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Article

Genesis of Dolomite Reservoir in Ediacaran Chigbrak Formation of Tarim Basin, NW China: Evidence from U–Pb Dating, Isotope and Element Geochemistry

by
Jianfeng Zheng
1,2,*,
Hui Wang
1,2,*,
Anjiang Shen
1,2,
Xianying Luo
1,2,
Zhao Cheng
3 and
Kun Dai
4
1
PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China
2
Key Laboratory of Carbonate Reservoirs CNPC, Hangzhou 310023, China
3
PetroChina Tarim Oil Field Company, Korla 841000, China
4
School of Earth Sciences, China University of Petroleum (Beijing), Beijing 102249, China
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(6), 725; https://doi.org/10.3390/min13060725
Submission received: 13 April 2023 / Revised: 18 May 2023 / Accepted: 23 May 2023 / Published: 25 May 2023
(This article belongs to the Special Issue Deposition, Diagenesis, and Geochemistry of Carbonate Sequences)
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Abstract

:
The Chigbrak Formation in the Upper Ediacaran is one of the important exploration targets in the Tarim Basin, NW China. However, no significant discoveries have been made in this field, and unclear reservoir genesis is one of the important factors restricting exploration breakthrough. This study examined the outcrops of the Aksu area in northwestern Tarim Basin by using systematic descriptions of petrologic features in the Upper Ediacaran Chigbrak Formation. Samples were selected for tests of stable carbon and oxygen isotopic compositions, strontium isotopic compositions, rare earth elements, LA–ICP–MS element mapping and U–Pb dating. It was found that (1) the Chigbrak Formation is mainly composed of dolomitic microbialite, with average values of δ13C (PDB), δ18O (PDB) and 87Sr/86Sr of 3.50‰, 2.95‰ and 0.709457, and has similar geochemical characteristics to the coeval seawater. The dolomites have also been characterized by a medium degree of cation ordering (avg. 0.68), a low content of ΣREEs (avg. 9.03 ppm) and a chondrite standardized curve of REEs showing enrichment of LREE and depletion of HREE. The U–Pb ages range from 538 to 618 Ma, corresponding to the age of Ediacaran period. (2) Dolomitization occurred in a marine diagenetic environment during the penecontemporaneous period, with seawater as the dolomitization fluid. (3) Vugs are the dominant pore type of Chigbrak Formation, and they are the products of the dissolution of meteoric water in penecontemporaneous period. (4) The main controlling factors of reservoir were lithofacies, meteoric water dissolution controlled by fourth– or fifth–order sequences and tectonic movement, and early dolomitization. The research results are of great significance to the dolomite reservoir prediction of the Upper Ediacaran Chigbrak Formation of the Tarim Basin.

1. Introduction

In recent years, dramatic breakthroughs have been achieved in the Precambrian hydrocarbon exploration globally. In Russia, about 50 oil and gas fields have been discovered in the Meso–Neoproterozoic Vendian and the Mesoproterozoic Riphean within the Siberian craton, with a total of proved and probable reserves of about 25.12 × 108 m3. In southern Oman, nine Precambrian carbonate reservoirs have been found within the Havel–Kloosterman area, with proved oil reserves of about 3.5 × 108 m3 [1]. In China, a Proterozoic buried hill reservoir has been found in Renqiu of the Jizhong depression, with proved oil and gas reserves of about 6.15 × 108 m3. The Anyue gas field producing from the Upper Ediacaran Dengying Formation has been found in the Leshan–Longnüsi paleo–uplift of the Sichuan Basin, with proved gas reserves of 5940 × 108 m3 [2].
In the Cambrian pre–salt ultra–deep (>6500 m) strata in the Tarim Basin, two sets of thick dolomite reservoirs have been identified, including the Upper Ediacaran Chigbrak Formation and the Middle–Lower Cambrian, which form favorable reservoir–caprock assemblages with the Cambrian–Precambrian high quality source rocks and the Cambrian gypsum–salt caprocks. Their oil and gas resources are expected to reach about 10.4 × 109 m3, showing huge exploration potential [3,4,5]. Breakthroughs in the Middle–Lower Cambrian were successful in obtaining economically viable oil and gas flows from Wells ZS1 and LT1 in 2012 and in 2019, respectively [6,7]. In contrast, the Upper Ediacaran is still in an exploration stage without any large scale reservoirs found. Only 12 wells have been drilled in the Upper Ediacaran Chigbrak Formation, including LT1, QT1, KT1, XH1, TD2 and DT1. The limited well data limit the geological understandings of the Chigbrak Formation. In recent years, an increasing number of studies have been conducted, and some important findings have been obtained. For example, the passive continental margin environment that was formed in the early Ediacaran continued in the uplifted zone of the Bachu–Central Tarim–Southeastern Tarim area during the late Ediacaran, and the Chigbrak Formation developed within carbonate ramp depositional systems [8,9,10]. In the outcrop area at the northwestern margin of this basin, the Chigbrak Formation is mainly composed of dolomitic stromatolites, thrombolites, etc., and is dominated by vuggy porosity [11,12,13]. Sedimentary facies, weathering crust karstification and hydrothermal dissolution have been identified as the main factors controlling the genesis of reservoirs [14,15,16]. Controversies on the genesis of dolomite reservoirs [17,18,19] and limited knowledge of the genetic mechanism of reservoir pores in the Chigbrak Formation restrict the prediction of favorable reservoirs.
This paper discusses the genesis of dolomite and the reservoir control factors in the Upper Ediacaran Chigbrak Formation in the Aksu outcrop area at the northwestern margin of the Tarim Basin by using 175 samples and through thin section observations, tests of stable carbon and oxygen isotopic compositions, strontium isotopic composition, rare earth elements, laser ablation inductively–coupled–plasma mass spectrometry (LA–ICP–MS) element mapping and U–Pb dating. The research results are of great significance in identifying the genesis and distribution of reservoirs in the Chigbrak Formation of the Tarim Basin.

