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. 2022 Dec 1;7(49):45287–45300. doi: 10.1021/acsomega.2c05830

Sedimentary Characteristics of the Permian Zhesi Formation in Eastern Inner Mongolia, China: Implications for Sedimentary Background and Shale Gas Resource Potential

Fei Hu †,‡,§, Jianpeng Wang †,*, Zhaojun Liu †,‡,§
PMCID: PMC9753208  PMID: 36530247

Abstract

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Eastern Inner Mongolia of China is located between the Siberian plate, the North China plate, and the Pacific plate and has a complex history of tectonic and sedimentary evolution. The sedimentary strata of the Zhesi Formation in the Middle Permian recorded rich environmental, structural, and petroleum geological information, which has great significance to the Paleozoic geological research in Northeast China. Through field outcrop observation and profile measurement, combined with geochemistry, mineralogy, and the reservoir physical property test, the sedimentary environment, tectonic setting, and shale gas resource potential of the Middle Permian Zhesi Formation are analyzed. The sedimentary facies of the Zhesi Formation are distributed in strips in the northeast direction, mainly developing littoral, shallow marine, and bathyal sedimentary environments. Clastic rock deposits are mainly developed in the littoral facies, carbonate platform and tempestite deposits are mainly developed in the shallow marine facies, and mudstone mixed with turbidite deposits are mainly developed in the bathyal facies. The sedimentary environment and chronological characteristics show that the Paleo-Asian Ocean was not completely closed in the Middle Permian, and its complete closure time should be later. The characteristics of source rocks and the shale gas resource potential in the Solon area are discussed. Controlled by the sedimentary environment, the Solon area mainly deposited thick dark shale mixed with turbidite sandstone, the accumulated thickness of the dark shale is more than 200 m, with an average vitrinite reflectance (Ro) of 2.44%, and the average residual total organic carbon (TOC) content is 0.85%. The average content of brittle minerals is 60.2% and the shale foliation fracture is developed, which is easy to form natural fractures and induced fractures, so the shale has good hydrocarbon generation potential, and the generated shale gas can exist in shale in an adsorbed state and a free state. In addition, the shale gas generated in the shale can migrate to the lenticular turbidite sand body in a short distance to form free shale gas. Therefore, there is a certain shale gas resource potential in the Solon area, and finding a favorable area with high TOC is the key to future exploration of Upper Paleozoic shale gas.

1. Introduction

Late Paleozoic sediments from eastern Inner Mongolia in Northeast China record the formation and evolution of the Paleo-Asian Ocean, which was located between the Siberia, North China, and Pacific plates.1−5 From the perspective of sedimentation, the types and characteristics of shale are the focus of shale oil and shale gas research in recent years,6−9 while the Paleozoic and older strata are the focus of shale gas research.10−13 The thick, fine-grained sediments of the Zhesi Formation in the Middle Permian have potential as a petroleum source rock and have not experienced regional metamorphism. The Zhesi Formation is one of four major source rocks in the late Paleozoic strata in northeast China.2,14 Previous work on the Zhesi Formation has focused on paleobiology, sequence stratigraphy,15−17 provenance, and tectonic setting,18−21 whereas few studies have considered the depositional characteristics and petroleum potential of the source rocks, and the study on the distribution of sedimentary environment only takes the Permian as the research unit.22−24 In addition, the closure time of the Paleo-Asian Ocean is controversial, some work has suggested that the Paleo-Asian Ocean closed in the Middle Permian or Early Carboniferous, before deposition of the Zhesi Formation,25−28 whereas other work suggests that the Paleo-Asian Ocean closed much later, in the late Permian.29−32 Combined with field work and geochemical analyses, the paleo environment and source rock potential of the Zhesi Formation have been studied. The results define the sedimentary distribution of the Zhesi Formation and provide constraints on the timing of the closure of the Paleo-Asian Ocean and yield information on the formation and potential of an important petroleum source rock of shale gas.

2. Geological Setting

Eastern Inner Mongolia is located in the northernmost part of China, which is connected with Mongolia and Russia in the north and is adjacent to Songliao Basin in the south. (Figure 1a). This area not only experienced the evolution of the Paleo-Asian Ocean tectonic system during the Paleozoic period but also was superimposed and transformed by the circum-Pacific tectonic system and the Mongolia-Okhotsk suture belt in the Mesozoic and Cenozoic periods, with a complex tectonic evolution history. Within the Jiamusi–Mongolia block, there is a band of late Paleozoic sediments. Late Carboniferous–Permian sediments are interpreted to have formed in a large marine sedimentary basin connected to the Paleo-Asian Ocean.1,33−35

Figure 1.

Figure 1

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Location and stratigraphic development of the study area. (a) Regional location and distribution of field geological survey points. (b) Generalized stratigraphic column.

Paleozoic strata in Eastern Inner Mongolia include rocks of Silurian, Devonian, Carboniferous, and Permian ages. Permian strata include the Shoushangou, Dashizhai, Zhesi, and Linxi formations.18,36,37 The Middle Permian Zhesi Formation consists of successive sedimentary deposits, including carbonates, sandstones, mudstones, and silty mudstones that show features of a passive continental margin (Figure 1b).

3. Samples and Methods

The research methods mainly include field geological surveys, profile measurements, and experimental testing. The field geological survey includes 25 observation outcrops, and there are mainly three measured profiles, which are located in West Ujimqin Banner, Horqin Right Front Banner, and Arong Banner (Figure 1a). The outcrops were photographed and described; in addition, the measured profiles were also stratified and sampled in detail. Among them, the Zhesi Formation is best exposed in the Solon profile (P1) of the Horqin Right Front Banner, an 821-m-thick section was measured and divided into 42 layers, and the petrology and sedimentary characteristics were described in detail. Fifty-four samples were taken from this profile, in which 29 sandstone samples were selected for micro slice analysis, 25 mudstone samples were selected for total organic carbon (TOC) and rock pyrolysis, and five samples were selected for vitrinite reflectance (Ro) analysis. At the same time, two shale samples were selected for scanning electron microscopy (SEM) and porosity and permeability tests (Figure 2). Typical mudstone, limestone, and sandstone samples from other outcrops were selected for TOC, slice, and paleontological identification.

Figure 2.

Figure 2

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Distribution positions and test items of the samples on the measured profile P1.

3.1. Slices

The preparation and microscopic observation of the sandstone slices and biological fossils were conducted in the Key Laboratory of Oil Shale and Coexistent Energy Minerals of Jilin Province, Changchun, China, following the standard of SY/T 5368-2000.

