Petrographic, Rare Earth Elemental and Isotopic Constraints on the Dolomite Origin: A Case Study from the Middle-Upper Cambrian Xixiangchi Formation in Eastern Sichuan Basin, Southwest China

Abstract

The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high ⁸⁷Sr/⁸⁶Sr ratios (0.71062 ± 0.00122). In combination with δ¹³C (−1.5‰ ± 0.2‰) and δ¹⁸O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ¹³C (−1.4‰ ± 0.4‰) compositions but higher δ¹⁸O (−8.9‰ ± 0.7‰) compositions, and lower ⁸⁷Sr/⁸⁶Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ¹⁸O compositions (−10.4‰ ± 0.4‰) and ⁸⁷Sr/⁸⁶Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ¹⁸O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the ⁸⁷Sr/⁸⁶Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs.

1. Introduction

Numerous studies have reported that dolomite reservoirs account for a considerable proportion of the global carbonate reservoirs [1,2,3]. Due to the abundant oil and gas resources in the dolomite reservoir and its uneven distribution in geological history, the mechanism of dolomitization has always been a research hotspot. [4,5,6]. Previous work suggests that identifying the origin of dolomitizing fluid is the key to discussing the origin of dolomite [7,8,9]. Dolomite formed via multiple stages of dolomitization with distinct petrographic and geochemical characteristics served as a response to the variation of fluid chemistry and burial conditions during the diagenetic processes of carbonates. Besides traditional sedimentary and petrographic analyses, numerous studies have been devoted to geochemical analyses, such as trace elements (e.g., Na, Fe, Mn, and Sr) and stable or unstable isotopes (e.g., C, O, and Sr) behaviors in carbonates during the past few decades [10,11,12]. However, the signal of C, O, and Sr isotopes as well as trace elements in carbonates are vulnerable to heterochthonous inputs or subsequent diagenesis, resulting in multiple possible explanations of dolomite origin [13,14].

In recent decades, REEs have been widely used to trace the sources of dolomitizing fluid [14,15,16,17,18]. Previous studies have demonstrated that without the influence of non-carbonate components, the REEs partition pattern in dolomite can inherit the information of the original seawater from the precursor limestone [19,20]. In addition, the REEs partition pattern of dolomite is obviously affected by diagenesis only when the water-rock ratios of some non-seawater-like diagenetic fluids exceed 10⁴ [15,21], while non-seawater-like diagenetic fluids, such as continental water and hydrothermal fluid, have different REEs characteristics [18] that can be identified. In general, it is a relatively reliable method to trace the source of dolomitizing fluid by REEs.

The carbonate rock of the Middle-Upper Cambrian Xixiangchi Formation, which is an important potential exploration stratum in the Sichuan Basin, experienced extensive dolomitization. Base on comprehensive analysis of petrology, C, O, and Sr isotopes as well as major and minor elements, the previous studies proposed that the Xixiangchi Formation dolomites were mainly formed in penecontemporaneous dolomitization or seepage-reflux dolomitization during early-stage diagenesis, and partially experienced further recrystallization during burial [22,23,24]. In addition, some studies suggest that the saddle dolomite of Xixiangchi Formation is associated with hydrothermal fluid [25,26]. However, the study on the origin of Xixiangchi Formation dolomite is still insufficient, especially from the perspective of REEs. Therefore, this paper attempts to investigate the origins of dolomitizing fluids and the dolomitization mechanism of the Middle-Upper Cambrian Xixiangchi dolomites in the Eastern Sichuan Basin. The integrated analyzing methods include detailed petrographic examinations, REEs characteristics and isotope (C, O, and Sr) geochemistry. Specially, this study aims to (1) present and interpret the petrological and geochemical characteristics especially the REEs characteristics of Xixiangchi Formation dolomite in the Eastern Sichuan Basin; (2) discuss the sources of dolomitizing fluids and the origin of corresponding dolomites; and (3) further discuss the effect of different dolomitizations on potential dolomite reservoirs.

2. Geological Setting

The Sichuan Basin is located in Southwest China, which covers a total area of 26.0 × 10⁴ km² approximately (Figure 1a). The Sichuan Basin is also one of the major basins that is rich in hydrocarbon resources in China [27,28]. The basin is diamond-shaped with tectonic boundaries of the Longmenshan fold belt, the Emeishan-Liangshan fold belt, the Hubei-Hunan-Guizhou fold belt, the Dabashan fold belt, and the Micangshan uplift. It is a superimposed basin with multicycle evolutions, which experienced the Caledonian cycle, Hercynian cycle, Indosinian cycle, Yanshanian cycle, and Himalayan cycle, lying on the western margin of the Yangtze Block [14,29]. These cycles include two tectonic hydrothermal events in the periods from the Late Sinian to the Early Cambrian and the Middle Devonian to the Middle Triassic, namely the Xingkai taphrogenic movement and the Emei taphrogenic movement, respectively [26,30]. The study area is mainly located in the Sanhui region, Eastern Sichuan Basin, close to the Eastern Sichuan high-steep structural belt.

From the bottom to top, the Cambrian strata have been subdivided into six intervals (Figure 1b), including the Qiongzhusi Formation, the Canglangpu Formation, the Longwangmiao Formation, the Gaotai Formation, and the Xixiangchi Formation, which is also referred to as the Loushanguan Group [22,31]. In the Early Cambrian, the sedimentary environment generally evolved from shallow shelf facies in the Qiongzhusi period to the delta and shore facies in the Canglangpu period in response to the decrease in the sea level [32]. From the Early Cambrian Longwangmiao Period to the Middle-Late Cambrian Xixiangchi Period, the basin gradually evolved from a carbonate gentle slope to a semi- to restricted carbonate platform setting [33,34,35]. Specially, the Longwangmiao and Xixiangchi formations are believed to have developed favorable carbonate reservoirs [34].

The lithology of the Xixiangchi Formation in Sichuan Basin mainly consists of dolostone, with a minor interaction of terrigenous clastics sediments that mainly occur at the top and bottom [36]. The Xixiangchi Formation is developed within a transgression-regression cycle, which can be further divided into five third-order sequences [36]. The overall thickness of the Xixiangchi Formation tends to increase from Northwest to Southeast, the maximum of which is approximately 800 m to 900 m in the Eastern margin of the Sichuan Basin. Due to the influence of paleogeographic pattern and the presence of the Leshan-Longnvsi syndepositional paleouplift, and the upper part of the Xixiangchi Formation was largely eroded in the Western margin of the Sichuan Basin [37,38,39]. The Xixiangchi Formation in the study area is in conformable contact with the underlying Middle Cambrian Gaotai Formation, whereas it exhibits an erosional contact with the overlying Lower Ordovician Tongzi Formation.