2. Geological Setting

The Tarim Basin in southern Xinjiang, China (Figure 1a), is a multi–cycle petroliferous basin superimposed by the Paleozoic craton basin and the Mesozoic and Cenozoic foreland basins [20], covering an area of 56 × 104 km2. According to the division of present–day structural units, the basin includes seven first–order structural units, i.e., the Tabei Uplift, the Central Uplift, the Southeast Uplift, the Kuqa Depression, the North Depression, the Southeast Depression and the Southwest Depression (Figure 1b).
Tectonic evolution of the Tarim landmass in the Precambrian period was closely related to the amalgamation–breakup cycles of the Colombia supercontinent and the Rodinia supercontinent [21,22]. At the end of the Neoproterozoic period, affected by the breakup of the Rodinia supercontinent, the continental extension that dominated the Cryogenian–early Ediacaran period led to the rift system development stage in the Tarim landmass, when continental rift basins were formed in its interior and margin and a set of thousand–meter–thick clastic rocks with basic igneous rock was deposited. In the late Ediacaran period, along with the continuous spreading of the rift, depression appeared due to the subsidence of the peripheral areas of the ocean basin, forming the carbonate platform–dominated depressed basin [23,24]. The vast areas to the north of the Central Uplift were covered by the ramp–type carbonate platform sedimentary system [25]. The thick dolomites of the Chigbrak Formation were formed penecontemporaneously, and their sedimentary differentiation was controlled by the earlier paleogeomorphology and differential subsidence of fault. At the end of the Ediacaran period, intensive uplifting within the Tarim landmass induced by the Kalpin movement led to weathering and subsequent dissolution of the dolomites at the top of the Chigbrak Formation. Thus, a regional unconformity was formed between Ediacaran and Cambrian.
Structurally, the study area is a part of the eastern segment of Kalpin fault–uplift zone in the North Tarim Uplift [26,27], and has a complete Ediacaran outcrop (Figure 1c). The Sugetbrak Formation is in conformable contact with the Chigbrak Formation (Figure 2a), the Lower Cambrian Yurtus Formation has a parallel unconformable contact with the Chigbrak Formation (Figure 2b) and the Scholbrak Formation is in conformable contact with the Yurtus and Usugar Formations [28]. In Aksu outcrop area, the Chigbrak Formation is about 173.5 m thick, and divided into four members from bottom to top according to color, lithology and sedimentary structure (Figure 3). Member Ch–1 at the bottom is 42.6 m thick, and mainly composed of gray and light gray dolomitic stromatolites with horizontal layered (Figure 4a), columnar (Figure 4b) and hummocky structures, intercalated with dolomitic oolite (Figure 4c), grainstone with a bonding structure (Figure 4d) and thin layers of yellowish gray and grayish purple argillaceous dolomite, quartz sandstone and argillite (Figure 4e,f). It shows the depositional characteristics of upper intertidal–supratidal environment. Member Ch–2 is 27.3 m thick, and mainly composed of medium–thin layers of grey dolomitic stromatolites with horizontal and wavy structures alternated with dolomitic oolite, grainstone with a bonding structure and dolomitic thrombolites. It shows the depositional characteristics of relatively high energy intertidal environment. Member Ch–3 is 93.1 m thick, and mainly composed of medium–thick layered dolomitic thrombolites with horizontal bedding (Figure 4g) alternated with dolomitic foamy microbialite (microscopically, it has no granular, clotted structure, but looks like foam) (Figure 4h), and transitions up–section into dolomitic grainstone with a bonding structure at its top. It has the depositional characteristics of a high energy subtidal environment. Member Ch–4 is 10.5 m thick, and mainly develops solution–brecciated dolomite as a result of strong weathering crust karstification (Figure 4i,j). Similar to member Ch–3, its original rocks mainly include dolomitic thrombolites and foamy microbialite. According to the gamma ray (GR), different members have different response characteristics (Figure 3): member Ch–1 has the highest GR values with obvious fluctuations, member Ch–2 and member Ch–3 have low GR values, and member Ch–4 has slightly higher GR values than member Ch–2 and member Ch–3. Pores are developed extensively in all four members, especially acicular–dissolved pores, millimeter–size dissolved pores and centimeter–decimeter–size vugs, which are common. The acicular–dissolved pores (Figure 4k) are developed in dolomitic grainstone and foamy microbialite, the millimeter–size dissolved pores and centimeter–size vugs (Figure 4l) are mainly observed in dolomitic stromatolites and dolomitic thrombolites, and the decimeter–size vugs are distributed in the brecciated dolomite in member Ch–4 (Figure 4m). In the upper of member Ch–3, the vugs are distributed along the layers, and often filled with multi–phase cements. Due to the fibrous dolomite cement and laminated dolomite cement have the characteristics of isopachous rim, the edges of vugs have a “laces structure” (Figure 4n,o).

3. Samples and Methods

A total of 104 hand specimens were taken from the Upper Ediacaran Chigbrak Formation in the Xigou section. All of them are fresh and have never been weathered or altered. Casting thin sections were prepared for all of the samples, correspondingly. Based on macroscopic characteristics of these hand specimens and the microscopic observation results of casting thin sections, 22 dolomite samples were selected for whole rock detection and analysis of stable carbon and oxygen isotopic compositions (δ13C and δ18O), strontium isotopic composition (87Sr/86Sr), degree of dolomite cation ordering and rare earth elements (REEs) content. In order to minimize the interaction between carbonate cements and host rocks, single structural components were drilled from all samples using a small micro–sampling drill and ground to a powder of 200 mesh with an agate mortar. Then, the sample powder was packaged with transparent drawing paper. In addition, 4 porous samples were chosen for δ13C, δ18O and 7Sr/86Sr analysis, laser ablation U–Pb dating and LA–ICP–MS element mapping (Figure 5). The elements of imaging are Mg, Ca, Mn, Fe, Sr, Ba, Th, U, Y, La, Ce, Eu and Lu. For δ13C and δ18O analysis, 60 μm–thick thin sections were prepared, and for 7Sr/86Sr analysis, laser ablation U–Pb dating and LA–ICP–MS element mapping, 100 μm–thick thin sections were prepared. Both of them were single–side polished and then purified in a super–clean laboratory.
Laser ablation δ13C, δ18O and 7Sr/86Sr analysis, whole rock δ13C, δ18O and 7Sr/86Sr measurements and the determination of order degree of dolomite were carried out in Key Laboratory of Carbonate Reservoirs, CNPC (China National Petroleum Corporation), Hangzhou, China. A Nd.YAG laser ablation system and a RESOlution S–155 laser ablation system were used for δ13C, δ18O and 7Sr/86Sr analysis, with laser spot diameters of 20 μm and 160 μm, respectively. Measurement of C and O isotopes was conducted using a Delta V Advantage IRMS with error ranges of ±0.06‰ and ±0.08‰, respectively, and the carbonate standard samples of GBW4405 and GBW4406 were used. Measurement of the Sr isotope was carried out using a Neptune Plus MC–ICP–MS, when two self–developed standard samples, Hailuo and XK1, were used, and a precision of 0.01% was obtained. Whole rock Sr isotopes were measured with a Isotope Ratio Mass Spectrometry (Triton Plus, Thermo Scientific, Germany), when the carbonate standard reference material of GBW04411 was used, and a precision better than 0.01% was obtained. Degree of dolomite cation ordering was measured using an X′pert Pro X–ray diffractometer, with a relative error < 10%, and the X–ray diffraction peak ratio of (015)/(110) was used to calculate cation ordering [29]. An ICP–MS method was used to analyze the REEs in Tongwei Analytical Technology Co., Ltd., Guizhou, China, using an inductively coupled plasma mass spectrometry (ICP–MS). Two international standard samples, W–2a and BHVO–2, were used, and a precision and an accuracy > 5% were obtained. Laser ablation U–Pb dating was performed in the Radiogenic Isotopes Laboratory, University of Queensland, Australia, using an ASI RESOlution SE laser ablation system with laser spot diameter of 100 μm, together with a Nu Plasma II MC–ICP–MS, the calibration is done by use of two international standard samples, NIST 614 and WC–1. Iolite 3.6 (University of Melbourne, Australia) was used for processing the LA–ICP–MS data acquired, and Isoplot3.0 (University of California Berkeley, CA, USA) was used to calculate the ages and map the Tera–Wasserburg concordia diagram. LA–ICP–MS trace element mapping was also performed in the Radiogenic Isotopes Laboratory, University of Queensland, Australia. An ASI RESOlution SE laser ablation system, where square laser spot with length of side of 50 μm and speed of movement of 0.05 mm/s, was applied together with a Thermo iCap–RQ ICP–MS, the calibration is done by use of International standard sample NIST 614. Element distribution maps were established with the raw data processed by Iolite 3.6.
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Figure 5. Photographs of test samples in the Chigbrak Formation (the red dotted boxes are the elements mapping area, the numbers with circles are the age, δ13C, δ18O and 87Sr/86Sr test points of different fabrics, corresponds to the numbers with circles in Table 1): (a) sample 56–1; (b) sample 58–1; (c) sample 76–1; (d) sample 81–4.
Figure 5. Photographs of test samples in the Chigbrak Formation (the red dotted boxes are the elements mapping area, the numbers with circles are the age, δ13C, δ18O and 87Sr/86Sr test points of different fabrics, corresponds to the numbers with circles in Table 1): (a) sample 56–1; (b) sample 58–1; (c) sample 76–1; (d) sample 81–4.
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4. Results

