Introduction
The diagenesis and pore evolution of shale reservoirs have been a hot research and difficulty in unconventional oil and gas geology in the past decade [1-2]. The mineral compositions and diagenetic evolution paths of shales from different petroliferous basins vary significantly [3], which result in various characteristics of shale reservoirs. For example, the authigenesis and dissolution processes of silica in marine black shale can affect the pore evolution and fracturing effectiveness of shale reservoirs [4-5], while the authigenesis and dissolution of calcite and dolomite in continental saline lake shale have a significant impact on shale reservoirs [6]. The Upper Paleozoic Fengcheng Formation in the Mahu Sag of the Junggar Basin contains high-quality alkaline lacustrine source rocks [7-8], and is the key target for shale oil exploration and development in recent years [9]. Although the oil shale of the Fengcheng Formation has shown good exploration prospects [9], the practice of exploration, development and production still faces various problems like “scattered sweet spots, unsatisfactory testing results of single-layer oil, and unclear production potential” [10]. The main reason for the above-mentioned problems is the insufficient understanding of the basic geological conditions of the Fengcheng Formation shale and the weak research on their diagenesis.
The shale diagenesis of the Fengcheng Formation is poorly studied. Yu et al. [11] focused on the alkaline diagenesis of the Fengcheng Formation, and identified zoned dolomite formation, quartz dissolution, chlorite and zeolite cementation. Guo et al. [8] proposed that organic matter degradation and thermal evolution could promote the dissolution of Mg-bearing clay minerals and formation of dolomite by analyzing the mineral compositions and diagenesis of shales with different organic matter abundances in the Fengcheng Formation. Shan et al. [12] summarized the most important diagenetic processes that control the formation and evolution of shale oil reservoirs in the Fengcheng Formation, which include compaction, devitrification, dissolution, recrystallization, and tectonic fracturing. Guo et al. [13] focused on the mineral analysis of the main felsic shale in the Fengcheng Formation, and proposed that its felsic shale includes three sub-types: detrital felsic shale, silica-rich felsic shale, and modified felsic shale, among which the modified felsic shale is primarily composed of detrital clays, feldspar, and volcanic materials that later underwent multiple stages of diagenetic transformation. The above studies on the shale diagenesis of the Fengcheng Formation either only considered the formation mechanism and diagenetic evolution process of a certain type of mineral, or focused on the whole Fengcheng Formation shale, lacking detailed comparison of shales from different sedimentary zones and stages, which still cannot meet the demand for predicting the “sweet spots” of shale oil. Laterally, several sedimentary zones were classified for the Fengcheng Formation [14], and water salinity in different zones varied greatly [9], resulting in distinctive evolution processes of diagenetic environments, which in turn led to significant differences in the diagenetic processes and reservoir characteristics of the shales from different sedimentary zones.
Considering the control of sedimentary environments on diagenesis and based on the previous analysis on the genesis of single minerals (e.g. siliceous, reedmergnerite and dolomite) in the Fengcheng Formation [8,13,15 -16], this study conducted thin section, backscatter, and argon-ion polishing scanning electron microscopy analysis on the shales of the Fengcheng Formation from different sedimentary zones. This study proposed a diagenetic model to stress the differential evolution of acid-alkaline diagenetic environments for alkaline lake shale reservoirs from different sedimentary zones. The diagenetic evolution sequences of shale reservoirs from different sedimentary zones were restored, and the influence of differential acid-alkaline diagenetic evolution on shale reservoir characteristics and its implications for shale oil exploration and development were explored and summarized.
1. Regional geological setting
The Junggar Basin, with an area of about 1.3×105 km2, is the second largest petroliferous sedimentary basin in northwestern China (Fig. 1a ). It is located in the triangle junction zone of Kazakhstan, Siberia and Tarim plates. This basin is now surrounded by several folded mountains: the Kelameili, Qinggelidi and Altay mountains to the northeast, the Hala’alate and Zaire mountains to the northwest, and the Yilin Heibiergen and Bogda mountains to the south. In the Late Paleozoic, the Junggar Basin was located in the Kazakhstan tectonic arc and was an important part of the Central Asian orogenic belt [17]. It initially received marine sediments, and then changed to receive lacustrine sediments as the ancient Tethys Ocean regressed from north to south within the basin [18]. The Mahu Sag has an area of about 5 200 km2, and is located in the northwestern Junggar Basin (Fig. 1a ). It is adjacent to the Wuxia Fault Belt and Kebai Fault Belt to the northwest, the Yingxi Sag, Sangequan Bulge and Xiayan Bulge to the east, and the Dabasong Bulge to the south (Fig. 1b ). The Fengcheng Formation is unconformably underlaid and overlaid by the Jiamuhe Formation and Xiazijie Formation, respectively, and has a thickness of 150-1 800 m and a burial depth of 2 600-6 500 m.
Fig. 1 Location of the Mahu Sag and distribution of key wells. |
During the deposition of the Fengcheng Formation, the Mahu Sag could be divided into three sedimentary zones based on stratigraphic lithology and thickness: the lake-central zone, marginal zone and transitional zone (Fig. 2 ). The lake-central zone had the most saline and alkaline lake water, where the Fengcheng Formation was greater than 800 m in thick and mainly consisted of Na-carbonate beds and Na-carbonate-rich shale, with a small amount of dolomitic shale. The lake water in the transitional zone was less saline and alkaline than that of the lake-central zone. The Fengcheng Formation in the transitional zone has a thickness of 450-800 m, and mainly contains dolomitic shale and minor siltstone and Na-carbonate-rich shale. The marginal zone had the least saline and alkaline lake water, and the Fengcheng Formation there has a thickness less than 450 m and is mainly composed of siltstone. The Fengcheng Formation shales from the above three sedimentary zones of the Mahu Sag all contain abundant organic matter, due to the unique organic matter preservation mechanism in shallow alkaline lake [7]. The Fengcheng Formation shale in the Mahu Sag mainly contains type-II1 and type-II2 organic matter. It has been mature and partly entered the hydrocarbon generation window (Ro value of about 1.0%)[19-20]. The Fengcheng Formation shale mainly contains clay, Na-carbonate, carbonate, and felsic minerals, among which felsic minerals are dominated [21-22]. The storage space of shale reservoirs mainly includes intergranular pores, dissolution pores, intercrystalline pores, drainage fractures, structural fractures, and bedding fractures [23]. Among them, residual intergranular pores, dissolution pores, and fractures are the most important storage space, and the fractures are prerequisite for high shale oil production.