3. Samples and Methods

A total of more than 100 samples were collected from the Sanhui section in the Eastern Sichuan Basin. During the Xixiangchi period, the Sanhui section was located in the restricted platform with relatively low-energy conditions as a whole. Its lithology was dominated by crystalline dolostone with a small amount of granular dolostone. It should be noted that, due to the low content of granular dolostone, and the fact that grains and cements may carry different geochemical information, this work will not discuss it as a separate type of dolostone. After careful description and macro-petrographic observation, 50 dolomite samples were selected for detailed investigation. Each sample was prepared into a thin section and a mirror-image slab for petrographic observation and micro-drill sampling when polished, respectively. The thin sections were first stained with Alizarin-Red S [41] for petrographic observation to differentiate dolomite from calcite. Then, Cathodoluminescence (CL) was conducted using a CL8200MK5 CL microscope with a beam voltage of ~15 kV and a beam current of ~330 μA, at the Chengdu Institute of Geology and Mineral Resources. A total of 15 selected samples were prepared into small cubes (~1 cm in diameter) for scanning electron microscopy (SEM) observation using a Quanta 250 FEG SEM equipment, with an accelerating voltage of 5 kV, at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology.

Based on the petrological observation above, approximately 1.1 g powdered sample was extracted from 37 mirror-image slabs using a low-speed microdrill for geochemical analyses. The stable isotope analysis and Sr isotope analysis were performed in the Experimental Center of the School of Geoscience, Yangtze University and the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, respectively, and the elemental geochemical analysis was completed at Nanjing Hongchuang Exploration Technology Service Co., Ltd., Nanjing, China.

For carbon and oxygen isotope determination, each powdered sample (~20 mg) was digested in concentrated (> 99%) orthophosphoric acid at 70 °C to extract CO₂ gas. Then the isotopic ratios of the CO₂ gas were measured with a DELTA V Advantage isotope ratio mass spectrometer. The isotope values were calculated according to the Pee Dee Belemnite (PDB) standard and recorded as conventional δ¹³C and δ¹⁸O notations, respectively. The analytical precision was monitored by repeated analysis of NBS-19. The analysis accuracies of C and O isotopes were better than 0.22‰ and 0. 37‰, respectively.

For strontium isotope analysis, each powdered sample (~50 mg) was digested in 2.5 N ultrapure HCl on an electric hot plate with 120 °C for approximately 24 h, then the Sr purification and separation of other cations was performed by via the quartz exchange column that packed with AG 50W-X 12 cation exchange resins. After that, the ⁸⁷Sr/⁸⁶Sr ratios were measured using a Thermo-Fisher Scientific Triton Plus mass spectrometer. The measured ⁸⁷Sr/⁸⁶Sr values were corrected according to the mass fractionation standard of ⁸⁶Sr/⁸⁸Sr = 0.1194 [42]. The analytical precision was monitored by repeated analysis of NBS 987. The mean measured value of NBS 987 was 0.710268 ± 0.000009 (2σ, n = 3).

For major element analysis, 0.6 g of drying sample powder mixed with Canadian Claisse flux (6.0000 g ± 0.3 mg 49.75Li₂B₄O₇:49.75LiBO₂:0.5%LiBr) was poured into the platinum crucible, and placed in the melting furnace at 1100 °C to melt. Then, the concentrations of the produced oxides were measured by an Axios^MAX X-ray fluorescence (XRF) spectrometry. The precision and accuracy of major element analysis is better than 6%.

For trace element analysis, 50 mg of sample powder was digested with 1.5 mL ultrapure HNO₃, 1.5 mL ultrapure HF, and 0.1 mL ultrapure HClO₄ in a Teflon bomb and evaporated to incipient dryness and then 3 mL HNO₃ was added and evaporated to dryness again. After that, 3 mL of 50% HNO₃ was added and the Teflon bomb was resealed and heated at 190 °C for 12 h. The final solution was transferred to a 100 mL polyethylene bottle, with the addition of 1 mL (Rh + Re) mixed standard solution (1 mg/L) and diluted with Milli-Q water to 100 g for determination by an inductively coupled plasma mass spectrometer (ICP-MS) (Elan DRC-e). The precision and accuracy of trace element analysis is better than 6%.

4. Results

4.1. Petrography

Based on the size, shape, and distribution of dolomite crystals [43,44], three types of matrix dolomites (D1, D2, and D3) and one type of cement dolomite (SD) were identified within the investigated Xixiangchi Formation in the Eastern Sichuan Basin. Combining the field observations with thin sections identification, the dominant matrix dolomite is D2, followed by D1, and the least is D3. In contrast, SD dolomite is relatively low in abundance and occurs primarily as fracture-filling in the matrix dolomite.

4.1.1. Very Fine to Fine Dolomite (D1)

In hand specimens, very fine to fine dolomite (D1) is generally dark gray to brown grey in color. According to detailed observation of thin sections under plane-polarized light (PPL) and SEM, D1 mainly consists of tightly packed planar-s to planar-e crystals with crystal size less than 30 μm (Figure 2a–d). Under CL, crystals exhibit non to very weak dull luminescence (Figure 2a). In places, D1 is characterized by horizontal laminas of dolomudstones in the thin sections (Figure 2b). Moreover, pyrites with a shape of cubic grain are locally present within the intercrystalline pores (Figure 2c).

4.1.2. Fine to Medium Crystalline Dolomite (D2)

In hand specimens, the fine to medium crystalline dolomite (D2) is generally gray to dark gray in color. From microscopic observation of thin sections, D2 is dominantly composed of planar-s crystals, with a crystal size ranging from 30 to 100 μm (Figure 2e–g). A portion of the crystals show dirty crystal surfaces (Figure 2e). Under SEM, some regular rhombohedral crystals and associated intercrystalline pores can be observed (Figure 2f). Furthermore, crystals show mottled dull-red luminescence under CL (Figure 2g).