3.2. TOC and Pyrolysis

The TOC and pyrolysis tests were conducted in the Key Laboratory of Oil Shale and Coexistent Energy Minerals of Jilin Province, Changchun, China. Mudstone samples were crushed to <250 μm and pretreated with concentrated HCl to determine TOC on a Leco CS-230, following the standard of GB/T 19145-2003. Pyrolysis was carried out using a Rock-Eval 6. The quantity of pyrolyzate generated from kerogen during gradual heating in a helium stream was normalized to TOC to calculate the hydrogen index (HI). The temperature of maximum generation (Tmax) was used as a maturation indicator.

3.3. Vitrinite Reflectance (Ro)

The Ro test was completed in the Laboratory of Changjiang University, Wuhan, Hubei Province, China. The vitrinite reflectance of 5 samples was measured by using a Leica MPV microscope under reflected white light and polarized light, and 50 randomly selected vitrinite points were measured for each sample. Before each vitrinite reflectance (VR) measurement, a sapphire (0.59% reflectance) and gadolinium gallium garnet standard (1.72% reflectance) were used to calibrate the instrument. The standard deviation of the mean of the vitrinite reflectance distribution was calculated following GB/T6948-2008.

3.4. Biomarkers

Gas chromatography–mass spectrometry (GC–MS) analyses were performed using a 6890N GC/5973B MS instrument equipped with an HP-5MS fused silica column. Representative portions of the selected samples were extracted using CH2Cl2 in a Dionex ASE 200 accelerated solvent extractor for about 1 h at 75 °C and 5 × 106 Pa. Then, the total solvent was evaporated to 1 mL in ZYMARK TurboVap 500 closed cell concentrator, and asphaltene was precipitated from the hexane–CH2Cl2 solution (80:1) and separated by centrifugation. Hexane-soluble organic matter mainly included saturated hydrocarbons, aromatic hydrocarbons, NSO compounds, and asphaltenes. The saturated hydrocarbons were further dissolved in hexane for GC–MS analysis.

3.5. Field Emission SEM

Using an Apreo field emission scanning electron microscope, the surface of shale was polished by argon ions with a Leica RES 102 ion polisher before observing the sample. It was smoother than mechanical polishing and was nondestructive to the sample, avoiding artificial pores caused by mechanical polishing, and carbon was plated on the surface of the sample with a thickness of 13 nm. When observing the sample, 5 kV voltage was used and 0.4 nA beam current was used, and the working distance was 4 mm.

3.6. Porosity and Permeability

The executive standard of porosity and the permeability data test is the determination of pore size distribution and porosity of solid materials by the mercury intrusion method and gas adsorption method Part 1: mercury intrusion method (GB/ T21650.1-2008), and the effective measurement pore size range is 0.0071–200 μm.

4. Results

4.1. Sedimentary Facies and Sediments

Based on the field outcrop observation and the profile measurement, the lithology, sedimentary structure, and sedimentary sequence are analyzed. Combined with slice identification, it is considered that the marine sedimentary environment is mainly developed in the study area, and the littoral, shallow marine, and bathyal facies can be further identified.

4.1.1. Littoral Facies

The littoral environment is equivalent to the upper part of the shelf and located above the wave base. Littoral facies mainly identify littoral clastic deposits in the study area, including conglomerate, medium sandstone, and fine sandstone, which are generally well-sorted and rounded. Typical outcrops are located in the East Ujimqin Banner (OC14, OC15) and the north of Solon town of Horqin Right Front Banner (OC4, OC5). Conglomerate and medium sandstone are mainly developed on the outcrop of the East Ujimqin Banner. The conglomerate is mainly variegated, with complex composition, a gravel diameter of 0.5–1.0 cm, medium sorting, and subrounded (Figure 3a). The medium sandstone is mainly grayish white, well-sorted, and subrounded, has developed parallel bedding, and is locally intercalated with thin layers of mudstone (Figure 3b). Medium sandstone and fine sandstone are mainly developed on the outcrop of Solon town. The medium sandstone is gray–white, the weathered surface is light yellow, well-sorted, and subrounded, in which low-angle oblique bedding is developed (Figure 3c). Fine sandstone is mainly gray, and the weathered surface is light brown, mainly block bedding (Figure 3d).

Figure 3.

Figure 3

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Characteristics of littoral clastic rocks of the Zhesi Formation. (a) Conglomerate is medium-sorted and subrounded; (b) medium sandstone with parallel bedding; (c) medium sandstone with low-angle oblique bedding; (d) fine sandstone with block bedding.

4.1.2. Shallow Marine

The shallow marine environment is equivalent to the lower part of the shelf and located between the wave base and storm wave base. Two types of carbonate platforms and tempestite deposits are mainly identified in the study area.

The platform originally refers to shallow water carbonate sedimentary environment with flat terrain, and the latter refers to all shallow water carbonate sedimentary environments, which can be further divided into a reef, beach, tidal flat, lagoon, limited platform, and open platform and other secondary environments.38,39 Beach and limited platforms are identified in the study area.

4.1.2.1. Beach

Beach refers to the environment with shallow water and high energy and mainly deposits allochthonic carbonate particles. It refers to the accumulation place of coarse particles (such as bioclastic, oolitic, sand debris, gravel debris, etc.). It is a high-energy sedimentary environment, which can be subdivided into gravel beach and sand debris beach.

Gravel beach: The formation of gravel beach is mainly related to storm and wave action. It is formed by the early deposited carbonate sediments being rolled up, broken, and accumulated by high-energy storms or waves. In the study area, it is mainly developed in Zhalantun (OC3, P2) and West Ujimqin Banner (OC17) outcrops. The arrangement of gravels in gravelly limestone is nondirectional, the diameter of gravels is 0.8–5 cm, the sorting is poor, and the grinding is mostly subangular–subcircular (Figure 4a). Chert nodules are developed in the gravelly limestone in Zhalantun outcrop (Figure 4b).

Figure 4.

