The exploration and development of Jimusar shale oil in the Junggar Basin has gone through four stages: exploration, pilot test, breakthrough and expansion. In September 2011, Well Ji 25 pumped an average daily oil production of 11.86 m³ after being fractured at 3403-3425 m of the second member of the Permian Lucaogou Formation. Subsequently, a series of exploration and appraisal wells (Ji 23, Ji 28, Ji 30, Ji 173, Ji 174, etc.) were deployed and tested for oil production, discovering the shale oil of the Permian Lucaogou Formation in the Jimsar Sag. After 9 years of test development with "horizontal well + volume fracturing", "straight well + conventional fracturing", and optimized development method, according to the conventional reservoir volume method, the well-controlled reserves in the favorable area were calculated at 11.12×10⁸ t, opening a new chapter in the exploration and development of large shale oil in western China. Since shale reservoirs feature continuous distribution, low porosity, low permeability and strong heterogeneity, single wells in shale reservoirs often have wide differences in production and poor continuous production capacity during development. How to predict and find the location of “sweet spots” and increase single well production is a practical means to realize economic and effective development of shale oil.

The distribution and quality of shale oil sweet spots are closely related to the sedimentary microfacies, diagenesis and other geological conditions. Shale reservoirs are often continuously affected by a variety of diagenetic processes, and the superposition and reformations of different diagenetic processes have a significant impact on shale porosity evolution and oil and gas accumulation inside the shale reservoirs^[1]. Fluids in different diagenetic environments with different pH values have widely different impacts on reservoirs. Because feldspar, carbonate and zeolite are unstable in an acidic environment and are prone to dissolution of inorganic-organic acidic medium, acidic dissolution pores are readily created and widely distributed. With the deepening of study on diagenesis, alkaline diagenesis has gradually attracted the attention of researchers. Studies have shown that feldspar can also be dissolved under certain alkaline medium conditions. The direct dissolution of quartz and the perfection of relevant theories have confirmed the existence of alkaline diagenesis in oil-gas bearing basins^[2,3,4,5]. Diagenetic fluids in a lake basin would change constantly with the evolution of diagenetic processes, and multiple alternations of acidic and alkaline diagenetic environments would end up with multiple pore development zones and significantly improve the reservoir physical properties^[6].

The previous study on diagenetic processes of the Permian Lucaogou Formation shale reservoirs in the Jimsar Sag was slow in progress^[7]. In this work, through a variety of experimental methods, diagenetic processes on the Lucaogou Formation shale reservoirs have been systematically investigated to find out the effects of acid-alkali alternation diagenetic processes on pore evolution and performance of the shale reservoirs.

The Jimusar sag is a secondary depression on the eastern uplift located in the secondary structure unit of the eastern Junggar Basin. It is bounded by thrust faults on the south, west and north (Fig. 1a) and is an irregular polygon on the plane with typical dustpan structure now (Fig. 1c). The Permian Lucaogou Formation in the sag was formed in a saline lake basin environment in the background of intracontinental rift valleys, and simultaneously affected by volcanic eruption and hydrothermal activity^[8,9,10], it is composed of fine-grained mixed sedimentary rocks of semi-deep lake-deep lake facies^{[11,12,13,14,15,16]}.

The Lucaogou Formation is divided into two members, P₂l₁ and P₂l₂ from bottom to top (Fig. 1b), and industrial oil flows have been obtained in "sweet spot sections" of the two members^[7]. The upper part of P₂l₂ is the upper sweet spot section and the most important set of reservoirs in the Lucaogou Formation in the Jimsar Sag. This set of reservoirs consists of (tuff-bearing) tuffaceous dolarenite, (tuff-bearing) tuffaceous feldspar detrital fine sandstone, (tuff-bearing) tuffaceous dolomite debris sandstone with gray mudstone, and dolomitic mudstone. The upper part of the P₂l₁ is the lower sweet spot section, and is mainly composed of (tuff-bearing) tuffaceous dolomitic siltstone, (tuff-bearing) tuffaceous argillaceous siltstone, and gray mudstone.