4.1. Stable Carbon and Oxygen Isotopic Compositions

Carbon and oxygen isotopes are related to the fluids causing dolomitization, which is mainly affected by salinity and temperature. Consequently, it serves as an effective indicator of dolomitization fluid properties and diagenetic environment. According to the whole rock δ13C and δ18O values (all data are PDB standard) of 22 samples (Table 2) and their scatter spots (Figure 6a), no significant correlation was shown between δ13C and δ18O values. This indicates that the samples have experienced weak transformation in the late diagenetic period and remained the basic information of the diagenetic fluids. For the Chigbrak Formation dolomites, δ13C ranges from 1.31‰ to 6.65‰, with an average of 3.50‰, and δ18O ranges from −4.81‰ to −1.23‰, with an average of −2.95‰. These values are generally similar to those of the late Proterozoic marine carbonates [30], and also distributed within the value ranges of the late Ediacaran seawater [15]. The values of δ13C reach the highest in member Ch–1 (4.21‰~6.18‰, avg. 5.50‰) and the lowest in member Ch–4 (1.31‰~2.40‰, avg. 1.93‰), and are almost the same in the other two intervals (1.68‰~3.48‰, avg. 2.47‰). The values of δ18O reach the lowest in member Ch–4 (–4.65‰~−4.05‰, avg. −4.34‰), are relatively high in member Ch–2 (−2.23‰~−1.39‰, avg. −1.76‰) and vary widely in member Ch–1 (–4.06‰~–1.23‰, avg. –2.56‰) and member Ch–3 (−4.81‰~−2.03‰, avg. −3.50‰).
According to the δ13C and δ18O values of four samples determined by the laser microsampling method (Table 1) and their scatter plots (Figure 6a), δ13C values of the host rocks and cements in the pores are generally consistent with those of the whole rock samples from Ch–3, and only slightly smaller in granular dolomite (avg. 1.48‰). The values of δ18O change greatly in the original rocks and different types of cements, and gradually decrease from the margin to the center of the pores. The averages of δ18O in the host rocks of dolomitic microbialite, laminated/fibrous dolomite cements, granular/bladed dolomite cements and saddle dolomite cements are −4.31‰, −5.09‰, −7.49‰ and −10.13‰, respectively.

4.2. Strontium Isotopic Composition

Strontium isotopic composition in carbonate rocks is affected by diagenetic environment and fluids, and is one of the important indicators of paleoclimate and diagenetic fluids [31]. According to the detected whole rock 87Sr/86Sr measurements of 22 samples (Table 2) and the scatter plot between 87Sr/86Sr and δ18O (Figure 6b), the values of 87Sr/86Sr range from 0.708745 to 0.710885, with an average of 0.709457. Similar to the values of δ13C, these values reach the highest in member Ch–1 (0.709334~0.710885, avg. 0.710069) and are almost consistent in the remaining intervals (0.708745~0.709419, avg. 0.709107). In general, the values of 87Sr/86Sr in the Chigbrak Formation in the study area are slightly higher than those of the coeval seawater, which range between 0.7087 and 0.7090 [32,33]. It is suggested that this is related to dolomite samples generally have higher values of 87Sr/86Sr than limestone ones [34]. The relatively high 87Sr/86Sr values of member Ch–1 samples may be explained by their sedimentary environment of a tidal flat, where crust–derived Sr was mixed into the sediments.
According to the 87Sr/86Sr measurements of the four samples determined by the laser microsampling method (Table 2) and the scatter plot between 87Sr/86Sr and δ18O (Figure 6b), the values of 87Sr/86Sr in the host rocks and other types of dolomite in the pores–except for saddle dolomite, whose values of 87Sr/86Sr are slightly higher (avg. 0.70936)–are generally distributed within the 87Sr/86Sr variation range of the coeval seawater. The values of 87Sr/86Sr are approximate in the host rocks of dolomitic microbialite (avg. 0.70883) and laminated/fibrous dolomite cements (avg. 0.70880), slightly higher in granular/bladed dolomite cements (avg. 0.70901) and exhibit an increasing trend from margin to center of the pores.

4.3. Degree of Dolomite Cation Ordering

Dolomite is a tripartite mineral whose lattice parameters are affected by composition, temperature and pressure [35]. The ideal crystal structure of dolomite is that Ca2+ and Mg2+ are arranged alternately along the C–axis and the molar percentage of Ca2+ and Mg2+ is the same, while in disordered dolomite, Ca2+ and Mg2+ are completely randomly distributed as the same structure of calcite. Generally, the order degree value greater than 0.8 is defined as high ordering, 0.6~0.8 is subordering, 0.4~0.6 is low ordering. Less than 0.4 is disordered [36,37], which can be used to indicate the crystallization rate, crystallization temperature and evolution of dolomite. According to the measurement results (Table 2), the degree of dolomite cation ordering in the Chigbrak Formation ranges from 0.60 to 0.77, and increases gradually from bottom to top, which is the opposite of the variation trend of δ18O (PDB) value. Its average value is 0.68, which belongs to the subordering range.

