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Article

Genesis and Related Reservoir Development Model of Ordovician Dolomite in Shuntogol Area, Tarim Basin

by
Liangxuanzi Zhong
1,2,
Leli Cheng
3,*,
Heng Fu
1,
Shaoze Zhao
4,
Xiaobin Ye
3,
Yidong Ding
3 and
Yin Senlin
3
1
College of Energy Resources, Chengdu University of Technology, Chengdu 610059, China
2
College of Further Education, Yangtze University, Jingzhou 434023, China
3
Institute of Logging Technology and Engineering, Yangtze University, Jingzhou 434023, China
4
College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(6), 545; https://doi.org/10.3390/min14060545
Submission received: 27 March 2024 / Revised: 6 May 2024 / Accepted: 22 May 2024 / Published: 25 May 2024
(This article belongs to the Special Issue Dolomitization, Recrystallization, and Cementation in Carbonate Sedimentary Rocks)
Browse Figures
Versions Notes

Abstract

:
The Ordovician thick dolostone in Shuntogol area of the Tarim Basin has the potential to form a large-scale reservoir, but its genesis and reservoir development model are still unclear. Starting from a sedimentary sequence, this study takes a batch of dolostone samples obtained from new drilling cores in recent years as the research object. On the basis of core observation and thin section identification, trace elements, cathodoluminescence, carbon and oxygen isotopes, rare earth elements, and X-ray diffraction order degree tests were carried out to discuss the origin of the dolomite and summarize the development model of the dolostone reservoir. The analysis results show that the Ordovician dolomite in the study area had a good crystalline shape, large thickness, high Fe and Mn values, and mostly showed bright red light or bright orange–red light under cathode rays. The ratio of δ18O values to seawater values at the same time showed a negative bias; the δCe values were negative anomalies, the δEu values were positive anomalies, and the order degree was high. This indicates that the dolomitization process occurred in a relatively closed diagenetic environment. The Ordovician carbonate rocks in the study area were low-lying during the sedimentary period, and with the rise of sea level, the open platform facies continued to develop. When the Middle and Lower Ordovician series entered the burial stage, the main hydrocarbon source rocks of the lower Cambrian Series entered the oil generation peak, and the resulting formation overpressure provided the dynamic source for the upward migration of the lower magnesium-rich fluid, and the dolomitization fluid entered the karst pore system in the target layer to produce all the dolomitization. This set of dolostone reservoirs is large in scale and can be used as a favorable substitute area for deep carbonate exploration for continuous study.

1. Introduction

High-quality oil and gas reservoirs can be developed in the dolostone formation, especially in the depth of more than 4500 m, and even in the depth of more than 6000 m, so the dolostone reservoir has been the focus of exploration [1,2,3]. Dolomitization is a multi-stage comprehensive process involving different geological conditions and different periods [4,5,6]. Although dolomites rarely precipitate in modern sediments [7,8,9,10], dolomites exist widely in ancient carbonate formations. In recent years, researchers have been unable to synthesize relatively ideal dolomites in a laboratory environment [11,12], which makes the genesis of dolomite a constant debate [13,14]. How to determine the fluid source and migration path of dolomitization is the core issue in studying the formation mechanism of dolostone reservoirs.
The Tarim Basin, as a large, superimposed composite basin in China, is rich in oil and gas resources [15] (Figure 1). In addition to a small amount of carbonate rocks preserved in the Carboniferous and Lower Permian series, most of the rest developed in the Cambrian–Ordovician series [16]. In recent years, the study of the Ordovician dolostone reservoir in the Tarim Basin has shown that the diagenesis of dolostone with different structures is different in the Tazhong area [17,18]. For example, the development of dolostone with a residual structure is related to seawater evaporation in the contemporaneous or quasi-contemporaneous period. The development of fine crystalline dolomite with a high idiomorphic degree is related to seawater reflux infiltration during the shallow burial period [19]. The development of meso-coarse-grained dolomite with other shapes is related to high-temperature hydrothermal conditions in the deep burial period. The dolostone reservoir of the Penglaiba Formation has undergone a comprehensive transformation in many diagenetic processes and formed a complex reservoir space type. Moreover, the reservoir is highly heterogeneous, and the porosity and permeability of different regions vary greatly, and the areas with good physical reservoir properties are concentrated in the areas subjected to severe weathering denudation and strong tectonic rupturing [20]. The Yingshan Formation dolostone reservoir was formed under the comprehensive effects of de-dolomitization, supergene karst, hydrocarbon charging, burial dissolution, and tectonic processes [21,22,23]. In the Bachu area, southwest of the basin, the Ordovician Penglaiba Formation dolostone reservoir has a vertical cycle related to high-frequency sequences, and dolomitization occurred in the same generation, which is different from traditional buried dolomitization [24]. However, in the Gucheng area in the eastern part of the basin, the Yingshan Formation dolostone reservoir experienced different stages of diagenesis, such as atmospheric freshwater dissolution in the same generation, burial dolomite in the middle period, and hydrothermal transformation in the late period; especially the dolostone near the fault has the potential to form high-quality reservoirs [25]. In recent years, with the exploration breakthrough in the Tabei area, the view of a fault-controlled reservoir has gradually emerged [26]. In the study of Ordovician carbonate rocks in the Shuntogol area, the formation model of dolomite in the Shuntogol area was different from that in other areas of the Tarim Basin [27]. This study began with the study of sedimentary sequences of Ordovician carbonate rocks in the Shuntogol area, combined with petrological and geochemical characteristics of dolomites, to explore the genetic model of dolomitization in this area, in order to provide reference for further exploration.