According to core description and thin section identification results, the Lucaogou Formation is thin and strong in longitudinal heterogeneity, and dominated by fine-grained mixed sedimentary rocks (Fig. 1b). XRD (X-ray diffraction) analysis of whole rock shows that the reservoirs are mainly composed of quartz, feldspar, carbonate minerals and clay minerals, and largely clastic rock, tuff and carbonate rocks^{[17,18,19,20,21,22,23,24]}. The sweet spot sections mainly include (tuff-bearing) tuffaceous dolarenite, (tuff-bearing) tuffy dolomitic sandstone, (tuff-bearing) tuffy dolomitic siltstone, (tuff-bearing) tuffaceous micrite microcrystalline algal dolomite, and (tuff-bearing) tuffaceous lithic feldspar siltstone-fine sandstone.

Analysis results of rock samples show that the Lucaogou Formation reservoirs have an overburden porosity of 6% to 16%, and overburden permeability of less than 0.1×10^-3 μm² in general, representing medium-low porosity and low-ultra low permeability and poor porosity-permeability correlation that are typical for shale reservoirs (Fig. 2). The results of cast thin section and physical property analysis of Well Ji 174 show the upper sweet spot section of the Lucaogou Formation has a porosity range from 2.25% to 18.80%, an average porosity of 11.26% (Fig. 1b), and mainly remaining intergranular pores, dissolution pores, and dolomite intercrystalline pores (Fig. 3a-3c); the lower sweet spot section has a porosity range from 1.85% to 20.60%, an average porosity of 9.85% (Fig. 1b), and mainly dissolved pores, remaining intergranular pores and dolomite intercrystalline pores (Fig. 3d). In general, the reservoirs have complex pore structure and strong heterogeneity. Pores in the reservoirs are largely in micrometer and nanometer scales, and a small amount is millimeter scale^[25,26].

Alkaline diagenesis refers to a series of diagenetic reactions in an alkaline diagenetic fluid environment. A large number of alkaline authigenic minerals and alkaline geochemical features suggest that the shale reservoirs in the study area experienced alkaline diagenetic environment during the diagenetic process. Influenced by the sedimentary environment and the nature of pore water, the study area is generally a diagenetic type in the background of alkaline diagenetic environment. The characteristics of alkaline diagenesis in this area mainly include quartz dissolution, carbonate cementation and metasomatism, and albite and analcime precipitation, etc.

3.1.1. Authigenic minerals in alkaline environment

3.1.1.1. Carbonate minerals

Carbonate minerals, sensitive to the pH, are likely to precipitate in slightly alkaline-alkaline environment (pH greater than 8), while likely to dissolve in acid environment^[6]. Therefore, alkaline environment is a necessary condition for the precipitation and stable existence of these minerals.

Carbonate minerals in the study area mainly include calcite, dolomite, and ankerite (Figs. 3e, 3f, 4a-4c, Table 1). Carbonate metasomatism is strong in an alkaline condition, mainly manifested as calcite replacing feldspar, dolomite replacing calcite, dolomite replacing feldspar, and ankerite replacing calcite, etc. According to the mineral metasomatic relationships and mineral growth order, the crystallization order of carbonate minerals is calcite-dolomite-ankerite-iron- bearing dolomite (Fig. 3g). This phenomenon indicates that the Mg²⁺ and Fe²⁺ concentrations in the pore water experienced a cyclic process from weak to strong to weak, which proves that the alkaline environment suitable for carbonate precipitation in the study area appeared in multiple stages. Related studies have shown that the dolomite and calcite in the Lucaogou Formation are rich in Sr, and have ⁸⁷Sr/⁸⁶Sr very close to ⁸⁷Sr/⁸⁶Sr in the mantle, suggesting the formation was affected by hydrothermal fluids or volcanic materials^{[9, 27]}.