4.4. Rare Earth Elements

The relative abundance of REEs in carbonate minerals mainly depends on the content and geochemical properties of REEs in fluid [38]. Thus, it has acted as an effective tool for determining the source of dolomitization fluid. The analysis results (Table 3) reveal that the total REEs (∑REEs) are remarkably higher in member Ch–1 (2.88~22.34 ppm, avg. 13.11 ppm) and member Ch–4 (2.88 ppm~22.34 ppm, avg. 13.11 ppm) than in member Ch–2 and member Ch–3 (0.36 ppm~2.51 ppm, avg. 1.22 ppm). The results were standardized by chondrite, and the standard normalized (SN) element anomalies were calculated by the following equations: δCe = 2 × CeSN/(LaSN + PrSN) and δEu = 2 × EuSN/(SmSN + GdSN) [39]. Calculating δCe and δEu as larger than 1.2 indicates a positive anomaly, and less than 0.8 indicates a negative anomaly. The standardized REE distribution pattern (Figure 7) shows that the Chigbrak Formation dolomites are characterized by enrichment of LREEs (La–Eu) and depletion of HREEs (Gd–Lu). Member Ch–4 has significant negative anomalies of δCe and δEu, and some samples from member Ch–1 and member Ch–3 have negative anomalies of δEu.

4.5. U–Pb Dating

U–Pb dating is used to determine the age of the host rocks of ancient carbonate rocks and their cements [40,41]. According to the dating results (Table 1 and Figure 8), for three samples of dolomitic microbialite, the ages of their host rocks range from 538 ± 28 Ma to 618 ± 37 Ma, corresponding to the Ediacaran period (538.8–635 Ma). The ages of laminated/fibrous dolomite cements vary from 558 ± 26 Ma to 576 ± 17 Ma, which is within the age range of the Ediacaran. The ages of granular/bladed dolomite cements range from 513 ± 20 Ma to 553 ± 20 Ma, corresponding to the age range between the second series of Cambrian (509–521 Ma) and the Ediacaran. The ages of saddle dolomite cements are 172.9 ± 3.9 Ma and 256 ± 12 Ma, corresponding to the geological ages of the middle Jurassic Aalenian (170.3–174.1 Ma) and the Upper Permian Lopingian (251.9–259.5 Ma), respectively. It was revealed that the age sequences of these dolomites with different textures in the three samples are consistent with their diagenetic sequences, that is, the age decreases from the host rocks of microbial dolomite, the dolomite at the margin of the pores to the dolomite in the pore center, which also reflected the reliability of all the measurement results.

4.6. Element Mapping

The composition of multi–phase cements in ancient carbonate rocks always exhibits high heterogeneity. A LA–ICP–MS–based element–mapping technique was applied on the samples to intuitively present the planar variation characteristics of elements within a millimeter–centimeter–scale area, which is crucial to understanding their forming process [42,43].
The plane distributions of Mg, Ca, Mn, Fe, Sr, Ba, Th, U, Y, La, Ce, Eu and Lu were analyzed for sample 58–1 (Figure 9). According to its plane–polarized light (PPL) and cathode luminescence (CL) characteristics and the plane distribution of Mg and Ca, three phases of dolomite cements–including fibrous dolomite (FD), bladed dolomite (BD) and granular dolomite (GD), and one phase of sparry calcite cement were identified in pores. MD has relatively high contents of Fe and Sr and low contents of other elements. FD has a high content of Fe; BD has high contents of Mn, Y, La, Ce, Eu and Lu; and GD has low contents of all kinds of trace elements. Sparry calcite has high content of Sr and low content of Mn, Ba, Th, U and Ce, and the extremely low content of Mn is the cause of its nonluminescence. In summary, the contents of trace elements in FD are close to those in MD.
The plane distributions of 13 elements were analyzed for sample 76–1 (Figure 10). According to the Mg and Ca plane distribution map, a weird banding effect can be seen which may be the result of imperfect data processing. Although the image resolution is not high, it is still possible to distinguish the Mg and Ca content of the main structure. Combining PPL and CL characteristics and plane distribution of Mg and Ca, three phases of dolomite cements in the pores, including fibrous dolomite (FD), laminated dolomite (LD) and saddle dolomite (SD), and fine dolomite cements in the micro–fractures were identified. The contents of all trace elements are low in MD. Similar to MD, FD also has low contents of all trace elements. LD has relatively high contents of Fe and Mn, and SD has high contents of Y and Eu. Dolomite in the micro–fractures contains abundant Fe, Mn, Y, La, Ce, Eu and Lu, and its cathodoluminescence characteristics have a correlation with the contents of Mn and Fe.