2. Geological Setting

The Tarim Basin is a superimposed basin consisting of an ancient continental crust basement, Paleozoic Marine cratonic sediments, and Meso-Cenozoic continental foreland basin sediments from the bottom to top [28] (Figure 1a). During the formation and evolution of the basin, the basin underwent multi-stage tectonic transformation and developed three near east–west uplifts (the Tabei uplift, Central uplift, and Tadongnan uplift) and four depressions (the Kuqa depression, Beibei depression, Taxian-southwest depression, and Tadongnan depression) [29].
The central uplift includes the Bachu uplift, Tazhong uplift, and Tadong uplift [27]. The Bachu uplift is connected with the Keping fault uplift to the west (Figure 1b) [21]. In the Sinian–Early Ordovician Period, the basin was in the regional extensional tectonic setting. In the Cambrian Period, the northern and southern margins of the basin developed into passive continental margins, and the western part of the basin was the inner depression of Tasikelatong, which was a set of carbonate platform facies deposits [30]. In the eastern part of the basin, the basin facies deposits of the Ku–Manocutan trough formed the sedimentary pattern of the Taxi platform–Tandong Basin, which was delimited by the Lenggu–Gucheng Xiangbian platform margin belt. With the change in sea level, the Xiangbian platform margin belt gradually accumulated eastward, and the scope of the Taxi platform expanded. After the deposit of the Penglaiba Formation, the Shuntogule area was dominated by continuous burial, but it experienced significant uplift in the Late Ordovician, Middle Devonian, Late Carboniferous, Late Permian, and other stages, and the burial depth exceeded 2000 m after the Silurian Period, and is now up to 7000 m. The Ordovician strata in this area are the Penglaiba Formation, Yingshan Formation, Yijianfang Formation, Chalbacke Formation, and Chalchalke Formation from the bottom–up, and carbonate deposits developed in the Middle and Lower Ordovician [31,32,33]. Under the influence of the Middle Caledonian Phase I movement, the sea level began to rise rapidly, and the study area was submerged as a whole, ending the carbonate deposits. Therefore, the target strata of this study were the Penglaiba Formation, Yingshan Formation, and Yijianfang Formation (Figure 1c).
After the end of the Cambrian Period, the sea level rose slowly, and the limited platform area of the Tarim Basin began to shrink, only remaining in the western Bachu area [34,35]. The Shuntogol area is located on the western platform, and its terrain is lower than the Kathak uplift and Tabai Saya uplift, and it is in the transition zone to the east into the basin [30]. During the Middle and Lower Ordovician sedimentary periods, the Shuntogol area was adjacent to the platform margin and developed an open platform facies continuously [36,37]. Taking Well GC9 as an example, the target strata in the study area can be divided into 8 third-order sequences and 29 fourth-order sequences. Figure 2 shows the division of third-order and fourth-order sequences in the lower member of Penglaiba Formation and Yingshan Formation. High-stage karst and low-stage karst occurred under the fourth- and third-stage sequence interfaces, respectively, forming the initial high-porosity system. The dolostone reservoir as a whole presents the characteristics of thick dolostone interspersed with thin limestone, showing regular changes with high-frequency cycles. Dolomitization is most developed under the quaternary sequence interface, while dolomitization is weak or not developed in the parts away from the quaternary sequence interface (Figure 2). Therefore, it is speculated that the dolomitization fluid channel migrates along the sequence interface.

3. Materials and Methods

The core samples were collected from the Gucheng well area, Shunnan well area, and Shunbei well area, all of which consist of dolostone from the Yingshan Formation and Penglaiba Formation, with a predominant presence of that from the Yingshan Formation. The geochemical characteristics of different types of Ordovician dolomites in the Shuntogol area were analyzed using cathode luminescence, electron probe analysis, carbon and oxygen isotopes, trace elements, and X-ray diffraction data. The petrographic observation and geochemical analysis were conducted at the State Key Laboratory of Reservoir Geology and Development at the Chengdu University of Technology. Cathode luminescence analysis was performed using a CL8200 MK5 instrument mounted on a Leica microscope platform. Initially, the wafer to be measured was placed in a vacuum chamber where it was evacuated to 0.3 Pa before exciting and stabilizing the beam at 100~500 μA. Luminescence conditions were observed and recorded while images were collected once the cathode’s highest voltage reached 10~15 kV.
Electron probe analysis utilized an EPMA-1720-h electron probe micro-analyzer to select representative dolomite minerals on marked wafers that were then numbered. A conductive carbon film approximately 20 nm thick was coated onto each tested wafer using a vacuum plating device before placing them into sample vacuum tanks for microelement quantitative analysis under accelerating voltage conditions of 15 kV with a beam current set at 10 nA.
Before conducting the carbon and oxygen isotope analysis, 200 samples of ground rock powder were introduced into a Labco Exetainer reaction bottle with a volume of 12 mL and sealed. The samples were then purged with high-purity helium gas (>99.999% purity) at a flow rate of 100 mL/min to remove any atmospheric contaminants. Subsequently, the released CO2 from the reaction was separated using a gas chromatographic column maintained at 70 °C. The purified CO2 was subsequently introduced into a Finnigan MAT-253 gas stable isotope mass spectrometer for final detection.
The pre-test processing procedure for rare earth element analysis involved grinding the sample to achieve a particle size of 200 mesh, followed by weighing out approximately 40 mg of the powdered sample. This weighed sample was then dissolved, dried, and extracted to obtain a constant volume solution. During analysis, the concentrations of various rare earth elements including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were determined using an inductively coupled plasma mass spectrometer (Varian 820).
X-ray diffraction order analysis was performed with an X’PERT Pro MPD X-ray powder diffractometer. Prior to testing, the pure white marble sample was ground to achieve a particle size of 200 mesh. The powdered sample weighing no less than 5 g was obtained using the tablet method. The anode target voltage and current used in this optical tube setup were set at 40 kV and 40 mA, respectively. The measured range for the 2θ angle spanned from 20° to 40°.

4. Results

4.1. Dolomite Petrology

The dolomite-bearing reservoir types of the target strata in the study area were mainly dolostone and limestone–dolostone transition rocks, among which the transition rocks could be divided into dolomite, dolomitic limestone, and gray and lime-bearing dolomite according to the relative content of lime and dolomite. According to the structure type, the dolomites could be divided into microcrystalline, fine-crystalline, medium-crystalline, and coarse-crystalline dolomite. Its porosity was not high, while its permeability varied greatly (Table 1), showing strong heterogeneity.
In terms of the distribution horizon, the crystalline dolomites were mainly distributed in the lower part of the Penglaiba Formation and Yingshan Formation (Figure 3A,B). These dolomites had good crystalline shape, large thickness, and developed intercrystalline pores and intercrystalline dissolution pores and fractures. The limestone–dolomite transition rocks were mainly distributed in the upper member of the Yingshan Formation and Yijianfang Formation, and the dolomites were scattered or porphyritically distributed in the limestone (Figure 3C). The dolomite was generally heteromorphic or hemidiomorphic, the intergranular contact was Mosaic, and some samples had pyrite development, residual micrite, and autogenous quartz crystalline qualities and occasionally black argillaceous qualities. In addition, most of the dolomites developed along the stylolites, which were common at all levels, were idiomorphic or semi-idiomorphic, and some of them could be seen with bright edges of fog centers (Figure 3D,E), indicating that they had undergone compaction and pressure dissolution. There was also a class of dolomite developed along fractures, the crystalline shape of which was rhombohedral, with a meso-crystalline or coarse crystalline structure. This kind of dolomite could be distributed across layers, and two phases of filling could be formed in some fractures with relatively developed dolomites. The first phase developed in a powdery shape at the edge of the fracture, and the second phase presented a coarse crystalline distribution inside the fracture (Figure 3F).

