East Sichuan Basin, China: towards prediction of fatal H2S concentrations. Mar. Petrol. Geol. 21, 1265–1279. Cross, M.M., Manning, D.A.C., Bottrell, S.H., Worden, R.H., 2004. Thermochemical sulphate reduction (TSR): experimental determination of reaction kinetics and implications of the observed reaction rates for petroleum reservoirs. Org. Geochem. 35, 393–404. Das, N., Horitaa, J., Holland, H.D., 1990. Chemistry of fluid inclusions in halite from the Salina Group of the Michigan basin: implications for Late Silurian seawater and the origin of sedimentary brines. Geochim. Cosmochim. Acta 54, 319–327. deVilliers, S., Nelson, B.K., Chivas, A.R., 1995. Biological controls on coral Sr/Ca and d18O reconstructions of sea surface temperatures. Science 269, 1247–1249. deVilliers, S., Shen, G.T., Nelson, B.K., 1994. The Sr/Ca-temperature relationship in coralline aragonite: influence of variability in (Sr/Ca)seawater and skeletal growth parameters. Geochim. Cosmochim. Acta 58, 197–208. Diener, A., Ebneth, S., Veizer, J., Buhl, D., 1996. Strontium isotope stratigraphy of the Middle Devonian: brachiopods and conodonts. Geochim. Cosmochim. Acta 60, 639–652. Ebneth, S., Diener, A., Buhl, D., Veizer, J., 1997. Strontium isotope systematics of conodonts: middle Devonian, Eifel Mountains, Germany. Palaeogeogr. Palaeoclimatol. Palaeoecol. 132, 79–96. Furter, W.F., 1978. Salt distillation review. Can. J. Chem. Eng. 25, 33–38. Gaswirth, S.B., Budd, D.A., 2007. The role and impact of regionally extensive freshwater–seawater mixing zones in the maturation of regional dolomite bodies within the proto Floridan Aquifer. Sedimentology 54, 1065–1092. Goldstein, R.H., Reynolds, T.J., 1994. Systematics of Fluid Inclusions in Diagenetic Minerals. SEPM Short Course 31, Tulsa. Hao, F., Guo, T.L., Zhu, Y.M., Cai, X.Y., Zou, H.Y., Li, P.P., 2008. Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China. Am. Assoc. Petrol. Geol. Bull. 92, 611–637. Hecht, L., Freiberger, R., Albert, G.H., Grundmann, G., Kostitsy, Y.A., 1999. Rare earth element and isotope (C, O, Sr) characteristics of hydrothermal carbonates: genetic implications for dolomite-hosted talc mineralization at Göpfersgrün (Fichtelgebirge, Germany). Chem. Geol. 155, 115–130.

2423

Heydari, E., 1997. The role of burial diagenesis in hydrocarbon destruction and H2S accumulation, Upper Jurassic Smackover Formation, Black Creek Field, Mississippi. Am. Assoc. Petrol. Geol. Bull. 81, 26–45. Heydari, E., Moore, C.H., 1989. Burial diagenesis and thermochemical sulfate reduction, Smackover Formation, southeastern Mississippi Salt Basin. Geology 12, 1080–1084. Huang, S.J., 1994. Carbon isotopes of Permian and Permian-Triassic boundary in Upper Yangtze platform and the mass extinction. Geochimica 23, 60–68 (in Chinese). Huang, S.J., Qing, H.R., Hu, Z.W., Wang, C.M., Gao, X.Y., Zou, M.L., Wang, Q.D., 2007a. The diagenesis and dolomitization of the Feixianguan carbonates of Triassic in NE Sichuan Basin: an overview. Adv. Earth Sci. 22, 495–503 (in Chinese). Huang, S.J., Qing, H.R., Hu, Z.W., Zou, M.L., Feng, L.W., Wang, C.M., Gao, X.Y., Wang, Q.D., 2007b. Influence of