5. Discussion

5.1. Genesis of Dolomite

As the Upper Ediacaran Chigbrak Formation is predominantly composed of dolomite; determining the origin of dolomite is critical for understanding the diagenetic evolution of reservoir. The dolomitization of the early diagenetic stage plays an important constructive role in the preservation of primary pores and early dissolved pores. However, dolomitization in the late burial stage had little effect on the early pore preservation, and even played a destructive role [44,45]. Therefore, it is very important to determine its occurrence time. In general, a preliminary estimate of the diagenetic stage when dolomitization occurred based on the petrographic characteristics is feasible. In the early diagenetic stage, dolomitization occurs quickly, the dolomite crystals formed are fine and the original depositional textures of rock was always retained. In the middle–late diagenetic stage, dolomitization occurred relatively slowly and the formed dolomites are coarse, usually larger than fine grain in size [46]. The Chigbrak Formation in the study area is mainly composed of micrite and powder crystal dolomites, and retained the original microorganisms or granular texture well, showing the characteristics of early diagenetic dolomite.
According to the geochemical data obtained by analysis and tests, the Chigbrak Formation has medium degree of dolomite cation ordering (avg. 0.68), indicating that its dolomitization process was characterized by rapid replacement and crystal growth, and occurred in the early burial stage at low temperature [47]. In general, the dolomites have low content of ΣREEs, no anomaly of δCe and negative anomaly or no anomaly of δEu, indicating seawater as the dolomitization fluid. High content of ΣREEs suggests the containing of mud or transformation by hydrothermal fluid. A negative anomaly of δCe is used to identify the mixture of meteoric water into dolomitization fluid, and positive anomaly of δEu is a typical indication of hydrothermal process [48]. The dolomites in the study area are generally characterized by low content of ΣREEs, and negative anomaly or no obvious anomaly of δCe and δEu. This reveals that the dolomitization fluid was seawater, and no hydrothermal fluid was involved in the dolomitization process. The exceptionally high content of ΣREEs (>20 ppm) in some samples from member Ch–1 and member Ch–4 was the result of the mud components in some layers in member Ch–1 and the mixture of atmospheric water into the dolomitization fluid in member Ch–4. This supports the conclusion that member Ch–1 was deposited in tidal flat environment and member Ch–4 experienced exposure in its late depositional stage.
The oxygen isotope composition of dolomite is determined by the salinity and temperature of diagenetic fluid. Therefore, the oxygen isotope value of dolomite is higher in high salinity evaporation environment; On the contrary, the oxygen isotope value of dolomite is lower in high–temperature and high–pressure buried environments [49]. As mentioned above, no obvious correlation was detected between the values of δ13C (PDB) and δ18O (PDB) in the samples, which was explained by the weak modification in the late diagenetic stage. These values were distributed within the variation range of δ13C (PDB) and δ18O (PDB) values of the late Ediacaran seawater, indicating that the Chigbrak Formation was formed at low temperature with seawater as the dolomitization fluid. Moreover, from bottom to top, the values of δ18O (PDB) decease first and then increase, corresponding to the depositional environment of a tidal flat with relatively strong evaporation for member Ch–1 and the supergene environment for member Ch–4, where it suffered from exposure and meteoric water dissolution.
Containing clay or the involvement of a hydrothermal process led to an increased ratio of 87Sr/86Sr [50]. The values of 87Sr/86Sr of the Chigbrak Formation are generally slightly higher than those of the coeval seawater. Two main reasons have been identified for this phenomenon. One is that dolomite samples usually have higher values of 87Sr/86Sr than limestone samples [34], and the other is that the excessively high values of 87Sr/86Sr in some samples from member Ch–1 prop up the overall average. The study area is located in Wensu Low Salient [25], and member Ch–1 probably contained some clay because of its depositional environment of tidal flat, which led to obviously higher 87Sr/86Sr values due to mixture of crust derived strontium. In addition, the slightly higher 87Sr/86Sr values in member Ch–4 are also probably related to the mixture of a small amount of crust derived strontium due to the exposure it suffered from in the late Ediacaran period. The average value of 87Sr/86Sr in the remaining intervals is 0.709064, which is close to that of the coeval seawater. This also indicated seawater as the dolomitization fluid for the Chigbrak Formation.
Pores are well developed in the dolomitic microbialite of Chigbrak Formation, and filled with dolomite cements of different phases and different types. Accurate determination of the formation age of these dolomites is achievable by an analytical comparison of the ages between the host rocks of dolomitic microbialite and the first phase dolomite cements at the margin of the pores [40]. It is revealed by the U–Pb dating data that ages of the host rocks of all the dolomitic microbialite samples (538 ± 28 to 618 ± 37 Ma) are larger than those of the first phase dolomite cements at the margin of the pores (558 ± 26 to 576 ± 17 Ma), and ages of the dolomite cements of the first phase are larger than those of other phases. This suggests a favorable consistency between age sequence and diagenesis sequence, and also reveals that dolomitization has already happened in the depositional stage of the Chigbrak Formation.
On the basis of the petrological, isotope geochemical, element geochemical and geochronological characteristics of carbonate minerals, it has been clarified that the Chigbrak Formation dolomites in the study area were formed in the epidiagenetic environment with seawater as the dolomitization fluid. The findings of the research on paleoenvironment and sedimentary model of the study area [12,19] demonstrate that the sedimentary sequence of inner ramp tidal flat and microbial mound–shoal complex in the setting of carbonate ramp was developed in the depositional stage of the Chigbrak Formation, when the whole study area was a near shore shallow water environment in warm and relatively dry climate. In this geological setting, the relatively strong evaporation led to an increased content of Mg2+ in seawater and allowed evaporative dolomitization of the sediments dominated by microorganisms in this near shore shallow water environment.

5.2. Pore Origin and Diagenesis Sequence

Based on the diagenetic process, pores in the carbonate rocks are mainly classified into primary pores formed in the depositional stage, secondary dissolved pores formed in the epidiagenetic stage and secondary dissolved pores formed in the burial stage. An accurate determination of origin of the pores is of great significance in clarifying the main controlling factors and distribution laws of reservoirs, especially that the identification of primary pores and penecontemporaneous epidiagenetic dissolved pores directly influences the prediction of facies controlled reservoirs. In the Chigbrak Formation in the study area, the intergranular pores in dolomitic oolite and grainstone with a bonding structure in member Ch–1 and member Ch–2 were easily identified as primary pores, and the framework pores in some dolomitic stromatolites and thrombolites were also identified as primary pores. Both of them have shared the characteristics of obvious fabric selectivity, homogeneous distribution and small pore size (<1 mm). As described above, the Kalpin Movement at the end of Ediacaran uplifted the study area tectonically, leading to atmospheric water dissolution and the resulted dolomite weathering crust. Therefore, the irregular shaped centimeter to decimeter–size vugs and fractures were clearly identified as the products of atmospheric water karstification near the unconformity surface. It is difficult to clarify the origin of the large pores and vugs predominantly developed in member Ch–3. However, these pores and vugs always have multiple phases of carbonate cements. Thus, based on the diagenesis sequences of cements of different phases within them, their forming mechanism and evolution history could be well understood by analyzing their geochemical characteristics and comparing with the host rocks.
Vugs in member Ch–3 have relatively falt, bedding–parallel occurrence and are developed by multi–cycles. From margin to center of the pores and vugs, one or multiple cements among fibrous dolomite, laminated dolomite, bladed dolomite, granular dolomite, saddle dolomite and sparry calcite occurred successively. Fibrous isopachous rim dolomites are recognized as the first phase cements. It is generally assumed that the carbonate cements with fibrous textures are the products of cementation in seawater in the early diagenetic stage and are originated from magnesium calcite and aragonite with fibrous texture [51]. According to the U–Pb dating data, fibrous isopachous rim dolomites in samples 56–1, 58–1 and 76–1 are slightly younger than the host rocks, and their geological ages vary within the age range of the Ediacaran (>541 Ma). The geochemical data indicate that the fibrous isopachous rim dolomites have similar values of δ13C (PDB), δ18O (PDB) and 87Sr/86Sr to the host rocks, and also show consistent distribution patterns of trace and rare earth elements as shown in Figure 8 and Figure 9. Based on the description above that the dolomitization fluid for the host rocks was seawater and the dolomitization occurred in the early burial stage, it has been confirmed that these pores and vugs were formed before cementation of the fibrous dolomites, and were the products of penecontemporaneous meteoric water dissolution.
Laminated isopachous rim dolomites have similar values of δ13C (PDB), δ18O (PDB) and 87Sr/86Sr and consistent U–Pb ages to fibrous dolomites but different contents of Mn and Fe, which resulted in different luminescence intensity under cathodic excitation. Specifically, laminated dolomites present intensive luminescence and fibrous dolomites seldom or never show luminescence, which comprehensively suggest that the cementation fluid was the mixture of seawater and atmospheric water. Compared with fibrous dolomites, bladed/granular dolomites have similar values of δ13C (PDB) and slightly higher 87Sr/86Sr, but more negative values of δ18O (PDB), which indicate their different diagenetic environments and warmer diagenetic fluid for fibrous dolomites. Bladed dolomites have higher contents of Mn and REEs, including Y, La, Ce, Eu and Lu, than granular dolomites, leading to higher luminescence intensity under cathodic excitation, and show the characteristics of a burial diagenetic environment. Judging from their U–Pb ages, bladed dolomites are determined as the products of the early burial diagenetic stage, and granular dolomites as the products of the middle burial diagenetic stage with formation water as the cementation fluid. Saddle dolomites have more negative values of δ18O (PDB) than the host rocks and other types of cements (avg. −10.13‰), indicating a higher temperature of diagenetic fluid. Their high values of 87Sr/86Sr (avg. 0.70936), high contents of Y and Eu and relatively high content of Mn comprehensively reflect that their diagenetic fluid was hydrothermal fluid. In addition, their U–Pb ages indicate they were formed in the late Permian–middle Jurassic period, corresponding to the late Hercynian movement and the tectonic active stage of the early Yanshan movement. This suggests that saddle dolomites are the products of the late burial diagenetic stage with a contribution of at least two hydrothermal processes.
Based on the above analyses, combined with the geological setting of the study area, the diagenesis sequence was established for each important stage that the Chigbrak Formation experienced after deposition (Figure 11). The early diagenetic stage mainly involved micritization under the mixture of seawater and atmospheric water, meteoric water dissolution Ⅰ, dolomitization, cementation Ⅰ of fibrous isopachous rim calcite, cementation Ⅱ of laminated calcite, mechanical compression in an early burial environment, cementation Ⅲ of bladed calcite and atmospheric water dissolution Ⅱ (Kalpin Movement period). The middle diagenetic stage mainly included cementation of granular dolomite and recrystallization of dolomite (according to the ordering degree of partial dolomite being relatively high, and coupled with the compositional zoning illustrated by the CL, it is possible that these dolomites have been recrystallized [52]; furthermore, the U–Pb age dates show a range of 80 million years which may potentially reflect dolomite recrystallization). The late diagenetic stage mainly included structural fracturing in a late burial environment, cementation Ⅱ of saddle dolomite, cementation Ⅳ of massive calcite, structural fracturing Ⅱ and cementation Ⅲ of saddle dolomite. In all the major diagenesis processes, two phases of meteoric water dissolution and dolomitization played the most important constructive role. The former formed vugs of layered distribution. Although it did not generate pores, the latter produced dolomites with higher compressive strength than limestone, which kept the primary pores and vugs formed in the early diagenetic stage from being destroyed by compaction throughout the long geological history. This understanding provided a reliable basis for clarifying the genesis of reservoirs. Specifically, sedimentary microfacies, penecontemporaneous meteoric water dissolution controlled by fourth– or fifth–order sequences, meteoric water dissolution controlled by tectonic movement and early dolomitization jointly control the development of dolomite reservoirs in the Upper Ediacaran Chigbrak Formation of the Tarim Basin.