4.1.3. Medium to Coarsely Crystalline Dolomite (D3)

In hand specimens, the medium to coarsely crystalline dolomite (D3) is light grey in color. According to microscopic observation of thin sections under PPL, D3 is chiefly dominated by nonplanar to planar-s crystals with crystal size between 100 and 300 μm (Figure 2h–j). D3 is commonly characterized by mosaic textures along the tightly packed dolomite crystals (Figure 2h). Locally, the grain ghosts of primary rock were retained (Figure 2h). Some crystals of D3 are truncated by stylolite (Figure 2i). Some crystals have a cloudy core with clear rims under PPL (Figure 2j). D3 resembles D2 under CL, displaying mottled dull-red luminescence in general (Figure 2j).

4.1.4. Saddle Dolomite (SD)

In hand specimens, the saddle dolomite (SD) is milk white in color. SD consists of nonplanar crystals, ranging in size from 0.3 to 4 mm, and presents cloudy crystal surfaces (Figure 2k). Under cross-polarized light (XPL), SD crystals exhibit undulatory extinctions (Figure 2l). Compared with the dull red luminescence of the matrix dolomites, SD commonly shows brighter red luminescence under CL.

4.2. Carbon and Oxygen Isotopes

The stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes of the selected 27 matrix dolomites and 10 saddle dolomites from the Xixiangchi Formation are summarized in

Table 1. Summary of δ¹³C, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, REE, and some element statistics of different types of dolomites in the Xixiangchi Formation. Note the δ¹³C, δ¹⁸O, and REE compositions of some D1, D2, and SD samples have been previously reported by Liang et al. [45]. See Tables S1–S4 for details.

Phase a	δ13CPDB/‰	δ18OPDB/‰	87Sr/86Sr	ΣLREE (μg/g)	ΣHREE (μg/g)	ΣREE (μg/g)	(Nb/Yb)SN	Eu/Eu*	Ce/Ce*	Mn (μg/g)	SiO2/%
D1
n	10	10	4	7	7	7	7	7	7	7	7
Mean	−1.5	−9.7	0.71062	7.2	0.8	8.0	1.3	0.65	0.84	121.4	3.4
St.dv	0.2	0.5	0.00122	2.3	0.3	2.6	0.1	0.03	0.02	61.5	1.6
Min.	−2.0	−10.3	0.70965	4.0	0.4	4.4	1.1	0.58	0.82	50.0	0.8
Max.	−1.2	−8.9	0.71270	10.5	1.2	11.5	1.5	0.69	0.87	260.0	6.3
D2
n	12	12	6	9	9	9	9	9	9	9	9
Mean	−1.4	−8.9	0.70992	6.5	0.7	7.2	1.2	0.85	0.87	191	2.1
St.dv	0.4	0.7	0.00035	2.0	0.2	2.2	0.2	0.04	0.04	45	1.0
Min.	−2.1	−10.0	0.70949	3.4	0.4	3.8	0.9	0.76	0.81	140	0.7
Max.	−0.9	−7.5	0.71064	11.0	1.2	12.2	1.4	0.90	0.94	270	3.9
D3
n	5	5	2	5	5	5	5	5	5	5	5
Mean	−1.4	−10.4	0.70989	5.1	0.6	5.6	1.2	0.88	0.88	208	1.7
St.dv	0.1	0.4	0.00048	1.3	0.1	1.4	0.2	0.02	0.02	88	0.8
Min.	−1.6	−11.0	0.70941	4.1	0.4	4.5	1.0	0.85	0.85	130	0.8
Max.	−1.2	−9.8	0.71037	7.7	0.7	8.4	1.5	0.92	0.91	380	3.2
SD
n	10	10	5	10	10	10	10	10	10	10	10
Mean	−2.1	−11.1	0.70941	4.8	0.8	5.6	0.8	0.70	0.83	272	1.3
St.dv	0.1	0.3	0.00003	1.4	0.3	1.7	0.0	0.01	0.01	55	0.7
Min.	−2.3	−11.6	0.70937	2.5	0.4	3.0	0.7	0.69	0.82	200	0.4
Max.	−2.0	−10.4	0.70945	8.0	1.4	9.4	0.9	0.72	0.85	340	2.8

^a D1: Very fine to fine dolomite; D2: Fine to medium crystalline dolomite; D3: Medium to coarsely crystalline dolomite; SD: Saddle dolomite.

(Table S1). The δ¹³C andδ¹⁸O values of D1 range from −1.2‰ to −2.0‰ (−1.5‰ ± 0.2‰, n = 10), and from −8.9‰ to −10.3‰ (−9.7‰ ± 0.5‰, n = 10), respectively. The δ¹³C and δ¹⁸O values of D2 lie within the range between −0.9‰ and −2.1‰ (−1.4‰ ± 0.4‰, n = 12) and between −7.5‰ and −10.0‰ (−8.9‰ ± 0.7‰, n = 12), respectively. The δ¹³C and δ¹⁸O values of D3 vary from −1.2‰ to −1.6‰ (−1.4‰ ± 0.1‰, n = 5) and from −9.8‰ to −11.0‰ (−10.4‰ ± 0.4‰, n = 5), respectively. For the cement SD, both of the δ¹³C and δ¹⁸O values are lower than those of the matrix dolomites, which cluster between –2.0‰ and −2.3‰ (−2.1‰ ± 0.1‰, n = 10) and between −10.4‰ and −11.6‰ (−11.1‰ ± 0.3‰, n = 10), respectively (Figure 3).

4.3. Strontium Isotope

The ⁸⁷Sr/⁸⁶Sr ratios from 17 dolomite samples are summarized in Table 1 (Table S2). A trend toward lower ⁸⁷Sr/⁸⁶Sr ratio is observed from D1 to D2, D3 and SD dolomites (Figure 4). The ⁸⁷Sr/⁸⁶Sr ratios of D1 range from 0.70965 to 0.71270 (0.71062 ± 0.00122, n = 4). D2 yields ⁸⁷Sr/⁸⁶Sr ratios from 0.70949 to 0.71064 (0.70992 ± 0.00035, n = 6). Later D3 shows ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70941 to 0.71037 (0.70989 ± 0.00048, n = 2). In contrast to the matrix dolomites, the ⁸⁷Sr/⁸⁶Sr ratios of the SD are lower, ranging from 0.70937 to 0.70945 (0.70941 ± 0.00003, n = 5).