Figure 4

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Characteristics of beach and open platforms of shallow marine facies in the Zhesi Formation. (a) Gravel-clastic limestone formed by storm; (b) chert nodule in gravel-clastic limestone; (c) parallel bedding in sand-clastic limestone; (d) brachiopod fossils; (e) crinoid stem fossils; (f) small, billed shellfish fossils

Sand debris beach: It is mainly developed in the West Ujimqin Banner area (OC16), which is composed of medium–fine sand carbonate particles. It is well-sorted, subangular–subcircular, and low-angle oblique bedding formed by the wave action which can be observed (Figure 4c). At the same time, a variety of marine fossils can be seen in arenaceous limestone, including brachiopods (Figure 4d), crinoids (Figure 4e), and small-billed shellfish (Figure 4f). They have a wide variety and are relatively well preserved. They are typical marine organisms of the Zhesi fauna.1,40

4.1.2.2. Limited platform

The limited platform refers to the relatively low-lying subtidal area where the movement of water is limited because of the shelter of shoals. The hydrodynamic conditions are weak, the energy is continuously low, and the salinity is slightly high, which are not conducive to the development of wide sea organisms, and the biological species and abundance are low. The main rocks are micritic limestone, argillaceous banded micritic limestone, and parasyngenetic dolomite. Their basic characteristics are the lack of high-energy particles, the common bioclastic in the rocks are mostly euryhaline organisms, and chert nodules can be developed. In the study area, it is mainly observed in Horingol (OC7) and Arun Banner (OC3, P2) in Inner Mongolia. The weathered color of micrite limestone is mostly earthy yellow (Figure 5a), gray, and the fresh color is dark gray. The thickness of the single layer is mostly more than 5 m, and the maximum thickness is 30 m (Figure 5b), and most of them are massive structures (Figure 5c), in which contemporaneous dolomite nodules can be seen (Figure 5d).

Figure 5.

Figure 5

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Sedimentary characteristics of the limited platform. (a) Micrite limestone with a weathered earthy yellow color; (b) thick micrite limestone; (c) massive micrite limestone with fresh dark gray color; and (d) contemporaneous dolomite nodules.

4.1.2.3. Tempestite

Storm deposits are mainly developed on the continental shelf between the normal wave base and the storm wave base, and the typical sedimentary rock type is storm rock.41 Tempestite is a series of sedimentary assemblages formed by storm waves under abnormal climate conditions. Therefore, the study of tempestite is of great significance for paleoclimate and paleoenvironment.42

The complete tempestite sedimentary sequence from the bottom to top is a coarse-grained retained layer section, graded bedding section, parallel bedding section, hummocky cross-stratification section, wave cross-bedding section, and mudstone section, which represents a gradual decline in water energy and a gradual transition from tempestite deposition to the normal shallow sea deposition process. In the West Ujimqin Banner (P3) area, except for the uppermost mudstone section, other sections are better observed (Figure 6).

  • (1)

    Coarse-grained retained layer section: Coarse-grained retained sediment is the retained sediment at the bottom of the storm peak, which is the product of storm winnowing. It is mainly gravelly limestone in the study area. The gravels are poorly sorted and mostly subangular–subcircular, with a particle size of 0.5–1.5 cm, and arranged directionally. They are the products of re-transportation and accumulation of sediments in the early stage of storm-crushing.

  • (2)

    Graded bedding section: The graded bedding section was formed in the storm energy decreasing stage. It is a layer section with positive progressive bedding formed by the transformation of the storm from the high-energy period to the low-energy period. It is developed on the coarse-grained residual layer section, which is manifested by the gradual transition from gravelly limestone to sandy limestone. From the bottom to top, the gravel content gradually decreases to disappear, and the particle size gradually becomes fine.

  • (3)

    Parallel bedding section: After the peak of the storm, the velocity of storm density flow began to decrease, fine debris materials quickly deposited from the suspended state, and the strong shear flow at the bottom formed parallel bedding. Lithology is mainly sandy limestone, which is developed on the graded bedding section.

  • (4)

    Hummocky cross-stratification section: With the attenuation of storm energy, the storm current is further weakened, but due to the oscillation of waves, the current forms hummocky cross-stratification, which is developed on the parallel bedding section, and the lithology is mainly sandy limestone.

  • (5)

    Wave cross-bedding section: In the late stage of storm evolution, the water flow gradually transformed to traction current, forming wave cross-bedding, and the bottom shape of wave marks was mainly observed in the study area.

  • (6)

    Mudstone section: After the storm period, the formation of normal shallow sea fine-grained suspended solid deposition, mainly mudstone. Not observed in the study area.

Figure 6.

Figure 6

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Characteristics of the tempestite sedimentary sequence in the West Ujimqin Banner.

4.1.3. Bathyal Facies

The bathyal facies are located below the storm wave base and mainly develop thick dark mud shale. It is observed in the Horqin Right Front Banner (P1), Ar Khorchin Banner (OC11), and Hexigten Banner (OC23) in the study area. The color is mainly grayish black, and the horizontal bedding is developed (Figure 7a). After weathering, the foliation is slightly developed and is papery (Figure 7b,c). Mudstone in some areas has high maturity and inclines to the slate (Figure 7d).

Figure 7.

Figure 7

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Shale development characteristics of the Zhesi formation in eastern Inner Mongolia. (a) Mud shale with horizontal bedding; (b) mud shale with foliation; (c) mud shale with foliation; and (d) mud shale slate inclined.

Among the dark shale, thin layers of fine sandstone and siltstone are often mixed, which are of turbidite origin. Turbidites are characterized by the development of the Bauma sequence. The complete Bouma sequence from the bottom to top including the progressive bedding segment (Section A), parallel bedding segment (Section B), ripple bedding segment (Section C), horizontal bedding segment (Section D), and massive bedding segment (Section E). The incomplete Bouma sequence can be identified in the study area as section A, section B, or section D of the independent section, and some combination sections can also be identified, such as the combination of section AD, section BD, and section CD of the Bouma sequence (Figure 8). It was observed in the west side of the Linxi County (OC23), Baiyinhua town of the West Ujimqin Banner (OC18), and Solon town of the Horqin Right Front Banner (P1) in the study area.

Figure 8.

Figure 8

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Characteristics of typical distant turbidites of Zhesi Formation.

Furthermore, thin section identification and particle size analysis of sandstone are carried out to explore the hydrodynamic conditions. Thin section identification shows that the sandstone mixed in the mudstone is poorly sorted and subangular, with a high content of rock debris, mainly volcanic rock debris, with the characteristics of turbidite (Figure 9a,b). Meanwhile, the grain-size cumulative frequency curve has a moderate slope, with a 10–25% suspension component (Figure 9c), indicating medium sorting and weak hydrodynamic conditions. The C–M diagram shows a linear trend parallel to the C–M baseline (Figure 9d), indicating turbidite deposition in a bathyal environment.

Figure 9.

Figure 9

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Sedimentary structure and hydrodynamic conditions of turbidite in the bathyal region. (a) Volcanic rock debris in the sandstone, Lv: Volcanic rock debris; (b) grains are poorly sorted, Lg: granite debris; (c) grain-size cumulative frequency curve of turbidite; (d) C–M diagram of turbidite.