6. Conclusions

(1)
The Upper Ediacaran Chigbrak Formation in the study area is mainly composed of dolomitic microbialite, the average values of δ13C (PDB), δ18O (PDB) and 87Sr/86Sr are 3.50‰ (1.31‰~6.65‰), −2.95‰ (−4.81‰~−1.23‰) and 0.709457 (0.708745~0.710885), which are similar to the characteristics coeval seawater. The dolomites are characterized by a medium degree of cation ordering (0.60~0.77, avg. 0.68) and low content of ΣREEs (0.36 ppm~35.40 ppm, avg. 9.03 ppm), and a chondrite standardized curve of REEs in a right wing type shows enrichment of LREE (La–Eu) and depletion of HREE (Gd–Lu). The ages determined by laser ablation U–Pb dating range from 538 to 618 Ma, corresponding to the age of the Ediacaran period.
(2)
Based on the isotope geochemical characteristics, element geochemical characteristics and U–Pb ages, combined with the petrologic characteristics (the dolomites have well preserved depositional textures, and their crystals are finer), it was identified that the dolomite of the Upper Ediacaran Chigbrak Formation was formed penecontemporaneously with marine seawater being the dolomitizing fluid.
(3)
Vugs are the dominant pore type of the Upper Ediacaran Chigbrak Formation. Based on the occurrence sequence of six types of cements (fibrous dolomite, laminated dolomite, bladed dolomite, granular dolomite, saddle dolomite and sparry calcite) in the vugs and their values of δ18O (PDB), 87Sr/86Sr, elements content and U–Pb ages, it has been concluded that the vugs are the products of penecontemporaneous dissolution of meteoric water.
(4)
Lithofacies, sedimentary microfacies, penecontemporaneous meteoric water dissolution controlled by fourth– or fifth–order sequences, meteoric water dissolution controlled by tectonic movement and early dolomitization are confirmed as the major controlling factors for the development of dolomite reservoirs in the Upper Ediacaran Chigbrak Formation.

Author Contributions

Conceptualization, J.Z. and A.S.; methodology, J.Z. and H.W.; software, X.L. and K.D.; validation, Z.C.; formal analysis, J.Z.; investigation, J.Z. and H.W.; resources, A.S. and Z.C.; data curation, H.W. and X.L.; writing–original draft preparation, J.Z. and H.W.; writing–review and editing, J.Z.; visualization, X.L. and K.D.; supervision, J.Z.; project administration, J.Z.; funding acquisition, J.Z. and H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was jointly funded by the Scientific Research & Technology Development Project of PetroChina Company Limited (Grant No. 2021DJ0501) and Basic science and strategic reserve technology fund project of CNPC (Grant No. 2022D–HZ02).

Data Availability Statement

Data are available upon reasonable request. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