4.4. Rare Earth Elements (REEs)

For comparison, the rare earth elements (REEs) concentrations of the investigated Xixiangchi dolomites are normalized by the post-Archaean Australian Shale (PAAS) [48], (Figure 5 and Figure 6; Tables S2 and S3). In addition, the Eu and the Ce anomalies of dolomites are calculated using the following equations: (1) Eu/Eu* = (3 × Eu_SN)/(2 × Sm_SN + Tb_SN) [49] and (2) Ce/Ce* = Ce_SN/(Pr_SN²/Nd_SN) [50] (Figure 7; Table 1 and Table S3).

4.4.1. REEs in Dolomite D1

The total REE concentrations (ΣREE) of D1 range from 4.4 to 11.5 μg/g, with an average of 8.0 ± 2.6 μg/g (n = 7), which is the highest among all types of dolomites measured (Table 1). The mean light REEs (LREEs, from 4.0 to 10.5 μg/g) concentrations and heavy REEs (HREEs, from 0.4 to 1.2 μg/g) concentrations are 7.2 ± 2.3 μg/g (n = 7) and 0.8 ± 0.3 μg/g (n = 7), respectively. The variation extent of (Nd/Yb)_SN ratio, reflecting the relative enrichment of LREEs and HREEs [20], varies from 1.1 to 1.5 (average 1.3 ± 0.1, n = 7), showing a slight enrichment of LREEs and depletion of HREEs. The REEs partition pattern of D1 is relatively flat (Figure 5 and Figure 6a). Moreover, D1 is characterized by slightly negative Ce anomalies of 0.84 ± 0.02 (from 0.82 to 0.87, n = 7) and obvious negative Eu anomalies of 0.65 ± 0.03 (from 0.58 to 0.69, n = 7) (Figure 7).

4.4.2. REEs in Dolomite D2

The ΣREE values of D2 range from 3.8 to 12.2 μg/g, with a mean value of 7.2 ± 2.2 μg/g (n = 9), which is lower than that of D1 (Table 1). The LREEs and HREEs concentrations of D2 are 6.5 ± 2.0 μg/g (between 3.4 μg/g and 11.0 μg/g, n = 9), and 0.7 ± 0.2 μg/g (between 0.4 μg/g and 1.2 μg/g, n = 9), respectively. The (Nd/Yb)_SN ratios of D2 are from 0.9 to 1.4 (1.2 ± 0.2, n = 9), indicating relatively enriched LREEs than HREEs. The REEs partition pattern of D2 also is relatively flat (Figure 5 and Figure 6b). The negative Ce anomaly is also weak with values ranging from 0.81 to 0.94 (0.87 ± 0.04, n = 9), and the negative Eu anomaly is weaker than that of D1 with values varying from 0.76 to 0.90 (0.85 ± 0.04, n = 9) (Figure 7).

4.4.3. REEs in Dolomite D3

The ΣREE values of D3 range from 4.5 to 8.4 μg/g, with a mean value of 5.6 ± 1.4 μg/g (n = 5), which is the lowest among all types of the investigated matrix dolomites (Table 1). LREEs and HREEs concentrations fall within the range of 4.1 μg/g to 7.7 μg/g (average 5.1 ± 1.3 μg/g, n = 5) and 0.4 μg/g to 0.7 μg/g (average 0.6 ± 0.1 μg/g, n = 5), respectively. The variation extent of (Nd/Yb)_SN ratios is from 1.0 to 1.5 (average 1.2 ± 0.2 μg/g, n = 5), indicating a slight LREEs enrichment, which is similar to that of D1 and D2 dolomites. The REEs partition pattern of D3 is relatively flat (Figure 5 and Figure 6c), which resembles the D1 and D2 dolomites above. The negative Ce anomaly is still weak, ranging from 0.85 to 0.91, with an average of 0.88 ± 0.02 (n = 5). The negative Eu anomaly is weaker than that of D1 and D2 dolomites, with values varying from 0.85 to 0.92 (average 0.88 ± 0.02, n = 5) (Figure 7).

4.4.4. REEs in Dolomite SD

The ΣREE concentrations of SD range from 3.0 to 9.4 μg/g (average 5.6 ± 1.7 μg/g, n = 10), which are close to those of dolomite D3. LREEs concentrations vary between 2.5 and 8.0 μg/g, with an average of 4.8 ± 1.4 μg/g (n = 10) (Table 1), while HREEs concentrations vary between 0.4 and 1.4 μg/g (n = 10), with an average of 0.8 ± 0.3 μg/g. Unlike the matrix D1 to D3 dolomites, the (Nd/Yb)_SN ratios of SD range from 0.7 to 0.9 with a mean value of 0.8 ± 0.04 (n = 10), showing the relative LREEs depletion and HREEs enrichment. The REEs partition pattern of SD is lightly left-leaning (Figure 5 and Figure 6d), which is obviously different from that of the matrix dolomites above. SD is characterized by a weakly negative Ce anomaly of 0.83 ± 0.01 (from 0.82 to 0.85, n = 10), and an obviously negative Eu anomaly of 0.70 ± 0.01 (from 0.69 to 0.72, n = 10) (Figure 7).

5. Discussion

5.1. Diagenetic Sequence

The petrological characteristics are used to reconstruct an appropriate diagenetic sequence in the Xixiangchi Formation (Figure 8). The pervasive distribution of the three types of matrix dolomites and some saddle dolomites suggest the limestone precursors probably experienced multiple dolomitization. Previous studies have shown that the Mn concentration can be used to provide constraints on diagenesis processes of carbonates [52,53]. From D1 to D2 and D3 dolomites, the coarser dolomite crystals combined with higher Mn concentrations (average 121.4 ± 61.5 μg/g, 191.1 ± 45.3 μg/g, 208.0 ± 88.0 μg/g, respectively) (Table 1 and Table S4) likely indicate the diagenetic environment changed from a near-surface environment to a burial environment with a higher temperature and pressure. At the same time, the gradual disappearance of original laminated textures (Figure 2b) and the presence of cloudy core with clear rims (Figure 2j) also agrees with this. It is generally believed that styolites are formed at a burial depth of 600–900 m [53]. Some crystals of D2 and D3 are truncated by stylolite (Figure 2i), suggesting that D2 and D3 dolomite were formed before entering the middle to deep burial environment. Moreover, the SD dolomite filling the fractures of matrix dolomites (Figure 2k), and yielding the highest Mn concentrations (average 272.0 ± 54.7 μg/g) (Table 1 and Table S4) and the brightest luminescence under CL, obviously postdates the matrix dolomites. Additionally, the pyrite (Figure 2c) filling the intercrystalline pores with a shape of cubic grain may be a product of the late diagenesis [54].

