In recent years， important exploration findings have been made in the Middle Triassic Leikoupo Formation in Sichuan Basin， and the large-scale reservoir is one of the key factors to control the hydrocarbon accumulation. Based on the data of outcrops， exploration wells， logging and seismic data， the lithofacies paleogeographic map of Leikoupo Formation is compiled and the characteristics of large-scale reservoirs are analyzed， and the following three understandings are obtained. （1） In Leikoupo stage， Sichuan Basin was a relatively closed carbonate platform， the south， east and north directions of Sichuan Basin were relatively closed， and seawater entered from the west. The first and second members of Leikoupo Formation are weakly rimmed carbonate platform， and the third and fourth members of Leikoupo Formation are rimmed carbonate platform. （2） The Leikoupo Formation can be divided into platform margin， open platform， restricted platform and evaporate platform， and the restricted platform is the main one， including lagoon， intra-platform shoal and tidal flat. There are two types of hydrocarbon reservoirs in Leikoupo Formation： Platform margin shoal and intra-platform shoal. （3） The Leikoupo Formation develops two sets of large-scale mound shoal reservoirs. One is the platform margin mound shoal of the third and fourth members of Leikoupo Formation in Northwest Sichuan， with large thickness （generally more than 50 m）. And the other is the intra-platform shoal in the lower part of the first member of Leikoupo Formation in Central Sichuan， which has the characteristics of small thickness， stable distribution， large area （more than 20 000 km2） and large scale. These two types of large-scale reservoirs are reformed by multi-stage karstification to form good quality reservoirs， which provide material basis for large-scale hydrocarbon accumulation. The results of this paper improve the understanding of the deposits and reservoirs of Leikoupo Formation in Sichuan Basin， and guide the hydrocarbon exploration in this field， which has important theoretical and practical significance.
Recently， the Devonian exploration wells in northwestern Sichuan Basin have obtained gas successively， which shows that the Devonian Guanwushan Formation in the area has great exploration potential. In order to further promote the exploration process of ultra-deep marine carbonate rocks in the Devonian Guanwushan Formation in northwestern Sichuan Basin， the field outcrops and cores were taken as the research objects， and the reservoir characteristics and formation mechanism were studied via core observation， thin section identification， trace element testing and isotope study. The results show that： A set of carbonate reservoirs mainly composed of bioclastic dolomite and breccia dolomite are developed in the Devonian Guanwushan Formation in northwestern Sichuan Basin， the reservoir space is mainly composed of dissolution pores， the physical property analysis data show the characteristics of low porosity and ultra-low permeability reservoir. Comprehensive analysis shows that the reservoir of Guanwushan Formation is the result of coupling action of many factors， among which beach facies sedimentary environment is the basis of reservoir formation， and contemporaneous dolomitization and multi-stage karst transformation are the main controlling factors of reservoir formation. The research results can provide important enlightenment for Devonian dolomite exploration in northwestern Sichuan Basin.
Industrial natural gas flows have been gained from the wells penetrated in the nodular limestone， shows a good potential for natural gas exploration. In the early stage， the research on the formation of this reservoir is not in-depth enough， which restricts the exploration of this type of gas reservoir. Based on core observation， thin section authentication， combined with the results of scanning electron microscope， nitrogen adsorption and desorption， rare earth elements， trace elements and energy spectrum analysis， three achievements were gained. The results show that the reservoirs mostly exist in the argillaceous limestone matrix， which is a set of tight carbonate fracture-pore reservoir with low porosity and low permeability. The reservoir space is complex and the talc diagenetic shrinkage fissure is the greatest contribution storage space to the reservoir. Talc is mainly transformed from sepiolite during diagenesis， the silicon element in sepiolite comes from the fault hydrothermal fluid of Maokou Formation in the same sedimentary period， and magnesium comes from bound pore water and conversion release of a metastable mineral. Sepiolite transforms into talc under the influence of pressure， temperature， buried time and grade of maturity during burial. Due to dehydration and silica precipitation， the crystal gets smaller， which formed large-scale of talc diagenetic shrinkage fissure that lays the material foundation for reservoir. Research results provide strong geological support for the nodular gas pool exploration of the first member of Maokou Formation of Permian in southern Sichuan Basin. It has a good guiding significance for oil and gas exploration.