Through temperature analysis of inclusions, the carbonate mineral inclusions have two homogenization temperature distribution intervals of 65-90 ℃ and 98-124 ℃, indicating that two stages of large-scale carbonate mineral cementation events occurred in the study area.3.1.1.2. AnalciteAnalcite is a sodium-rich silicate mineral developed in alkaline lake basins or alkaline water bodies affected by hydrothermal and volcanic substances^[28]. In terms of formation environment, the conditions conducive to zeolites precipitation are high salinity alkaline medium conditions (high pH value) rich in Ca^{2 +}, Na⁺, K⁺ ions, in addition, appropriate partial pressure by carbon dioxide is also required^{[6, 29]}. Zeolites can’t exist stably in acidic environment, so zeolites are good indicators of alkaline sedimentary and diagenetic environment.The analcite is in grain shape under monopolarized light and fully extinct under orthogonal light (Fig. 3h). The analcite grains in the backscatter images are dark gray, and have irregular bay shape edge due to acidic dissolution, and are in point-line contact or floating stage (Fig. 4d). The results of electron probe analysis show that the analcite samples in Lucaogou Formation have Si/Al values of 2.51-2.73, and it is generally believed that the high silicon content analcite with Si/Al value of about 2.7 is generated by the reaction of siliceous volcanic glass and alkaline water^[30] (Table 2). Therefore, the analcite in the study area is related to pyroclastic alteration in the early diagenetic period.

3.1.1.3. Authigenic albite

Albite is an extremely pure terminal component of plagioclase solid solution series. In the diagenetic process, the formation modes of albite mainly include: (1) albitization of feldspar clast caused by ionic metasomatism; (2) secondary growth of albite at the edge of feldspar clast; and (3) newly generated albite associated with the dissolution of feldspar clast^[31]. All the three kinds of albite formation modes require alkaline diagenetic fluid environment.

Through analysis of cast thin sections and observation with scanning electron microscope, authigenic albite cement was found in every well section of the study area. Under the scanning electron microscope, the authigenic albite grains are mostly plate-shaped automorphic crystal with a clean surface and simple double crystals, and are in columnar shape vertically distributed along the edge of feldspar moldic hole (Fig. 4e). Electron probe and X-ray diffraction analysis show that the dissolved feldspars are all alkaline feldspars.

3.1.1.4. Clay minerals

Through whole rock XRD analysis of 403 samples, the shale reservoirs of the Lucaogou Formation have a low content of clay minerals on the whole (on average 12.5%). The XRD results of clay minerals show that the clay minerals of the Lucaogou Formation are mainly illite/montmorillonite mixed layer and chlorite/montmorillonite mixed layer, with relative contents of 40.3% and 27.6%, respectively. The kaolinite content is extremely low, with a relative content of only 0.5% (Fig. 5a, Table 3). The ratio of illite/montmorillonite mixed layer mainly ranges from 70% to 100%, and the ratio of chlorite/montmorillonite mixed layer is mainly 20% to 40% (Fig. 5b).

Authigenic kaolinite is formed under the condition of acidic medium, while montmorillonite and illite/montmorillonite minerals are clay minerals formed in alkaline medium, and are mostly altered from volcanic materials (Fig. 4f). In alkaline medium conditions, when the pore water is rich in K⁺, at a certain temperature, montmorillonite would transform into illite or feldspar transform directly to illite; when pore water is rich in Mg²⁺ and Fe²⁺, in alkaline medium condition, chlorite can directly precipitate, or montmorillonite can convert into chlorite through chlorite/montmorillonite layer. Illite is formed in weakly alkaline pore water rich in K⁺ or is transformed from other minerals during diagenesis. For example, kaolinite can gradually transform into illite in slightly alkaline or alkaline solution rich in K⁺. Therefore, the appearance of illite indicates the existence of weakly alkaline-alkaline environment^[6] (Fig. 4g).

3.1.2. Quartz dissolution

Previous studies suggested that there is direct dissolution of silica^[32]. When the pH value is less than 9, SiO₂ basically does not dissolve; when the pH value is greater than 9, SiO₂ begins to dissolve. Meanwhile, as the pH value increases, the solubility of SiO₂ also increases accordingly. Therefore, SiO₂ can only precipitate in acidic or weakly alkaline environment. Quartz dissolution indicates that there were alkaline fluids during the diagenetic process, and the reservoirs have been reformed to some extent. Through observation and analysis of reflected light rock slices and SEM, the alkaline dissolution of quartz mainly appears in irregular bay shape at edges of the particles or honeycomb shape dissolution on quartz grain surface (Fig. 4h).