4.2. Geochemical Characteristics

4.2.1. Cathode Luminescence Characteristics and Electron Probe Results

In the diagenetic process of carbonate rocks, trace elements are generally rich in Fe and Mn and poor in Na, Mg, and Sr [32,33], in which Sr can be used as a tracer element of seawater, and Na can indicate the salinity of diagenetic fluid [34,35]. The test results showed that the dolomite in fine-powdery dolostone and dolomitic limestone had a lower Mn value and higher Sr value (Figure 4a,b,e–g), while the dolomite in medium-coarse crystalline dolomite and the dolomite filled in solution holes, caverns, and tectonic fractures had higher Fe and Mn values and medium Sr values (Figure 4c,i). The dolomites distributed along the stylolites had high Mn values and medium high Sr values (Figure 4d,h). The dolomite filled in cracks had high Mn and Sr values and high Na values (Figure 4j–l).
In addition, the cathodic luminescence characteristics of the dolomites in the study area were quite different. The existing crystalline dolomites with Fe/Mn values lower than 7 showed bright red light or bright orange–red light under cathode rays (Figure 5A). There were also dolomites with Fe/Mn values between 8.33 and 79, which were distributed along the stylolites, and most of their fog cores emitted no light or dim light, while the bright edges emitted dark–brighter orange light (Figure 5B).

4.2.2. Carbon and Oxygen Isotope

According to the available samples, the distribution of δ13C (PDB) ‰ and δ18O (PDB) ‰ in the Middle and Lower Ordovician dolomite ranged from −0.10‰ to 0.19‰ and from −9.99‰ to −13.47‰, respectively. The distribution of δ13C (PDB) ‰ and δ18O (PDB) ‰ in fine-grained dolomite ranged from −2.59‰ to 0.24‰ and from −13.47‰ to −6.44‰. The δ13C (PDB) ‰ and δ18O (PDB) ‰ of the mesocratic dolomite were −2.59‰ and −12.35‰, respectively (Figure 6).

4.2.3. Rare Earth Element

The analysis results showed that the total REE content of Ordovician carbonate rocks in the study area was not high; the highest value was only 10.18 × 10−6, while the dolomite ∑REE value was higher than that of limestone, reflecting the difference between dolomitization fluid and marine fluid when limestone was formed. The LREE was enriched in the samples, and ∑LREE/∑HREE also showed a rule that the higher the degree of dolomitization, the greater the value. The δCe values were all negative anomalies, while the δEu values of fine-powdered porphyritic and glomerular dolomite were positive anomalies (Figure 7).

4.2.4. Order Degrees of Dolomite

The order degrees of Ordovician dolomites in the study area varied greatly, and the coarse-grained and medium-grained dolomites as a whole showed a high order degree, while the fine-grained and powdery dolomites had a low order degree (Figure 8). Among them, the order degree of powdery dolomite was 0.56~0.68, with an average of 0.61. The order degree of fine-grained dolomite was 0.52~0.74, with an average of 0.62. The order degree of mesocrystalline dolomite was 0.62~0.94, with an average of 0.79. The order degree of coarse crystalline dolomites was 0.72~0.97, with an average of 0.83, and the order degrees of the four crystalline types of dolomites were quite different.

