Pore structure characteristics are the main factor affecting shale reservoir， and its qualitative and quantitative characterization and main controlling factors are key issues in shale reservoir research. In order to explore the differences in microscopic pore structures and main controlling factors of different sedimentary microfacies of deep shale reservoir， this paper selects the Wufeng-Longmaxi formations in Well Z301 of Zigong area in southern Sichuan Basin as an example， based on systematic experiments such as core， thin section， scanning electron microscopy observations， X-ray diffraction analysis， organic geochemical analysis， N₂/CO₂ adsorption， high-pressure mercury injection， the vertical heterogeneity of pore structure in the O₃ w-S₁ l ₁ shale reservoir is analyzed. The research results indicate that the sedimentary microfacies of the O₃ w-S₁ l ₁ shale reservoir in the study area can be divided into three categories from bottom to top： strong reducing， high carbon， calcium-rich， and silicon rich deep-water continental shelves （microfacies ①）， weak reducing-medium carbon-calcium containing-silicon mud mixed-deep water continental shelves （microfacies ②）， and weak reducing-weak oxidizing-low carbon-siliceous mud-semi deep water continental shelves （microfacies ③）； among the three types of microfacies， macropores are mainly inorganic pores， while mesopores and micropores are mainly organic pores. Mesopores and micropores are also the main pore types that control the volume and specific surface area of shale pores； the development degree of different pore types varies among the three microfacies； mesopores and micropores are the most important pore type that controls reservoir physical properties and gas content； TOC and the content of clay minerals are the key factors affecting the pore structure of deep shale. Quartz has a slightly weaker controlling effect on nanoscale pores， while carbonate minerals have no significant controlling effect on nanoscale pores； the characteristics of high TOC， low clay minerals， and high brittleness minerals in microfacies ① determine that it is the most commercially valuable lithofacies for mining. The relevant conclusions can provide guidance for enriching the high-yield patterns of deep shale gas enrichment.

The deep shale gas reservoirs in the north Luzhou District of the Sichuan Basin are affected by multiple phases of tectonic movements， and are characterized by complex tectonic conditions， rapid changes in in-situ stress， which leading to the difficulties of in-situ stress prediction. A specific in-situ stress seismic approach based on pre-stack azimuthal anisotropic inversion is proposed. Pre-stack AVAZ inversion approach applied azimuthal anisotropy AVA equation under Bayes theory framework. The elastic and anisotropic parameters of the shale gas reservoir are inverted from pre-stack Offset Vector Tile （OVT） gather. Meanwhile， a formula Differential Horizontal Stress Ratio （DHSR） based on the fracture density and Poisson's ratio representation is derived， which is utilized to estimate the DHSR of shale gas reservoir. Wufeng Formation-Long 1 submember and favorable areas for shale gas exploration and development were delineated. This provides reliable geophysical evidence for reserve evaluation， well trajectory design， and reservoir stimulation in the study area. Using this method， deep shale gas in the Wufeng Formation–Long11 subsection of the northern Luzhou area in the Sichuan Basin was predicted by DHSR， and favorable areas for shale gas exploration and development were delineated. This provides a reliable geophysical basis for the reservoir evaluation， well trajectories designment， and reservoir modification.

The Xingkai rifting resulted in the formation of an extensional trough in the Deyang-Anyue area of the Sichuan Basin. The deposition of the Lower Cambrian Qiongzhusi Formation is obviously controlled by the trough pattern， and many sets of black carbonaceous shale and gray-black silty shale are developed inside it. It has been proved by exploration that it has great development potential. However， the understanding of sedimentary facies， sedimentary model and favorable facies belt of Qiongzhusi Formation is still unclear， which restricts the long-term deployment of exploration and development. Therefore， with the help of a large number of macro and micro observation techniques such as core， thin section and scanning electron microscope， combined with geophysical data and key geological parameter analysis， this study summarizes the sedimentary facies signs， geophysical facies signs and quantitative facies signs of Qiongzhusi Formation in the middle part of Deyang-Anyue rift trough， finely dissects the sedimentary facies belt distribution of Qiongzhusi Formation， establishes the sedimentary evolution model of Qiongzhusi Formation shale in the middle part of Deyang-Anyue rift trough， and compares the mineral composition， TOC， U/Th， Y/Ho， shale thickness and other indicators to optimize the favorable facies belt. The results show that： The Qiongzhusi Formation can be divided into three subfacies： Continental uplift， continental slope and continental shelf. The continental shelf is further divided into deep-water shelf facies in the trough and shallow-water shelf facies outside the trough with the trough as the boundary. According to the sedimentary characteristics and environmental differences， it is further divided into siliceous mud shelf facies， （including） silty mud shelf facies， muddy silty sand shelf facies and muddy silty sand shelf facies.