4.2. Characteristics of Source Rocks

4.2.1. Source Rock Evaluation

The evaluation of source rocks mainly includes three aspects: the abundance of residual organic matter, the type of organic matter, and the maturity of organic matter. Twenty-five mudstone samples were collected from the solon town profile (P1) and tested (Table 1). The vitrinite reflectance (Ro) test results of five mudstone samples range from 2.05 to 2.75%, with an average of 2.44%, indicating that the source rocks in the study area are in an over-mature stage.

Table 1. Geochemical Test Results of Source Rocks of Zhesi Formation.
no. samples TOC (%) S1 (mg/g) S2 (mg/g) Ro (%) Por (%) Per (10–3 μm2) isoprenoid
 steroid
Pr/nC17 Ph/nC18 C27 C28 C29
1 B1603-05 1.37 0.02 0.08                
2 B1603-08 0.88 0.07 0.11                
3 B1603-09 0.91 0.02 0.07                
4 B1603-10 0.95 0.02 0.08 2.05 1.59 0.0033 0.31 0.17 35 13 52
5 B1603-12 0.84 0.07 0.13       0.32 0.18 40 8 52
6 B1603-13 0.78 0.02 0.08                
7 B1603-15 0.55 0.02 0.09 2.75 1.61 0.0237 0.28 0.19 34 15 51
8 B1603-17 0.80 0.02 0.10       0.45 0.21 38 9 53
9 B1603-19 0.91 0.05 0.13       0.55 0.39 38 10 52
10 B1603-20 1.21 0.02 0.33                
11 B1603-21 0.67 0.03 0.08 2.67     0.65 0.41 38 12 50
12 B1603-22 0.98 0.03 0.11                
13 B1603-27 0.59 0.02 0.08                
14 B1603-30 0.73 0.02 0.09 2.45     0.66 0.48 28 15 57
15 B1603-31 0.77 0.02 0.09                
16 B1603-32 0.50 0.03 0.08                
17 B1603-37 0.67 0.02 0.09       0.52 0.45 31 21 48
18 B1603-39 0.69 0.01 0.06                
19 B1603-41 0.60 0.02 0.10 2.28     0.65 0.54 29 19 52
20 B1603-42 0.94 0.02 0.07                
21 B1603-43 0.82 0.02 0.07                
22 B1603-45 1.33 0.03 0.10                
23 B1603-47 0.80 0.02 0.07                
24 B1603-48 0.74 0.03 0.10       0.64 0.72 30 19 53
25 B1603-51 0.81 0.02 0.09                
maximum 1.37 0.07 0.33 2.75 1.61 0.0237 0.65 0.72 40 21 53
minimum 0.48 0.01 0.06 2.28 1.59 0.0033 0.28 0.17 28 8 48
average 0.85 0.027 0.10 2.44 1.60 0.0135 4.96 0.72 34 14 52
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The residual TOC test results show that the TOC of the Zhesi Formation is mainly between 0.5 and 1.0%, with a total of 22 samples, accounting for 88% of the total test samples, and the residual TOC of 3 samples is between 1.0 and 2.0%, accounting for 12% of the total test samples. The maximum residual TOC is 1.37%, the minimum is 0.50%, and the average is 0.85%. According to the evaluation standard of source rock with TOC,43 the dark mudstone of the Zhesi Formation in the study area is a medium source rock.

The organic matter type is an important indicator to reflect the quality of source rocks and an important factor to determine the hydrocarbon generation capacity and hydrocarbon generation attributes of organic matter. Through the pyrolysis test and analysis of dark mudstone, the free hydrocarbon S1 is between 0.01–0.07 mg/g, with an average of 0.026 mg/g; pyrolysis hydrocarbon S2 is between 0.06–0.16 mg/g, with an average of 0.097 mg/g (Table 1). However, because of the high degree of thermal evolution of organic matter in the study area, the pyrolysis parameters and the related diagrams are inaccurate in distinguishing the types of organic matter.38

Biomarkers can reflect the type of organic matter in over-mature source rocks.44,45 According to the isoprenoid alkane (Pr/nC17, Ph/nC18) data, the organic matter-type discrimination diagram shows that the source rock samples of the Zhesi Formation are located in type II area (Figure 10a); in the sterane triangle diagram, the samples are located in the typeII2 area (Figure 10b). Through the above comprehensive analysis, the organic matter type of the Zhesi Formation in the study area should be mainly type II.

Figure 10.

Figure 10

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Discrimination diagram of the organic matter type of source rock of the Zhesi Formation. (a) Pr/n-C17-Ph/C18 diagram; (b) C27–C28–C29 diagram.

4.2.2. Shale Reservoir Characteristics

Shale reservoir space types are divided into inorganic pores and organic pores, in which inorganic pores are divided into intergranular pores, intragranular pores, and dissolution pores.46 Intergranular pores exist between mineral particles, which are flaky, oblate, and irregular in shape. They are common in the study area, and the pore size is generally 1–4 μm (Figure 11a). Intragranular pores are developed in the interior of mineral particles, which are relatively isolated. They are less developed in the study area, and the pore size is more than 1 μm (Figure 11b). Dissolution pores are formed by chemical dissolution at the edge or inside of unstable minerals such as feldspar and carbonate minerals. They are mainly feldspar dissolution pores, which are relatively developed in the study area, and the pore size is generally 2–5 μm (Figure 11c). Organic pores are uncommon in samples, relatively isolated, round, oval, or concave, with smooth edges, and the pore diameter is generally less than 1 μm (Figure 11d).

Figure 11.

Figure 11

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SEM characteristics of shale reservoir space in the Zhesi Formation. (a) Intergranular pores exist between mineral particles; (b) intragranular pores in the interior of mineral particles; (c) dissolution pores exist between mineral particles; (d) organic pores exist in the organic particles.

The porosity and permeability of the two samples were tested. The porosity values of the two samples were 1.59 and 1.61%, respectively, and the permeability values were 0.0033 × 10–3 and 0.0237 × 10–3 μm2, respectively.

5. Discussions

5.1. Distribution of Sedimentary Facies

Based on the sedimentary facies analysis of 25 field outcrops and 3 measured profiles, the plane sedimentary facies of the Middle Permian Zhesi Formation in eastern Inner Mongolia are restored. It can be seen that the sedimentary facies in the study area are mainly distributed in a northeast strip. The coastal facies are distributed in the northwest, mainly composed of coarse clastic rocks, adjacent to shallow marine facies, mainly composed of carbonate rocks mixed with fine clastic rocks, and the southeast is bathyal facies, mainly composed of dark mudstone mixed with fine clastic rocks (Figure 12).