5.2. Assessing Sample Validity

It is indispensable to evaluate the validity of the samples before analyzing geochemical data, as diagenetic alteration and non-carbonate components would affect the accuracy of geochemical data for carbonates. Previous studies have shown that with the progress of diagenesis, Sr is gradually depleted in carbonates, while Mn is gradually enriched [52,53]. Therefore, it is generally believed that when the Mn/Sr ratio is less than 3, meaning that the carbonate has not undergone obvious diagenetic alteration [55]. When Mn/Sr ratios are less than 10.0, although carbonates have undergone a certain degree of diagenetic transformation, they usually retain the original marine information [56]. The Mn/Sr ratios of the matrix dolomites in this work range from 0.8 to 5.6 with an average of 3.0 (Table S4). Therefore, it can be inferred that these dolomites almost retain the original carbon and oxygen isotopic signatures. If the δ¹⁸O values of carbonates are less than −10‰, they may have undergone intense diagenetic alteration [57]. However, considering the δ¹⁸O values in the Middle-Late Cambrian seawater range from −10.5‰ to –6.9‰ [46], it can be inferred that carbonates may still preserve a primary environmental signal, even if the δ¹⁸O values are less than −10‰ [58]. Additionally, if the δ¹³C and δ¹⁸O values of marine carbonates are not significantly positively correlated, they are considered to retain the original carbon and oxygen isotopic signatures [11,46]. The result shows that there is no significant covariation of the δ¹³C and δ¹⁸O values of samples, suggesting that the effects of diagenesis on the isotopic values are minimal (Figure 3).

The REEs composition of carbonate is vulnerable to contaminated non-carbonate components during the depositional or diagenetic stage. These non-carbonate components, such as terrigenous detritus [59,60] and Fe-Mn oxides/hydroxides [61], commonly increase the ΣREE concentrations of carbonate. Consequently, such contamination may influence the REEs partition pattern of carbonates, displaying a flat pattern in general [20,62]. Therefore, it is necessary to assess the contamination of non-carbonate components on the REEs composition, while discussing its implication for the origin of dolomitizing fluids.

Firstly, the mean ΣREE concentrations of different types of dolomites from the Xixiangchi Formation are all far below 100 μg/g, demonstrating little impact of terrigenous detritus [63]. In addition, the concentrations of Zr, Sc, Th, and Hf can also be used as indicators for evaluating terrigenous detritus contamination [20,60,64]. The average concentrations of Zr, Sc, Th, and Hf of the investigated Xixiangchi dolomites are 0.05 μg/g, 0.52 μg/g, 0.21 μg/g, and 0.003 μg/g, respectively (Table S4), suggesting that the impact of terrestrial detritus is very limited [17,65]. Furthermore, dolomites contaminated by terrigenous detritus would result in a positive correlation between the (Nd/Yb)s_N values and ΣREE concentrations [66]. In the presented work, there is no significant covariation of the (Nd/Yb)s_N values and the ΣREE concentrations (Figure 9a), which excludes the possibility of terrigenous detritus contamination. Additionally, the correlation analysis of Sc concentrations and ΣREE concentrations can also be used to evaluate the degree of terrigenous detritus contamination of carbonates [20,67]. The result (Figure 9b) combined with the above analysis, shows that samples were slightly affected by terrigenous detritus. The mean SiO₂ contents of D1, D2, D3, and SD dolomites are 3.4%, 2.1%, 1.7%, and 1.1%, respectively (Table 1 and Table S4), indicating that their terrigenous detritus are dominated by silicate minerals, and D1 is most likely to be weakly affected by silicate minerals.

Secondly, Fe-Mn oxides/hydroxides are able to adsorb REEs [68,69], as a result of which the Fe and Mn concentrations may exhibit a positive correlation with REE concentrations. However, for the examined Xixiangchi dolomites in this work, the Fe and Mn concentrations display unobvious correlations with ΣREE concentrations (Figure 9c,d), indicating that the contamination derived from Fe-Mn oxides/hydroxides is limited or negligible. In summary, the REEs contamination by non-carbonate components is insignificant for the investigated Xixiangchi dolomites, which makes it feasible to discuss the origin of dolomitizing fluids and dolomites in the following section.

5.3. Ce Anomaly

The Ce anomaly in carbonates is widely used to reflect the redox condition of seawater due to its sensitivity to redox condition [19,51,70]. Under oxidizing conditions, soluble Ce³⁺ in seawater is easily oxidized to insoluble Ce⁴⁺ with a weak mobility. The Ce⁴⁺ tends to be adsorbed on the surface of sedimentary particles to fractionate Ce from other trivalence REEs, thus leading a negative Ce anomaly of the seawater from which carbonates are precipitated [51]. In addition, Ce⁴⁺ can be preferentially absorbed and incorporated into Fe-Mn oxides, organic matters, or clay particles [68,71], thus resulting in negative Ce anomalies in other marine sediments [65].

In the presented work, all the D1, D2, D3 and SD dolomites have weakly negative Ce anomalies of 0.84 ± 0.02, 0.87 ± 0.04, 0.88 ± 0.02 and 0.83 ± 0.01, respectively (Figure 7, Table 1). However, the Ce anomalies of D1, D2, and D3 dolomites are very close, indicating that the dolomitizing fluids may be of similar origin and the diagenetic condition may be weak oxidizing, such as a near-surface to shallow burial environment. In addition, the value of Ce anomalies increased from D1 to D2 and D3 dolomites, indicating a gradual weakening oxidizing trend at burial. In contrast, the negative anomaly of SD dolomite is most obvious, which contradicts the petrographic evidence if it indicates that SD was formed in a near-surface environment. Therefore, it is referred that the formation of SD dolomite is associated with other origins of fluids.

5.4. Eu Anomaly

Under reducing conditions, Eu³⁺ in the fluid can commonly be reduced to Eu²⁺, which is more likely to replace Ca²⁺ into the carbonate lattice. While the redox potential of Eu^{2 +}/Eu³⁺ is mainly controlled by the temperature [72], which is characterized by decreasing trend with an increasing temperature at burial. Generally, there are two needed conditions for the presence of positive Eu anomalies in carbonates, namely a reducing diagenetic environment and the temperature of diagenetic fluids over 200 °C [72]. Consequently, a positive Eu anomaly in dolomites is usually adopted as an indicator for the hydrothermal dolomitization event [73,74]. Notably, negative Eu anomalies can also be observed in dolomites when the hydrothermal fluids are related to the intermediate-acidic magma [75,76].