sulfate reduction on diagenesis of Feixianguan carbonate in Triassic, NE Sichuan Basin of China. Acta Sedimentol. Sin. 25, 815–824 (in Chinese). Huang, S.J., Qing, H.R., Hu, Z.W., Zou, M.L., Feng, L.W., Wang, C.M., Gao, X.Y., Wang, Q.D., 2007c. Closed-system dolomitization and the significance for petroleum and economic geology: an example from Feixianguan carbonates, Triassic, NE Sichuan basin of China. Acta Sedimentol. Sin. 23, 2955–2962 (in Chinese). Huang, S.J., Qing, H.R., Pei, C.R., Hu, Z.W., Wu, S.J., Sun, Z.L., 2006. Strontium concentration, isotope composition and dolomitization fluids in the Feixianguan Formation of Triassic, Eastern Sichuan of China. Acta Petrol. Sin. 22, 2123–2132 (in Chinese). Jiang, X.F., Gu, Z.D., Zhao, R.R., Zuo, Y.A., 2009. Geochemical characteristics and orign of formation water in Feixianguan formation around Kaijiang-Liangpingt Trough, Sichuan Basin. Natural Gas Explor. Dev. 32 (5–7), 17 (in Chinese). Kasprzyk, A., 2003. Sedimentological and diagenetic patterns of anhydrite deposits in the Badenian evaporite basin of the Carpathian Foredeep, southern Poland. Sediment. Geol. 158, 167–194. Korsch, R.J., Mai, H.Z., Sun, Z.C., Gorter, J.D., 1991. The Sichuan basin, southwest China: a late proterozoic (Sinian) petroleum province. Precamb. Res. 54, 45– 63. Korte, C., Jasper, T., Kozur, H.W., Veizer, J., 2005. d18O and d13C of Permian brachiopods: a record of seawater evolution and continental glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 224, 333–351. Korte, C., Jasper, T., Kozur, H.W., Veizer, J., 2006. 87Sr/86Sr record of Permian seawater. Palaeogeogr. Palaeoclimatol. Palaeoecol. 240, 89–107. Korte, C., Kozur, H.W., Bruckschen, P., Veizer, J., 2003. Strontium isotope evolution of Late Permian and Triassic seawater. Geochim. Cosmochim. Acta 67, 47–62. Krouse, H.R., Viau, C.A., Eliuk, L.S., 1988. Chemical and isotopic evidence of thermochemical sulfate reduction by light hydrocarbon gases in deep carbonate reservoirs. Nature 333, 415–419. Lacazette, A., 1990. Application of linear elastic fracture mechanics to the quantitative evaluation of fluid-inclusion decrepitation. Geology 18, 782–785. Lee, S.G., Lee, D.H., Kim, Y.J., Chae, B.G., Kim, W.Y., Woo, N.C., 2003. Rare earth elements as indicators of groundwater environment changes in a fractured rock system: evidence from fracture-filling calcite. Appl. Geochem. 18, 135–143. Li, G.J., 2008. The Research of Dolostone Reservoir Characteristics of the Feixianguan Formation of Lower Triassic in the NE Sichuan Basin. Chengdu University of Technology, Chengdu, Sichuan (in Chinese). Li, J., Xie, Z.Y., Dai, J.X., Zhang, S.C., Zhu, G.Y., Liu, Z.L., 2005. Geochemistry and origin of sour gas accumulations in the NE Sichuan Basin, SW China. Org. Geochem. 