Figure 12.

Figure 12

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Sedimentary facies distribution of the Middle Permian Zhesi Formation in eastern Inner Mongolia.

5.2. Constraints on the Closing Time of the Paleo-Asian Ocean

During the late Paleozoic period, the Siberian plate rapidly drifted southward and the North China plate drifted northward. The two plates collided during the late Paleozoic period, resulting in the closure of the Paleo-Asian Ocean and the formation of the Xing’an–Mongolia orogenic belt.1,47,48 The closure of the Paleo-Asian Ocean is thought to have occurred from the west to east in a scissor-like manner.49,50 However, the timing of the closure of the Paleo-Asian Ocean remains debated. Closure might have occurred during the Early Permian or Carboniferous period25−28 or as late as the late Permian period.29−32

In the Early Permian period, the North China plate and the Siberia plate subducted each other. The study area is a typical marine deposit of terrigenous clastic rocks of the Shoushangou Formation mixed with a small amount of bioclastic limestone, which is a typical active continental margin deposit. Meanwhile, a large number of volcanic rocks of the Dashizhai Formation are developed at the same time.19,51

During the Middle Permian period, this study found that the Zhesi Formation mainly developed marine sediments such as littoral clastic rocks, gravel and sand debris beaches, tempestite and carbonate platforms of shallow marine, and dark mudstone with turbidite of bathyal, indicating that the eastern part of Inner Mongolia was still a typical continental margin marine sedimentary environment during the Middle Permian period, and there should also be an ocean basin toward the Songliao Basin, although the oceans are shrinking and the northern margin had changed from an active to a passive continental margin (Figure 13). At the same time, the existence of brachiopods in the Zhesi Formation (Figure 4) belongs to the nature of northern biogeography, and its cold-water type attribute indicates that there is a deep-sea ocean basin between the main body of the Siberian plate and the North China plate at this time,1,34 which proves that the Paleo-Asian Ocean was not completely closed in the Middle Permian at least, and its complete closure time should be later than the Middle Permian.

Figure 13.

Figure 13

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Depositional model of the Middle Permian Zhesi Formation in eastern Inner Mongolia.

The late Permian Linxi Formation in the Xing ‘an-Mongolia orogenic belt records the transition from marine to continental lacustrine deposition in eastern Inner Mongolia and the continental lacustrine deposition was found in the lower part of Linxi Formation.36,49,52 In addition, detrital zircons in the Linxi Formation yield old ages, with peaks at ∼1800 and ∼2500 Ma, consistent with the ages of basement rocks of the North China block.53−57 This suggests that the late Permian Linxi Formation received a large volume of sediment from the North China block, confirming that this block had collided with the Jiamusi-Mongolia block by the late Permian. In contrast, these two age peaks are not recorded by the Middle Permian Zhesi Formation. We conclude that the Paleo-Asian Ocean was fully closed by the early Late Permian.

5.3. Shale Gas Resource Potential

Shale gas refers to the unconventional natural gas that is attached to the organic-rich shale and its interlayer and mainly exists in the adsorption and free state.34 Most of the natural gas generated by shale is still stored in shale intervals to form shale gas reservoirs. In the process of shale gas evaluation, organic matter abundance, organic matter maturity, mineral composition, porosity and permeability, thickness, burial depth, and gas content are the most basic parameters to characterize the degree of shale gas enrichment. It is generally believed that shale gas can be formed when the TOC is greater than 0.5%, the Ro is greater than 0.4%, the total porosity of rock is greater than 3%, and the permeability is greater than 0.001 × 10–3 μm2.58−61

According to the test results of the measured profile samples in the Solon area (P1), the oil and gas resource potential in the Solon area is evaluated. The Ro test results range from 2.05 to 2.75%, with an average of 2.44%, from the perspective of the organic genetic evolution stage of natural gas, it is in the stage of pyrolysis gas, and the main gas component generated is methane. Therefore, the shale in the study area has the basic conditions for the formation of shale gas.

Controlled by the sedimentary environment, the solon area is the bathyal environment, mainly depositing thick dark shale mixed with turbidite sandstone (Figure 14). The accumulated thickness of the extremely thick dark shale deposited in the bathyal area is more than 200 m, the residual TOC content of the shale is between 0.48 and 1.37%, with an average of 0.85% (Table 1), the shale has good hydrocarbon generation potential, and the generated shale gas can exist in shale in an adsorbed state. In addition, the shale foliation fracture is developed, and the content of brittle minerals is 44.2–68.1%, with an average value of 60.2%,62 which is easy to form natural fractures and induced fractures, and the average permeability is 0.0135 × 10–3 μm2; these parameters are higher than the evaluation standard of shale gas (Table 1), and shale gas can also exist in shale in a free state. At the same time, the shale gas generated in the shale can migrate to the lenticular turbidite sand body in a short distance to form free shale gas (Figure 14). Therefore, the comprehensive research shows that there is a certain shale gas resource potential in the Solon area. However, in general, the average TOC of shale is low, and the high TOC value is relatively small and concentrated, which restricts the high abundance enrichment of shale gas. At the same time, the porosity of the two shale samples tested is relatively low, which may be related to the low development of organic matter pores caused by a low TOC. Therefore, looking for favorable areas with high TOC is the key to the future exploration of Upper Paleozoic shale gas in eastern Inner Mongolia.

Figure 14.

Figure 14

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Shale gas accumulation model of the Zhesi Formation of the Solon area.

6. Conclusions

  • 1.

    The sedimentary facies of the Middle Permian Zhesi Formation in eastern Inner Mongolia are distributed in a NE trending strip, mainly developing littoral facies, shallow marine, and bathyal facies. Coarse clastic rock deposits are mainly developed in the littoral facies, carbonate platforms and tempestite deposits are mainly developed in the shallow marine facies, and mudstone mixed with turbidite deposits is mainly developed in the bathyal facies.

  • 2.

    The marine sedimentary environment and chronological characteristics show that the Paleo-Asian Ocean was not completely closed in the Middle Permian, and its complete closure time should be later than the Middle Permian.

  • 3.

    The Zhesi Formation develops extremely thick marine shale. Comprehensive research shows that the solon area in eastern Inner Mongolia has certain potential for shale gas resources. Looking for favorable areas with a high TOC is the key to the exploration of shale gas in the future of the study area.

Acknowledgments

The authors thank the Opening Foundation of Key Laboratory for Oil Shale and Paragenetic Energy Minerals, Jilin Province, for support. This study received financial assistance from the China Geological Survey (1211302108025-5-1).