Fig. 2 Differential evolution model for the sedimentary and diagenetic environments (mainly pH value) of the Fengcheng Formation from different sedimentary zones of the Mahu Sag (the section location is shown in |
2. Differential diagenetic environments in various sedimentary zones of alkaline lake
The Fengcheng Formation in the Mahu Sag is a typical alkaline lake deposit in China, and its sedimentary-diagenetic environment evolution differs significantly from those of freshwater and non-alkaline saline lake deposits. Firstly, the original sedimentary environment is very alkaline (pH>9), which can enhance the solubility of quartz [24] and the instability of silicate minerals (clay minerals) [25]. Quartz, clay minerals, and tuffaceous materials are prone to be dissolved or transformed in alkaline lakes, resulting in higher concentrations of soluble silica and Al(OH)3 in the lake water [26]. Secondly, compared to gypsum and glauberite precipitated in non-alkaline saline water environments and to calcite and dolomite formed in brackish water environments, the Na-carbonates deposited in alkaline lakes have greater solubility and stronger acid-sensitivity. Na-carbonates can quickly neutralize the acidic fluids and gases that were generated during organic matter degradation and hydrocarbon generation, keeping the diagenetic environment alkaline as a whole. Thirdly, due to the occurrence of unique bacteria and algae in alkaline lakes and the favorable preservation conditions for organic matter in shallow water areas, shales with high organic matter abundance can also be deposited in the transitional and marginal zones [7].
The Ro values of the organic matter in the Fengcheng Formation range from 0.7% to 1.1% [19], and the homogenization temperature of the primary inclusions in the reedmergnerite varies between 90 °C and 110 °C [16]. According to the petroleum and natural gas industry standard of the People's Republic of China, “The division of diagenetic stages in clastic rocks” (SY/T 5477-2003), the Fengcheng Formation shale is currently in the middle diagenetic A stage. The organic acid theory proposes that within the temperature of 80 °C to 120 °C in the middle diagenetic A stage, the acids that are expelled from organic matter reach the peak [27], which can transform the diagenetic environment into acidic environment. Therefore, comprehensively considering the salinity and alkalinity of lake water and organic matter enrichment in different sedimentary zones of the alkaline lake, this study suggests that the sedimentary-diagenetic environment of the Fengcheng Formation varies greatly among different sedimentary zones in the Mahu Sag (Fig. 2 ). The shale in the lake-central zone (represented by wells FN5, AK1, and HS5) contains a large amount of organic matter and Na-carbonates. Although organic matter in the shale can generate abundant organic acids and CO2 during the mature stage, they can be quickly neutralized by Na-carbonates, resulting in the diagenetic environment always being alkaline. The shale in the transitional zone (represented by wells FN1, FN2, and FN14) contains abundant organic matter but a few Na-carbonates. The early inherited alkaline diagenetic environment will be gradually transformed into acidic environment with the continuous expelled acidic fluids from organic matter during the hydrocarbon generation process. The marginal zone, represented by wells MY1 and X40, is characterized by weakly alkaline sedimentary environment due to being far from the lake-central zone. The diagenetic environment of the shale can be changed from early weakly alkaline to late acidic due to the continuous expelled acidic fluids during the hydrocarbon generation process by organic matter.
3. Main diagenesis of shale in the lake-central zone
3.1. Dissolution and transformation of minerals in alkaline environments during depositional to syn-depositional stage
The lake water in the lake-central zone is very alkaline with pH value greater than 9 or even exceeding 10, which will lead to the dissolution of terrestrial fine-grained quartz after entering the alkaline lake. Therefore, the shale in the lake-central zone has rare quartz and a high content of K-feldspar and albite. Few terrestrial clay minerals exist within the shale in the lake-central zone, also due to the dissolution and transformation in highly alkaline water. For example, detrital montmorillonite is unstable in alkaline water and is prone to be dissolved or transformed into new silicate minerals, such as Fe-illite, Mg-bearing montmorillonite, K-feldspar, and analcime [28-29]. The dissolution and transformation of terrestrial detrital minerals in strongly alkaline environments provide the main material sources for authigenic silicate minerals in alkaline lakes [28].
Volcanic materials are also unstable in alkaline water, prone to be dissolved or transformed into new silicates and siliceous minerals [30-31]. Most alkaline lakes in the world are directly related to volcanic activity and associated hydrothermal activity [32]. The synchronous volcanic materials are important sources for authigenic silicate minerals in alkaline lake sediments [26,30]. For example, the volcanic ash found in the Fengcheng Formation cores of Well HS5 in the lake-central zone has been altered into authigenic silicate minerals such as albite and zeolite. The transformation of volcanic and terrestrial detrital materials can continue until the early diagenetic stage, both mainly forming similar authigenic silicate minerals such as zeolite, searlesite, and feldspar. The matric K-feldspar of the Fengcheng Formation shale in the lake-central zone often coexists with northupite (Fig. 3a ), which is the result of multiple dissolution and precipitation (Fig. 3b ). The matric K-feldspar is characterized by varying size, irregular shape and blurred boundaries, with a few owning regular crystal shapes (Fig. 3c ). There are also abundant authigenic K-feldspar between Na-carbonates and plentiful intercrystalline pores are developed between K-feldspar crystals (Fig. 3d-3f ).