In the presented work, D1, D2, and D3 dolomites all show negative Eu anomalies with mean values of 0.65 ± 0.03, 0.85 ± 0.04, and 0.88 ± 0.02, respectively (Table 1). This is suggestive of their dolomitization processes, which were mainly influenced by original seawater instead of hydrothermal fluids. From D1 to D2 and D3 dolomites, the negative anomalies exhibit a weakening trend (Figure 7), indicating the increasing temperature of diagenetic fluids at burial. In addition, it should be pointed out that SD also shows no significant positive Eu anomalies (0.70 ± 0.01). However, combined with their petrological characteristics, the influence of hydrothermal fluids cannot be ruled out for SD dolomites. Furthermore, there are two possibilities accounting for the negative Eu anomaly for the SD dolomites in this work. The first one is that the temperature of the basinal fluid (less than 200 °C) is not high enough. The second is that the hydrothermal fluid itself is characterized by a negative Eu anomaly. If the second is the case for the Xixiangchi Formation SD dolomites, the fluid may be related to the contribution of magma associated with the Emei taphrogenic movement [76].

5.5. Origins of Dolomitizing Fluids

REE partition patterns of the normal marine carbonates, which is most similar to modern seawater, usually displays obvious left-leaning characteristics and negative Ce anomalies [20]. While the REEs partition patterns of D1, D2, and D3 dolomites show similar near-flat characteristics with weak LREE enrichments, weakly negative Ce anomalies and negative Eu anomalies (Figure 5 and Figure 6). This partition feature of the PAAS-normalized may be related to the variable REE atomic radii. It is indicated that the atomic radii decrease with the increasing atomic number, which means the LREEs have larger atomic radii than the HREE and are easier to replace Ca ions in the CaCO₃ lattice during diagenesis [77]. This may explain the depletion of HREE in the investigated dolomites relative to modern seawater. At the same time, from D1 to D2 and D3 dolomites, the degree of negative Eu anomalies decreased, indicating that the reducibility of dolomitizing fluids gradually increased. Meanwhile, this trend is coupled with decreasing ΣREE concentrations, which is indicative of a gradual evolution of the diagenetic environment from a near-surface to a burial environment. In addition, the SD dolomites have a left-leaning partition pattern which looks similar to that of seawater (Figure 5 and Figure 6d), which seems to be contradicted with their petrological characteristics. Therefore, a possible explanation for the fluid source is the dolomitizing fluid for SD dolomites is a mixture of seawater and hydrothermal fluid.

C, O, and Sr isotopes of dolomites are also commonly used to trace the origin of their dolomitizing fluids. Compared with the O isotopes, the C isotopic compositions are less susceptible to diagenetic alterations [78,79], which generally retain the signals of coeval seawater. In specific, O isotopic compositions have different fractionation coefficients in different mineral phases and are controlled by temperature and salinity [78]. As a result, the original signals of coeval seawater in dolomites tend to be overprinted by during diagenetic processes. The reported δ¹³C and δ¹⁸O values of the Middle-Upper Cambrian marine carbonate by Veizer et al. [46] varies from −2.1‰ to 0.7‰ and from −6.9‰ to −10.5‰, respectively. The δ¹³C values of D1, D2, and D3 dolomites (−1.5‰ ± 0.2‰, −1.4‰ ± 0.4‰, and −1.4‰ ± 0.1‰, Table 1) are bracketed by range of the Middle-Upper Cambrian marine carbonates, indicating that the diagenetic fluid may be seawater or seawater-derived fluid. The SD dolomites yield slightly lower δ¹³C values (−2.0‰~−2.3‰) than the Middle- Upper Cambrian carbonates, which may indicate the influx of some other diagenetic fluids. In contrast, the δ¹⁸O values of the D1, D2, D3, and SD dolomites (−9.7‰ ± 0.5‰, −8.9‰ ± 0.7‰, −10.4‰ ± 0.4‰ and −11.1‰ ± 0.3‰, Table 1) show relatively negative excursion characteristics than those of the Middle- Upper Cambrian marine carbonates. The negative excursion of D1 may be caused by the influence of fresh water in a near-surface environment, while the negative excursion of D2, D3, and SD dolomites may represent a diagenetic process with gradually increasing temperature. Previous studies have shown that, for the Phanerozoic carbonate rocks, δ¹⁸O is approximately 0, the meteoric water is approximately −5, and the burial fluids is −10 and lower [80]. If the Cambrian seawater is assumed as −8‰, burial fluids shall be assumed as −18‰ and lower. In general, the δ¹⁸O values of the Xixiangchi Formation dolomites show no significant negative excursion compared with those of the coeval seawater. This indicates that the formation of Xixiangchi Formation dolomites was mainly influenced by seawater rather than buried fluids.

The ⁸⁷Sr/⁸⁶Sr ratios of least-altered marine carbonates are considered to be identical to those of the coeval seawater [81]. The deviation in the ⁸⁷Sr/⁸⁶Sr ratios of dolomites from the global Sr isotopic variation curve indicates the existence of post-depositional diagenetic modifications, or a non-marine fluid source [82]. The variation in ⁸⁷Sr/⁸⁶Sr ratios of the Cambrian seawater is from 0.70840 to 0.70915 [47]. The ⁸⁷Sr/⁸⁶Sr ratios from D1 to D2, D3 and SD dolomites (0.71062 ± 0.00122, 0.70992 ± 0.00035, 0.70989 ± 0.00048 and 0.70941 ± 0.00003, Table 1), exhibiting a decreasing trend, are all higher than the value of coeval seawater (Figure 4). It is noted that such phenomenon may be caused by the incorporation of silicate minerals, which can be represented by the contents of SiO₂, Al, Ti, and Sc in bulk rocks. Overall, this is not the case for the investigated Xixiangchi dolomites since no significant correlation is reflected in Fig 10. However, the abnormally high ⁸⁷Sr/⁸⁶Sr value of D1 was clearly influenced by silicate minerals (Figure 10a,b).