The authors declare no competing financial interest.

References

  1. Wang C. W.; Jing W.; Zhang X. Z.; Ma Z. H.; Chi X. G.; Liu Y. J.; Li N. New understanding of the Late Paleozoic tectonics in northeastern China and adjacent areas. J. Stratigr. 2008, 32, 119–136. [Google Scholar]
  2. Zhang X. Z.; Qiao D. W.; Chi X. G.; Zhou J. B.; Sun Y. W.; Zhang F. X.; Zhang S. Q.; Zhao Q. Y. Late-Paleozoic tectonic evolution and oil-gas potentiality in northeastern China. Geol. Bull. China 2011, 30, 205–213. [Google Scholar]
  3. Ming C. D.; Hou D. J.; Zhao X. M.; Song J.; Deng J.; Wu S. S.; Li J. H. The geochemistry records of paleoenvironment and sea-level relative movement of Middle Permian Zhesi Formation in eastern Inner Mongolia. Acta Geol. Sin. 2015, 89, 1484–1494. [Google Scholar]
  4. Chai H.; Ma Y. F.; Santosh M.; Hao S. L.; Luo T. W.; Fan D. F.; Gao B.; Zong L. B.; Mao H.; Wang Q. F. Late Carboniferous to Early Permian oceanic subduction in central Inner Mongolia and its correlation with the tectonic evolution of the southeastern Central Asian Orogenic Belt. Gondwana Res. 2020, 84, 245–259. 10.1016/j.gr.2020.02.016. [DOI] [Google Scholar]
  5. Lu L.; Qin Y.; Zhang K. J.; Han C. Y.; Wei T.; Li Z. F.; Qu Z. H. Provenance and tectonic settings of the Late Paleozoic sandstones in central Inner Mongolia, NE China: Constraints on the evolution of the southeastern Central Asian Orogenic Belt. Gondwana Res. 2020, 77, 111–135. 10.1016/j.gr.2019.07.006. [DOI] [Google Scholar]
  6. Liu B.; Wang H.; Fu X.; Bai Y.; Bai L.; Jia M.; He B. Lithofacies and depositional setting of a highly prospective lacustrine shale oil succession from the Upper Cretaceous Qingshankou Formation in the Gulong sag, northern Songliao Basin, northeast China. AAPG Bull. 2019, 103, 405–432. 10.1306/08031817416. [DOI] [Google Scholar]
  7. Song Y.; Li S.; Hu S. Warm-humid paleoclimate control of salinized lacustrine organic-rich shale deposition in the Oligocene Hetaoyuan Formation of the Biyang Depression, East China. Int. J. Coal Geol. 2019, 202, 69–84. 10.1016/j.coal.2018.11.016. [DOI] [Google Scholar]
  8. Liu B.; Wang Y.; Tian S.; Guo Y.; Wang L.; Yasin Q.; Yang J. Impact of thermal maturity on the diagenesis and porosity of lacustrine oil-prone shales: Insights from natural shale samples with thermal maturation in the oil generation window. Int. J. Coal Geol. 2022, 261, 104079 10.1016/j.coal.2022.104079. [DOI] [Google Scholar]
  9. Song Y.; Ye X.; Shi Q. R.; Huang C. Y.; Cao Q.; Zhu K.; Cai M. J.; Ren S. C.; Sun L. L. A comparative study of organic-rich shale from turbidite and lake facies in the Paleogene Qikou Sag (Bohai Bay Basin, East China): Organic matter accumulation, hydrocarbon potential and reservoir characterization. Palaeogeogr. Palaeocl. 2022, 594, 110939 10.1016/j.palaeo.2022.110939. [DOI] [Google Scholar]
  10. Liu B.; Bechtel A.; Sachsenhofer R. F.; Gross D.; Gratzer R.; Chen X. Depositional environment of oil shale within the second member of Permian Lucaogou Formation in the Santanghu Basin, Northwest China. Int. J. Coal Geol. 2017, 175, 10–25. 10.1016/j.coal.2017.03.011. [DOI] [Google Scholar]
  11. Liu B.; Song Y.; Zhu K.; Su P.; Ye X.; Zhao W. C. Mineralogy and element geochemistry of salinized lacustrine organic-rich shale in the Middle Permian Santanghu Basin: Implications for paleoenvironment, provenance, tectonic setting and shale oil potential. Mar. Petrol. Geol. 2020, 120, 104569 10.1016/j.marpetgeo.2020.104569. [DOI] [Google Scholar]
  12. Shi X. W.; Kang S. J.; Luo C.; Wu W.; Zhao S. X.; Zhu D.; Zhang H. X.; Yang Y.; Xiao Z. L.; Li Y. Shale gas exploration potential and reservoir conditions of the Longmaxi Formation in the Changning area, Sichuan Basin, SW China: Evidence from mud gas isotope logging. J. Asian Earth Sci. 2022, 233, 105239 10.1016/j.jseaes.2022.105239. [DOI] [Google Scholar]
  13. Zhang J. C.; Shi M.; Wang D. S.; Tong Z. Z.; Hou X. D.; Niu J. L.; Li X. Q.; Li Z. M.; Zhang P.; Huang Y. Q. Fields and directions for shale gas exploration in China. Nat. Gas Ind. B 2022, 9, 20–32. 10.1016/j.ngib.2021.08.014. [DOI] [Google Scholar]
  14. Ren S. M.; Qiao D. W.; Zhang X. Z.; Liu Y. J.; Wang N.; Sun Y. W.; Tang Z. X.; Cui Y. Q. The present situation of oil & gas resources exploration and strategic selection of potential area in the Upper Paleozoic of Songliao Basin and surrounding area, NE China. Geol. Bull. China 2011, 30, 197–204. [Google Scholar]
  15. Jiang G. Q.; Gao D. Z.; Zhang W. J.; Luo Z. H.; Li S. J.; Gao D. Z. Patterns of sedimentary associations and evolution of sequence of Zhesi Formation in Sonid Zuoqi, Inner Mongolia. Geoscience 1995, 9, 162–169. [Google Scholar]
  16. Li S. L.; Wang X. L.; Xu Y. H.; Zhang H. J.; Liu X. D.; Li J. W. Characteristics of the Maokou’an Reef in the Zhesiaobao, Inner Mongolia. Acta Sediment. Sin. 2004, 22, 434–442. [Google Scholar]
  17. Song T. S.; Zhu Z. P.; Qu X. Y.; Maimaitiming D.; Liu L.; Wang D. H.; Zhang X. Z.; Ma Z. H. Organic geochemistry characteristics of marine dark mudstone in the Middle Permian Zhesi Formation in Suolun area of Inner Mongolia, China. Geol. Bull. China 2011, 30, 300–307. [Google Scholar]
  18. Yao J.; Hu D. Q.; Sun G. S.; Zhan N. C.; Li Y.; Dong Q. S.; Sun G. S. Raman spectrum characteristics of organic materials of Middle Permian Zhesi Formation in Suolun area, Inner Mongolia and their geological implication. Global Geol. 2016, 35, 760–770. [Google Scholar]
  19. Zhao Y. L.Late Paleozoic tectonic evolution of the central and southern Grate Xing’an Ranges: Constrains from provenance characteristics of Permian sandstones. Doctoral Dissertation, Jilin Univ., 2010.
  20. Dong C.; Zhou J. B. Geochemical characteristics analysis and provenance tracing of sandstone in Middle Permian Zhesi Formation, northeast Inner Mongolia. Acta Petrol. Mineral. 2012, 31, 663–673. [Google Scholar]
  21. Hu F.; Liu Z. J.; Wang J. P. Provenance and tectonic setting of the Middle Permian Zhesi Formation in eastern Inner Mongolia, China: Implications from petrology, mineralogy, and element geochemistry. ACS Earth Space Chem. 2022, 6, 1823–1835. 10.1021/acsearthspacechem.2c00090. [DOI] [Google Scholar]
  22. Zhu R. K.; Xu H. X.; Deng S. H.; Guo H. L. Lithofacies palaeogeography of the Permian in northern China. J. Paleogeogr. 2007, 19, 133–142. [Google Scholar]
  23. Cui J. P.; Ren Z. L.; Shi Z.; Li J. X.; Wang W. Q. Analysis on Permian depositional characteristics and proto-type Basin of Northeast China. Geoscience 2013, 27, 260–268. [Google Scholar]
  24. Wang X. J.; Li S. J.; Xu C.; Zhang S.; Xu F. L.; Duan J. W. Lithofacies palaeogeography of Middle Permian in northeast China. J. Shandong Univ. Sci. Technol. 2014, 33, 19–27. [Google Scholar]
  25. Tang K. D. Tectonic development of Paleozoic foldbelts at the north margin of the Sino-Korean Craton. Tectonics 1990, 9, 249–260. 10.1029/TC009i002p00249. [DOI] [Google Scholar]
  26. Xu B.; Chen B. Framework and evolution of the Middle Paleozoic orogenic belt between Siberian and north China Plate in northern Inner Mogonia. Sci. China Ser. D 1997, 40, 463–469. 10.1007/BF02877610. [DOI] [Google Scholar]
  27. Tong Y.; Jahn B.M.; Wang T.; Hong D. W.; Smith E.I.; Sun M.; Gao J.F.; Yang Q.D.; Huang W. Permian alkaline granites in the Erenhot-Hegenshan belt, northern Inner Mongonia, China: Model of generation, time of emplacement and regional tectonic significance. J. Asian Earth Sci. 2015, 97, 320–336. 10.1016/j.jseaes.2014.10.011. [DOI] [Google Scholar]
  28. Zhu J. B.; He Z. J. Detrital zircon records of Upper Permian-Middle Triassic sedimentary sequence in the Linxi area, Inner Mogonia and constraints on timing of final closure of the Paleo-Asian Ocean (Eastern segment). Acta Geol. Sin. 2017, 91, 232–248. [Google Scholar]
  29. Li C. Y. A preliminary study of plate tectonics of China. Bull. Chinese acad. Geol. Sci. Ser. I 1980, 2, 11–20. [Google Scholar]
  30. Wang H. Z. The main stages of crustal development of China. Earth Sci. J. Wuhan Coll. Geol. 1982, 3, 155–178. [Google Scholar]
  31. Wang Y. J.; Fan Z. Y. Discovery of Permian Radiolarians in Ophiolite Belt on northern side of Xar Moron River, Inner Mongolia and its geological significance. Acta Palaeontol. Sin. 1997, 36, 58–66. [Google Scholar]
  32. Robinson P. T.; Zhou M. F.; Hu X. F.; Reynoids P.; Bai W. J.; Yang J. S. Geochemical constraints on the origin of the Hegenshan Ophiolite, Inner Mongolia, China. J. Asian Earth Sci. 1999, 17, 423–442. 10.1016/S1367-9120(99)00016-4. [DOI] [Google Scholar]
  33. Yu H. Z.; Cai X. Y.; Han S. H.; Xu Y. J. The Carboniferous-Permian source rocks in the Songliao Basin. Sediment. Geol. Teth. Geol. 2003, 23, 62–66. [Google Scholar]
  34. Zhang X. Z.; Zhou J. B.; Chi X. G.; Wang C. W.; Hu D. Q. Late Paleozoic tectonic-sedimentation and petroleum resources in northeastern China. J. Jilin Univ. Earth Sci. Ed. 2008, 38, 719–725. [Google Scholar]
  35. Li F. L.; Qu X. Y.; Liu L.; Yang D. M.; Wang D. H.; Zhao G. X. Sedimentary environment on Upper Permian Linxi Group in Inner Mongolia. Acta Sediment. Sin. 2009, 27, 265–272. [Google Scholar]
  36. Chen B.Sedimentary characteristics and tectonic setting of Permian in the Xing’an region of Inner Mongolian Autonomous region. Master dissertation, China Univ. Geosci., 2011.
  37. Li Y. K. Detrital zircon U-Pb dating from the Permian Zhesi Formation, Suolun Area, eastern Inner Mongolia, China and its tectonic implications. Sci. Technol. Eng. 2012, 12, 6269–6277. [Google Scholar]
  38. Read J. E. Carbonate platform facies models. AAPG Bull. 1985, 69, 1–21. 10.1306/AD461B79-16F7-11D7-8645000102C1865D. [DOI] [Google Scholar]
  39. Tucker M.E.Shallow-marine carbonate facies and facies models; Blackwell Sci. Pub: London, 1985. [Google Scholar]
  40. Yang B.; Zhang X. H.; Yang X. J.; Xia H. D. Brachiopod Faunas from the Permian Jisu Honguer (Zhesi) Formation in Xilinhot, Inner Mongolia, and its significance. Geol. Bull. China 2017, 36, 1683–1690. [Google Scholar]