5. Discussion

5.1. Origin of Dolomites

Trace elements in carbonate rocks are rich in Fe and Mn and poor in Na, Mg, and Sr, among which Sr can be used as a tracer element in seawater, and the content of Na can also indicate the salinity of diagenetic fluid [36,37]. In the Ordovician dolostone in the Shuntogol area, the dolomites in fine-powdery dolostone and dolomites in dolomitic limestone had lower Mn values and higher Sr values (Figure 4a,b,e–g), indicating that dolomitization occurred in a relatively open diagenetic environment [38,39,40]. The medium-coarse crystalline dolostone and dolomite filled in solution holes, caverns, and tectonic fractures had high Fe and Mn values and medium Sr values (Figure 4c,i), indicating that dolomite was formed by late diagenesis [41,42,43]. The dolomites distributed along the stylolites line had high Mn value and medium high Sr value (Figure 4d,h), indicating that dolomitization was involved in marine flow and occurred in a relatively closed diagenetic environment [44]. The dolomite filled in fractures had high Mn and Sr values and high Na values (Figure 4j–l), which were related to the participation of hydrothermal fluids in diagenesis [45,46].
In addition, the cathodic luminescence characteristics of dolomites in the study area were quite different. The existing crystalline dolomites with Fe/Mn values lower than 7 showed bright red light or bright orange–red light under cathode rays (Figure 5A), indicating a deep buried reduction environment. There were also dolomites with Fe/Mn values between 8.33 and 79. These dolomites were distributed along the stylolites, and most of their fog cores were non-luminous or dim-luminous, reflecting the weak oxidation or near-surface oxidation environment of their original sediments, while the bright edges were dark–brighter orange-red (Figure 5B), indicating that these dolomites occurred in shallow burial environments [47].
From the geochemical index, the δ13C of the dolomites was near the normal seawater range, while the δ18O values were negative. Under the burial condition, the oxygen isotope will migrate to the negative direction due to the high temperature of the underground brine [48], so the dolomite of the Middle and Lower Ordovician Period in the Shuntogol area occurred in the closed burial diagenetic stage. In terms of rare earth elements, the total REE content was not high; the highest value was only 10.18 × 10−6, while the dolomite ∑REE value was higher than that of the limestone, reflecting that the dolomitization fluid was different from the marine fluid when the limestone was formed [40,41,42]. The LREE was enriched in the samples, and ∑LREE/∑HREE also showed a rule that the higher the degree of dolomitization, the greater the value [49]. The δCe values were all negative anomalies, indicating that the dolomitization fluid was reductive. The δEu values of fine-powder porphyritic and clump dolostone were positively abnormal, reflecting the presence of hydrothermal fluid in diagenetic fluid (Figure 6). The order degree of the dolomite as a whole showed that the order degrees of the coarse- and medium-crystalline dolostone were high, and the order degrees of fine and powdery dolostone were low. Powdery and fine crystalline dolostone are mainly found in the upper part of Yingshan Formation and Yijianfang Formation [43]. They are mainly heteromorphic and semi-idiomorphic, with a poor idiomorphic degree and low-order degree, reflecting that they were formed by rapid crystallization under high-salinity, medium conditions, and are related to quasi-syngenetic dolomitization [50,51,52]. Most of the mesocrystalline and coarse-grained dolostones are located in the lower part of Penglaiba Formation and Yingshan Formation, and the order degree is higher, which indicates that the dolomitization is more thorough and crystallization temperature is higher.
Based on the above analysis, the medium-coarse crystalline dolomite in the thick-layer dolostone was the origin of deep-buried dolomitization. The dolomites distributed along the stylolites had an origin in dolomitization. The fine dolomite and the fine dolomite in the dolomitic limestone were the origins of the “evaporation pump” dolomitization in the same generation. The medium-coarse crystalline dolomite filled with caverns was of late-hydrothermal dolomitization origin. According to the above analysis, dolomitization in the study area occurred in a relatively closed diagenetic environment, which was the result of buried dolomitization.

5.2. Dolomite-Related Reservoir Development Model

The Ordovician thick-layer dolostone in the study area was the product of a multi-period buried dolomitization superimposed process, and its development process was controlled by a multilevel sequence [53]. Due to the limited scale, the upper member of the Yingshan Formation and Yijianfang Formation, in which are distributed a small amount of limestone, were not included in this model discussion. With the high-frequency and short-period sea level rise and fall, multiple sets of high-stage karst pore systems developed in the target strata in the study area [54]. At this time, the possibility is not excluded that the platform beach height in the sedimentary area is enriched by the limited water body due to evaporation, which causes the quasi-syngenetic dolomitization of the newly deposited limestone (Figure 9A).
These different types of dolomite-related reservoirs underwent burial diagenesis and metasomatic dolomitization in the later stage, so the thick dolomitization in the study area was the product of the superimposed process of multi-stage buried dolomitization [41]. Combined with other aspects of understanding, the development model of the dolomite-related reservoir was summarized [48].
At the beginning of the Ordovician period, the structure around the Tarim Basin was stable, the sea level rose slowly. The restricted platform area began to shrink, only remaining in the western Bachu area, and the Shuntogule area began to develop an open platform, main platform, and beach limestone deposits [39]. Along with the six–four high-frequency short-period sea level rise and fall, multiple sets of high-stage karst pore systems developed in the Ordovician carbonates in the Shuntogole area [40]. At this time, Mg2+ enrichment in the restricted water body due to evaporation was not excluded in the platform beach height of the deposition area, which promoted the quasi-syngenetic dolomitization of the newly deposited limestone (Figure 9A). When the depth of the Penglaiba Formation and the lower member of the Yingshan Formation reached 100 m and entered the shallow burial stage until 1000 m, the upper member of the Yingshan Formation and the Yijianfang Formation received deposition. Previous studies [53] obtained the ages of 464 ± 12 Ma and 441 ± 16 Ma from two dolostone samples of the Penglaiba Formation in the Yonganba section by using laser U-Pb dating analysis; they were from the Early to Middle Ordovician and Late Ordovician to Early Silurian Periods. Seawater dolomitization could occur in the shallow burial stage, and its dolomitization fluid may have been the mixed fluid of convection of hot seawater and the return of slightly evaporated seawater in the upper part, or it may have been the in-layer fluid retained by mixing fresh water and seawater in the exposed stage [54]. The original pore system superimposed karsts in the lower stage on the basis of the karst in the upper stage [55]. This provided a channel for the lateral migration of dolomitization fluid (Figure 9A). Dolomitization occurred in places where the original pores are rich, and the dense mud micrite developed during the transgression period, which did not develop secondary pores due to the lack of channels for dissolution fluids to enter, so dolomitization did not occur.
When the depths of the lower part of the Penglaiba Formation and Yingshan Formation were 1000 m and 4600 m, they entered the medium-deep burial stage [56]. At this time, the strata were completely detached from seawater, and it was the stage of large-scale dolomite diagenesis. The basic and geochemical characteristics of the thick Ordovician dolostone in the Shuntogol area showed that the development environment of this set of dolomites had reduction conditions [48]. The dolomites had good crystalline shape, large thickness, high Fe and Mn values, and mostly showed bright red light or bright orange light under cathode rays [49]. The δ18O values all showed a negative bias, the δCe values were negative anomalies, and the δEu values were positive anomalies. The high degree of order indicated that the dolomite was formed in a closed burial environment [56]. When the Penglaiba Formation and the lower part of the Yingshan Formation entered the middle burial period, the main source rocks of the lower Cambrian series entered the oil generation peak, and the resulting formation overpressure provided the power source for the upward migration of the lower magnesium-rich fluid [57,58,59]. The dolomitization fluid channel was still dominated by the original pores, continuously amplified by the late-low stage and the superimposed karst stage [60]. Overall dolomitization occurred along the network of pores, fractures, and caverns in the upper region, resulting in the formation of the middle-coarse crystalline dolomites in the thick layers of the Penglaiba Formation and the lower Yinghe Formation, and the recrystallization of the whole or part of the dolomites of the mud-microcrystalline dolomites that formed in the quasi-syngenetic and shallow burial periods was also completed (Figure 9B). In this thick layer, the intercrystalline pores and intercrystalline dissolved pores were relatively developed (Figure 9C), inheriting the primordial biological advantages of granular limestone in the upper region and are the most favorable set of Ordovician dolostone reservoirs in the study area [61]. In the late Hercynian movement, extensive magmatic movement occurred in the Tarim Basin, and hydrothermal dolostone could have also been formed in some parts of the study area (Figure 9B). Although magnesium-rich hydrothermal fluid activity could have carried out dissolution transformation along the fracture and formed high-quality reservoirs in local areas, most of the hydrothermal recrystallized dolomites of other and idiomorphic origin were formed near the fracture. These dolostones were mostly with enlarged edges, causing damage to the original porosity and permeability channels, which was not conducive to the formation of high-quality reservoirs.
Minerals 14 00545 g009
Figure 9. Development model of dolomitization of Ordovician carbonate rocks in the Shuntogol area (sequence classification was modified after [62]). (A) Dolomitization controlled by the third order, (B) dolomite formed in the penosyngenetic and shallow burial periods, (C) intercrystalline pores and intercrystalline dissolved pores of dolomite.
Figure 9. Development model of dolomitization of Ordovician carbonate rocks in the Shuntogol area (sequence classification was modified after [62]). (A) Dolomitization controlled by the third order, (B) dolomite formed in the penosyngenetic and shallow burial periods, (C) intercrystalline pores and intercrystalline dissolved pores of dolomite.
Minerals 14 00545 g009

6. Conclusions

(1) After the Cambrian Period, the limited platform area in the Tarim Basin began to shrink, and the study area was low in the Early and Middle Ordovician Periods. It continued to develop open platform facies, and the thick-layer dolostone of the Middle and Lower Ordovician Periods was not formed in the quasi-contemporaneous period.
(2) Combined with the analysis of petrographic and geochemical characteristics, the thick-layer dolostone that developed in the Middle and Lower Ordovician series in the study area was the result of buried dolomitization, and the fluid channel of dolomitization was the granular beach porosity system formed by karsts at the multilevel sequence interface.
(3) When the Middle and Lower Ordovician Series entered the burial period in the study area, the main hydrocarbon-source rocks of the Lower Cambrian series entered the oil generation peak, and the resulting formation overpressure provided the power source for the upward migration of the lower magnesium-rich fluid, and the dolomitization fluid entered the karst pore system in the target layer to produce all the dolomitization.

Author Contributions

Conceptualization, L.Z.; formal analysis, H.F. and Y.S.; investigation, S.Z.; resources, X.Y. and Y.D.; writing—review and editing, L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing (No. PRP/open-2104).

Data Availability Statement

The datasets presented in this article are not readily available because the well data is confidential to commercial companies.

Acknowledgments

We would like to thank the Sinopec Northwest Oilfield Exploration and Development Research Institute for their technical input throughout the period of this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Location and structural feathers of the Tarim Basin; (b) the division of tectonic units of the study area; (c) stratigraphic column of the Ordovician in the Shuntogol area.
Figure 1. (a) Location and structural feathers of the Tarim Basin; (b) the division of tectonic units of the study area; (c) stratigraphic column of the Ordovician in the Shuntogol area.
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Figure 2. Characteristics and sequence sedimentary evolution of Ordovician dolostone in Well GC9.
Figure 2. Characteristics and sequence sedimentary evolution of Ordovician dolostone in Well GC9.
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Figure 3. Microscopic characteristics of dolomite (A) Well GC7, O1p, 6540 m, dolomite intercrystalline dissolution hole (-); (B) Well SN5, O1-2y, 7073.75 m, mud-powdery dolomite (-); (C) Well SN5, lower O1-2y, 7178.31 m, dolomitic limestone (-); (D) Well GL2, upper O1-2y, 5984.25 m, strong dolomitization along the stylolite (-); (E) Well SB2, O2yj, 7441.05 m, dolomite distribution along the stylolite (-); (F) Well SN5, O1-2y, 7073.20 m, fine-grained dolostone, two-stage dolomite filled in the solution cavity (-).
Figure 3. Microscopic characteristics of dolomite (A) Well GC7, O1p, 6540 m, dolomite intercrystalline dissolution hole (-); (B) Well SN5, O1-2y, 7073.75 m, mud-powdery dolomite (-); (C) Well SN5, lower O1-2y, 7178.31 m, dolomitic limestone (-); (D) Well GL2, upper O1-2y, 5984.25 m, strong dolomitization along the stylolite (-); (E) Well SB2, O2yj, 7441.05 m, dolomite distribution along the stylolite (-); (F) Well SN5, O1-2y, 7073.20 m, fine-grained dolostone, two-stage dolomite filled in the solution cavity (-).
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Figure 4. Characteristics of trace elements and cathodic luminescence intensity in dolomite: (a) bright dolomite portion of powdery dolomite; (b) dark dolomite portion of powdery dolomite; (c) extrapolated powdery dolomite; (d) dolomite bright edges distributed along the stylolites of sparry arenaceous limestone; (e) dolomite-bearing micrite, same occurrence as that in (d); (f) the same lithology as that in (e), dolomite fog core distributed along the stylolites; (g) lithology is the same as that in (d), dolomite in the matrix; (h) dolomite clumps along the stylolites of dolomite-bearing sparry arenaceous limestone; (i) pebbly sparry arenite limestone, same occurrence as that in (h); (j) dolomite filled with dissolution holes of siliceous dolomite; (k) same lithology as that in (d), dolomite filled in fractures; (l) the lithology is the same as that in (i), and the occurrence is the same as that in (k).