Although the influence of terrigenous components on REES is slight, the ⁸⁷Sr/⁸⁶Sr ratios are very sensitive to terrigenous components [83]. The mean ⁸⁷Sr/⁸⁶Sr ratio of D1 dolomite is the highest in the matrix dolomites, which may be influenced by terrigenous components. This is consistent with REEs of D1 affected by a minor content of terrigenous components in a near-surface condition. By contrast, the mean ⁸⁷Sr/⁸⁶Sr ratios of D2 and D3 dolomites are lower than D1 dolomites and closer to those of seawater. Such phenomenon is attributed to the decreasing influence of terrigenous components, as the diagenetic environment is gradually closed at burial. Besides, the mean ⁸⁷Sr/⁸⁶Sr ratio of SD dolomites resembles to that of seawater, indicating that diagenetic fluids may be influenced by seawater-derived fluids and are barely affected by terrigenous components.

5.6. Petrogenesis of Dolomites

5.6.1. Petrogenesis of D1 Dolomite

D1 is composed of planar-s to planar-e, very fine to fine dolomite crystals (<30 μm), with a minor content of horizontal to low-angle algel laminas and some pyrites (Figure 2a–c). These laminas usually form in low-energy tidal-flat environments [84]. Such D1 dolomite (crystal size less than 20 μm) has been previously interpreted as primary dolomite [85]. However, due to the absence of gypsum and the non to very weak luminescence (Figure 2a), a significant evaporation can be excluded for D1 in a near-surface formation environment in the present work. D1 is characterized by the highest mean ΣREE value (8.0 ± 2.6 μg/g) of all the examined Xixiangchi Formation dolomites, flat partition patterns with slight LREE enrichments ((Nb/Yb)_SN = 1.3 ± 0.1) and weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02) (Figure 6a; Table 1). Such phenomenon suggests that dolomitizing fluid for D1 is associated with seawater-derived fluids. In addition, the weakly negative Ce anomaly, coupled with the strongly negative Eu anomaly (Eu/Eu* = 0.65 ± 0.03), reflects a weakly oxidized and low-temperature diagenetic environment. The δ¹³C values (−1.5‰ ± 0.2‰) are consistent with the Middle-Upper Cambrian carbonate signature, while the relatively negative δ¹⁸O values (−9.7‰ ± 0.5‰) may imply the influx of fresh water in the near-surface environment. The highest mean ⁸⁷Sr/⁸⁶Sr value (0.71062 ± 0.00122) may be attributed to the presence of silicate minerals in a near-surface environment. Therefore, based on the characteristics above, D1 is thought to be formed by penecontemporaneous dolomitization in the near-surface environment in which the Mg^{2 +} -rich fluids seem to be derived from seawater (Figure 11a).

5.6.2. Petrogenesis of D2 Dolomite

The D2 dolomite is the most abundant type of the investigated Xixiangchi dolomites. D2 is featured by planar-s, fine to medium dolomite crystals (30 to 100 μm). The nature of dirty crystal surfaces may be caused by incomplete metasomatism or residual microcrystalline dolomite formed by recrystallization [17] (Figure 2e). According to the mottled dull-red luminescence (Figure 2g), there exists a possibility of metasomatism for D2 in a near-surface or shallow burial environment. The mean ΣREE value (7.2 ± 2.2 μg/g) of D2 is slightly lower than that of D1, while the almost flat partition pattern is similar to that of D1 ((Nb/Yb)_SN = 1.2 ± 0.2; Ce/Ce* = 0.87 ± 0.04) (Figure 6b; Table 1), indicating similar dolomitizing fluids. The higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04) than those of D1, suggesting a relatively higher diagenetic temperature at burial than D1. In addition, the negative excursion of δ¹⁸O values of D2 (−8.9‰ ± 0.7‰) than D1 may be the result of fresh water interaction, which has less influence than that on D1 due to its deeper burial regime. A relatively low mean ⁸⁷Sr/⁸⁶Sr value (0.70992 ± 0.00035) indicates decreasing contamination of terrigenous components. Collectively, this nature of low mean ⁸⁷Sr/⁸⁶Sr value for D2 further confirms its more closed diagenetic environment than D1. Consequently, D2 dolomite is thought to be formed in relation to the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment with Mg²⁺-rich seawater-derived fluids migrating downwards (Figure 11b).

5.6.3. Petrogenesis of D3 Dolomite

Compared with D1 and D2 dolomites, the D3 dolomite is less widespread in the presented work. D3 is dominated by nonplanar to planar-s, medium to coarsely crystalline dolomite crystals (100 to 300 μm). The grain ghost of some D3 dolomites (Figure 2h) suggests that it may be formed by recrystallization of granular dolomite. The nature of the cloudy core with clear rims in D3 crystals indicates the replacement of a precursor carbonate (cloudy core) and cementation (clear rim) [3,87] (Figure 2j). Together with the relatively brighter mottled dull-red luminescence than D2 (Figure 2j), the incomplete metasomatism and recrystallization of the host carbonate rock can be inferred in D3. D3 has the lowest mean ΣREE value (5.6 ± 1.4 μg/g) among the three types of matrix dolomites, and similar REE partition patterns ((Na/Yb)_SN = 1.2 ± 0.2; Ce/Ce* = 0.88 ± 0.02) to those of D1 and D2 dolomites (Figure 6c; Table 1). In this way, the dolomitizing fluids for D1, D2, and D3 dolomites seem to evolve from similar origins. The less obviously negative Eu anomaly of D3 (Eu/Eu* = 0.88 ± 0.02) is close to that of D2, which suggests D3 was formed in a condition with slighter higher diagenetic temperatures than D2 at burial. This is supported by the more negative excursion of δ¹⁸O values (−10.4‰ ± 0.4‰) than D1 and D2. At the same time, the mean ⁸⁷Sr/⁸⁶Sr value (0.70941 ± 0.00048) is closer to the coeval seawater signature, reflecting the relatively closed diagenetic environment. Subsequently, D3 is thought to be formed by the overgrowth or recrystallization of the pre-existing dolomites in a shallow to medium burial environment in which the Mg²⁺ may derive from basinal fluids stored in the intercrystalline pores (Figure 11c).