36, 1703–1716. Li, K., Cai, C., He, H., Jiang, L., Xiang, L., Huang, S., Zhang, C., 2011. Origin of palaeofluids in the Ordovician carbonates in Tahe oilfield, Tarim Basin: constraints from fluid inclusions and Sr, C and O isotopes. Geofluids 11, 71–86. Lin, Y.T., Pan, Z.R., 2001. Study on density and torming dassification of gas field brine. J. Salt Lake Res. 9 (3), 1–7 (in Chinese). Lin, Y.T., Yao, Y.C., Kang, Z.H., Wang, N.J., 2004. Study on the geochemical characteristics and resource significance of the highly mineralized potassiumrich brine in the Sichuan Xuanda salt basin. J. Salt Lake Res. 12 (1), 8–18 (in Chinese). Liu, D.H., Xiao, X.M., Xiong, Y.Q., Geng, A.S., Peng, P.A., Shen, J.G., Wang, Y.P., 2006. Origin of natural sulphur-bearing immiscible inclusions and H2S in oolite gas reservoir, Eastern Sichuan. Sci. China: Ser. D Earth Sci. 49, 242–257. Liu, W.H., Tenger, G.B., Zhang, Z.N., Zhang, J.Y., Zhang, D.W., Fan, M., Fu, X.D., Zheng, L.J., Liu, Q.Y., 2010. H2S formation and enrichment mechanisms in medium to large scale natural gas fields (reservoirs) in the Sichuan Basin. Petrol. Explor. Dev. 37, 513–522. Lowenstein, T.K., Timofeeff, M.N., Kovalevych, V.M., Horita, J., 2005. The major-ion composition of Permian seawater. Geochim. Cosmochim. Acta 69, 1701–1719. Lüders, V., Moller, P., Dulskl, P., 1993. REE fractionation in carbonates and fluorite. In: Moller, P., Lüders, V. (Eds.), Monograph Series on Mineral Deposits, vol. 30, pp. 133–150. Ma, Y.S., Fu, Q., Guo, T.L., Yang, F.L., Zhou, Z.Y., 2005. Pool forming pattern and process of the Upper Permian–Lower Triassic, the Puguang gas field, northeast Sichuan Basin, China. Petrol. Geol. Exp. 27, 455–461 (in Chinese). Ma, Y.S., Guo, T.L., Zhao, X.F., Cai, X.Y., 2007a. Formation mechanism of high-quality deep burial dolomite in Puguang Field. Sci. China Earth Sci. 37 (Suppl. 2), 43–52 (in Chinese). Ma, Y.S., Guo, X.S., Guo, T.L., Huang, R., Cai, X.Y., Li, G.X., 2007b. The Puguang gas field: new giant discovery in the mature Sichuan Basin, southwest China. Am. Assoc. Petrol. Geol. Bull. 91, 627–643. Ma, Y.S., Mou, C.L., Tan, Q.Y., Yu, Q., Wang, R.H., 2007c. Reef-bank features and their constraint to reservoirs of natural gas, from permian Changxing Formation to

2424

K. Li et al. / Applied Geochemistry 27 (2012) 2409–2424

Triassic Feixianguan Formation in Daxian-Xuanhan area of Sichuan Province. South China. Earth Sci. Front. 14, 182–192. Ma, Y.S., Zhang, S.C., Guo, T.L., Zhu, G.Y., Cai, X.Y., Li, M.W., 2008. Petroleum geology of the Puguang sour gas field in the Sichuan Basin, SW China. Mar. Petrol. Geol. 25, 357–370. Machel, H.G., 2001. Bacterial and thermochemical sulfate reduction in diagenetic settings-old and new insights. Sediment. Geol. 140, 143–175. Machel, H.G., 2004. Concepts and models of dolomitization—a critical reappraisal. In: Braithwaite, C., Rizzi, G., Darke, G. (Eds.), The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs. Geological Society (London) Special Publication 235, pp. 7–63. Machel, H.G., Krouse, H.R., Sassen, R., 1995. Products and distinguishing criteria of bacterial and thermochemical sulfate reduction. Appl. Geochem. 