To address the unclear understanding of the fracture propagation behavior during zipper fracturing between multiple wells in deep shale gas reservoirs affected by small faults， a flow-solid coupling model for horizontal well zipper fracturing fracture propagation， considering the influence of small faults， was established based on the finite discrete element method （FDEM）. A joint model with initial width， zero cohesion， and zero tensile strength was used to characterize small faults， and the fracturing model was validated using on-site microseismic data. Taking the deep shale gas reservoir in southern Sichuan as an example， the zipper fracturing fractures propagation and inter-well fractures intersection behavior under the influence of small faults were simulated and studied. The research results indicate that compared to natural fractures， small faults have a stronger ability to capture hydraulic fractures and block their crossing and propagation. Under the same conditions， the response well pressure increase through natural fractures is greater and the pressure response time is longer due to the influence of natural fractures with low flow conductivity and small faults with high flow conductivity and high filtration loss. As the approach angle of the small fault decreases， the distance between the small fault and the response well increases， the number and bandwidth of the small fault layers decrease， and the misplacement distance of the perforation holes in the two wells increases. Consequently， the increase in response well pressure decreases， and the corresponding pressure response time shows a decreasing trend. This study provides important insights for the zipper fracturing design of deep shale gas wells with small fault development.

Significant breakthroughs were made in the exploration and development of the Weirong area. Taking the Longmaxi shale as the object， we analyzed its gas-bearing characteristics and main controlling factors by means of XRD and on-site gas content test. The results are as follows： the area develops low-carbon-carbon rich shale， which is at the high-over-mature stage. The average total gas content is 2.78 m³/t， and the present desorption gas is mainly free gas， with an average of 2.25 m³/t， and the average adsorption gas is 0.53 m³/t. Carbon-rich mixed siliceous shale is the dominant lithology. There is a positive correlation between the TOC content， pore structure， and their respective gas contents， and at the same time the TOC content affects the pore structure， so that the TOC content is the main controlling factor. Therefore， TOC content is the main controlling factor and pore structure is the direct factor. Clay content is negatively correlated with gas content， and R _O content is positively correlated with gas content.

It is greatly beneficial for shale gas development and increased production by characterizing the connectivity of natural fractures in shale reservoirs and evaluating the mechanical effectiveness under in-situ stress condition. Taking the Wufeng-Longmaxi formations in the Dingshan-Dongxi area， southeastern margin of Sichuan Basin as an example， the characteristics of natural fracture were identified by core observation， imaging logging， and 3D laser scanning， and their connectivity and mechanical effectiveness were characterized and analyzed. The results show that the natural fractures are mainly medium-high dip angle shear fractures， interlayer slip fractures， and bedding fractures with dominant NE and near EW strike， and the unfilled percentage is about 9.7%. Unfilled fractures mainly exist in interlayer slip fractures and high dip angle（>45°） shear fractures. The natural fracture types in the Dingshan-Dongxi area can be well distinguished by the image characteristics from imaging loggings. The topological analysis of the fracture network shows that the number of lines （C_L） and the number of branches （C_B） are generally lower than 2 and 1.5， respectively， indicating that fracture network is poorly connected. Furthermore， there are many fractures with more sets and good connectivity in the overlapping areas in Dingshan nose shaped folds. The connected fractures network mainly exists in the 1-3 sub-members and the longitudinal C_L and C_B are 1-3 times different. The joint roughness coefficient （JRC） of the natural fracture surface in the study area is between 2.2 and 14.1， which shows a decreasing trend with the increase of clay content. Under in-situ stress， all natural fractures are in mechanical ineffectiveness state. Among them， in the section of argillaceous and argillaceous-rich shale （clay content >30%） with small minimum principal stress and large two-directional stress difference， the low-high dip angle natural fracture with 30°-75° to the maximum principal stress has a better mechanical effectiveness. Our research can provide a reference for natural fracture fine model and “sweet pot” prediction of shale reservoirs.

Abundant shale gas resources are stored in marine Longmaxi Formation in Sichuan Basin， large-scale commercial development has been achieved， and significant progress has been made in the study of shale gas accumulation theory. However， shale reservoir shows the characteristics of multiple lithofacies types， rapid vertical and horizontal changes， and strong heterogeneity， which restricts the precise evaluation of reservoir and the effective evaluation of resources. The characterization of micro-pore development characteristics of shale is the key to reservoir evaluation and optimization. Especially， as an important space for the occurrence and enrichment of shale gas， it is of great significance to clarify the differences of pore development characteristics of shale organic matter in different lithofacies. In order to deal with the energy challenge， the exploration and development of shale gas resources has gradually moved from medium and shallow layer to deep and ultra deep layer. Sichuan Basin is rich in deep shale gas resources， however， the research on the geological theory of shale gas accumulation is weak， which restricts the progress of deep shale gas exploration. In this paper， Longmaxi Formation shale in Luzhou block in southern Sichuan is selected as the research object. Based on X-ray diffraction mineral analysis and cast thin section identification， six lithofacies including siliceous shale， clay shale， calcium silicon mixed shale， mixed shale， siliceous rock and siliceous calcareous shale are identified in small layers from 1 to 3 of S₁l₁¹ sub-member. Results show that there are great differences in micro reservoir characteristics of different lithofacies types. In addition， based on the full view mosaic scanning （MAPS） technology， organic matter and organic matter pores are effectively extracted for fine characterization， and it is found that there are significant differences in the development characteristics of shale organic matter pore forming efficiency， organic matter pore face ratio， roundness and aspect ratio of different lithofacies types. It is pointed out that siliceous shale has high organic matter pore forming efficiency， high porosity of organic matter pores， and good organic matter pore roundness， and high gas content and brittleness index， which are the favorable lithofacies types for shale gas occurrence and enrichment. The secondary favorable lithofacies are clay-bearing siliceous shale and siliceous shale， whereas calcareous shale facies is a disadvantaged type of lithofacies. The research results can provide scientific theoretical basis and support for effective reservoir evaluation and optimization， prediction of shale gas enrichment areas and exploration and development deployment.

The southern Sichuan Basin is the hot spot of shale gas exploration and development in China， where there are many types of shale gas reservoirs and abundant data. In order to summarize the basic geological characteristics and laws of enrichment and high yield in that area， by using seismic， drilling， logging， testing data， we analyzed key geological factors of shale gas reservoir in the aspects of sedimentation， tectonic deformation， preservation conditions， and also analyzed characteristics， differences of enrichment condition and implications of typical shale gas reservoirs. The results show， （1）The sedimentary process controls the formation thickness， quality and thickness of reservoir in O₃w-S₁l^1-1； （2）The influence of tectonic deformation on shale gas reservoir reflects in structural style， burial depth， geostress field， characteristics of natural fracture； （3）The preservation conditions of shale gas reservoirs are affected by many factors such as the intensity of structural reconstruction， sedimentation and burial depth； （4）Shale gas reservoirs in southern Sichuan Basin can be divided into four types： type of slope， type of syncline， type of low-steep anticline with wide-gentle syncline and type of faulted anticline， while different types of gas reservoirs have different geological characteristics， enrichment conditions； （5）The geological and engineering characteristics of different types of shale gas reservoirs are analyzed and corresponding technical countermeasures are provided， which are helpful to improve the production of single well and the construction of shale gas production capacity. The research results enrich the theory of shale gas enrichment and high yield in Sichuan Basin， and provide technical reference for the large-scale and cost-efficient development of shale gas in other areas.

In order to discuss the geological characteristics of deep Longmaxi Formation shale reservoir in Zhaotong Shale Gas Demonstration area in the southwest margin of the Upper Yangtze River， taking Leibo area as an example， on the basis of field geological survey and indoor experimental data， and through regional geological data， a systematic study was carried out on the petrological characteristics， sedimentary facies distribution， organic geochemical parameters， and reservoir space types and characteristics of Longmaxi Formation shale in the study area. The mineral components in the study area are mainly quartz and clay minerals， with an average of 36.3% and 34.85% respectively， and contain a small amount of carbonate minerals， feldspar and pyrite； TOC content is 0.12%-5.94%， with an average of 2.12%； The two areas near Zhongdu town-Nan'an town and the east of Leibo county are high TOC value areas and sedimentary centers in the study area. The sedimentary facies are mainly deep-water continental shelf， which evolved to the West into shallow water continental shelf. TI value is 36.5-82.5， mainly type Ⅱ₁ kerogen； R_O is 2.34%-2.72%， with an average of 2.48%， which is at over mature stage； The average porosity is 8.99% and the average pore diameter is 5.682 nm. The pore types are mainly organic matter pores， mineral dissolution pores and micro fractures. The pore morphology is mainly nano-sized open quadrilateral parallel plate pores and densely stacked spherical intergranular pores， with a small amount of open cylindrical pores and fracture structures. The porosity of shale shows a positive correlation with permeability. Compared with the major shale gas producing areas at home and abroad， the lower shale of Longmaxi Formation in the southwest margin of Sichuan Basin shows the characteristics of high TOC content， high organic matter abundance， high maturity， high content of brittle minerals and development of organic matter pores. According to its hydrocarbon generation capacity and exploitability， it is divided into source rock section and reservoir dominated section. The upper member of Longmaxi Formation is characterized by low TOC content， high clay mineral content and developed microcracks. It is a non-source rock and a cap dominated interval.

In recent years， deep shale gas exploration breakthroughs have been achieved in the Wufeng -Longmaxi formations in the complex tectonic region of the Sichuan Basin and southeastern margin， showing good prospects for deep shale gas exploration and development. Based on the data from drilling wells and experimental analysis tests， the study of deep shale gas enrichment characteristics is carried out in the Nanchuan area of the complex tectonic region of the southeastern Sichuan Basin margin， focusing on the role of changes in the formation environment such as formation temperature and pressure on deep shale gas enrichment. The study concludes that： （1） The dominant sedimentary phase zone is the basis for hydrocarbon formation in shale gas reservoirs. The Wufeng Formation - the first member of Longmaxi Formation in the study area was formed in a deep-water shelf sedimentary environment with high-quality shale development， which has good material conditions for the formation of shale gas reservoirs. （2） Organic carbon content controls the degree of development of nanoscale organic matter pores， and the high-pressure-ultra-high-pressure environment is conducive to the maintenance of pores and it plays a positive role in improving the physical properties of deep shale. （3） Deep shale gas has the typical geological characteristics of high temperature， high ground stress， and exceptionally low permeability. The influence of temperature on the adsorption capacity of shale is more obvious than that of pressure， and the deep shale gas is mainly free gas. High pressure can slow down or inhibit the gas flow， which is beneficial to shale gas preservation. （4）The changes of temperature and pressure have complex effects on gas diffusion. High temperature will increase the diffusion coefficient of gas， aggravate the migration and escape of gas， while high pressure can slow down or inhibit the flow of gas， which is beneficial to the preservation of shale gas. （5） The burial depth and pressure coefficient show a certain positive correlation， and the burial depth has a more significant effect on the pressure coefficient of syncline shale gas， indicating that the preservation conditions of deep syncline shale gas reservoirs have a tendency to become better. The residual syncline core with larger depths， inner-sag uplift， and slopes with reverse faults can be favorable targets for shale gas exploration in complex tectonic zones.

Due to the significant increase in plasticity of deep marine shale reservoir in southern Sichuan Basin under high temperature and high pressure conditions， recently， single brittleness evaluation method is difficult to accurately characterize its fracability， which severely restricts the selection of sweet spots and engineering transformation in the area. As a case from deep marine shale reservoir of the Wufeng-Longmaxi formations in the southern Sichuan Basin， through triaxial high-temperature and high-pressure experiments， fracture toughness and X-ray diffraction experiments， the mechanical properties and influencing factors of shale reservoir are studied， and the rock fracture morphology under different loading conditions is quantified. According to the morphological characteristics of shale， the analysis of brittleness influencing factors and comprehensive quantitative evaluation have been carried out. It is believed that deep marine shale reservoir in southern Sichuan Basin is characterized by the high elastic modulus and low type I fracture toughness. The brittleness is mainly controlled by the mineral composition， temperature and pressure conditions and the degree of bedding development， and the morphological characteristics of rock fracture are quantified by the fractal dimension of cracks， and in the case of high quartz mineral content （>50%）， low confining pressure （<20 MPa）， medium and low temperature （<60 ℃） and high physical and chemical degree， the fractal dimension of the sample after the experiment is high， and the fracture cracks are mainly complex shear cracks， and the corresponding brittleness degree is high. Through the correlation analysis between the normalized rock mechanical parameters and the brittleness index of the stress-strain curve and the fractal dimension， a comprehensive evaluation index is established using the analytic hierarchy process. This evaluation index can better characterize brittleness of deep marine shale. It is shown that the 3¹ sublayer of the first Member of Longmaxi Formation has a high brittleness index， which is the main target layer for later shale gas development.

Taking Well W， a typical coring well of Wufeng-Longmaxi formations in Weirong area， south Sichuan as the research object， based on core observation， thin section， argon ion polishing， scanning electron microscope， X-diffraction， organic geochemistry， tight physical properties and pore structure， the development characteristics and controlling factors of shale reservoir in the study area are discussed. The research shows that： （1） The shale of Wufeng-Longmaxi formations in Weirong area is mainly calcareous silicon rich carbonaceous shale， calciferous siliceous shale and calcium containing silicon rich carbonaceous shale. The organic matter type is type I， which is in the over mature stage. The black shale is characterized by medium to high organic carbon content， with an average clay mineral content of 40.0%， an average quartz content of 34.7% and an average carbonate mineral content of 18.2%. The mineral content of carbonate rock is relatively high. （2） The shale of Wufeng-Longmaxi formations in the study area is generally a low porosity and low permeability reservoir， mainly developing organic pores， interlayer pores （fractures） of clay minerals， inter-granular pores of pyrite， feldspar alteration pores （fractures） and grain edge fractures （pore） equal reservoir space. （3） The average value of micropores in Wufeng-Longmaxi formations is 0.009 73 mL/g， the average value of mesopores is 0.018 00 mL/g， and the average value of macropores is 0.003 77 mL/g. The reservoir space is mainly composed of micropores and mesopores. Organic matter is the main contributor to micropores， mesopores and specific surface area， and quartz minerals are the main contributor to macropores. Micropores and mesopores develop together， and they are specific surface area. Macropores are different from micropores and mesopores on the basis of development materials， and the contribution of macropore to relative surface area is limited. （4） From top to bottom， the proportion of organic pores increases gradually. At the bottom of Wufeng-Longmaxi formations， the proportion of organic pores can reach 76.73%， with an average of 55.73%. In the upper part of the sampling section， inorganic pores are relatively developed， accounting for an average of more than 80%. The main contributor of organic pores is organic matter， and the main contributor of inorganic pores is clay minerals. （5）Under the deep high temperature and high pressure conditions of Wufeng-Longmaxi formations in South Sichuan， it can develop and maintain organic rich shale reservoir and high-quality shale reservoir. The contribution of carbonate minerals to shale reservoir space is limited. Finding thick organic rich shale will still be the key to shale gas dessert.