Figure 4. Characteristics of trace elements and cathodic luminescence intensity in dolomite: (a) bright dolomite portion of powdery dolomite; (b) dark dolomite portion of powdery dolomite; (c) extrapolated powdery dolomite; (d) dolomite bright edges distributed along the stylolites of sparry arenaceous limestone; (e) dolomite-bearing micrite, same occurrence as that in (d); (f) the same lithology as that in (e), dolomite fog core distributed along the stylolites; (g) lithology is the same as that in (d), dolomite in the matrix; (h) dolomite clumps along the stylolites of dolomite-bearing sparry arenaceous limestone; (i) pebbly sparry arenite limestone, same occurrence as that in (h); (j) dolomite filled with dissolution holes of siliceous dolomite; (k) same lithology as that in (d), dolomite filled in fractures; (l) the lithology is the same as that in (i), and the occurrence is the same as that in (k).
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Figure 5. Dolomite cathode luminescence characteristics: (A) Well SN5, O1p, 7075.30 m, powdery dolostone, overall red; (B) Well SN4, O2yj, 6333.49 m, sparry sand arenite limestone, dolomite distribution in the seam line, dolomite orange–red light, fog core bright-edge phenomenon is obvious.
Figure 5. Dolomite cathode luminescence characteristics: (A) Well SN5, O1p, 7075.30 m, powdery dolostone, overall red; (B) Well SN4, O2yj, 6333.49 m, sparry sand arenite limestone, dolomite distribution in the seam line, dolomite orange–red light, fog core bright-edge phenomenon is obvious.
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Figure 6. Carbon and oxygen isotope characteristics of dolomite.
Figure 6. Carbon and oxygen isotope characteristics of dolomite.
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Figure 7. Characteristics of rare earth elements in dolomite. The data were standardized for chondrite rare earth elements [32]. The marine limestone in the table is used for data comparison only.
Figure 7. Characteristics of rare earth elements in dolomite. The data were standardized for chondrite rare earth elements [32]. The marine limestone in the table is used for data comparison only.
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Figure 8. Dolomite order degree characteristics.
Figure 8. Dolomite order degree characteristics.
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Table 1. Statistical table of physical property parameters of dolostone in the study area.
Table 1. Statistical table of physical property parameters of dolostone in the study area.
StrataLithologyPorosity Max/%Porosity Min/%Porosity Mean/%Permeability Max/mdPermeability Min/mdPermeability Mean/md
Lower member of Yingshan FormationDolostone2.30 0.20 1.13 14.50 0.001 1.19
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Zhong, L.; Cheng, L.; Fu, H.; Zhao, S.; Ye, X.; Ding, Y.; Senlin, Y. Genesis and Related Reservoir Development Model of Ordovician Dolomite in Shuntogol Area, Tarim Basin. Minerals 2024, 14, 545. https://doi.org/10.3390/min14060545

AMA Style

Zhong L, Cheng L, Fu H, Zhao S, Ye X, Ding Y, Senlin Y. Genesis and Related Reservoir Development Model of Ordovician Dolomite in Shuntogol Area, Tarim Basin. Minerals. 2024; 14(6):545. https://doi.org/10.3390/min14060545

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Zhong, Liangxuanzi, Leli Cheng, Heng Fu, Shaoze Zhao, Xiaobin Ye, Yidong Ding, and Yin Senlin. 2024. "Genesis and Related Reservoir Development Model of Ordovician Dolomite in Shuntogol Area, Tarim Basin" Minerals 14, no. 6: 545. https://doi.org/10.3390/min14060545

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