Acknowledgments

We would thank Yongquan Chen and Jingao Zhou for their valuable suggestions, Anping Hu and Feng Liang for their guidance in the experiments. We would also like to thank Yongjin Zhu, Guang Yu, Rong Xiong, Jianhua Dong and Yu Liu for the help in field work. We acknowledge this generous funding from PetroChina Company Limited.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 00725 g001 550
Figure 1. Geological setting map of study area: (a) location of the Tarim Basin in northwestern China indicated with pink zone; (b) tectonic location of the study area indicated with a red line rectangle; (c) stratigraphic sequence from Ediacaran to middle Cambrian.
Figure 1. Geological setting map of study area: (a) location of the Tarim Basin in northwestern China indicated with pink zone; (b) tectonic location of the study area indicated with a red line rectangle; (c) stratigraphic sequence from Ediacaran to middle Cambrian.
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Figure 2. Photographs of Chigbrak Formation in the outcrop: (a) bottom characteristics (the red line is the boundary between Chigbrak Formation and Sugetbrak Formation); (b) top characteristics (the red curve is the boundary between Chigbrak Formation and Yurtus Formation).
Figure 2. Photographs of Chigbrak Formation in the outcrop: (a) bottom characteristics (the red line is the boundary between Chigbrak Formation and Sugetbrak Formation); (b) top characteristics (the red curve is the boundary between Chigbrak Formation and Yurtus Formation).
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Figure 3. Stratigraphic column of Chigbrak Formation.
Figure 3. Stratigraphic column of Chigbrak Formation.
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Figure 4. Photographs of dolomite in the Chigbrak Formation: (a) dolomitic stromatolite (vertical perspective), member Ch–1, outcrop image; (b) dolomitic stromatolite, with framework pores, member Ch–1, blue casting thin section PPL image; (c) dolomitic oolite, with intergranular pores, member Ch–1, blue casting thin section PPL image; (d) dolomitic grainstone with bonding structure, with intergranular pores and intragranular pores, member Ch–1, blue casting thin section PPL image; (e) stromatolite dolomite is interbedded with thin layered yellowish–gray, grayish–brown argillaceous dolomite and quartz sandstone, member Ch–1, outcrop image; (f) micrite dolomite is interbedded with dolomitic quartz sandstone, blue casting thin section PPL image; (g) dolomitic thrombolite, with framework pores, member Ch–3, blue casting thin section PPL image; (h) dolomitic foamy microbialite, with dissolved pores, member Ch–3, blue casting thin section PPL image; (i) brecciated dolomite, with interbreccia pores and fractures, member Ch–4, outcrop image; (j) brecciated dolomite, breccias are better rounded, member Ch–4, outcrop image; (k) dolomitic foamy microbialite, with acicular–dissolved pores, member Ch–3, outcrop image; (l) dolomitic thrombolite, vugs with layered distribution, member Ch–3, outcrop image; (m) brecciated dolomite, with decimeter–size vugs, member Ch–4, outcrop image; (n) dolomitic thrombolite, vugs with “lace structure” are developed along the bedding, member Ch–3, blue casting thin section PPL image; (o) dolomitic thrombolite, vugs with “lace structure” are developed along the bedding, member Ch–4, sample section image.
Figure 4. Photographs of dolomite in the Chigbrak Formation: (a) dolomitic stromatolite (vertical perspective), member Ch–1, outcrop image; (b) dolomitic stromatolite, with framework pores, member Ch–1, blue casting thin section PPL image; (c) dolomitic oolite, with intergranular pores, member Ch–1, blue casting thin section PPL image; (d) dolomitic grainstone with bonding structure, with intergranular pores and intragranular pores, member Ch–1, blue casting thin section PPL image; (e) stromatolite dolomite is interbedded with thin layered yellowish–gray, grayish–brown argillaceous dolomite and quartz sandstone, member Ch–1, outcrop image; (f) micrite dolomite is interbedded with dolomitic quartz sandstone, blue casting thin section PPL image; (g) dolomitic thrombolite, with framework pores, member Ch–3, blue casting thin section PPL image; (h) dolomitic foamy microbialite, with dissolved pores, member Ch–3, blue casting thin section PPL image; (i) brecciated dolomite, with interbreccia pores and fractures, member Ch–4, outcrop image; (j) brecciated dolomite, breccias are better rounded, member Ch–4, outcrop image; (k) dolomitic foamy microbialite, with acicular–dissolved pores, member Ch–3, outcrop image; (l) dolomitic thrombolite, vugs with layered distribution, member Ch–3, outcrop image; (m) brecciated dolomite, with decimeter–size vugs, member Ch–4, outcrop image; (n) dolomitic thrombolite, vugs with “lace structure” are developed along the bedding, member Ch–3, blue casting thin section PPL image; (o) dolomitic thrombolite, vugs with “lace structure” are developed along the bedding, member Ch–4, sample section image.
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Figure 6. Binary plots of geochemistry in the Chigbrak Formation dolomite: (a) δ13C versus δ18O; (b) 87Sr/86Sr versus δ18O.
Figure 6. Binary plots of geochemistry in the Chigbrak Formation dolomite: (a) δ13C versus δ18O; (b) 87Sr/86Sr versus δ18O.
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Figure 7. Normalized distribution patterns of rare earth elements in the Chigbrak Formation dolomite: (a) member Ch–1 and member Ch–2; (b) member Ch–3 and member Ch–4.
Figure 7. Normalized distribution patterns of rare earth elements in the Chigbrak Formation dolomite: (a) member Ch–1 and member Ch–2; (b) member Ch–3 and member Ch–4.
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Figure 8. LA–MC–ICP–MS U–Pb concordia diagram of dolomite in the Chigbrak Formation: (a) MD of sample 58–1; (b) FD of sample 58–1; (c) BD of sample 58–1; (d) GD of sample 58–1; (e) MD of sample 76–1; (f) FD of sample 76–1; (g) LD of sample 76–1; (h) SD of sample 76–1; (i) MD of sample 81–4; (j) GD of sample 81–4; (k) SD of sample 81–4.
Figure 8. LA–MC–ICP–MS U–Pb concordia diagram of dolomite in the Chigbrak Formation: (a) MD of sample 58–1; (b) FD of sample 58–1; (c) BD of sample 58–1; (d) GD of sample 58–1; (e) MD of sample 76–1; (f) FD of sample 76–1; (g) LD of sample 76–1; (h) SD of sample 76–1; (i) MD of sample 81–4; (j) GD of sample 81–4; (k) SD of sample 81–4.