10, 373– 389. McArthur, J.M., Howarth, R.J., Bailey, T.R., 2001. Strontium isotope stratigraphy: LOWESS version 3: best fit to the marine Sr-isotope curve for 0–509 Ma and accompanying look-up table for deriving numerical age. J. Geol. 109, 155–170. Meng, X.X., 2003. Diagenesis and reservoirs of Lower Triassic Feixianguan Formation in NE Sichuan Basin. Southwest Petroleum University, Chengdu, Sichuan (in Chinese). Miiller, G., 1962. Zur Geochemie des Strotiums in Ozeanen Evaporiten unter pesonderer Beruoksichtigung der Sedimentaren Colestin-la-gerstatte von Hemmelte-West (Sub-Oldenburg). Geology 35, 1–90. Milliman, J.D., 1974. Marine Carbonates. Springer-Verlag, NewYork, p. 375. Minissale, A., Magro, G., Vaselli, O., Verrucchi, C., Perticone, I., 1997. Geochemistry of water and gas discharges from the Mt. Amiata silicic complex and surrounding areas (central Italy). J. Volcol. Geotherm. Res. 79, 223–251. Muchez, P., Nielsen, P., Sintubin, M., Lagrou, D., 1998. Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic-Cretaceous. Sedimentology 45, 845–854. Noth, S., 1997. High H2S contents and other effects of thermochemical sulfate reduction in deeply buried carbonate reservoirs: a review. Geol. Rundsch. 86, 275–287. Nothdurft, L., Webb, G.E., Kamber, B., 2004. Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones. Geochim. Cosmochim. Acta 68, 263–283. Orr, W., 1977. Geologic and geochemical controls on the distribution of hydrogen sulfide in natural gas. In: Campos, R., Goni, J. (Eds.), Advances in Organic Geochemistry. Enadisma, Madrid, Spain, pp. 571–597. Orr, W.L., 1982. Rate and mechanism of non-microbial sulfate reduction. Geol. Soc. Am. Ann. Conv., Abstr. Prog., 580. Patterson, R.J., Kinsman, D.J.J., 1982. Formation of diagenetic dolomite in coastal sabkha along Arabian (Persian) Gulf. Am. Assoc. Petrol. Geol. Bull. 66, 28–34. Qin, Z., 1998. Metallogenic characteristics of the celestite-type Sr deposit in Chongqing area. J. Chengdu Univ. Technol. 25, 224–232 (in Chinese). Rickabym, R.E.M., Schragm, D.P., Zondervan, I., Riebesell, U., 2002. Growth rate dependence of Sr incorporation during calcification of Emiliana huxleyi. Global Biogeochem. Cycles 16. http://dx.doi.org/10.1029/2001GB001408. Sandberg, P.A., 1983. An oscillating trend in Phanerozoic nonskeletal carbonate mineralogy. Nature 305, 19–22. Schiebel, W., 1978. New Strontium Deposit in Iran. Industrial Minerals No. 132. Simpson, G., Yang, C., Hutcheon, I., 1996. Thermochemical sulfate reduction: a local process that does not generate thermal anomalies. In: Ross, G.M. (Comp.), 1995 Alberta Basement Transects Workshop, Lithoprobe Report # 51, Lithoprobe Secretariat, Univ. British Columbia, pp. 241–245. Snorre, O., 1981. Formatian of celestite in the Wenlock, Osloregian Norway: evidence for evaporitic depositional environments. J. Sediment. Petrol. 60, 397– 410. Souissi, F., Sassi, R., Dandurand, J., Bouhlel, S., Hamda, S.B., 2007. Fluid inclusion microthermometry and rare earth element distribution in the celestites of the Jebel Doghra ore deposit (Dome Zone, northern Tunisia): towards a new genetic model. Bull. Soc. Geol. 178, 459–471. Stoessell, R.K., Moore, C.H., 1983. Chemical constraints and origins of four groups of Gulf Coast reservoir fluids. Am. Assoc. Petrol. Geol. Bull. 67, 896–906. Stoll, H.M., Schrag, D.P., 2000. Coccolith Sr/Ca as a new indicator of coccolithophorid calcification and growth rate. Geochem. Geophys. Geosyst. 1, 1999GC000015.

