The K₁sh₂¹ Fm. is a set of hydrocarbon-rich high-quality shale with high organic matter abundance, good organic matter type, and appropriate thermal evolution degree of organic matter. It has a TOC value of 1%-6%, with an average of 2.5% (Fig. 3), and contains a certain amount of residual hydrocarbons. S₁ value can reach 2.40 mg/g, with an average of 0.89 mg/g, and the hydrocarbon generation potential (S₁+S₂) value reaches 4.79 mg/g. The shale has high gas content, and the bubbles escape continuously from the shale in the water. The total gas content analyzed on-site can reach 1.52 m³/t, with an average value of 1.21 m³/t. The total gas content from well logging interpretation can reach 2.68 m³/t. The organic matter content of mudstone developed in the shale section is generally lower, the TOC value is less than 0.5%, and the (S₁+S₂) value is less than 1.0 mg/g, and there is almost no gas, and the soaked core has no bubbles.

The statistical results of 131 shale samples from the K₁sh Fm. in Lishu fault depression show that Type I organic matter accounts for 5.4%, Type II₁ organic matter content is the highest and accounting for 44.6%, Type II₂ organic matter accounts for 33.9%, and Type III organic matter accounts for 16.1%. The analysis results of stable carbon isotopic composition and organic maceral characteristics of shale kerogen in K₁sh Fm. of Well JLYY-1 show that there are significant differences in organic matter types between gas-bearing shale and gas-free mudstone. Shale has lighter carbon isotopic composition of kerogen with δ¹³C₁ values of -28.4‰ to -25.0‰, and microscopic analysis of organic matter macerals shows that organic matter is mainly amorphous body from low aquatic algae, rich in content and layered. It is comprehensively determined that the types of organic matter types mainly II₁ and I, with a small amount of Type II₂. The carbon isotope composition of kerogen in mudstone is relatively heavy, δ¹³C₁ is greater than -24‰, with the characteristics of typical Type III kerogen. Under the microscope, the microscopic components are mainly vitrinite components from higher plants. The isotopic data of natural gas produced by Well JLYY-1 also shows that the gas comes from types I and II₁ organic matters, which also proves that this set of hydrocarbon-rich shale is mainly composed of primary organic matter such as algae.

The K₁sh Fm. shale is relatively deep in the whole Songliao Basin, especially in the north of the basin, such as the Xujiaweizi fault depression, where it is buried more than 5000 m deep, and the degree of organic matter evolution is relatively high. In the Lishu fault depression in the south of the basin, due to the larger tectonic uplift during the later stage, the buried depth of K₁sh Fm. is 1000-6000 m with a large buried depth span. The thermal evolution degree of source rocks is also quite different, with a R_o value between 0.6%-3.0% (Fig. 5). For example, in the Shuanglong subsag, the K₁sh Fm. shale has the lowest degree of organic evolution at the mature stage and is dominated by the oil generation process. In Sangshutai deep-lying area, the K₁sh Fm. shale is buried more than 5000 m, the evolution degree of organic matter is expected to exceed 3%, which is at the over mature stage, mainly generating dry gas. On the southeast slope where Well JLYY-1 is drilled, the buried depth of K₁sh Fm. shale is 3000-3240 m, and the R_o value is 1.2%-1.5% with an average of 1.3%. It belongs to the high mature evolution stage and the products are mainly wet gas and condensate gas. Microscopic analysis shows that the mineral asphalt matrix formed by algae degradation in shale has no fluorescence and is bright white (micrite) under the partial reflection of light, indicating that the oil generation process has been completed and entered the high-maturity evolution stage. In addition, the methane content of the natural gas produced in Well JLYY-1 is 83%, the ethane content is 8.59%, and the drying coefficient is about 0.88, which belongs to typical wet gas. There was 0.23 m³ of condensate produced during the test. The color of the crude oil is light yellow to yellowish-white, and the oil is clear and transparent. In summary, the thermal evolution degree of organic matter in the shale section at a buried depth of 3000-3500 m in the Lishu fault depression is at a high mature stage, mainly generating wet gas. The evolution degree of shale organic matter at a buried depth of more than 3500 m is at an over-mature stage, mainly generating dry gas.