In order to find out the tectonic-sedimentary characteristics of the Ordovician Kelimoli-Wulalike formations in the Ordos Basin， and provide technical support for the prediction and target optimization of favorable exploration zones in this area， the paleotectonic pattern and sedimentary characteristics of the Ordovician Kelimoli-Wulalike stages in the western edge of the Ordos Basin are studied systematically under the guiding ideology of tectonic controlling deposition， based on seismic， well logging， drilling cores， microscopic thin section and outcrops. The results indicate that： （1）The paleotectonic pattern of the Kelimoli-Wulalike stages reflects the characteristics of “one land， one uplift and one depression， and high and sag alternate with each other along the northeast direction of transition zone”； （2）The deposition pattern of the Kelimoli-Wulalike stages is rimmed shelf and the types of sedimentary facies include platform edge， slope， deep-water shelf and basin； （3）The paleotectonic pattern controls the plane distribution of sedimentary facies. The platform margins are distributed along the central paleo-uplift and the NE-trending bulge on the uplift transition zone， the slope facies are mainly distributed along the NE-trending sag on the uplift transition zone； the deep-water shelf facies and basin facies are mainly distributed along the western depression.
Tight oil has become a research focus in the field of global unconventional petroleum. The dense oil resources of Chang 7 member in Ordos Basin are abundant and have great exploration and development potential. In this study， different influence of diagenesis on micro pore-throat characteristics of tight sandstone reservoirs in Chang 7 member in Jiyuan and Zhenbei areas of Ordos Basin were studied by using the test and analysis data of rock slice， casting slice， scanning electron microscope （SEM）， mercury injection test， porosity-permeability analysis and X-ray diffraction analysis. The results showed that： The sandstone of Chang 7 member in Jiyuan and Zhenbei areas of Ordos Basin is dominated by feldspathic and lithic arkose. The main clastic components in Jiyuan area are feldspar， followed by quartz； while the dominate components for Zhenbei area are quartz， followed by feldspar. The compositional maturity is generally low with medium sorting for these two areas. Reservoir pore types are mainly residual intergranular pore and feldspar dissolved pore for Jiyuan area， while Zhenbei area are mainly feldspar dissolved pore and residual intergranular pore. The radius of pore and throat for these two areas are similar， but the porosity， throat sorting coefficient and pore throat connectivity show obvious differences. Different influence of diagenesis is the main reason for the heterogeneity of reservoir in these areas. Because of the development of biotite and clay matrix in Jiyuan area， the compaction leads to the loss of primary pores， while the dissolution is weak. Chlorite hinders the formation of other authigenic minerals， thus preserving part of the remaining primary intergranular pores， and the reservoir connectivity is relatively better than Zhenbei area. The content of plastic volcanic rocks in Zhenbei area is high， and the compaction leads to more loss of primary pores. Illite is well developed， which is easy to be eroded and broken by high-speed fluid in the pore throat. After migration， the pore throat is blocked， resulting in the decrease of reservoir permeability and the number of effective pore throat and effective pore throat radius. Although Zhenbei area experienced stronger dissolution， it has no obvious improvement on reservoir connectivity.