The acidic diagenesis in the study area is dominated by acidic dissolution of feldspar, associated with a small amount of carbonate dissolution. Carbonic acid and organic acids formed when organic matter matures are the main sources of acidic fluids in the study area^[33]. Affected by the carbonic acid and organic acids discharged by the two stages of organic matter maturation, feldspar crystal fragments in the reservoirs are significantly dissolved. Observation of cast thin sections shows the surface or edge of feldspar crystal fragments are dissolved to various degrees to form dissolution intergranular pores, dissolution intragranular pores and moldic pores (Figs. 3d and 4i). Under the SEM, it is found the acid dissolution of feldspar mainly goes along cleavages, and there is a gradual dissolution expansion along the cleavages. Calcite usually occurs in connected crystal cement or filling of cracks in the rock, with no obvious signs of dissolution. Only in the local porphyry dolomite interval, dissolution of calcite particles has been seen.

The burial depth of the Lucaogou Formation in the study area is 2500 to 4000 m, the R_o value of source rock is 0.70% to 1.30% (on average 0.78%), and the maximum peak temperature of pyrolysis T_max is 428 ℃ to 459 ℃ (on average 440 ℃). According to the oil and gas industry standards of the People’s Republic of China^[34], the shale reservoirs of the Lucaogou Formation are currently in stage A of middle diagenesis^[35,36] (Fig. 6).

During the sedimentary period of the Lucaogou Formation, the lake water was alkaline, the average value of Sr/Ba in the reservoirs was 1.43, the average value of Ba/Ga was 7.75, the average value of Th/U was 1.61^[36,37], and δ¹³C and δ¹⁸O in dolomite were higher and relatively stable^[38,39,40]. There are three possible reasons for the alkaline sedimentary water body: (1) Volcanic activity was strong in the Junggar Basin in Permian^[37], the tuff material formed had high contents of alkaline feldspar and medium-based tuff etc., these materials released Na⁺, K⁺, Mg²⁺, and Fe²⁺ during hydrolysis, leading to an alkaline depositional environment. (2) Hydrothermal activity brought about a large amount of alkaline materials. (3) The weathering and leaching of volcanic material around the basin formed alkaline cations like Mg²⁺ and Fe²⁺ which were brought into the lake by rivers around the basin.

The water nature of alkaline saline lake basin in the sedimentary period determined that the formation water in the penecontemporaneous to early diagenetic stage A was alkaline, and the main diagenetic processes include gypsum, dolomite and calcite precipitation, and feldspar and quartz alkaline dissolution. Carbonate minerals such as dolomite and calcite in the slope area of the basin have a δ¹³C of 6.8‰ to 9.7‰, 8.3 ‰ on average, which is higher than that of lacustrine primary carbonate rocks^[41], so they are of penecontemporaneous period evaporative lacustrine facies origin affected by volcanic materials or hydrothermal fluid^[42]. In addition, microbial genesis is also an important formation mechanism of dolomite in deep depressions of the basin.

From late period of early diagenesis B stage to early period of middle diagenesis A stage, affected by burial thermal effect and the thermal catalysis of tuffaceous material, organic matter reached low maturity and started to generate oil, and the CO₂ and organic acids produced in association with oil generation dissolved feldspar and carbonate minerals. This stage was the period of acidic diagenesis.

In the late period of middle diagenesis A stage, as organic acids and carbonic acid dissolved feldspar and other minerals, they consumed themselves, so the pore water gradually changed from acidic to weakly alkaline and entered alkaline diagenetic environment again. The dolomite formed at this stage is burial origin and has iron-bearing characteristic. Cementation of iron-bearing dolomite and iron-bearing dolomite replacing calcite occurred. In addition, there are a series of alkaline diagenetic features such as illite precipitation and dissolution of secondary quartz overgrowth edge.

The above phenomena indicate that the shale reservoirs of Lucaogou Formation have characteristics of alternate acid and alkaline diagenetic processes, and alkaline diagenesis took dominance, and acid dissolution mainly occurred after the early diagenetic stage (Fig. 6).

In addition to depositional conditions, diagenesis is also an important factor affecting reservoir physical properties and pore evolution, among which compaction, dissolution, and cementation are the three most important factors. Different from the traditional acidic diagenesis, when the main diagenetic environment is alkaline, the reservoirs generally undergo multiple alternations of the acidic and alkaline diagenetic environments, which ultimately significantly improve the physical properties of the reservoirs^[6]. Affected by mineral composition, texture, different rocks have different types and sizes of pores. Based on burial history, diagenesis and their impact on pores, the pore evolution of the Lucaogou Formation reservoirs in the study area has three stages, alkaline compaction and porosity reduction-porosity increase stage, porosity increase stage by acid dissolution, and porosity increase-porosity reduction stage by alkaline dissolution (Fig. 7).