The paleo-environment control factors during shale deposition mainly include paleoclimate, paleo-water depth, paleo-redox, and paleo-salinity^[28]. The variation law of paleo-environment indicators in the vertical sequence of Well JYYY-1 reflects the paleo-environment for forming the set of organic-rich shale in K₁sh₂¹ Fm. The research results show that compared with the mudstone in overlying K₁sh₂² Fm., this set of shale has the characteristics of relatively medium V/Ni value, relatively high Fe/Mn value, relatively high Sr/Ba value, relatively high Al/Ti value, and relatively high pyrite content (Fig. 6), and these five indicators are exactly the key indicators for paleo-water depth, paleo-climate, paleo-water salinity, paleo-productivity, and paleo-redox conditions. It is inferred that the set of organic-rich shale in K₁sh₂ Fm. was formed in a closed sedimentary water environment under a semi-arid paleo-climate. The development period of the Late Jurassic-Early Cretaceous fault depression in the Songliao Basin was accompanied by multiple volcanic activities. Multiple sets of tuffs found in the gas-bearing shale section of Well JLYY-1 indicate the multi-stage volcanic activities during the deposition of high-quality shale in K₁sh₂¹ Fm. Volcanic ash deposition can promote the eutrophication of water and the proliferation of authigenic algae. In the process of volcanic ash eruption, a large number of carbon sources (CO₂, CH₄), other nutrients (N, Si, P) and trace metal elements (Fe, Zn, Mn, Ni, V) were carried. These substances entered the sedimentary water body, promoted the prosperity of aquatic organisms and the improvement of primary productivity, and created a favorable material basis for forming organic matter ^[29]. At the same time, the paleo-productivity indexes (Fe/S value and P element content) of the gas- bearing shale section of K₁sh Fm. in Well JLYY-1 are also significantly higher than those of the overlying mudstone section with undeveloped tuff. The enrichment of pyrite and organic sulfur elements refers to a good reduction environment ^[30], indicating that the addition of volcanic ash in the lacustrine basin promoted the explosion of organic matter and created a favorable reduction environment for the enrichment of organic matter in a sedimentary basin. The pyrite content in the shale section of K₁sh Fm. in Well JLYY-1 increases significantly, most of which exceed 1.2%, and the highest value reaches 5%. In the core and under a scanning microscope, it can be seen that a large number of pyrite aggregates are distributed in strips and layers, and mainly in spherical-ellipsoidal shapes. Moreover, the shale section has significantly high sulfur content, more than 0.50%, up to 1.68%, while the pyrite content in the overlying mudstone of K₁sh² Fm. is significantly reduced, basically less than 0.5%, and the sulfur content is very low, basically less than 0.1%. In the shale section, the changing trends of pyrite and sulfur contents are consistent with the change in TOC value. This means that in the shale above the tuff interlayer, the sulfur content and pyrite content show an obvious increase trend. Through the U/Th value and U value, it is also confirmed that the water body had obvious reducibility during the deposition period of the shale section ^[31-32]; while during the deposition period of upper mudstone, the water body changed into a preferential oxidation environment, which is not conducive to the preservation of organic matter (Fig. 8). The above analysis confirms that during the period of shale deposition, the reducibility of the water body was significantly enhanced, which provided a favorable environment for the preservation of organic matter. The research shows that this reduced water environment was controlled by volcanism ^[33]. A large number of sulfur-containing gases such as SO₂ and H₂S emitted by the volcano entered the lake basin in the form of an aerosol, which provided a sufficient sulfur source for bacterial sulfate reduction (BSR), enhanced the reducibility of water and promoted the formation of reduction environment in the lake basin.

Additionally, carbonatization in the saltwater environment promotes hydrocarbon generation of organic matter. The addition of volcanic ash increases the salinity of water in sedimentary basins ^[34]. The Sr/Ba value is generally positively correlated with paleo-salinity, which is an effective parameter for distinguishing the paleo-salinity of lake water ^[35]. The Sr/Ba value in the shale section of K₁sh₂¹ Fm. exceeds 1 (Fig. 6), indicating a salt water sedimentary environment. The Sr/Ba value of the overlying mudstone section in K₁sh₂² Fm. decreases rapidly, which is less than 0.1, indicating a freshwater sedimentary environment. Based on a large number of microscopic and energy spectrum analyses, this study found that under this environment, there is a special symbiotic relationship between carbonate minerals and organic matters, and there may be carbon element exchange between carbonate crystals and associated organic matters (Fig. 7). The abnormally high-value distribution range of Ca element in porous organic matter can be defined according to the element energy spectrum scan. This phenomenon is mainly controlled by carbonatization. This means that carbonate rock-organic matter symbiosis has high sensitivity in an alkaline formation water environment, which is easy to induce the decarburization effect of original organic groups and obvious hydroxylation at the same time, and promotes the generation and transformation of nitrogen-containing organic matter and phenyl^[36-37].

The gas-bearing shale in K₁sh₂¹ Fm. has the characteristics of calcium-rich minerals. The content of carbonate minerals in the shale is up to 48%, with an average of 15%; the content of clastic minerals such as feldspar and quartz is 20%-50%, with an average of 34%; and the content of clay minerals is 24%-60%, with an average of 49%. Multiple sets of tuff interlayers are developed in the shale section. Multiple tuff development sections can be identified in the shale section of Well JLYY-1 with a thickness of 51 m (Fig. 3). The tuff is grayish-yellow, with fine-grained and massive structure. Under the optical microscope, the particle size is very fine, which is volcanic glassy. It can be seen that feldspar crystal pyroclasts are replaced by calcium and a small amount of quartz crystal pyroclasts (Fig. 8). The gas-bearing shale section has well-developed lamellation and a significant laminar structure under the microscope. The gas-free mudstone in K₁sh₂² Fm. is thick and massive, with undeveloped lamellation, hard and dense. Under the microscope, it is mainly an argillaceous scale structure, evenly distributed, almost free of carbonate minerals (the average content is less than 1%), with a high content of clay minerals (larger than 50%), and no tuff interlayer.

Based on the measured core data and nuclear magnetic resonance (NMR) logging data of K₁sh Fm. in Well JLYY-1, the porosity and permeability characteristics of shale are systematically analyzed. The results show that the overall physical properties of the shale reservoir in K₁sh₂¹ Fm. are better. The main porosity is 2%-8% with an average of 6.37%, and the maximum porosity is 10.8%. The permeability is mainly distributed in (0.001-0.010)×10^-3 μm² (Fig. 9). Compared with shale, the reservoir physical properties of mudstone in K₁sh₂² Fm. are worse. The analysis results of NMR logging show that the effective porosity of clayey mudstone is less than 1%, and the permeability of NMR logging is less than 0.001×10^-3 μm².

The K₁sh₂¹ Fm. shale has relatively good porosity and permeability conditions mainly due to the development of inorganic pores, organic pores, and bedding fractures. Inorganic pores mainly include clay mineral intercrystalline pores and carbonate mineral dissolution pores. The intercrystalline pores of clay minerals are mainly micropores and mesopores with mainly chlorite intercrystalline petaloid pores and illite intercrystalline needle pores in morphology, generally less than 100 nm. With the largest number, intercrystalline pores of clay minerals are the main matrix pores (Fig. 10a). The dissolution pores in carbonate minerals are mainly round and dominated by mesopores, the pore diameter is generally about 100 nm (Fig. 10b). The organic pores are mainly developed in the organic matter filled between the particles of inorganic minerals, and the pore size is generally about 350 nm, up to 1 μm (Fig. 10c), with better pore connectivity. This kind of organic matter has no fixed form and belongs to the amorphous body. It is the product of liquid hydrocarbon migration, filling, cracking and solidification, i.e.,the post-oil asphalt, with a large number of residual gas pores developed in it. The shale section also develops a large number of bedding fractures with a width of 0.01-0.15 mm (Fig. 10d), which are also the major reservoir spaces of shale gas. They can be divided into two types: contraction fractures of hydrocarbon generation in organic matter, and interlayer contraction fractures in clay minerals. Organic pores, inorganic pores, and bedding fractures together constitute the effective reservoir space.

Based on CO₂ and N₂ adsorption experiments and high-pressure mercury injection experiments, the full pore size distribution maps of shale reservoirs in K₁sh Fm. are obtained. According to the high-pressure mercury injection curve (Fig. 11a), macropores above 100 nm are mainly microfractures, and the peak width of microfractures is 5-10 μm. For meso-micropores within 100 nm, carbon dioxide and nitrogen adsorption are used to characterize the pore volume and pore size distribution curves (Fig. 11b). The peak pore size is mainly distributed at 10-50 nm, which is mainly provided by organic pores and dissolution pores in carbonate minerals. For micropores below 10 nm, the pore size distribution has no obvious peak value, generally provided by clay mineral pores.

During the burial process, shale formation has undergone the joint transformation of inorganic and organic diagenesis. Inorganic diagenesis is mainly compaction, cementation, dissolution, and metasomatism, while organic diagenesis is mainly hydrocarbon generation and evolution of organic matter, with the interaction with each other^[38]. The formation of high-quality shale reservoir space in K₁sh Fm. is controlled by diagenesis and hydrocarbon generation evolution.

Based on the development characteristics and genesis of shale pores in K₁sh Fm., combined with the comprehensive analysis of diagenetic evolution sequence and organic hydrocarbon generation evolution history, the evolution model of high-quality shale reservoir in the K₁sh₂¹ Fm. is established under the control of hydrocarbon generation and diagenesis (Fig. 12), which can be divided into four stages: pore reduction by compaction, salinization and cementation, organic acid dissolution and pressurization by hydrocarbon generation, and pressure relief by structural uplift with fractures developed.

Stage 1: Pore reduction by compaction. The strata settled rapidly and a large amount of organic matter was deposited and preserved, which laid a material foundation for later hydrocarbon generation and accumulation. At the same time, the primary pores were reduced rapidly under the influence of compaction.

Stage 2: Salinization and cementation. The basin was in a stable subsidence and depression stage. The water body presented a saltwater environment. The nature of the formation water was alkaline. Ion differentiation and exchange occurred in the deposition process. Calcium and magnesium ions in shale migrated to the adjacent sandy lamina along the microfractures, and local carbonate cementation and sedimentation occurred to form the calcium-rich and siliceous lamina.

Stage 3: Organic acid dissolution and pressurization by hydrocarbon generation. The buried depth of shale increased, the ground temperature rose to 90- 140°C and the organic matter reached the mature stage and began to generate hydrocarbon. At this time, the organic acid began to be produced and fill the reservoir, the early carbonate minerals were dissolved to form a large number of dissolution pores. With the increase in evolution degree, a large number of organic matters were transformed, resulting in many contraction fractures by hydrocarbon generation ^[39]. At the same time, abundant hydrocarbons were generated to produce abnormal high pressure, which induced many bedding overpressure fractures ^[40]. The middle diagenetic stage began, clay minerals such as montmorillonite were transformed into illite and dehydrated, and many contraction fractures were formed along the bedding plane. Under the joint constraints of the maximum pressure gradient and the minimum fracture path, they converged, crossed, and connected, and finally formed a bed-parallel and interconnected fracture network.

Stage 4: Pressure relief and fracture formation by tectonic uplifting. Due to plate subduction, the Songliao Basin experienced large-scale uplift and denudation movement, and the strata suffered uplift and denudation. Due to the reduction of overlying formation pressure, the overpressure of shale strata was gradually released. Moreover, the bedding fractures were further opened along the boundary of the mineral lamina and developed widely.