5.6.4. Petrogenesis of SD Dolomite

A minor content of saddle dolomite (SD) cements are identified, which occlude large fractures of the matrix dolomites above. SD consists of nonplanar, coarsely dolomite crystals with undulatory extinctions (Figure 2k–l). Combined with brighter red luminescence compared to matrix dolomites (Figure 2l), the dolomitizing fluid for SD is likely different from that of matrix dolomites. This is supported by the different REE partition patterns of SD (Figure 6d; Table 1). If the hydrothermal dolomitization is adopted for D3, the resulted negative Eu anomaly (Eu/Eu* = 0.7 ± 0.01) is suggestive of a mixed dolomitizing fluid of seawater and hydrothermal fluids yielding negative Eu anomalies. Alternatively, the hydrothermal fluid may be associated with the contribution of magma produced during the period of Emei taphrogenic movement. In addition, the further negative excursion of δ¹⁸O values (−11.1‰ ± 0.3‰) than the matrix dolomite also indicates the existence of hydrothermal fluids. At the same time, the mean ⁸⁷Sr/⁸⁶Sr value (0.70941 ± 0.00003), which is closest to coeval seawater signature, suggests that the seawater accounts for a certain proportion of the dolomitization fluid. It is referred that the mixture of seawater may be facilitated by the connection of tectonic fractures, which may also explain the similarity of the Ce anomaly of SD dolomite to that of D1 dolomite. Based on the geochemical characteristics above, SD is thought to be formed by low-temperature hydrothermal dolomitization with the mixed fluid of seawater and hydrothermal fluids (Figure 11c).

5.6.5. Implications for Hydrocarbon Exploration

Dolomitization generally plays an important role in the development of carbonate reservoirs [88]. Compared with D1 and D3 dolomites, intercrystalline pores are more commonly observed in D2 dolomite, which has important implications for hydrocarbon exploration in the Xixiangchi Formation in the Eastern Sichuan Basin. The difference in molar crystal volume of dolomite and calcite and possibly dolomitization under closed or semi-closed conditions may account for the formation of the porous layer [89,90]. The D1 dolomite is formed in an open near-surface environment with sufficient Mg²⁺-rich fluids, where enables rapid dolomitization. This may be the explanation for its fine crystal size and tightly packed structure. In contrast, D2 dolomites formed in semi- to closed environments with Mg²⁺-rich seawater-derived fluids migrating downwards, making its crystals coarser than those of D1, which likely had a major role in the development of the D2 associated porosity of the few porous layers. At the same time, D2 may also be recrystallized from the pre-existing D1 dolomite, while no significant volume change is expected during recrystallization of dolomite. In this way, some of the D2 dolomite is relatively compact [89,90,91]. Additionally, since D3 dolomite is formed in a burial environment, it is related to the overgrowth or recrystallization of the pre-existing D1 and D2 dolomites. Thus, the D3 dolomite, which is larger in crystal size than D2 dolomite, has mosaic textures along the tightly packed dolomite crystals, and almost without intercrystalline pores. Therefore, the quality of potential dolomite reservoir in the Xixiangchi Formation is strongly controlled by the dolomitizing history of carbonates.

6. Conclusions

Petrographic examination of the investigated Xixiangchi Formation in the Eastern Sichuan Basin reveals three types of matrix dolomites (D1, D2, and D3) and one type of cement dolomite (SD), namely very fine to fine dolomite (D1), fine to medium crystalline dolomite (D2), medium to coarsely crystalline dolomite (D3) and saddle dolomite (SD). The matrix dolomites have similar near-flat REE partition patterns, while SD dolomite has a left-leaning REE partition patterns. From D1 to D2 and D3 dolomites, the degree of negative Eu anomalies decreased, the Mn concentrations and the degree of negative Ce anomalies increased in response to a gradual evolution of the diagenetic environment from a near-surface to a relative burial environment. Integrating the analyses of the regional geological setting, petrography, REE and C, O and Sr isotopes, the current work suggests:

(1)

D1 dolomite is composed by planar-s to planar-e crystals (< 30 μm), showing non to very weak luminescence. It is characterized by weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), which is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. This agrees with its higher ⁸⁷Sr/⁸⁶Sr ratios (0.71062 ± 0.00122) with possible contamination from silicate minerals. This is supported by its δ¹³C (−1.5‰ ± 0.2‰) and δ¹⁸O (−9.7‰ ± 0.5‰) compositions, which are almost bracketed by range of the Middle- Upper Cambrian marine carbonates, even though the influx of fresh water may lead to its slightly negative shift of δ¹⁸O values as compared;

(2)

D2 dolomite, featured by planar-s crystals (30–100 μm) with dirty surfaces, exhibits mottled dull-red luminescence. D2 dolomite has similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04) compared with D1, thus suggesting a relatively higher diagenetic temperature at burial. The lower ⁸⁷Sr/⁸⁶Sr ratios (0.70992 ± 0.00035) than D1 imply a decreased influence from terrigenous components with a deeper burial regime. Consequently, D2 is thought to be formed by the post-penecontemporaneous seepage-reflux dolomitization or be formed by recrystallization of D1 dolomite in a near-surface or a shallow burial environment. The dolomitizing fluids mainly come from seawater, which is confirmed by the δ¹⁸O excursion (−8.9‰ ± 0.7‰) within the range of the Middle-Upper Cambrian marine carbonates;

(3)

D3 dolomite is dominated by nonplanar to planar-s crystals (100–300 μm) with some cloudy core with clear rims, displaying relatively brighter mottled dull-red luminescence than D2. In contrast to D1 and D2 dolomites, the D3 dolomite has more negative δ¹⁸O compositions (−10.4‰ ± 0.4‰), similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) and slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02), implying a relatively higher diagenetic temperature at burial, which conforms to the lower ⁸⁷Sr/⁸⁶Sr ratios (0.70989 ± 0.00048). Consequently, D3 dolomite is regarded to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment;

(4)

SD dolomite, occurred as fracture filling (0.3–4 mm) cements, exhibits undulatory extinctions and brighter red luminescence. Both of the negative δ¹³C values (−2.1‰ ± 0.1‰) and δ¹⁸O values (−11.1‰ ± 0.3‰) than the Middle- Upper Cambrian marine carbonates suggest diagenetic products. The lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01), SD dolomite is interpreted to be formed by hydrothermal dolomitization with possible diagenetic fluids associated with the magma during the period of Emei taphrogenic movement. Moreover, the similar ⁸⁷Sr/⁸⁶Sr ratios (0.70941 ± 0.00003) as to those of coeval seawater and left-leaning REE partition patterns suggest a mixed origin of seawater as the dolomitizing fluid;

(5)

Variation of dolomitizing histories affects the possibility of different types of dolomite as potential reservoirs. Among the three types of the Xixiangchi matrix dolomites, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir.