Introduction

The Permian Lucaogou Formation on the northern margin of the Bogda Mountains in the Junggar Basin, China, has been identified as a significant source of hydrocarbons (Chen et al. 2016). This formation has been identified as a promising area for shale oil exploration (Zhang et al. 2018a, b, c, d). Previous researchers have conducted extensive exploration of this layer, including the sedimentary sequences (Cheng et al. 2022; Liang et al. 2012), oil shale characteristics (Zhang et al. 2018b, c, d), source rocks (Shan et al. 2021; Zhang et al. 2018b, c, d), and reservoir formation characteristics (Li et al. 2016; Jiao et al. 2007). However, the majority of research has focused on the Jimusa Depression, with the periphery of the Bogda Mountains only receiving limited attention. In particular, there are no published reports on the development characteristics of diagenesis and its impact on the physical properties of the Lucaogou Formation shale reservoir near the northern margin of the Bogda Mountains front structural belt. This limits the exploration and understanding of the Lucaogou Formation shale oil and gas reservoir. This paper therefore attempts to analyse the characteristics and major controlling factors of the Lucaogou Formation shale reservoir on the northern edge of the Bogda Mountains in the Junggar Basin in China. To this end, the rock and mineral characteristics, source rock geochemistry, and development characteristics of the Lucaogou Formation shale reservoir will be investigated in order to provide a basis for decision-making for shale oil exploration in this area.

The northern margin of the Bogda Mountains is defined as the area of the folded thrust belt in front of the northern Tianshan Mountains, which is adjacent to the Ilin Heibiergen Mountain in the south and extends to the Changji Depression in the basin in the north (Fig. 1). The northern margin of the Bogda Mountains has undergone four phases of regional tectonic movements from the Paleozoic to the present, including the Hercynian, Indosinium, Yanshan and **shan stages (Wang et al. 2008; Zheng et al., 2020a), which have produced different patterns of basin evolution. Among them, the Permian is a key period for the formation of the Junggar Basin. The basin has been influenced by plate movement, which has resulted in the development of extensional rifting (P1) and compressional flexural basin (P2-3) (Wang et al. 2018; Wartes et al. 2000; Zhang et al., The area has undergone a sedimentary evolution process, which can be divided into three main stages: marine facies, marine continental transitional facies and lake facies (**ao et al. 2008; Wu et al. 2005; Liu et al. 2023). In the Middle Permian, the Junggar Basin was characterised by pan-basinal sedimentation. At this time, the Bogda Mountains were not yet uplifted, and the sedimentation of semi-deep lake facies was widespread on the southern margin (Qu et al. 2019), providing a favourable tectonic sedimentation environment for the formation of shales.

Fig. 1
figure 1

Location map of the study area on the northern edge of Bogda Mountain

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Materials and methods

A total of 59 samples were selected from the drill holes and some field profiles for TOC analysis. A further 98 samples were selected for mineral composition examination, 28 samples were selected for scanning electron microscopy examination and analysis, 39 samples were collected from the bottom up for mercury intrusion examination and analysis, and 31 samples were selected for thin section identification. Finally, 31 samples were selected for porosity testing. All tests were conducted in the laboratory of China College of Petroleum (East China). Prior to analysis and testing, the fresh samples are ground to a particle size of less than 0.2 millimeters for TOC content analysis under pollution-free conditions. For scanning electron microscopy examination, the KYKY-2800 scanning electron microscope is employed, with the detection standard being SY/T5162-1997. The analysis was conducted using a BRUKER D2 X-ray diffractometer, with the test standard being JY/T 016-1996. The pore structure of the reservoir was analysed using a high-pressure mercury porosimeter/PoreMaster 33, with block samples being used, aggregates and direct impact parts being avoided, and the samples being dried at 50℃ for 6 h. The thin section was identified using an Olympus polarising microscope, with the identification standard SY/5368.2–2019. The effective porosity and permeability of the lithology were then tested using the PDP-200 ultra-low permeability tester.

Result and discussion

Stratigraphic development characteristics

The thickness of the Lucaogou Formation (P2l) is 400–1200 m, and its lithology is quite variable in profile. It is divided into four sections from bottom to top (Fig. 2): the soil is mainly grey-green sandstone and grey siltstone, with a thin layer of grey-black oil shale. The sedimentary structure is characterised by parallel stratification, cross-stratification, and undulating stratification, which can be observed in fan-delta front sedimentation. The middle part is composed of gray, dark gray clayey siltstone, silty shale, and dolomitic shale, with a thin layer of gray-black shale intercalated. The single layer is thin, and occasionally a thick layer of gray fine sandstone can be observed. Horizontal and undulating stratifications are developed. Ancient cod fossils have been identified in the Lucaogou Formation shale in the **g**gzigou section, as well as in outcrops and boreholes in the adjacent area. In the geological past, ancient cod primarily inhabited coastal lakes on the continental margin and exhibited high salinity requirements, typically inhabiting a saline to semi-saline sedimentary environment. The upper part of the Lucaogou Formation is composed of dark grey, grey-black mudstone and clay shale, interbedded with several layers of dolomitic clay shale. Horizontal intercalations and micaceous intercalations are observed, as well as plant remains and biological fault structures. It is likely to be a semi-deep sea-deep-sea facies deposit. The Lucaogou Formation is in conformable contact with the underlying **gzigou Formation and non-conformable contact with the overlying Hongyanchi Formation. In the Guodikeng area, the total thickness of the Lucaogou Formation is 458.5 m. It is thick in the north and thin in the south within the secondary Dabancheng depression, with a thickness of 800–2000 m. The Yongfeng secondary depression is characterised by an average thickness of 800–1200 m.

Fig. 2
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Comprehensive histogram of the study area on the northern edge of Bogda Mountain

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Reservoir characteristics

Petrology characteristics

There are considerable differences in the lithology between the upper and lower parts of the Lucaogou Formation on the northern edge of the Bogda Mountains, which are reflected in significant differences in the mineral composition. In order to clarify the rock type, mineral composition, and variation pattern of the Lucaogou Formation, a series of microscopic analyses were conducted, including X-ray diffraction, scanning electron microscopy, and mineral composition. These analyses involved the examination of 114 sample sections.

The lithological development of the Lucaogou Formation is relatively complex, with different lithological formations often interconnected, resulting in a variety of rock types in the deposit. Different scholars have different standards for classification and different views (Qu et al. 2019; Tao et al. 2022). Based on outcrops, drillings, cores, and microscopic observations, the rock types of the Lucaogou Formation are divided into three categories: The Lucaogou Formation comprises shale, siltstone, and dolomite (Fig. 3). The main types of shale are slate, dolomitic shale, and silty shale (Fig. 3A, B), while siltstone is composed mainly of dolomitic siltstone and clayey siltstone (Fig. 3C, D). The particle size of the siltstone is 25–95 μm, with quartz and feldspar accounting for 50–90%. The dolomites are mainly clayey siltstone, silty dolomite, and micrite dolomite (Fig. 3E, F).

Fig. 3
figure 3

Petrological features of the Lucaogou Formation on the northern edge of the Bogda Mountains. (A) Borehole Qi1, 2492 m, dark grey shale, blocky result. (B) Bocan 1 well, 199.1 m, dark grey shale, with dolomite visible under the microscope. (C) Hongyanchi section, dolomitic siltstone. (D) JZK1 well, 313.6 m, dolomitic siltstone, exhibiting oil stains on the fracture surface. (E) Zhunye 4 well, 1204 m, mudstone dolomite, containing light oil within cracks. (F) **dalongkou section, oolitic mudstone dolomite, displaying visible intergranular pores

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With regard to the mineral composition (Tables 1 and 2), the content of clay minerals in the Luyi Member is relatively high, ranging from 88.2 to 95.7%, with an average of 92.4%. The most significant minerals are kaolinite and illite, followed by chlorite and timething interlayer. The distribution range of illite and kaolinite is 30.4–95.4%, with an average of 68.5%. The distribution range of chlorite and the Iran/Mongolia interlayer is 16.7–47.7%, with an average of 31.4%. The content of brittle minerals is relatively low, with a range of 4.3–11.8%, with an average of 7.6%. The mineralogy is dominated by quartz and feldspar, which account for over 71% of the total. The carbonate mineral content is relatively low, with a percentage of < 29%.

Table 1 Mineral content statistics of the Permian Lucaogou formation at the northern edge of Bogda Mountain
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Table 2 Statistical table of clay mineral content of the Permian Lucaogou formation at the northern edge of Bogda Mountain
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In comparison to the Lu1 Member, the clay minerals in the Lu2 Member and Lu3 Member are reduced, with a distribution range of 45 ~ 75%, with an average value of 59.1%. The main minerals present are kaolinite and chlorite, followed by illite and a timing interlayer. The content of brittle minerals ranges from 25 to 55%, with an average value of 37.8%. Dolomite is the most prevalent mineral, followed by felsic minerals and calcite, which reflects the increase in dolomitic rocks.