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Figure 9. Photomicrographs and element maps of sample 58–1.
Figure 9. Photomicrographs and element maps of sample 58–1.
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Figure 10. Photomicrographs and element maps of sample 76–1.
Figure 10. Photomicrographs and element maps of sample 76–1.
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Figure 11. Diagenetic sequence of Chigbrak Formation.
Figure 11. Diagenetic sequence of Chigbrak Formation.
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Table
Table 1. δ13C, δ18O, 87Sr/86Sr and U–Pb age values of different fabrics in the Chigbrak Formation dolomite.
Table 1. δ13C, δ18O, 87Sr/86Sr and U–Pb age values of different fabrics in the Chigbrak Formation dolomite.
SampleMemberFabricsδ13C
‰ (PDB)		δ18O
‰ (PDB)		87Sr/86SrU–Pb Age
Ma
56–1Ch–3① Dolomitic Microbialite (MD)1.87−4.260.70885/
② Fibrous dolomite in the pore (FD)2.40−4.810.70880 /
③ Granular dolomite in the pore (GD)2.28−7.34 0.70893/
58–1Ch–3①Dolomitic Microbialite (MD)2.21−4.730.70882605.4 ± 6.9
② Fibrous dolomite in the pore (FD)2.46−5.080.70871576 ± 17
③ Bladed dolomite in the pore (BD)1.59−6.61 0.70895553 ± 20
④ Granular dolomite in the pore (GD)0.95−7.860.70902538 ± 25
76–1Ch–3① Dolomitic Microbialite (MD)2.69−4.290.70877618 ± 37
② Fibrous dolomite in the pore (FD)2.12−5.570.70876558 ± 26
③ Laminated dolomite in the pore (LD)2.83−4.90 0.70894559 ± 22
④ Saddle dolomite in the pore (SD)2.01−10.390.70925256 ± 12
81–4Ch–4① Dolomitic Microbialite (MD)3.33−3.950.70889538 ± 28
② Granular dolomite in the pore (GD)1.09−8.140.70913513 ± 20
③ Saddle dolomite in the pore (SD)1.95−9.870.70948172.9 ± 3.9
Table
Table 2. δ13C, δ18O, 87Sr/86Sr and order degree values of dolomite (whole rock) in the Chigbrak Formation.
Table 2. δ13C, δ18O, 87Sr/86Sr and order degree values of dolomite (whole rock) in the Chigbrak Formation.
SampleLithofaciesMemberδ13C
‰ (PDB)		δ18O
‰ (PDB)		87Sr/86Sr Ordering Degree
1–2Dolomitic Stromatolite (SD)Ch–15.86−2.670.7099920.68
4–1Dolomitic Grainstone with clay (GDC)Ch–16.18−2.710.7107580.62
11–1Dolomitic Stromatolite (SD)Ch–15.34−3.050.7096340.61
12–4Dolomitic Stromatolite (SD)Ch–16.65−4.060.709334 0.67
14–1Dolomitic Grainstone with bonding structure (GDBS)Ch–15.93−2.200.7093590.60
18–1Dolomitic Grainstone with bonding structure (GDBS)Ch–14.49−2.550.709877 0.62
21–1Dolomitic Stromatolite (SD)Ch–15.35−1.230.710885 0.66
23–1Dolomitic Oolite (OD)Ch–14.21−1.990.710710 0.64
26–2Dolomitic Stromatolite (SD)Ch–22.89−1.440.7092420.63
29–1Dolomitic Thrombolite (TD)Ch–22.21−1.390.709077 0.68
29–3Dolomitic Stromatolite (SD)Ch–22.35−2.230.709059 0.70
36–1Dolomitic Stromatolite (SD)Ch–22.17−1.980.708906 0.70
39–1Dolomitic Thrombolite (TD)Ch–32.64−2.030.708745 0.67
42–1Dolomitic Foamy Microbialite (FMD)Ch–33.03−4.450.709093 0.68
47–2Dolomitic Foamy Microbialite (FMD)Ch–33.48−2.650.7092140.71
55–1Dolomitic Thrombolite (TD)Ch–32.15−2.460.709086 0.67
65–1Dolomitic Foamy Microbialite (FMD)Ch–31.95−4.810.7090150.69
70–1Dolomitic Foamy Microbialite (FMD)Ch–31.68−4.400.709178 0.70
76–2Dolomitic Foamy Microbialite (FMD)Ch–32.65−3.670.709097 0.71
81–1Brecciated Dolomitic Thrombolite (BTD)Ch–42.09−4.330.709132 0.73
81–3Brecciated Dolomitic Foamy Microbialite (BFMD)Ch–41.31 −4.65 0.709419 0.73
81–5Brecciated Dolomitic Thrombolite (BTD)Ch–42.40−4.050.7092410.77
Table
Table 3. Rare earth elements content values of dolomite in the Chigbrak Formation.
Table 3. Rare earth elements content values of dolomite in the Chigbrak Formation.
SampleLa
(ppm)		Ce
(ppm)		Pr
(ppm)		Nd
(ppm)		Sm
(ppm)		Eu
(ppm)		Gd
(ppm)		Tb
(ppm)		Dy
(ppm)		Ho
(ppm)		Er
(ppm)		Tm
(ppm)		Yb
(ppm)		Lu
(ppm)		ΣREEs
(ppm)
1–24.0378.3030.9853.7460.7300.1880.6650.1000.5620.1080.3040.0440.2830.04320.10
4–13.2656.8070.7552.7950.5130.1070.4780.0760.4650.1010.3050.0460.2990.04516.06
11–13.7468.6401.1684.5590.8820.2040.8510.1380.8430.1780.5200.0750.4670.07022.34
12–42.0394.7730.5251.8980.3350.0860.3100.0480.2790.0550.1530.0210.1270.01810.67
14–13.8058.5940.9223.2880.6290.1320.5680.0940.5710.1170.3440.0500.3180.04519.48
18–10.8421.6310.2240.8290.1620.0510.1760.0300.1860.0400.1120.0170.1080.0174.43
21–10.5981.1880.1470.5010.0990.0290.0890.0140.0860.0180.0510.0080.0470.0072.88
23–11.5463.6570.4421.7210.3380.0800.3310.0520.3080.0620.1760.0250.1510.0228.91
26–20.4571.1020.1320.4460.0860.0260.0800.0120.0680.0130.0410.0060.0380.0062.51
29–10.3000.7700.0890.3270.0580.0170.0530.0080.0460.0080.0250.0030.0210.0031.73
29–30.2220.6450.0740.2830.0510.0160.0480.0070.0410.0080.0230.0030.0210.0031.45
36–10.0700.1570.0160.0560.0110.0030.0130.0020.0130.0030.0090.0010.0080.0010.36
39–10.1990.4370.0410.1550.0310.0080.0320.0050.0330.0070.0240.0040.0210.0031.00
42–10.1820.3200.0330.1330.0270.0070.0310.0050.0330.0080.0240.0040.0220.0030.83
47–20.1230.2040.0230.0960.0200.0050.0270.0040.0280.0070.0230.0030.0210.0030.59
55–10.2180.3600.0450.1820.0360.0110.0450.0070.0450.0110.0340.0050.0310.0041.03
65–10.2200.3880.0450.1880.0360.0100.0460.0070.0460.0110.0360.0050.0310.0051.08
70–10.4570.8880.0990.3580.0700.0170.0710.0110.0720.0170.0470.0070.0420.0072.16
76–20.1590.2280.0300.1210.0260.0060.0270.0060.0330.0090.0300.0050.0290.0050.71
81–17.1728.9651.6737.7381.9560.4362.1630.3231.9270.4301.2540.1751.0340.15035.40
81–34.5075.6081.0554.3420.8930.1920.9140.1400.8360.1770.5250.0760.4580.06819.79
81–55.3266.7111.2385.4651.2840.2831.3850.2081.2420.2720.7980.1120.6700.09825.09
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Zheng, J.; Wang, H.; Shen, A.; Luo, X.; Cheng, Z.; Dai, K. Genesis of Dolomite Reservoir in Ediacaran Chigbrak Formation of Tarim Basin, NW China: Evidence from U–Pb Dating, Isotope and Element Geochemistry. Minerals 2023, 13, 725. https://doi.org/10.3390/min13060725

AMA Style

Zheng J, Wang H, Shen A, Luo X, Cheng Z, Dai K. Genesis of Dolomite Reservoir in Ediacaran Chigbrak Formation of Tarim Basin, NW China: Evidence from U–Pb Dating, Isotope and Element Geochemistry. Minerals. 2023; 13(6):725. https://doi.org/10.3390/min13060725

Chicago/Turabian Style

Zheng, Jianfeng, Hui Wang, Anjiang Shen, Xianying Luo, Zhao Cheng, and Kun Dai. 2023. "Genesis of Dolomite Reservoir in Ediacaran Chigbrak Formation of Tarim Basin, NW China: Evidence from U–Pb Dating, Isotope and Element Geochemistry" Minerals 13, no. 6: 725. https://doi.org/10.3390/min13060725

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