  41. Feng Z.Z.Sedimentary petrology; Petrol. Ind. Press: Beijing, 1993. [Google Scholar]
  42. Wei Q. L.; Zheng R. C.; Zhou G.; Xiao L. Clastic tempestite in Xiejiawan Member of Ganxi Formation within Longmenshan area and its significance. Geol. China 2011, 38, 1282–1288. [Google Scholar]
  43. Chen J.P.; Huang D.F.; Chen J.J.; Zhang D.J.; Cheng K.M.; Wang T.G.. Hydrocarbon generation and migration in Jiudong Basin; Petrol. Ind. Press: Beijing, 1996. [Google Scholar]
  44. Zhao Q. F.; Wang J. Q.; Chen J. W.; Zhao Y. D.; Dong H. P.; Zhang Y. G.; Liang J. Optimization of evaluation index of Paleozoic high mature marine source rock in the Lower Yangze region. Geol. Bull. China 2021, 40, 330–340. [Google Scholar]
  45. Tang Y. J.; Zhang K.; Su F.; Zhang J. Evaluating index for over-matured source rocks-By taking the source rock in Upper Permian Linxi Formation of Lu D2 for example. J. Oil Gas Technol. 2013, 35, 35–40. [Google Scholar]
  46. Loucks R. G.; Reed R. M.; Ruppel S. C.; Hammes U. Spectrum of pore types and networks in mud rocks and a descriptive classification for matrix-related mudrock pores. AAPG Bull. 2012, 96, 1071–1098. 10.1306/08171111061. [DOI] [Google Scholar]
  47. Han G. Q.; Liu Y. J.; Wen Q. B.; Li W.; Wu L. N.; Zhao Y. L.; Ding L.; Zhao L. M.; Liang C. Y. LA-ICP-MS U-Pb dating of detrital zircons from the Permian sandstones in north side of Xar Moron River suture belt and its tectonic implications. Earth Sci. J. China Univ. Geosci. 2011, 36, 687–702. [Google Scholar]
  48. Zhou J. B.; Zeng W. S.; Cao J. L.; Han J.; Guo X. D. The tectonic framework and evolution of the NE China: from ∼ 500 Ma to ∼ 180 Ma. J. Jilin Univ. Earth Sci. Ed. 2012, 42, 1298–1316. [Google Scholar]
  49. Li F. L.; Xiao F.; Meng F. C.; Ren Z. Y. Geochemical characteristics and implication for provenance of Upper Permian Linxi Formation clastic rocks in Solonker area, Inner Mongolia. J. Jilin Univ. Earth Sci. Ed. 2016, 46, 1769–1780. [Google Scholar]
  50. Li J. H.; Li W. B.; Wang H. H.; Mao X.; Yang J. Y.; Fu C. J. Global tectonic setting during the Late Paleozoic convergence of Pangea: Discussion of Plate Vortex Trajectory. Acta Geol. Sin. 2014, 88, 980–991. [Google Scholar]
  51. Wang C. W.; Sun Y. W.; Li N.; Zhao G. W.; Ma X. Q. Tectonic implications of Late Paleozoic stratigraphic distribution in Northeast China and adjacent region. Sci. China Ser. D: Earth Sci. 2009, 52, 619–626. 10.1007/s11430-009-0062-7. [DOI] [Google Scholar]
  52. Su F.; Li X. H.; Zheng Y. J.; Bian X. F.; Zhang H. H.; Zhang J. Paleosalinity analysis of Upper Permian Linxi Formation in Solonker Area, Inner Mongolia. J. Yangtze Univ. Nat. Sci. Ed. 2014, 11, 31–33. [Google Scholar]
  53. Li Y. L.; Zhou H. W.; Ge M. C.; Liao Q. A.; Zhang X. H.; Zhong Z. Q. LA-ICP-MS zircon U-Pb dating of migmatite from north Shuangjing schist in Linxi, Inner Mongolia. J. Miner. Petrol. 2008, 28, 10–16. [Google Scholar]
  54. Mao D. B.; Zhong C. T.; Chen Z. H.; Lin Y. X.; Li H. M.; Hu X. D. The isotope ages and their geological implications of high-pressure basic granulites in north region to Chengde, Hebei province, China. Acta Petrol. Sin. 1999, 15, 524–531. [Google Scholar]
  55. Liu S. W.; Lv Y. J.; Feng Y. G.; Liu X. M.; Yan Q. R.; Zhang C.; Tian W. Zircon and monazite geochronology of the Hongqiyingzi Complex, northern Hebei, China. Geol. Bull. China 2007, 26, 1086–1100. [Google Scholar]
  56. Feng Y. G.; Liu S. W.; Lv Y. J.; Zhang C.; Shu G. M.; Wang C. Q. Monazite age mapping of Longhua S-type granites in the northern margin of the north China Craton. Acta Petrol. Sin. 2008, 24, 104–114. [Google Scholar]
  57. Hu B.; Zai M. G.; Guo J. H.; Peng P.; Liu F.; Liu S. LA-ICP-MS U-Pb geochronology of detrital zircons from the Huade Group in the northern margin of the North China Craton and its tectonic significance. Acta Petrol. Sin. 2009, 25, 193–211. [Google Scholar]
  58. Li Y. J.; Liu H.; Liu J. X.; Cao L. C.; Jia X. C. Geological regional selection and an evaluation method of resource potential of shale gas. J. Southwest Petrol. Univ. Sci. Technol. Ed. 2011, 33, 28–34. [Google Scholar]
  59. Wang P. W.; Chen Z. L.; He X. Y.; Dong L.; Xu Z. Y.; Ma L. Q.; Dong Y.; Huang L. Shale gas accumulation conditions and play evaluation of the Devonian in Guizhong Depression. Oil Gas Geol. 2012, 33, 353–363. [Google Scholar]
  60. Wang S. J.; Yang T.; Zhang G. S.; Li D. H.; Chen X. M. Shale gas enrichment factors and the selection and evaluation of the core area. Eng. Sci. 2012, 14, 94–100. [Google Scholar]
  61. Zhang J. C.; Lin L. M.; Li Y. X.; Jiang S. L.; Liu J. X.; Jiang W. L.; Tang X.; Han S. B. The method of shale gas assessment: Probability volume method. Earth Sci. Front. 2012, 19, 184–191. [Google Scholar]
  62. Li S. Z.; Zhou X. G.; Wang D. D.; Zhang J. D.; Lin Y. H.; Zeng Q. N.; Zhang W. H. Micro-structure analysis of mudstone in the Middle Permian Zhesi Formation in Solun area, Inner Mongolia. Petrol. Geol. Exp. 2016, 38, 502–508. [Google Scholar]

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