The content of clay minerals in the fourth section of the Lujiang River is the lowest at 10.2 ~ 45.3%, with an average of 34.2%. It is mainly composed of kaolinite, illite, and Iran/Mongolia interlayer, followed by chlorite. The content of brittle minerals has increased and is between 55.7 and 90.8%, with an average of 65.8%. The most important minerals are felsic minerals and dolomite, which account for 81–95%, with an average of 87.2%, followed by calcite, which accounts for less than 5%, with an average of 2.5%.

In general, the main minerals of the Lucaogou Formation are clay minerals (36%), felsic minerals (32%), and dolomite (22%) (Fig. 4). The calcite content is less than 2.8%, indicating that the Lucaogou Formation is dominated by brittle minerals that easily form cracks and are a potentially favourable reservoir for shale oil.

Fig. 4
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Triangle diagram of shale rock deposit of Lucaogou Formation on the northern edge of Bogda Mountain

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Physical property

The concept of “high porosity and low permeability” is the general understanding of the sediments in the Lucaogou Formation by various scholars (Zha et al. 2017; Kuang et al. 2012), but there are still significant differences in the physical properties of the three types of lithological sediments. For example, the porosity of shale is 3–10%, with an average value of 7.2%, and the permeability is between 0.001 and 1 mD, with an average value of 0.012 mD. The porosity of siltstone is 6 ~ 18%, with an average of 8.9%, and the average permeability is 0.076 mD. The porosity of dolomite is 4–14%, with an average of 6.1%, and the average permeability is 0.053 mD. The statistical data show that among the three types of deposits, the physical properties of siltstone deposits are the best, followed by dolomite and shale deposits are the worst.

Under the microscope, it was found that the deposit types of the Lucaogou Formation are mainly dissolution pores, bedding fractures and structural fractures, followed by primary intergranular pores and intergranular pores (Fig. 5), which are characterised by significant changes in deposit size and different types. The shale is rich in organic matter and forms a large number of dissolved organic matter pores during thermal evolution, but the pore size is small, most of them smaller than 2 mm (Fig. 5A, B). Organic acids generated during the thermal evolution of the shale lead to the dissolution of feldspar, carbonate minerals, volcanic ash and other soluble components in the siltstone and dolomite deposits (Fig. 5C, D), resulting in a large number of intragranular and intergranular pores with a pore diameter of 10 ~ 60 μm. Under the influence of a sedimentary environment, laminated fractures develop mainly in the parts with gradual alteration and mutation of three kinds of lithology (Fig. 5E), while structural fractures develop mainly in siltstone and dolomite deposits with relatively developed brittle minerals (Fig. 5D, E, F).

Fig. 5
figure 5

Microscopic characteristics of Lucaogou formation in the northern edge of Bogda Mountain. (A) West Dalongkou section, distribution of organic matter pores in shale. (B) Baoming mining section, pores of dissolved clay minerals in shale. (C) Lucaogou section, calcite intergranular microcracks are developed and filled with carbon. (D) Hongyanchi section, dissolution of feldspar in siltstone. (E) In the Baiyanggou section, clay minerals and organic matter are frequently interbedded, and bedding joints are developed. (F) In the **g**gzigou section, structural fractures are developed in the shale

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Fig. 6
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Histogram of pore throat distribution of Lucaogou formation reservoir on the northern edge of Bogda Mountain

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From the analysis results of the mercury intrusion tests (Fig. 6), the nanoscale pore throat radius accounts for about 97% and is mainly distributed between 50 and 540 nm, with an average of 150 nm. Therefore, the smaller pore throat radius is the main reason for the low permeability.

Diagenesis

The Bogda Mountains have undergone complex Piedmontese tectonic movements, with multiple uplifts and subsidence. The division of diagenetic stages is also complex. Microscopic observations and analyses indicate that the Lucaogou Formation is currently in the early stages of intermediate diagenesis (Fig. 7). Secondary hydrocarbons are formed in the source rock, a high organic content can be seen under the microscope, and a large number of feldspar and carbonate minerals have dissolved.

Fig. 7
figure 7

Stages of constituent rocks in Lucaogou, north edge of Bogda Mountain

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During the early stages of diagenesis, clastic sedimentary materials and organic material were compacted to form rock. Due to the small particle size of the clastic particles, it was difficult to form a support, resulting in a large reduction in primary pores, which had a more destructive effect on the physical properties of the reservoir. The primary pores are only preserved in areas with larger debris particles or earlier independent mineral cementation inclusions such as chlorite (Fig. 8A), and these early cementation processes slow to some extent the destructive effect of compaction on reservoir space. At this stage, with increasing burial depth, organic matter also begins to enter the hydrocarbon formation threshold, leading to the development of dissolution pores around the organic matter (Fig. 8B).

By the middle of diagenesis, the burial depth continues to increase and compaction has weakened. The organic matter enters the mature stage and with the production of a large amount of hydrocarbon fluids, acidic fluids from the organic matter penetrate the interlayers and pores, leading to further dissolution (Fig. 8C), especially in the interlayers of the cloud rock and fine sandstone. Dissolution occurs mainly because the interlayer contains not only organic matter and other hydrocarbon-forming substances, but also microlayers and pores that form fluid channels so that the products can be easily transported away after dissolution, forming a larger corrosion space. Microscopically, pores formed by the dissolution of tuff, feldspar, carbonate rock and other debris are widely distributed, and these dissolution pores have played a constructive role in improving the physical property space of the shale. Because the fluid contains a large number of minerals, cementation occurs simultaneously between cracks and pores. Under the microscope, the secondary enlargement of quartz (Fig. 8D), carbonate cement blocks such as calcite (Fig. 8E) and a large number of clay cements such as chlorite and zeolite can be observed. Compared to the constructive effect on the reservoir space in the early stages (Fig. 8F), cementation at this stage has occupied the originally small secondary space, further reducing the physical properties of the reservoir and playing a destructive role.

Fig. 8
figure 8

Microscopic characteristics of Lucaogou constituent rocks at the northern edge of Bogda Mountain. (A) Qi1 well, 2213.20 m, P2l, with leaf-like chlorite and plate-like zeolite minerals on the particle surface. (B) Lucaogou interval, P2l, has a small number of dissolution pores and is filled with carbonate and authigenic clay between the particles. (C) Qi1 well, 2273.2 m, P2l, feldspar particles dissolve along the cleavage to form a small amount of sheet-like clay (D) Bocan 1 well, 321.2 m, P2l, quartz secondary enlargement. (E) Ji 172 well, 2927.9 m, P2l, calcite crystal and leaf-shaped chlorite filled with quartz secondary enlargement and intergranular filling. (F) **dalongkou interval, P2l, zeolite minerals and intergranular pores in the matrix

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Previous trace element studies indicate that during the sedimentation period of the Lucaogou Formation, the ancient salinity was high and the water had the characteristic of alkaline salinisation. This also led to the cementation of the Lucaogou Formation during the early diagenetic period due to its high mineralisation. In the late early and middle diagenetic stages, the organic matter gradually evolved into hydrocarbons due to the increasing geothermal gradient. At this time, the fluid environment gradually changes from alkaline to weakly alkaline or weakly acidic due to the neutralisation of acidic fluids, and dissolution and cementation are more developed (Fig. 9).

Fig. 9
figure 9

Staged diagenetic process of the reservoir in Lucaogou formation on the northern edge of Bogda Mountain

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Influence of diagenesis on reservoir physical properties

Compaction, cementation and dissolution in diagenesis are important factors that influence the development and evolution of the physical properties of the deposit (Shan et al. 2022; Yu et al. 2022). By analysing the diagenesis of the deposit at the northern edge of Bogda Mountain, we can identify the developmental stage of diagenesis and the effects of different diagenesis on the deposit.

The maturity of the rock composition of the Lucaogou Formation at the northern edge of Bogda Mountain is relatively low, and the compressive strength is weak. Microscopy shows that the elongated debris particles, clay minerals, plant debris, organic fragments, etc. are arranged in a certain direction along the direction of lower compressive stress. Among them, the debris and mineral particles exhibit linear contact properties during compaction. This also leads to a significant reduction in primary pores, which improves the physical properties of the deposit.

The statistical results show that in the early stage of diagenesis, the content of carbonate minerals increases, while the primary pores and residual intergranular pores also increase (Fig. 10A, B). This is mainly because carbonate rocks have a strong anti-compaction effect in the early diagenetic stage, preserving some of the original pores. During middle diagenesis, the precipitation of carbonate cement blocked the pore throats and compacted the reservoir. Later dissolution of the carbonate rocks was relatively weak, and the dissolution pores of calcite and other carbonate rocks were poorly developed. According to the analysis diagram of the relationship between carbonate cement content and porosity, surface porosity (Fig. 10C, D), as the carbonate content increased, the porosity and surface porosity decreased, and the physical properties became worse. At the same time, the physical properties of the reservoir deteriorated with the increase in plastic particle debris content, and there was a negative correlation with porosity, indicating that plastic particles are easily affected by compaction and primary pores are destroyed during the continuous burial process. Cementation of clay minerals has no obvious effect on reservoir porosity (Fig. 10E, F). Yimeng mixed bed and illite have a certain destructive effect on reservoir permeability. The higher the content, the lower the permeability and the worse the physical properties of the reservoir (Fig. 10G, H).

Fig. 10
figure 10

Correlation between carbonate, rock debris, clay content, and porosity of the Lucaogou Formation on the northern edge of Bogda Mountain

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In summary, there is a strong positive correlation between the depth of burial of the Lucaogou Formation and the reduction in its compaction porosity. As the burial depth increases, the porosity gradually decreases by 10–15%, which is the most important factor negatively affecting its physical properties. The second most important factor is cementation. During the early stages of diagenesis, cementation can prevent the physical properties of the reservoir from being destroyed. The rate of porosity reduction is low, with porosity decreasing by only 2–3%. However, during the middle stages of diagenesis, the reduction in pore space due to cementation of carbonate rocks is significantly higher than in the early stages. Porosity decreases by 2–6%. Although dissolution can increase pore space, experiments show that the rate of increase is only 3–5%. Compared to compaction and cementation, dissolution has a relatively small effect on increasing porosity (Fig. 11).

Fig. 11
figure 11

Variation curve of the influence of different diagenesis on the porosity of the Lucaogou Formation on the northern edge of Bogda Mountain

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Conclusions

(1) The rock types of the Lucaogou Formation reservoir are divided into three categories: Shale, siltstone and dolomite. The main types of shale are shale, dolomitic shale and sandy shale; the main types of siltstone are dolomitic siltstone and argillaceous siltstone; and the main types of dolomite are argillaceous siltstone, sandy dolomite and muddy crystalline dolomite. The main minerals are clay minerals (36%), felsic minerals (32%) and dolomite (22%). The calcite content is less than 2.8%, indicating that the Lucaogou Formation is mainly composed of brittle minerals that are prone to fracturing and are potentially favourable reservoirs for shale oil.

(2) The Lucaogou Formation consists of three types of reservoirs, namely mud shale, sandstone and dolomite. The mud shale has a porosity range of 3–10%, with an average of 7.2%, and a permeability range of 0.001 to 1 mD, with an average of 0.012 mD. Sandstone porosity ranges from 6 to 18% with an average of 8.9% and average permeability of 0.076 mD. The porosity of the dolomite ranges from 4 to 14% with an average of 6.1% and an average permeability of 0.053 mD. The physical properties of the reservoirs vary according to their type, with the sandstone type having the best properties, followed by the dolomite type and the shale type having the worst properties. The main types of fractures are dissolution pores, laminated fractures and structural fractures, followed by intergranular pores. The nanoscale pore throat radius accounts for about 97%, mainly distributed between 50 and 540 nm, with an average of 150 nm. The low permeability of the reservoirs is mainly due to the smaller pore throat radius.

(3) At present, the shale of the Lucaogou Formation is in the intermediate diagenetic stage. It mainly undergoes compaction, cementation (carbonate, clay mineral and siliceous) and dissolution. Compaction (with a porosity reduction rate of 2-3%) and cementation (with a porosity reduction rate of 2-6%) negatively affect the physical properties of the shale. However, subsequent dissolution has a constructive effect on the physical properties, increasing porosity by 3–5%. The diagenetic environment has gradually changed from an alkaline to a weak alkaline/weak acid stage.

This paper elucidates the rock and mineral properties, source rock geochemistry and shale reservoir development characteristics of the Lucaogou Formation through geological and geochemical experimental methods. This study reveals for the first time the characteristics and main controlling factors of shale reservoirs in the study area. The results provide a decision-making basis for shale oil exploration in the region and can serve as a reference for the study of complex mountain front shale oil reservoirs.