The alkaline compaction porosity reduction-porosity increase stage occurred in early diagenetic stage A to early diagenetic stage B from the early stage of Late Permian to Late Triassic (200-220 Ma). Based on the quantitative simulation of staged paleo-porosity evolution of reservoir diagenesis, the Lucaogou Formation sediments had an initial porosity of 50% to 55%, as subsidence went on, the Lucaogou Formation reduced to a porosity of about 20% by 200 Ma (Late Triassic) due to mechanical compaction (Fig. 7). The whole rock XRD results of Well Ji 174 in the study area show that the upper and lower sweet spot sections of the shale reservoirs in the Lucaogou Formation have higher tuff content than the intervals not in sweet spots. In the interval of tuffaceous feldspar detrital silt fine sandstone, affected by the sedimentary environment and alkaline fluid, and the quartz suffered alkaline dissolution according to equation (1). The alkaline environment is conducive to the dissolution and migration of siliceous minerals as quartz etc. and aluminosilicate minerals, and silicon and aluminum elements are transported in the form of negatively charged organic complex ions. These complex ions are stable in alkaline liquids and likely to precipitate in acidic liquids^[43]. Owing to the porosity reduction caused by mechanical compaction and porosity increase caused by alkaline dissolution, the porosity is higher than the normal compaction curve (Fig. 7).

In contrast, the tuff and organic matter-bearing micritic dolomite and micritic algal dolomite intervals have fewer primary pores and well-developed intercrystalline pores. Due to the cementation of calcite in the reservoirs and the increase of dolomite crystals in the rock framework, these reservoirs have higher resistance to compaction.

The porosity increase stage by acid dissolution occurred in early Middle diageneis A stage from the early Jurassic to Early Cretaceous (100-200 Ma ago), when the Lucaogou Formation shale reservoirs further reduced in porosity to 10% (Fig. 7). The source rock of the Lucaogou Formation in the Jimsar Sag has entered the oil-generating stage (with R_o value reaching 0.5%) since the Late Triassic, and entered massive hydrocarbon generation stage in the Middle Jurassic (at R_o value of 0.7%)^[44]. To the late Middle Jurassic, before the formation of liquid hydrocarbons by the source rock, oxygen-containing functional groups removed from kerogen formed water-soluble organic acids (such as formic acid, acetic acid, propionic acid, and oxalic acid), and reached the peak of organic acid production. The acidic fluids in the Lucaogou Formation were mainly organic acids, followed by carbonic acid generated by dissolution of carbon dioxide from organic acids dissociation at high temperatures in water^[45]. The alkaline feldspars in the tuff of the tuff-bearing feldspar detrital fine sandstone and tuff and organic matter-bearing micrite dolomite dissolved massively according to equation (2), increasing porosity. With the increase of tuff, the secondary pores become more developed.

The porosity increase-porosity reduction stage by alkaline dissolution occurred in the middle diagenesis A stage from late Early Cretaceous (100 Ma from now) to the present, when the porosity of the tight reservoirs of the Lucaogou Formation reduced to 9%. Due to the consumption of organic acids and carbonic acid, the diagenetic environment started to change from acidic to weakly alkaline or alkaline. Under the condition of alkaline medium, iron-bearing calcite and iron-bearing dolomite precipitated in the pores, and the porosity reduced. Fine quartz particles in tuff dissolved in the alkaline environment. Tuff contained more alkaline feldspar, and the alkaline feldspar reacted with kaolinite from acid dissolution to form illite and illite/montmorillonite minerals^[46]. Therefore, the reservoir rocks have characteristics of porosity increase caused by alkaline dissolution and porosity reduction caused by compaction and cementation. Now the main body of the reservoirs is in the middle diagenesis A stage, and has primarily secondary pores formed by dissolution of feldspar crystal fragments and intercrystalline pores of dolomite. The physical property analysis data shows that the reservoirs have an overall average porosity of 5% to 12% (Fig. 7).

In the alternate acid-alkali diagenetic environments, three secondary pore zones developed in the Lucaogou Formation of the study area, namely porosity reduction-porosity increase by alkaline compaction secondary pore development zone at the depth of 2415-2655 m, acidic dissolution porosity increase secondary pore development zone at 3115-3870 m depth, and alkaline dissolution porosity increase and porosity reduction secondary pore development zone at the depth of 4015 to 4280 m. But at the present stage, under stronger compaction, the reservoirs have low porosities (Fig. 8).

The results of cast thin section, scanning electron microscopy and high-pressure mercury intrusion experiments show that the primary intergranular pores are 1-30 µm in diameter, the intergranular dissolution pores are 10-50 µm in diameter (up to 100 µm), and the intragranular dissolved pores are 5-20 µm in diameter, and the intercrystalline pores are 100-1000 nm in diameter, and mainly found in dolomite and dolomitic siltstone (fine sandstone). The tuffaceous feldspar detrital fine sandstone contains pores largely 0.036-5.000 µm in size, with micropores, submicron pores and nanopores account for 77%, 20%, and 3% respectively. This kind of pore structure is similar to that of conventional siltstone-fine sandstone reservoirs. This kind of reservoir has higher content of anti-compacting minerals (quartz, feldspar), and mainly remaining intergranular pores and acidic dissolution pores (Figs. 9 and 10). Tuffaceous and organic matter-bearing micrite dolomite contains tuff dissolution pores and dolomite intercrystalline pores. The pores are wide in size distribution range, and dominated by submicron pores (79%). This kind of rock has higher retained hydrocarbon content^[35]. The porosity characteristics of this kind of rock are the reformation results of alternate alkaline diagenesis and acid diagenesis. Alkaline dissolution improved the rock's overall resistance to compaction and enabled some primary pores to be preserved, providing channels for later acid dissolution (Figs. 9 and 10). The micritic algal dolomite has mostly intercrystalline pores of submicron scale (90%), and a small proportion of micron-scale pores (only 5%), showing a relatively poor pore structure. Compared with tuffaceous and organic matter-bearing micrite dolomite, it has fewer micron-scale dissolution pores, which is related to the low content of acidic dissolution substances in such rock and the weak reformation of acidic diagenesis (Figs. 9 and 10).

Diagenetic facies is the core element that determines the storage performance and oil and gas enrichment of clastic reservoirs, and represents the combined effect of diagenetic environment and diagenetic minerals^{[7, 47]}. Accurate evaluation of diagenetic facies enables effective prediction of favorable reservoirs and diagenetic traps. Researchers in China and abroad have different understandings on and divisions of diagenetic facies^[7], which are mainly reflected in the connotation and naming of diagenetic facies, but most of them involve diagenetic processes and their products. According to the idea of combining the source rock type and the dominant pore type, the diagenetic facies of the Lucaogou Formation shale reservoirs is divided into tuffaceous residual intergranular pore-dissolution pore facies, tuffaceous and organic matter-bearing micrite dolomite mixed pore facies, and micrite algal dolomite intercrystalline pore facies. Affected by differences in lithology, different lithofacies in the study area exhibit different diagenetic characteristics.

5.1.1. Tuffaceous residual intergranular pore and dissolution pore facies

The silt-fine sandstone samples with higher tuff content have higher contents of alkaline feldspar and quartz crystal debris, and are mainly mud-fine silt-sand grade in granularity. These descending volcanic materials were carried into the lake basin by gravity or wind^[48]. Experiencing less transport and grinding, the crystal debris is mostly in sharp angular shape, and in particle-supported, point-line and line contact. The main pore types in this facies are residual intergranular pore, intergranular dissolved pore, feldspar intragranular pore and tuff dissolved pore. The remaining intergranular pores are mostly triangular and quadrilateral, with uniform size and distribution (Fig. 11a, 11b).

Diagenesis occurred under the condition of alkaline media, and both quartz and feldspar crystal fragments suffered dissolution, resulting in irregular bay-shaped edges of quartz and feldspar crystal fragment in the reservoirs, and a large number of secondary dissolution pores which provides storage space for oil (Fig. 3i). In the early alkaline diagenetic environment, alkali feldspar and quartz were dissolved to various degrees, increasing the pore volume. In the late alkaline diagenetic environment, the oil generated entered into the remaining intergranular pores, inhibiting mechanical compaction and protecting the reservoir pores. During diagenesis in acidic medium, the pH of reservoir fluid was controlled by organic acids, and feldspar crystal fragments and tuff were dissolved to various degrees (Fig. 11c). This diagenetic facies is constructive diagenetic facies influenced by acid and alkali dissolutions.

5.1.2. Tuffaceous and organic matter-bearing micritic dolomite mixed pore facies

This type of diagenetic facies is characterized by the mixing of tuff and organic micrite dolomite in different proportions, including tuffy micrite dolomite and tuffaceous micrite dolomite (Fig. 11d). This kind of diagenetic facies has oil generation mechanism different from the traditional model, when the tuff material mixes with dolomitic rock containing organic matter, the trace elements in the tuff has catalytic effect to hydrocarbon generation of source rock, among which, radioactive elements with high heat flow values, like U, Th, K make the formation temperature rise, and thus promoting the thermal evolution of hydrocarbon source rock^[49]. Clearly, tuff material played an important role in facilitating the maturity of organic matter^[9]. The sections in Lucaogou Formation with higher contents of volcanic material have R_o values of above 0.8% and T_max values of above 440 °C, reaching the limit of oil and gas maturity. In contrast, the horizons with lower contents of volcanic material have R_o and T_max values not reaching the standard of source rock maturity. In addition, the horizons with higher volcanic material content have an average value of C₂₉ααα20S/(S+R) of 0.49, and average C₂₉αββ/(ααα+ αββ) value of 0.32, which also reflect that the organic matter has reached maturity. Therefore, under the thermal catalysis of volcanic activity, the early diagenetic organic matter could enter a low-maturity period. The formation of organic acids led to the dissolution of crystal fragments and the increase of secondary pores. The generated oil and gas are not only stored in intergranular pores and dissolved pores, but also intercrystalline pores rich in dolomite itself. There are many intergranular pores and organic matter pores observed in the micrite dolomite under SEM (Fig. 11e, 11f).

In this type of diagenetic facies, there is a kind of micrite dolomite-microbial dolomite with high oil content^[50,51]. The dolomite has largely micro-nano-scale intercrystalline pores, a porosity range of 3.89%-8.26%, and a permeability range of (0.101 2-0.371 9)×10^-3 μm². Although poor in physical properties, these rocks are widely distributed laterally and vertically, and uniform in lithology, so they have good oiliness. There are "bay-like" decomposition and oil generation characteristics at the edge of organic matter. The oil appears in thin film at the edge of intercrystalline pores or filling in pores, so this kind of rock is a set of effective reservoirs with self-generation and self-storage characteristics.

5.1.3. Micritic algal dolomite intercrystalline pore facies

Under the condition of early alkaline medium, the algal bloomed in the saline lake basin under the influence of hydrothermal fluid and tuff, providing conditions for mass production and deposition of organic matter. The brine in the bottom layer of the lake basin was isolated from air, providing ideal preservation conditions for organic matter.

The micrite algal dolomite intergranular pore facies is distributed in deep lake facies and is the main source rock type in the basin^[51], in which organic pores, laminar algal layers and fractures are the main media for in-situ hydrocarbon expulsion and accumulation (Fig. 11g). It has mainly dolomite intercrystalline pores, and tight rocks. Although poorest in physical properties in the study area, it has fairly good oil-bearing property and self-generation and self-storage characteristic (Fig. 11h, 11i).

Most of the source-reservoir reservoirs in the study area belong to the tuffaceous and organic matter-bearing micritic dolomite mixed pore facies, followed by micritic algal dolomite intercrystalline pore facies. Due to the development and preservation requirements of large amounts of organic matter, the tuffaceous and organic matter-bearing micritic dolomite mixed pore facies and micritic algal dolomite intercrystalline pore facies are mostly distributed in the semi-deep lake-deep lake facies with high basic tuff content. Due to the frequent periodic rise and fall of the lake level, the relatively coarse- grained lithofacies, such as the tuffaceous remaining intergranular pore-dissolution pore facies, is widely distributed in the sedimentary lake basin.

The distribution of diagenetic facies has control effect on pore structure, and is of great guiding significance for the reservoir development. The upper and lower oil-bearing sections have higher contents of tuff, microbial dolomite and laminar algal than the non-oil-bearing sections, and better mixing degree of tuff and microbial dolomite than the non-oil-bearing sections. The upper and lower oil-bearing layers have better physical properties and higher oil content than the non-oil-bearing layers. Moreover, statistical results show that there is a positive correlation between tuff content and dissolution.

Taking Well Ji 174 as an example, the layers with oil saturation greater than 50% are mostly concentrated in the upper and lower oil-bearing sections, and come largely in tuffaceous remaining intergranular-dissolution pore facies and tuffaceous and organic matter-bearing micritic dolomite mixed pore facies. These two types of diagenetic facies mostly occur in the upper and lower oil layers with higher tuff content with interstratified pattern vertically.

The Lucaogou Formation is saline lake facies sediment, with two cycles in lithology. The upper and lower sweet spot sections correspond to two salinization peaks of the lake basin. From the lower sweet spot section to the upper sweet spot section, the lake basin turned closed, shallower in water body, and higher in salinity. On the plane, the tuffaceous remaining intergranular pore-dissolution pore facies in the second member of the Lucaogou Formation is distributed in the southern and middle parts of the sag, and is the main type of lithofacies in the upper sweet spot section. Wells encountering tuffaceous remaining intergranular pore-dissolution pore facies have good oil test results. For example, Well Ji 172-H had a daily oil production of 56.32 t after 15-stages of fracturing in formation testing. The tuffaceous and organic matter-bearing micritic dolomite mixed pore facies is distributed in the eastern part of the sag. Well Ji 23 drilling into this facies had a production capacity of 0.24 t/d after acid fracturing (Fig. 12a). The first member of Lucaogou Formation is composed of shallow lacustrine facies, shallow lacustrine facies with semi-deep lacustrine facies and semi-deep lacustrine facies sediments. The main body of this member is tuffaceous remaining pore-dissolution pore facies. Well Ji 36-H encountering this facies had an oil production of 20.2 t/d after 20-stage fracturing (Fig. 12b). The micritic algal dolomite intercrystalline pore facies in the first and second members of Lucaogou Formation are mainly distributed near Well Ji 174, which is deep lacustrine deposit with low oil production.

Overall, the tuffaceous remaining intergranular pore-dissolved pore facies and tuffaceous and organic matter-bearing micritic dolomite (or microbial dolomite) mixed pore facies are the diagenetic facies with the best porosity-permeability correlation in the study area. With low saturation median pressure and high oil recovery efficiency, they are favorable diagenetic facies of the Lucaogou Formation.

The Permian Lucaogou Formation reservoirs in the Jimsar Sag, Junggar Basin have mainly primary intergranular pores, alkaline dissolution pores, and acid dissolution pores. The tuffaceous remaining intergranular pore-dissolution pore facies has mainly primary intergranular pores and acid and alkali dissolution pores. The tuffaceous and organic matter-bearing micritic dolomite mixed pore facies contains pores from acid dissolutions and dolomite intercrystalline pores from an alkaline environment. The micrite algal dolomite intercrystalline pore facies contains dominantly dolomite intergranular pores formed in an alkaline environment.

The pore evolution of the shale reservoirs is comprehensively controlled by mechanical compaction and alternate acid-alkali diagenesis, and has experienced three stages, alkaline compaction porosity reduction-porosity increase stage, acid dissolution porosity increase stage and alkaline dissolution porosity increase-porosity reduction stage. There are three secondary pore development zones in the reservoirs: alkaline compaction porosity reduction-porosity increase secondary pore development zone at 2500 m; acid dissolution porosity increase secondary pore development zone at 3250- 3750 m; and alkaline dissolution porosity increase-porosity reduction secondary pore development zone at 4100-4200 m.

The tuffaceous remaining intergranular pore-dissolution pore facies features wide distribution and good pore structure, and the tuffaceous and organic matter-bearing micritic dolomite mixed pore facies has the characteristics of higher contents of oil and dissolution pore, so they are favorable diagenetic facies for "sweet spots".