Shale oil in Chang 7 member （Abbr. as Chang 7） mainly developed as gravity flow deposits in semi-deep to deep lacustrine environment. Compared to the marine shale rocks in North America， continental shale oil in Chang 7 is thin in single sand body， poor in lateral continuity， strong in reservoir heterogeneity， tight in its reservoir and low in formation pressure index. These characteristics made it more difficult to develop shale oil in Chang 7 and thus shale oil has driven much attention as it might be another significant superseding area for national energy safety in China. After years of research and field practice， “volume development” theory was proposed based on the unique characteristics of the shale oil in China. Guided by the theory， complex artificial fracture systems are established after multi-section fracturing in horizontal wells， and thus forms a composite flow pattern including nonlinear seepage and imbibition replacement processes between multi-sized artificial fractures and the matrix. Application of the theory has dramatically optimized the seepage environment and created “artificial oil and gas reservoir” underground， and thus established a key technology characterized with long horizontal well， short well spacing， large well cluster， stereoscopic fracture and subdivision volumetric fracturing. This technology has made commercial development of shale oil in Chang 7 possible and a demonstration area with one-million tons of annual shale oil production has been established. It also provides theoretical basis and technical support for an overall development of the continental shale oil in China.
With the comprehensive development of Fuling shale gas field， the analysis of production dynamics， production characteristics and development evaluation of shale gas wells has become an urgent problem to be solved. In the view of the production dynamics of conventional and unconventional shale gas reservoirs， scholars have proposed different classical decline curve analysis methods for evaluation. These methods， from empirical to semi-empirical and theoretical， all have assumptions and limitations， all have certain assumptions and limitations， and are not universal. By using the law of decline found in actual production data， a new model of decline calculation is established， and the analytical results are compared with the classical model. The research results show that： （1） The relationship between the monthly decline rate of 60， 120 and 180 days calculated from the actual production data and the production time is well fitted with the power law exponential model， which indicates that the decline rate at the stage of gas well production decline presents a power law decline； （2） On the basis of establishing the model of the decline rate by using the power law exponential model， the constant decline rate is changed into variable decline rate by optimizing the differential calculation method， and a new simplified decline curve model is proposed； （3） Compared with the existing Arps harmonic decline model， Hsieh decline model， PLE decline model and Duong decline model， the new calculation model shows better fitting effect， with relative error within 2%-4%.
The Sinian Dengying Formation gas reservoir in Sichuan Basin is an ancient， deep， low-porosity and strongly heterogeneous karst carbonate gas reservoir. Due to the limitation of the existing industry standards on reservoir types， the response of dynamic and static data can not be one-to-one correspondence. Therefore， based on the static data， the reservoir of Sinian Dengying Formation gas reservoir in Sichuan Basin was subdivided into two stages from the perspective of development， the dynamic response of various reservoirs were clarified， and the technical countermeasures for gas wells in different production stages of different types of reservoirs are formulated. The technical countermeasures include：（1） The production organization strategies for five types of reservoirs are defined，i.e. the strategy of high production and high pressure should be implemented for fractured-vuggy I and porous-vuggy I，the ratio of porous-vuggy I is optimized to 1/4-1/3；the strategy of low production and low pressure should be implemented for fractured-vuggy II，porous-vuggy II and pore I，the ratio of fractured-vuggy II is optimized to 1/12-1/10. According to this strategy， the gas wells with established reservoir type should be optimized and the production flow sections should be divided and the early dynamic reserve evaluation should be carried out. （2） For those that have been put into production but the reservoir type is not clear， it is necessary to evaluate whether the production of gas wells is stable，preliminarily determine the reservoir type，carry out dynamic monitoring， implement reservoir subdivision types， and optimize gas well production allocation according to countermeasures （1）. （3） For production building wells， they should be divided into different types according to static data after oil testing， and production allocation should be designed according to the scheme after production. Dynamic data should be accurately enrolled to prepare for the determination of reservoir types. The research results provide an effective support for the upper production of Dengying Formation gas reservoir and the long-term stable production of 60×108 m3/a scale.
Based on Langmuir slip condition， properties of rock and gas， storage mechanisms of fluid， an apparent permeability for gas transport in shale matrix is established. Coupling influence of ad/desorption effect， stress sensitivity effect， water saturation， real gas effect and volume content of organic pores is considered in proposed model， which is used to investigate dynamic change characteristics of gas transport capacity of shale matrix in reservoir condition. Then， the reliability of the proposed model is verified by experimental and LBM simulation data from literatures. During reservoir depletion， the apparent gas permeability of shale matrix initially decreases and then increases. What’ more， gas slippage effect， radius， stress sensitivity effect and water saturation have significant impact on apparent gas permeability. The apparent gas permeability of shale matrix decreases with increase of stress sensitivity coefficient and irreducible water saturation. The apparent gas permeability of shale matrix impacted by irreducible water saturation and stress effective effect decreases by 87.56% when water saturation reaches 0.3.The proposed model provides a certain help for evaluating apparent gas permeability of shale matrix in reservoir conditions.
Thick layers of dark shale are developed in Shennongjia area of northwestern Hubei Province in the Early Cambrian， which has good geological conditions for shale gas occurrence. However， there are few investigations and studies， and the reservoir characteristics and controlling factors are still not clear. Based on the shale gas field investigation in this area， core and outcrop samples were systematically collected to carry out analysis and test. After defining basic evaluation parameters， micro reservoir space type and feature identification， micro pore structure quantitative characterization were carried out， and the influencing factors of reservoir performance were analyzed. The research results show that： The shale has unique mineral composition characteristics and dominant lithofacies， and has fairly good hydrocarbon generating material base， which is in the over mature stage； The rock is quite dense， the apparent porosity and permeability are very low， and the micro reservoir space is mainly composed of nano inorganic pores， organic pores and micro fractures； Micropores with pore diameter less than 10 nm have large specific surface area and complex pore structure， which are the main space for natural gas adsorption and storage. The results show that： Different minerals have different effects on the pore development of shale， organic matter is the main component of micropores， and organic matter plays a construction role mainly by compounding with inorganic minerals such as quartz and pyrite； Slope type shale has the best reservoir and adsorption capacity， followed by deep water type and platform type； Organic matter pore is damaged to a certain extent， but still has a certain degree of damage and reservoir capacity； Different types of microfractures play different roles in shale gas preservation， and dissolution fractures mainly play a destructive role； Poor gas bearing capacity of the most abundant organic matter section is related to the underdevelopment of organic matter pores in siliceous shale facies under the background of overheated evolution.
At present， the method of organic carbon and silicon content is often used to optimize shale gas enrichment and high-yield intervals. However， when selecting layers according to organic carbon content， the differences of different types of organic matter and their controlled organic pores are usually ignored， while the consideration of biogenic silica content is ignored when selecting layers according to silica content. Organic pores determine shale gas reservoir capacity， and biogenic silica determines formation ability of shale high-quality reservoirs. In this paper， an optimization method of shale gas enrichment and high production interval based on organic pore and biogenic silica content is proposed. Based on the porosity of different types of organic matter and the composition of organic matter types， the calculation method of organic pore proportion of different hydrocarbon generating organisms is proposed， and the calculation method of shale biogenic silica content is proposed based on the silica content and logging parameters in shale. Through field application analysis， it is found that under the dual constraints of organic pore ratio of more than 3% and biogenic silica content of more than 30%（i.e. "3+30" method for optimization of shale gas enrichment and high-yield interval based on organic pore and biogenic silica）， the optimized shale interval from Wufeng Formation to Longmaxi Formation has good consistency with the current shale gas high-yield interval. This method realizes the organic unity of evaluation of the reservoir capacity and the transformation ability of shale gas enrichment and high-yield layers， and has certain application value in the optimization of favorable intervals and shale gas reserves prediction.
The development background， geochemical characteristics， reservoir physical and gas bearing characteristics and preservation conditions of Cambrian marine source rocks in Tadong area of Tarim Basin are studied by means of drilling coring and logging methods， combined with maceral identification， organic carbon analysis， rock pyrolysis， vitrinite reflectance test， whole rock mineral analysis， uniaxial compression and porosity permeability test. The favorable area is predicted. The results show that the organic macerals of Cambrian Xidashan-Xishanbulake shale in Tadong area are characterized by rich marine vitrinite and high organic carbon content. The main types of organic matter are type I and type II1. The vitrinite reflectance is relatively moderate. The organic matter is in the high mature to over mature stage， and the thermal evolution lasts for a long time. It is found that the structural conditions in this area are not conducive to the preservation of shale gas. The gas content and organic carbon content are the two main controlling factors of the shale gas resources by using the grey correlation method and the weight analysis method. Based on the relevant parameters of the shale， the favorable shale gas area in the Tadong low uplift area is predicted， and two core areas and one favorable area are divided. The development background， geochemical characteristics， reservoir physical properties and gas bearing characteristics of Cambrian marine source rocks in Tadong area of Tarim Basin are studied by using drilling coring and logging methods， combined with maceral identification， organic carbon analysis， rock pyrolysis， vitrinite reflectance test， whole rock mineral analysis， uniaxial compression and porosity permeability test， and the favorable shale gas areas are also studied forecast. The results show that the organic macerals of Cambrian Xidashan-Xishanbulake formations shale in Tadong area are characterized by rich marine vitrinite and high organic carbon content. The types of organic matter are mainly typeⅠandⅡ1.The vitrinite reflectance is moderate. The organic matter is in the stage of high maturity and over maturity.The characteristics of long thermal evolution，high content of brittle minerals and well-developed porosity and permeability are also found. The physical properties are poor condition， large elastic modulus and small Poisson's ratio. Based on the conclusion， this paper uses the grey correlation method and weight analysis method to determine that gas content and organic carbon content are the two main controlling factors of the shale gas resources. Based on the relevant parameters of the shale， the favorable shale gas area in Tadong low uplift area is predicted， and two core areas and one favorable area are divided.
The carboniferous gas reservoir in Shapingchang is fissure-pore carbonate gas reservoir with large reserves， deep reservoir burial， developed fractures， strong heterogeneity and features of medium and low porosity， medium and low permeability. It is in the middle-late stage of development. In order to meet the needs of large-scale construction of gas storage in China， it is considered possible to rebuild the gas storage by screening and evaluating in the early stage. However， it is necessary to further carry out the preliminary evaluation research， especially under the production condition of gas storage alternately swallowing and spitting. The change of reservoir physical property and seepage characteristics needs to be strengthened. Through a series of experimental studies， combined with the field production test data， this paper established a reservoir stress sensitivity research method under alternating pressure. It analyzed the reservoir physical property change characteristics after the drop of formation pressure in gas reservoir development， as well as the physical property change law under alternating pressure change after the reconstruction of gas storage. The results show that the permeability of Shapingchang Carboniferous reservoir is a power function with effective pressure. The stress sensitivity of fracture reservoir is strong. The gas reservoir has decreased from the original formation pressure of about 55 MPa to the present about 10 MPa. The permeability of the three types of reservoirs has decreased to different degrees. The porosity type， pore （hole） seam type and fracture type decreased by about 5%， 15% and 41% respectively. However， if the gas storage is assumed to operate between 30 and 55 MPa， the permeability changes between 3% and 28%.
At present， there are great differences in the evaluation methods of underground gas storage traps in different regions and types， especially in the aspect of fault dynamic sealing， there is no systematic and complete evaluation method and system. Based on the investigation of a large number of literatures on sealing evaluation of gas storage at home and abroad， combined with the sealing evaluation examples of 27 built gas storages in China， this paper puts forward a method to evaluate the dynamic sealing property of gas storage traps by using stress field. From the perspective of geomechanics， the dynamic sealing property of gas storage traps is evaluated by analyzing the change of stress field in the process of injection and production. Firstly， the in-situ stress parameters of single well are calculated， the current three-dimensional in-situ stress field is established， and the spatial distribution of principal stress and effective stress is analyzed. Secondly， the Coulomb failure criterion is used to judge the fault stability under the present in-situ stress state. Finally， the main purpose of this paper is to establish a three-dimensional numerical model to simulate the dynamic change of reservoir pressure and the cyclic pressure of gas injection reservoir. The process of dynamic change （effective stress） of in-situ stress caused by the change can be used to evaluate the stress risk of fault sealing. This method can be used to determine whether the gas storage can be operated safely under different injection production periods.
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