In order to explore the pore structure and fractal characteristics of shale reservoirs in the Jingmen exploration area and quantitatively evaluate the heterogeneity and complexity of pores， taking Wufeng-Longmaxi formations of Well YT3 as the research object， the pore structure characteristics of shale reservoir are analyzed by low-pressure nitrogen adsorption experiments， organic carbon content testing， X-ray diffraction analysis， etc. and the FHH fractal model is established based on the low temperature nitrogen adsorption fractal geometry method. The relationship between fractal dimension and shale composition， pore structure， physical property， gas-bearing property and burial depth is discussed. The results show that： （1） The organic carbon （TOC） content of the lower Longmaxi Formation and Wufeng Formation is significantly higher than that of the upper Longmaxi Formation. With the increase of burial depth， the silicate minerals increase gradually， while clay minerals decrease； （2）The on-site desorption gas content of shale shows that the gas content of the upper section of Longmaxi Formation with low organic matter abundance is lower than that of the lower section of Longmaxi Formation and Wufeng Formation. In terms of shale gas composition， the upper section of Longmaxi Formation is dominated by nitrogen， while the lower sections of Wufeng Formation and Longmaxi Formation are dominated by methane； （3）The isothermal curve of shale under low-pressure nitrogen is close to type IV classified by IUPAC， and the adsorption hysteresis loop is similar to types H₃ and H₄. It is flat and slit pores， and the pores are mainly micro medium pores distributed below 50 nm. The adsorption volume of the lower Longmaxi Formation and Wufeng Formation is significantly larger than that of the upper Longmaxi Formation， and the high abundance of organic matter provides a large amount of organic pore storage space； （4）The BET specific surface area and BJH total pore volume in the lower Longmaxi Formation and Wufeng Formation with high organic matter are significantly larger than those of upper Longmaxi Formation with low organic matter， while the average pore size is significantly smaller； （5） Shale pores have obvious fractal characteristics， and the fractal dimension D₂ is greater than D₁， indicating that the complexity of pore structure is greater than that of pore surface. Fractal dimension has a significant positive correlation with total organic carbon（TOC） content， BET specific surface area and burial depth， a weak positive correlation with quartz content and BJH total pore volume， a significant negative correlation with clay mineral content and average pore diameter， but almost no correlation with porosity and permeability. There are many factors that affect the fractal dimension. Correlation analysis reveals that the mass fraction of organic carbon and clay mineral， specific surface area， average pore diameter and burial depth are the main controlling factors. The fractal dimension can be used to quantitatively evaluate the complexity and heterogeneity of pore structure， which provides an idea for studying the distribution characteristics of shale pore structure and reservoir evaluation.

The deep shale gas is rich in resources and has great potential for exploration and development. However， in the process of complex geological evolution， the material composition， mechanical properties and phase occurrence of the reservoir have changed， which has certain particularity and an important influence on the development of reservoir pores. Taking the deep shale of Wufeng-Longmaxi formations in Luzhou block as an example， based on the experiments of reservoir material composition and pore structure characterization， this paper obtained the material composition and space characteristics of the reservoir， and studied the evolution law and the pore contribution of reservoir material. The results show that： （1）Vertically， from shallow to deep， the organic matter content of Wufeng-Longmaxi formations shale increases firstly and then decreases， the content of brittle minerals increases while the content of clay minerals decreases. （2）The pore contribution of organic matter in Longmaxi Formation increases with the increase of burial depth， but decreases with the increase of burial depth in Wufeng Formation. The change trend of the pore contribution of clay minerals is opposite to that of organic matter. （3）The hard minerals， such as quartz and pyrite， have the function of “anti-compaction and pore preservation”， and are the key factors for the preservation of organic matter pores in the bottom of Wufeng-Longmaxi formations. （4）The upper part of Wufeng Formation-1^st submember of 1st Member of Longmaxi Formation （Long1₁ submember） is a high-quality reservoir in the study area， in which the upper part of Wufeng Formation-3 sublayer of Long1₁ submember is a type I reservoir. In order to realize economic development， it is suggested to optimize the fracturing technology scheme according to the characteristics of high temperature and pressure in deep reservoir and high tensile strength of rock.

Shale reservoir characteristics are the fundamental factors affecting shale gas resource evaluation， and shale reservoir properties are of great significance to shale gas occurrence. Through X-Ray diffraction， field emission scanning electron microscopy， porosity and other experimental analysis of downhole core samples， the reservoir characteristics and gas content of deep shale in Wufeng-Longmaxi formations in Luzhou area of southern Sichuan Basin are studied， and the shallow shale in Longmaxi Formation in Changning area is compared. The results show that total organic carbon （TOC） content of Wufeng-Longmaxi formations deep shale in Luzhou area of southern Sichuan Basin is high （average， 3.37%）， maturity is in the over-mature stage （average， 2.24%）. Reservoir porosity is relatively high （average， 4.45%）， and various types of pores such as inorganic mineral pores and organic matter pores are developed. In the mineral composition of the shale reservoirs， quartz content is relatively high （average， 44.25%）， low clay content （average， 29.03%）， high brittle mineral content （average， 57.35%）， and good brittleness conditions （average brittleness indexes I and II are 55.35% and 67.44%， respectively）. The shale reservoir is overpressure （pressure coefficient > 2.0） and gas-bearing capacity is good （gas content is 4.10-7.90 m³/t）， which is different from the shallow shale of Longmaxi Formation in Changning area. Overall， the deep shale of Wufeng-Longmaxi formations in Luzhou area has good reservoir properties， which is beneficial to the occurrence and enrichment of deep shale gas.