Fig. 3 Authigenic silicate minerals and related pores formed in the strongly alkaline diagenetic environment of the Fengcheng Formation shale in the lake-central zone. (a) The Na-carbonate-rich shale matrix composed of northupite and K-feldspar, Well AK1, 5 668.89 m, backscattering image; (b-c) Diverse and dense K-feldspars in the Na-carbonate-rich shale matrix, Well AK1, 5 665.99 m, secondary electronic imaging, argon-ion polishing backscattering images; (d-f) Authigenic K-feldspar between Na-carbonate crystals, developing intercrystalline pores, Well HS5, d and e are 4 798.98 m, f is 4 799.50 m. Kf—K-feldspar; Nor—northupite; St—shortite; Q—quartz. |
3.2. Authigenesis, replacement and dissolution processes in alkaline environments during early and middle diagenetic stages
In the inherited high-alkaline environment during early diagenetic stage, abundant authigenic hydrous Na-carbonate and borate minerals will be formed within the shale in the lake-central zone [33]. Due to the old age of the Fengcheng Formation, the early formed hydrous Na-carbonates, e.g. pirssonite and gaylussite, will dehydrate to form more stable minerals, such as shortite (Fig. 4a ), eitelite (Fig. 4b ), and northupite (Fig. 4c ). Shortite replacing eitelite and northupite replacing shortite can be found in the Fengcheng Formation shale in the lake-central zone. Such authigenesis and replacement of Na-carbonate minerals mainly occur during early and middle diagenetic stages.
Fig. 4 Authigenic Na-carbonate minerals and related pores formed in the strongly alkaline diagenetic environment of the Fengcheng Formation shale in the lake-central zone. (a) Shortite crystals in shale, Well FN5, 4 072.90 m, under cross-polarized light; (b) Eitelite crystals in shale, Well FN3, 4 129.60 m, under cross-polarized light; (c) Northupite crystals in shale, Well FN5, 4 071.05 m, under plane-polarized light; (d) Reedmergnerite replacing shortite in shale, Well FN3, 4 128.00 m, under cross-polarized light; (e) Reedmergnerite replacing eitelite and shortite in shale, Well FN3, 4 128.00 m, under cross-polarized light; (f) Residual shortite on the surface of reedmergnerite crystals, Well FC011, 3 862.20 m, under cross-polarized light; (g) Dissolution pores in shortite crystals, Well HS5, 5 341.45 m, secondary electronic imaging; (h) Particle-edge cracks between reedmergnerite crystals, Well HS5, 5 130.00 m, secondary electronic imaging; (i) Intercrystalline pores between trona and wegscheiderite crystals, Well HS5, 4 796.70 m, secondary electronic imaging. |
The maturity of organic matter in shale increases with the growing burial depth. In addition to hydrocarbons, the organic matter in the shale will also generate abundant organic acids (e.g. carboxylic acids and phenolic acids) and acidic gases (e.g. CO2 and H2S), which have a significant impact on the diagenetic environment and mineral composition of petroleum systems [34]. Due to the occurrence of abundant Na-carbonate minerals in the shale of lake-central zone, the acidic fluids generated during organic matter maturation were quickly neutralized by the surrounding Na-carbonate minerals, so the diagenetic environment of the shale can always keep alkaline. The above neutralization process should have been characterized by the dissolution of Na-carbonate minerals, which would produce a large amount of dissolution pores. However, most Na-carbonate minerals within the shale matrix of lake-central zone were replaced by reedmergnerite (Fig. 4d-4f ), and only a few minerals were locally dissolved to form secondary dissolution pores (Fig. 4g ). The dissolution pores have circular or elliptical shapes, varying sizes, with the maximum size of 20 μm and the minimum size less than 1 μm. Except for a few dissolution pores connected by microcracks, most pores are isolated. Reedmergnerite is an important authigenic borate mineral in the Fengcheng Formation shale (Table 1 ). It was mainly formed by replacing Na-carbonate minerals in the lake-central zone, which can be evidenced by the rich residual Na-carbonate minerals left in reedmergnerite-rich zones (Fig. 4d-4f ). Previous studies have shown that the main formation temperature of reedmergnerite in the Fengcheng Formation ranges from 90 °C to 110 °C, corresponding to the large-scale hydrocarbon generation stage[16]. Therefore, the reedmergnerite was the main product of Na-carbonate minerals neutralizing acidic fluids.
Table 1 Characteristics, occurrence, and genesis of main authigenic minerals in the alkaline lake shale of the Fengcheng Formation [13,15 -16,35] |
| Type | Mineral name | Mineral identification characteristics | Occurrence | Genesis | |
|---|---|---|---|---|---|
| Ca/Mg-Na- carbonate | Shortite | Orthorhombic, wedge-shaped or short prismatic, biaxial negative mineral, birefringence of 0.039, interference color reaching III level blue, can be dyed red with alizarin red. | Mainly present in dolomitic shale and Na-carbonate-rich shale; occurring as dispersed euhedral crystals, patches, and nodules. | (1) During the burial process, the salinity of pore water in shale increases due to compaction and filtration, resulting in the displacive growth of shortite crystals, distorting the surrounding lamina and sediments; (2) Transformation from early crystallized pirssonite and gaylussite. | |
| Eitelite | Trigonal, colorless in thin section, large crystals often occurring as pseudo hexagons, uniaxial negative mineral, birefringence of 0.155, high-grade white interference color, can be dyed red with alizarin red. | Mainly present in dolomitic shale and Na-carbonate-rich shale; occurring as dispersed euhedral crystals, patches, and nodules. | During the burial process, the salinity of pore water in shale increases due to compaction and filtration, resulting in the displacive growth of eitelite crystals, distorting the surrounding lamina and sediments. | ||
| Interbedded with shale, occurring as thin layers | Primary sediments | ||||
| Northupite | Isometric, octahedron or didodecahedron crystals, low negative protrusion, total extinction under crossed polarized light, can be dyed red with alizarin red. | Occurring as dispersed euhedral crystals, nodules, and irregular shapes in Na-carbonate-rich shale. | During the burial process, the salinity of pore water in shale increases due to compaction and filtration, resulting in the precipitation of northupite crystals | ||
| Interbedded with shale, occurring as layers or thin layers | Primitive sediments, or replacing eitelite layers | ||||
| Na- carbonate | Trona | Monoclinic, prismatic, tabular or radiating fibrous crystals, low negative protrusion, birefringence of 0.128, high-grade white interference color, can be dyed red with alizarin red. | Layered, with acicular single crystal and clustered or radial aggregates. | A common salt mineral in saline and alkaline lake sediments, precipitating from high CO2 solutions. | |
| Wegschei- derite | Triclinic, mainly fibrous, acicular or prismatic crystal, biaxial negative mineral, birefringence of 0.095, high-grade white interference color, can be dyed red with alizarin red | Layered, with fibrous, acicular or tabular crystals, coexisting with trona. | Controlled by temperature, with a minimum temperature of 89.5 °C for synthesis in laboratory; forming by replacing trona. | ||
| Na-B silicate | Reedmerg- nerite | Monoclinic, biaxial negative mineral, birefringence of 0.018 4, interference color reaching I level yellow, clear {001} cleavage. | Wedge-shaped, tabular or butterfly shaped, often coexisting with shortite and eitelite in Na-carbonate-rich shale and filling cracks or dispersed in dolomitic shale matrix. | Controlled by temperature, and mainly formed by replacing Na-carbonates, calcite, dolomite and feldspar during hydrocarbon generating stage. | |
| Searlesite | Monoclinic, biaxial negative mineral, birefringence of 0.019, interference color reaching II level purple, mainly identified by backscattering images, secondary electron imaging, and energy spectrum analysis. | Nodular, with blade-shape, tabular or spherical crystal. | An alteration product of volcanic materials in alkaline lakes, often coexisting with authigenic K-feldspar in the shale of lake-central zone. | ||
| Ca/Mg- carbonate | Calcite | Orthorhombic, high-grade white interference color, can be dyed red with alizarin red; rarely coexisting with Na-carbonate minerals and often present in sediments deposited in the low salinity water. | Mainly filling the Na-carbonate mold pores and cracks. | Authigenic minerals when diagenetic fluids with a low salinity. | |
| Dolomite | Orthorhombic, high-grade white interference color; dolomite cannot be stained with alizarin red, which is different from eitelite. | Dispersed in shale matrix, or filling the Na-carbonate mold pores and cracks. | The dolomite dispersed in shale matrix is an authigenic mineral, while that filling in the mold pore and cracks is a diagenetic mineral formed by replacing Na-carbonate or calcite. | ||
| Authigenic feldspar | K-feldspar | Authigenic K-feldspar can be identified by backscattering images, secondary electron imaging, and energy spectrum analysis. | Dispersed in shale matrix. | An alteration product of volcanic materials and detrital clay minerals in alkaline lakes, often coexisting with searlesite in the shale of the lake-central zone. | |
| Albite | Triclinic, biaxial positive mineral, interference color reaching I level yellow; authigenic albite can be identified by backscattering images, secondary electron imaging, and energy spectrum analysis. | Dispersed in shale matrix. | An alteration product of volcanic materials and detrital clay minerals in alkaline lakes; can be transformed from analcime and K-feldspar. | ||
| Authigenic clay minerals | Sepiolite | Orthorhombic, fibrous or earthy, biaxial negative mineral, interference color reaching III level green. | An authigenic Mg-bearing clay mineral, mainly dispersed in shale matrix. | An alteration product of volcanic materials and detrital clay minerals in alkaline lakes, usually distributed in the shale of the marginal zone. | |
It is the reedmergnerite, not the secondary dissolution pores, that formed during the process of Na-carbonates neutralizing acidic fluids, which is related to the high content of boron in volcanic alkaline lacustrine environments. The primary strata deposited in volcanic alkaline lake environments contain a large amount of boron ions, particularly abundant in the lake-central zone [35]. The chemical activity of boron ions increases with the growing burial depth and temperature, and they will enter the acidic fluids generated during organic matter generating hydrocarbons to form boric acid fluids. When interacting with such boric acid fluids, the Na-carbonate minerals will be dissolved to release abundant sodium ions, promoting the formation of reedmergnerite together with boron and silicon. This is the reason why the replacement of Na-carbonate minerals by reedmergnerite is the main diagenesis for the shale in the lake-central zone during the middle diagenetic stage. Na-carbonate minerals and reedmergnerite generally have diameter of larger than 1 mm and can form multiple types of storage space such as particle-edge cracks (Fig. 4h ), intercrystalline pores (Fig. 4i ), as well as a few intracrystalline dissolution pores (Fig. 4g ).
4. Main diagenetic processes of shale in alkaline-acid conversion environment of the transitional zone
4.1. Dissolution and authigenesis of minerals in alkaline environments during syn-depositional to early diagenetic stage
The transitional zone has less alkaline lake water than the lake-central zone. The dissolution and transformation processes that occurred during depositional and syn-depositional stage in the lake-central zone also existed in the transitional zone, but with weaker intensity. Large-scale bedded Na-carbonates do not exist in the transitional zone, also due to the lake water with relatively weak alkalinity there. However, during the low water levels and strong evaporation of lakes, the salinity and alkalinity of lake water increased, and dewatering fractures could be formed in shale matrix (Fig. 5a ), mainly filled with Na-carbonate minerals, dominated by shortite (Fig. 5b ) and eitelite [8]. The above-mentioned Na-carbonate minerals cannot be preserved during high water levels of lakes and were prone to be dissolved to form mold pores, which were later filled with calcite and dolomite (Fig. 5c ).
Fig. 5 Main diagenetic processes of the Fengcheng Formation shale in the transtional zone during the early diagenetic stage. (a) Abundant Na-carbonate pseudocrystals and dewatering fractures in shale, and Na-carbonate pseudocrystals distorting the surrounding lamina, indicating their formation in the early diagenetic stage, Well FN2, 4 040.64 m, thin section scanning image; (b) Residual shortite in the pseudocrystal indicating that the original Na-carbonate mineral is shortite, Well FN1, 4 329.40 m, backscattering image; (c) Dolomite replacing shortite in the pseudocrystals, Well FN1, 4 329.40 m, backscattering image; (d) Dolomite replacing calcite nodules, Well FN2, 4 040.64 m, backscattering image; (e) Dolomite crystals in shale, Well FN2, 4 040.64 m, backscattering image; (f-g) Dolomite crystals in shale, Well FN1, 4 184.00 m, f is under plane-polarized light, g is under cross-polarized light. |
During the syn-depositional and early diagenetic stage in the transitional zone, besides the dissolution or authigenesis of Na-carbonate minerals, abundant microcrystalline dolomite (crystal size of 15-70 μm) can also be formed in the shale matrix (Fig. 5 d-5g). The dolomite crystal size in the Fengcheng Formation shale is significantly larger than that in non-alkaline saline lake deposits in China. For example, the crystal size of dolomite in the Upper Xiaganchaigou Formation of the Qaidam Basin is generally less than 10 μm [36]. The microcrystalline dolomite in the Fengcheng Formation shale of the transitional zone exhibits zoned fluorescence and dark-red cathodoluminescence, whose formation can be divided into an initial nucleation period and a sustained growth period [9,37]. During the syn-depositional stage, a large amount of HCO3- and CO32- ions exist in the inherited alkaline environment of alkaline lakes, while the Mg2+ ions show a zoned distribution. In the lake-central zone, due to the precipitation of large amounts of eitelite and northupite, most Mg2+ ions have been consumed, hindering the formation of dolomite. Lake water of the marginal zone had an overall low salinity, so Mg2+ ion content there is low, also hindering the formation of dolomite. Hence, dolomite mainly nucleated in the transitional zone, where lake water salinity was relatively high and little magnesium competition from Mg-bearing Na-carbonate minerals occurred, leading to the highest Mg2+ ion content there [37]. During the subsequent burial process, the tuff materials and clay minerals were further dissolved or transformed, which can continuously release Mg2+ ions, causing the continuous growth of dolomite. Therefore, dolomite formed in the transitional zone is characterized by large crystal size.
4.2. Dissolution and replacement of minerals in acidic environments during middle diagenetic stage
During the early period of middle diagenesis stage, organic matter became mature and generated a large amount of hydrocarbons and acidic fluids, resulting in the diagenetic environment of shale in the transitional zone gradually changing from alkaline to acidic. The early formed authigenic minerals such as carbonate minerals and feldspar became unstable and prone to be dissolved or replaced. In the Fengcheng Formation shale of the transitional zone, the replacement of carbonate nodules by reedmergnerite (Fig. 6a-6b ) is common. With the replacement area increasing, euhedral reedmergnerite crystals will be formed (Fig. 6a ). In addition, reedmergnerite can also replace feldspar and albite nodules (Fig. 6c-6d ). When feldspar and carbonate minerals coexist, reedmergnerite preferentially replaces feldspar (Fig. 6d ). Scattered, butterfly-shaped reedmergnerite can also be seen in the shale of the transitional zone, with a diameter of approximately 1-5 mm (Fig. 6e-6f ). Different from the reedmergnerite in the replaced nodules, the butterfly-shaped reedmergnerite in shale matrix was mainly formed by replacing matrix minerals such as albite, K-feldspar, searlesite and dolomite. Because pyrite and organic matter in the matrix cannot be replaced by reedmergnerite, they were gradually pushed to the growth front of reedmergnerite. Therefore, a large amount of pyrite and organic matter were accumulated in the growth front of butterfly-shaped reedmergnerite [8].
Fig. 6 Reedmergnerite replacement in the Fengcheng Formation shale in the transitional zone during the middle diagenetic stage. (a) Reedmergnerite replacing (Fe-)dolomite nodules, Well FN14, 4 065.04 m, backscattering image; (b) Reedmergnerite replacing calcite and (Fe-)dolomite nodules simultaneously, Well FN14, 4 065.04 m, backscattering image; (c) Reedmergnerite replacing K-feldspar nodules, Well FN14, 4 065.14 m, backscattering image; (d) Reedmergnerite replacing albite nodules, Well FN2, 4 038.35 m, backscattering image; (e) Butterfly-shaped reedmergnerite, Well FN1, 4 340.54 m, under plane-polarized light; (f) Butterfly-shaped reedmergnerite, Well FN2, 4 040.64 m, backscattering image. |
In the shale samples of the transitional zone where reedmergnerite is underdeveloped, the acidic diagenesis was dominated by the dissolution and replacement of feldspar. Under acidic diagenetic environments, albite in shale was dissolved along cleavage (Fig. 7a ), forming many small albite crystals (Fig. 7b ). However, the dissolution of feldspar in the shale of the transitional zone was relatively weak, and instead, feldspar was mostly replaced by cryptocrystalline quartz (crystal size less than 1 μm) (Fig. 7c-7e ), between which abundant intercrystalline pores were developed. In addition, detrital K-feldspar can also be replaced by albite, which mainly occupied the central position of K-feldspar (Fig. 7f ).
Fig. 7 Dissolution and replacement of detrital feldspar in the Fengcheng Formation shale in the transitional zone during the middle diagenetic stage. (a) Albite dissolution along cleavage, Well FN2, 4 038.35 m, backscattering image; (b) Albite grain transformed into cryptocrystalline albite by dissolution along cleavage, Well FN2, 4 038.35 m, secondary electronic imaging; (c) Dissolution and silicification of detrital albite, Well FN2, 4 038.35 m, secondary electronic imaging; (d) Silicification of detrital K-feldspar in the center, Well FN2, 4 038.35 m, backscattering image; (e) Silicification of detrital K-feldspar in matrix, Well FN1, 4 361.36 m, secondary electronic imaging; (f) Albite replacing detrital K-feldspar in the center, Well FN2, 4 038.35 m, backscattering image. |
5. Main diagenesis of shale in weakly alkaline-acid conversion environment of the marginal zone
5.1. Authigenesis of minerals in weakly alkaline environments during depositional to early diagenetic stage
The marginal zone has the least saline and alkaline lake water among different sedimentary zones of alkaline lake, so rare Na-carbonate minerals but massive chert bands and nodules occur in the shale there (Fig. 8a ). In the alkaline environment (pH>9), the solubility of SiO2 increases exponentially with growing pH and detrital quartz is dissolved in the alkaline water, causing a significant increase in soluble SiO2 content. Due to the weak alkalinity and small depth of the lake water in the marginal zone, the pH value of the lake water there was easily affected by external conditions such as precipitation, causing a sharp solubility decrease of SiO2. This will lead to the soluble SiO2 in the water to precipitate and form colloids such as opal, which was later dehydrated to form chert [32]. In addition, clay mineral aggregates or interstitial materials can also be seen in the shale of the marginal zone (Fig. 8b ), which are speculated to be sepiolite based on the energy spectrum. Sepiolite is a common authigenic Mg-rich and Al-poor clay mineral in alkaline lake sediments, with a number of intercrystalline pores developed (Fig. 8c ).
Fig. 8 Main diagenetic processes in the Fengcheng Formation shale in the marginal zone. (a) Abundant chert bands and nodules in shale, Well MY1, 4 798.50 m, under plane-polarized light; (b) Sepiolite aggregates and associated cracks (arrow) in shale, Well MY1, 4 828.65 m, backscattering image; (c) Abundant intercrystalline pores between sepiolite crystals in shale, Well MY1, 4 828.65 m, backscattering image; (d-e) Detrital K-feldspar dissolution, Well X40, 4 580.38 m, backscattering image; (f) Albite dissolution, Well X40, 4 582.15 m, backscattering image; (g-h) Dolomite dissolution, Well MY1, 4 810.93 m, backscattering image; (i) Calcite dissolution, Well MY1, 4 794.31 m, backscattering image. |
5.2. Dissolution of minerals in acidic environments during the middle diagenetic stage
The Fengcheng Formation shale in the marginal zone also contains high abundance of organic matter, which can generate massive hydrocarbons and acidic fluids in the early period of middle diagenesis stage, leading to the transformation from weakly alkaline to acidic diagenetic environment. In the acidic environment during the middle diagenetic stage, diagenesis was dominated by mineral dissolution, such as local dissolution of detrital feldspar and carbonate minerals. Unlike the dissolution of feldspar in the shale of the transitional zone, the dissolution of detrital K-feldspar and albite in the shale of the marginal zone is more intense (Fig. 8d-8f ), with many dissolution pores developed on the surface. The difference in feldspar dissolution between the above two zones is controlled by the closure degree of the diagenetic system [38]. In a closed diagenetic system, the products of feldspar dissolution cannot migrate out and therefore they precipitate in the form of quartz. However, in a relatively open diagenetic system, the products of feldspar dissolution can migrate out to other areas and the dissolution pores can be effectively preserved. The shale in the marginal zone is mainly composed of siltstone, with a higher original porosity and more open diagenetic system compared to the dolomitic shale in the transitional zone. Therefore, the feldspar was mainly dissolved in the marginal zone but mainly replaced by quartz in the transitional zone. In addition, carbonate minerals such as dolomite and calcite in the shale of the marginal zone can also be dissolved in the acidic environment during the middle diagenesis stage, forming many dissolution pores with the pore size up to micrometers (Fig. 8g-8i ).
6. Differential diagenetic evolution and controlling mechanism for shale reservoirs in different sedimentary zones of alkaline lake
According to the above analysis of main diagenesis occurring in different diagenetic stages of the shale in different sedimentary zones (the lake-central zone, transitional zone, and marginal zone) of the alkaline lake, it is found that due to the various sedimentary-diagenetic environment evolution in different sedimentary zones of the Mahu Sag, main diagenetic processes occurring in different diagenetic stages also differ greatly. This leads to significant differences in the reservoir characteristics of shales in different sedimentary zones, which in turn affects the later exploration and development of shale oils.
6.1. Differential authigenesis of minerals during depositional to early diagenetic A stage
During the depositional to syn-depositional stage, a large amount of Na-carbonate minerals were deposited in the lake-central zone due to the high salinity and alkalinity of lake water (Fig. 4a-4e ). At the same time, the detrital quartz, clay minerals, and volcanic materials that transported or fell into this zone were dissolved or transformed and difficult to be preserved (Figs. 9 and 10 ). Meanwhile, in the transitional and marginal zones where water salinity and alkalinity were relatively low, only a few or rare Na-carbonate minerals were deposited, and terrestrial minerals such as quartz can be well preserved. Moreover, due to the susceptibility of the salinity and alkalinity of lake water to external conditions, dissolved silica in the water often precipitated to form opal, which was further transformed into chert bands and nodules (Figs. 8a , 9 and 10 ).
Fig. 9 Diagenetic evolution sequence of the Fengcheng Formation shale in different sedimentary zones of the Mahu Sag. |
During early diagenetic A stage, the diagenetic environments of shales in different sedimentary zones inherited characteristics from the original sedimentary environments. However, due to the strata compaction and the gradual discharge of interstitial water, pore water of the shale in the early diagenetic A stage had higher salinity than the original lake water, which promoted the nucleation and growth of interstitial Na-carbonate minerals. The interstitial Na-carbonate minerals were most abundant in the lake-central zone, which had the highest salinity of pore water (Figs. 9 and 10 ). However, in the transitional and marginal zones, due to the unstable water level, newly generated interstitial Na-carbonate minerals would suffer dissolution and calcification during water level rise or meteoric freshwater leaching (Figs. 5a , 9 and 10 ). Due to the “magnesium competing” from Mg-bearing Na-carbonate minerals in the lake-central zone and the low magnesium content in the marginal zone, the nucleation and growth of interstitial dolomite mainly occurred in the shale of transitional zone. In addition, due to the different salinity and alkalinity of pore water during the early diagenetic A stage, the alteration products of volcanic ashes and detrital clay minerals in the shale matrix in different sedimentary zones varied greatly: K-feldspar mainly in the lake-central zone, zeolites mainly in the transitional zone, and Mg-bearing clay minerals mainly in the marginal zone (Figs. 3 , 8b-8c , 9 and 10 ).
Fig. 10 Mineral composition evolution sequence of the Fengcheng Formation shale in different sedimentary zones of the Mahu Sag. |
6.2. Differential replacement and dissolution of minerals during early diagenetic B to middle diagenetic A stage
During the early diagenetic B stage, as the formation temperature increased, the clay minerals (mainly montmorillonite) in the shales of the transitional and marginal zones gradually underwent illitization (Fig. 9 ). Since the detrital clay minerals in the shale of the lake-central zone have been dissolved during the depositional to syn-depositional stage, diagenesis related to clay minerals did not occur there (Fig. 9 ). The zeolites formed during the early diagenetic A stage in the shale of the transitional zone were gradually transformed to feldspars with growing formation temperature. In addition, a large amount of Mg2+ ions were released from the alteration of clay minerals and volcanic ashes in the shale of the transitional zone, leading to the continued growth of interstitial dolomite and the dolomitization of calcite (Figs. 5d-5g , 9 and 10 ).
During the middle diagenetic A stage, the organic matter in the shale entered the mature stage, and began to generate large amounts of hydrocarbons, organic acids and acidic gases. Due to the different contents of Na-carbonate minerals, the diagenetic environments of the shales in different sedimentary zones during the middle diagenetic A stage differed greatly: an alkaline environment for the lake-central zone, and an acidic environment for the transitional and marginal zones (Fig. 2 ). Reedmergnerite crystals contain abundant hydrocarbon inclusions and their formation was closely related to the migration of hydrocarbons. Considering the low porosity and permeability of shale reservoirs, hydrocarbons generated by organic matter could only migrate in a short distance. Therefore, the short distance migration of boron ions along with the hydrocarbons generated by the shale could lead to the replacement of Na-carbonate minerals by reedmergnerite in the shale of the lake-central zone (Fig. 4d-4f ), and also can cause the replacement of dolomite, calcite, and feldspar by reedmergnerite in the shale of the transitional zone (Fig. 6 ). Compared to the lake-central and transitional zones, the marginal zone has a low content of boron [29], and reedmergnerite replacement rarely occurred there (Figs. 9 and 10 ).
The acidic fluids generated by organic matter can also cause local dissolution of Na-carbonate minerals in the shale of the lake-central zone, forming a certain amount of intracrystalline dissolution pores (Figs. 4g , 9 and 10 ). Due to the varying closure degrees of diagenetic system, significant differences in the dissolution intensity of shale between the transitional and marginal zones exist. Affected by acidic fluids, the transitional zone shale with a relatively closed diagenetic system mainly underwent silicification of detrital albite and K-feldspar (Fig. 7c-7e ), while the marginal zone shale with a relatively open diagenetic system mainly underwent the local dissolution of carbonate minerals and feldspar, forming a large amount of dissolution pores (Figs. 8d-8i , 9 and 10 ).
6.3. Controlling mechanism of differential diagenetic evolution on reservoirs and its implications for shale oil exploration and development
During the depositional to middle diagenetic A stage, the diagenetic differences of shales in different alkaline lake sedimentary zones mainly lie in the types and intensities of mineral authigenesis, replacement, and dissolution. From the perspective of reservoir-controlling mechanisms, the above-mentioned diagenesis can be divided into two types. The first type is the diagenesis that controls the mineral compositions of shale reservoirs, including the authigenesis and replacement of minerals. The second type is the diagenesis that controls the storage space of shale reservoirs, mainly including mineral dissolution.
During the depositional to middle diagenetic A stage, the first type of diagenesis for the shale in the lake-central zone mainly includes authigenesis of Na-carbonate minerals and their partial replacement by reedmergnerite, alteration of volcanic ashes and detrital clay minerals into K-feldspar. Considering that most detrital minerals such as quartz have been dissolved in the primary lake water with a high alkalinity, the shale in the lake-central zone is mainly composed of Na-carbonate minerals, feldspar, and reedmergnerite (Fig. 10 ). During the depositional to middle diagenetic A stage, the first type of diagenesis for the shale in the transitional zone mainly includes authigenesis of dolomite and chert, alteration of volcanic ashes and detrital clay minerals into zeolites and further into feldspars, replacement of carbonate minerals and feldspars by reedmergnerite, partial silicification of feldspars, and illitization of montmorillonite. Since some detrital minerals such as quartz can still be preserved in the sediments due to the relatively moderate alkalinity of primary lake water, the shale in the transitional zone is mainly composed of dolomite, reedmergnerite, quartz and felspar (Fig. 10 ). However, the first type of diagenesis for the shale in the marginal zone during the depositional to middle diagenetic A stage mainly include authigenesis of chert, alteration of volcanic ashes and detrital clay minerals into Mg-bearing clay minerals (mainly montmorillonite), and illitization of montmorillonite. Since most detrital minerals such as quartz can be preserved in the sediments due to the low alkalinity of primary lake water, the shale in the marginal zone is mainly composed of quartz, feldspar, dolomite, calcite and illite (Fig. 10 ).