Tekin, E., Varol, B., Ayan, Z., Zeynep, A., Muharrem, S., 2002. Epigenetic origin of celestite deposits in the Tertiary Sivas Basin: new mineralogical and geochemical evidence. Neues Jahrbuch für Mineralogie 30, 289–318. Toland, W.G., 1960. Oxidation of organic compounds with aqueous sulfate. J. Am. Chem. Soc. 82, 1911–1916. Tritlla, J., Alonso-Azcarate, J., Bottrell, S.H., 2000. Molten sulphur-dominated fluids in the origin of a native sulphur mineralization in lacustrine evaporites from Cervera del Rio Alhama (Cameros Basin, NE Spain). J. Geochem. Explor. 69–70, 183–187. Veizer, J., 1989. Strontium isotopes in seawater through time. Ann. Rev. Earth Planet. Sci. 17, 141–167. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Goddéris, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater. Chem. Geol. 161, 59–88. Wang, M., Lu, S.F., Li, J.J., Xue, H.T., Song, J.Y., 2010. Generation history of the two gas-sources of Feixianguan Formation gas-pools in the NE Sichuan Basin and their relative contribution. Petrol. Explor. Dev. 37, 167–173. Wang, S.J., 2004. Digenesis and Reservoirs of Lower Triassic Feixianguan Formation in Tieshan area in NE Sichuan Basin. Southwest Petroleum University, Chengdu, Sichuan (in Chinese). Ward, W.C., Halley, R.B., 1985. Dolomitization in a mixing zone of near-seawater composition, late Pleistocene northeastern Yucatan Peninsula. J. Sediment. Res. 55, 407–420. Warren, J., 2000. Dolomite: occurrence, evolution and economically important associations. Earth-Sci. Rev. 52, 1–81. Wei, G.Q., Chen, G.S., Du, S.M., Zhang, L., Yang, W., 2008. Petroleum systems of the oldest gas field in China: neoproterozoic gas pools in the Weiyuan gas field, Sichuan Basin. Mar. Petrol. Geol. 25, 371–386. Wei, G.Q., Yang, W., Zhang, L., Jin, H., Wu, S.X., Shen, Y.H., 2005. Dolomization geneticmodel of Feixianguan Group oolitic beach reservoir in Northeast Sichuan Basin. Natural Gas Geosci. 16, 162–166. Worden, R.H., Smalley, P.C., 1996. H2S-producing reactions in deep carbonate gas reservoirs: Khuff formation, Abu Dhabi. Chem. Geol. 133, 157–171. Worden, R.H., Smalley, P.C., Cross, M.M., 2000. The influence of rock fabric and mineralogy on thermochemical sulfate reduction: Khuff Formation, Abu Dhabi. J. Sediment. Res. 70, 1210–1221. Worden, R.H., Smalley, P.C., Oxtoby, N.H., 1996. The effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate reservoirs. Geochim. Cosmochim. Acta 60, 3925–3931. Xu, X.G., Liao, G.Y., 1994. Geology and genesis of the Sr deposits in East Sichuan. Geol. Chem. Miner. 16 (1), 29–39 (in Chinese). Yang, C., Hutcheon, I., Krouse, H.R., 2001. Fluid inclusion and stable isotopic studies of thermochemical sulphate reduction from Burnt Timber and Crossfield East gas fields in Alberta, Canada. Bull. Can. Petrol. Geol. 49, 149–164. Zhang, B., 2010. Integrated Study on Reef and Shoal Facies Reservoir of the Changxing Formation in Eastern Sichuan-Northern Chongqing Area. Chengdu University of Technology, Chengdu, Sichuan (in Chinese). Zhang, S.C., Zhu, G.Y., Chen, J.P., Liang, Y.B., 2007. A discussion on gas sources of the Feixianguan Formation H2S-rich giant gas fields in the NE Sichuan Basin. Chin. Sci. Bull. 52 (1), 113–124 (in Chinese). Zhao, W.Z., Luo, P., Chen, G.S., Cao, H., Zhang, B.M., 2005. Origin and reservoir rock characteristics of dolostones in the Early Triassic Feixianguan Formation, NE Sichuan Basin, China: significance for future gas exploration. J. Petrol. Geol. 28, 83–100. Zhao, W.Z., Wang, Z.C., Wang, Y.G., 2006. Formation mechanism of highly effective gas pools in the Feixianguan Formation in the NE Sichuan Basin. Geol. Rev. 52, 708–718 (in Chinese). Zheng, R.C., Shi, J.N., Luo, A.J., Li, S., Li, G.L., 2007. Geochemical characteristics of dolomite reservoirs in NE Sichuan Basin. Nat. Gas. Ind. 28 (11), 1–6 (in Chinese). Zhu, C.Y., Wu, W.H., 1999. Stable isotopic characteristics and metallogenic mechanism of Huayingshan strontium mineralization belt. Geochimica 28, 197–202 (in Chinese). Zhu, G.Y., Zhang, S.C., Liang, Y.B., Ma, Y.S., Zhou, G.Y., Dai, J.X., 2006. Characteristics of Gas Reservoirs with high content of H2S in the NE Sichuan Basin and the consumption of hydrocarbons due to TSR. Acta Sedmentol. Sin. 24, 299–308 (in Chinese).