During the formation and evolution of a fault depression lake basin, the factors such as the change of accommodation space, sediment supply, and deposition rate under the original sedimentary sequence framework control the scope of the lake, the depth of the water body, the lithology combination and sediment distribution, and then control the generation and preservation of shale gas vertically. Generally speaking, the balanced compensation period of the basin is most conducive to the formation of high-quality source rocks, and the overcompensation period is conducive to the formation of good caprocks. The spatial matching of the two is conducive to the preservation of shale gas. The sequence stratigraphy of Shahezi Foramtion in the Lishu fault depression is divided according to the lithologic combination, well-logging element geochemistry, and other data. The results show that the water level of the lake basin rose gradually and the sediment supply rate was the highest during the deposition period of the lowstand systems tract of K₁sh₁ Fm. Under the control of the main controlling faults of the fault depression basin and the shape of the basin basement, a set of mudstone layers near the sedimentary center of the basin was formed as the basin floor. Shale was deposited in the development period of the lake transgressive systems tract in the K₁sh₂¹ Fm. At this time, the sediment supply rate and the growth rate of accommodation space were in equilibrium, and the lake basin was in the equilibrium compensation stage, which was the most favorable period for the development of high-quality source rocks. Moreover, the area of the lake basin reached the largest, and high-quality shale with large thickness and continuous distribution was deposited in the whole fault depression lake basin. During the development period of the highstand systems tract in K₁sh₂² Fm., the water body began to retreat, the sediment supply rate was higher and was greater than the growth rate of accommodation space, and the lake basin was in the stage of overcompensation. At this time, the lake basin area began to shrink, the prosperity degree of organic matter decreased, and the hydrocarbon generation capacity decreased. In addition, the sediments were still dominated by fine-grained sediments such as clay minerals. Affected by later compaction, the reservoir space decreased, the porosity and permeability were extremely poor, the pore throat was finer, and there was larger displacement pressure to form good caprock and natural roof, which was sealed over the high-quality shale in K₁sh₂¹ Fm. This is conducive to the in-situ preservation of the natural gas generated by shale.

In the Lishu fault depression, a set of the stratigraphic framework of a floor + organic-rich shale + a roof formed by mudstone of lowstand systems tract of K₁sh₁ Fm., organic-rich shale in lacustrine transgressive systems tract of K₁sh₂¹ Fm. and mudstone of highstand systems tract of K₁sh₂² Fm. widely exists in this region. Therefore, the core problem of the preservation conditions of this set of organic-rich shale in K₁sh₂¹ Fm. is the hydrocarbon generation process during the later stage of fault depression, the fault-depression transformation and later uplift process of structures, and whether the pre-existing faults are strongly active to destroy the existing sealing conditions of the roof and floor layers.

The K₁sh Fm. in the Lishu fault depression develops a series of nearly NNE-trending normal faults. The plane extension length of these faults is generally 4-7 km, the fault throw is generally 15-25 m, the longitudinal extension distance is shorter, and most of these faults disappear in the Yingcheng Formation. According to the evolution mechanism of fault activity and deformation properties, combined with the analysis of the structural evolution of the Lishu fault depression, it is determined that these faults are early strike-slip extensional faults. The main growth activity was in the fault depression period. They were formed during the sedimentary period of the Huoshiling Formation, and the activity was strong during the sedimentary period of K₁sh Fm. At the end of the sedimentary period of the Yingcheng Formation, under the regional sinistral strike-slip background, the strata were locally uplifted and denuded, and the fault activity was significantly weakened. During the sedimentary period of the Denglouku Formation, the strata subsidence began to get rid of the control of faults, and the fault activity was further weakened during the fault depression period. After entering the depression period, the Lishu fault depression entered the basin-wide type sedimentation. During this period, the fault activity stopped basically, and the seismic reflection characteristics are mainly overlapped and parallel contact ^[41-42].

According to the hydrocarbon generation history and fluid inclusion temperature measurement of K₁sh Fm. shale, the main hydrocarbon generation and expulsion period of Shahezi shale were determined during the late deposition of the Denglouku Formation and the deposition period of the Quantou Formation. Therefore, the active period of faults was before the main hydrocarbon generation and expulsion period of shale gas, and the faults were inactive after shale gas accumulation, which provided stable structural conditions for in-situ enrichment of shale gas.

To sum up, the original sedimentary mudstones of the lowstand systems tract of K₁sh₁ Fm. and highstand systems tract of K₁sh₂ Fm. as well as stable fault conditions, constitute favorable preservation and sealing space for hydrocarbon generation and enrichment of organic-rich shale in K₁sh₂ Fm. of the Lishu fault depression.