The Fengcheng Formation shales in different sedimentary zones of the Mahu Sag all contain rich organic matter, and therefore organic matter pores are an important type of storage space for the Fengcheng Formation shale reservoirs (Fig. 10 ). The second type of diagenesis for the shale in the lake-central zone during the depositional to middle diagenetic A stage mainly includes weak dissolution of Na-carbonate minerals, which can form a few intracrystalline and intercrystalline dissolution pores (Figs. 4g and 10 ). Therefore, the main storage spaces of shale in the lake-central zone include organic matter pores and a small number of intracrystalline and intercrystalline dissolution pores. The second type of diagenesis for the shale in the transitional zone during the depositional to middle diagenetic A stage mainly includes relatively weak dissolution of carbonate minerals and feldspar due to a closed diagenetic system, which can form some dissolution pores (Figs. 7a-7c and 10 ). Since some intercrystalline pores can be developed between the quartz crystals formed by the silicification of feldspar (Fig. 7e ), the main storage spaces of shale in the transitional zone include organic matter pores, quartz intercrystalline pores and some dissolution pores of calcite, dolomite and feldspar. The second type of diagenesis for the shale in the marginal zone during the depositional to middle diagenetic A stage mainly includes relatively strong dissolution of carbonate minerals and feldspar due to an open diagenetic system, which can form abundant dissolution pores (Figs. 8d-8i and 10). Therefore, the main storage spaces of shale in the marginal zone include organic matter pores, and dissolution pores of calcite, dolomite and feldspar.
Due to the success of shale oil revolution in the United States, shale oil has become a hot research field and attracted widespread attention from scholars both domestically and internationally in recent years [39]. Shale oil has two occurrences in reservoirs, adsorbed and free shale oils. Under the recent technological conditions, free shale oil is the main extraction target, and its content directly affects the oil fluidity in shale, thereby determining whether the shale oil development wells can achieve high and stable production [40]. The relative contents of adsorbed and free shale oils are closely related to pore size, and the free shale oil is mostly distributed in large pores[40-41]. Therefore, the amounts of large pores directly affect the extraction efficiency of shale oil. The Na-carbonate-rich shale in the lake-central zone only has small intercrystalline or intracrystalline dissolution pores and organic matter pores, so it can only contain a small amount of free shale oil. The dolomitic shale in the transitional zone has small intercrystalline pores, organic matter pores and some dissolution pores with large aperture, so it can contain a certain amount of free shale oil. However, the siltstone in the marginal zone has many large-aperture dissolution pores, so it can contain massive free shale oil. Horizontal well and volume fracturing are key technologies for shale oil development [42], and the fracturing effectiveness of shale depends on the relative content of brittle minerals [43]. The dolomitic shale in the transitional zone and siltstone in the marginal zone are mainly composed of brittle minerals such as felsic minerals, calcite and dolomite, with a low content of plastic minerals such as Na-carbonates and clays. Therefore, the dolomitic shale and siltstone have a good fracturing effectiveness. The Na-carbonate-rich shale in the lake-central zone contains many plastic Na-carbonate minerals, with a low content of brittle minerals such as carbonate and felsic minerals. Therefore, the Na-carbonate-rich shale has a poor fracturing effectiveness. In addition, Na-carbonate minerals have strong fluidity under high temperature and pressure conditions. They are easy to block perforations, which poses great difficulties for the smooth exploitation of shale oils.
Finding high-quality “sweet spots” is one of the most important successful experiences for shale oil revolution in the United States [42]. From the perspective of exploration and development, the siltstone in the marginal zone contains high content of free shale oil and has a good fracturing effectiveness, so it is the best “sweet spot” for shale oil extraction. The dolomitic shale in the transitional zone contains a certain amount of free shale oil and has a good fracturing effectiveness, which is also the favorable “sweet spot” for shale oil extraction. A small amount of dolomitic shale deposited in the lake-central zone has similar reservoir characteristics to that in the transitional zone, which may be a potential “sweet spot” for shale oil extraction.
7. Conclusions
The shale in the lake-central zone was in an alkaline diagenetic environment during the depositional to middle diagenetic stage due to its inherited highly saline and alkaline environment and the presence of massive Na-carbonate minerals. The diagenesis of the shale mainly includes the precipitation of Na-carbonate minerals and dissolution or alteration of volcanic ashes and terrestrial minerals into K-feldspar during the depositional to early diagenetic stage, and the replacement of Na-carbonate minerals by reedmergnerite during the middle diagenetic stage.
Due to hydrocarbon generation process by organic matter, the early inherited alkaline diagenetic environment of the shale in the transitional zone was transformed into acidic diagenetic environment during the middle diagenetic stage. The diagenesis of the shale mainly includes the nucleation and growth of interstitial dolomite, authigenesis and dissolution of Na-carbonate minerals and dissolution or alteration of volcanic ashes and terrestrial minerals into zeolites during the depositional to early diagenetic stage, and the replacement of feldspar and carbonate minerals by reedmergnerite and silica during the middle diagenetic stage.
The diagenetic environment of the shale in the marginal zone underwent a transition from early weakly alkaline to late acidic during the depositional to middle diagenetic stage. The diagenesis of the shale mainly includes the authigenesis of silica and Mg-bearing clay minerals during the depositional to early diagenetic stage, and the dissolution of feldspar and carbonate minerals during the middle diagenetic stage.
The differential acidic-alkaline diagenetic environment evolutions in different sedimentary zones of the alkaline lake result in significant differences of shale reservoir characteristics. The Na-carbonate-rich shale in the lake-central zone only develops small intercrystalline or intracrystalline dissolution pores and organic matter pores, and owns many plastic Na-carbonate minerals, so it contains a small amount of free shale oil and has a poor fracturing effectiveness. The dolomitic shale in the transitional zone develops intercrystalline pores, organic matter pores and some dissolution pores, and owns dominant brittle minerals, so it contains a certain amount of free shale oil and has a good fracturing effectiveness. The siltstone in the marginal zone develops many dissolution pores and owns dominant brittle minerals, so it contains massive free shale oil and has a good fracturing effectiveness. Comprehensively considering the free shale oil content and fracturing effectiveness, the siltstone and dolomitic shale are the “sweet spots” for the extraction of shale oils in alkaline lake deposits.