Ordos Basin is the second largest sedimentary basin in China with abundant oil and gas resources. After more than 50 years of exploration practice， PetroChina Changqing Oilfield Company has innovated reservoir-forming geological theories such as large delta of inland depression lake basin， continental tight sandstone gas and Ordovician karst paleo-landform. In 2022， the oil and gas equivalent has exceeded 65 million tons， and the China's largest oil and gas production base and largest natural gas production area have been built. In recent years， in order to further consolidate the resource base for continuous and stable production of oil and gas， Changqing Oilfield Company has continuously deepened comprehensive geological research， strengthened technical research， and increased risk exploration efforts in new areas. Oil and gas exploration has achieved fruitful results. Through innovative theoretical understanding of the formation of the Mesozoic Chang 73 laminated shale oil and the new series Chang 82 beach bar tight oil reservoir， the new oil reserves increased by 400 million tons； Natural gas exploration has innovated the theory of oil and gas accumulation in Ordovician undersalt， Taiyuan Formation biolimestones and Ulalik shale in the western margin of the basin， increased the scale of reserves by more than 400 billion cubic meters. At the same time， a series of new unconventional oil and gas exploration technologies such as geophysics and fracturing have been formed， and major discoveries have been made in oil and gas exploration. It is estimated that the new oil resource potential of the basin is 2-4 billion tons， and the natural gas resource potential is 2-3 trillion cubic meters. It has laid a solid resource foundation for ensuring long-term stable production of more than 60 million tons of Changqing Oilfield Company.
In order to accurately characterize the pore throat structure of tight reservoir in the Chang 8 Member of the Wuqi-Zhidan area of the Ordos Basin， based on the analyses of cast thin section， scanning electron microscopy， X-ray diffraction， high-pressure mercury injection， constant velocity mercury injection， and oil-water phase infiltration， the pore throat fractal characteristics and throat fractal characteristics of tight reservoirs in Chang 8 Member were studied by using fractal theory. The relationship between fractal characteristics and reservoir porosity， permeability， and microscopic pore throat characteristics was systematically analyzed. The results show that the lithology of the study area is dominated by lithic feldspar sandstone， with an average porosity of 8.2% and an average permeability of 0.16×10-3 μm2. The reservoirs are generally tight. The pore types of the reservoir are mainly intergranular pores and dissolution pores. According to the morphology of capillary pressure curve and high-pressure mercury injection parameters， the pore throat structure of the reservoir is divided into two types： type I and type II. The constant rate mercury intrusion experiment showed that the mercury saturation of the samples in the study area was mainly contributed by the throat. The fractal dimension of throat calculated by constant velocity mercury injection has two characteristics：Fine throat section DT1 and coarse throat section DT2. The full aperture fractal dimension and throat fractal dimension have good negative correlation with physical properties， mercury injection saturation and mercury removal efficiency. The throat fractal dimension has a good negative correlation with the mercury injection saturation of the throat and the radius of the mainstream throat. The more homogeneous the pore throat is， the more difficult it is for the oil phase fluid to pass through the water-bearing reservoir and the lower the relative permeability of the oil phase.
Strike-slipping is a prevalent tectonic process in petroliferous basins. A systematical research on the effect of strike-slipping on tectonic fractures is significant to the prediction of tectonic fractures. Taking the Bozi-1， Keshen-5 and Keshen-13 gas reservoirs in the Kelasu structural belt of Kuqa Depression as examples， the characteristics and contribution of tectonic fractures in the presence of strike-slipping effect were analyzed using cores， image logging and drilling fluid leakage. At last， a probable dynamics mechanism of strike-slipping was discussed. The Bozi-1， Keshen-5 and Keshen-13 gas reservoirs all develop sinistral strike-slipping， which leads to the weakening of relationship between the distribution of tectonic fractures and the structural form， but the variation of tectonic fractures is not simply corresponding to the direction of strike-slipping， and the quantitative mechanical mechanism has yet to be deeply researched. To anticlinal gas reservoir in compressional tectonic region， the universal fracture development mode of anticline is not suitable for the determination of fracture development areas and the deployment of drilling wells when strike-slipping exists， while some correction should be made using actual geological data.
Volcano-hydrothermal effect has an important influence on the sedimentary evolution and biological source of Permian Lucaogou Formation in Jimusar Sag. The core samples of Well J305 in Jimusar Sag of Junggar Basin were collected and systematically studied by element geochemistry and organic geochemistry methods. The specific contents include geochemical characteristics of source rocks， evolution of sedimentary paleoenvironment and biological sources of organic matter. The results show that： （1）The major and trace elements in the source rock samples of the Lucaogou Formation show some differences between P2l1 and P2l2. The P2l1 is relatively rich in Sr， U， Fe2O3， MgO and CaO， and the P2l2 is relatively rich in V， SiO2 and Al2O3.（2）The sedimentary period of the Lucaogou Formation was generally in a saline water environment. The water body of the P2l1 was deep， and the water depth of the P2l2 fluctuated greatly and was relatively shallow. From P2l1 to P2l2， the salinity gradually decreased. The paleo-oxygen phase is a relatively stable reducing environment， and the enrichment of organic matter is controlled by paleo-productivity， that is， biological source. Biological sources are related to paleosalinity， paleoclimate and volcanic-hydrothermal activities to a certain extent.（3）The overall characteristics of biological source composition are that the abundance of bacteria and algae is greater than that of higher plants， the abundance of bacteria is greater than that of algae， and algae are mainly green algae. Among them， cyanobacteria account for a certain proportion of bacteria，and the proportion of cyanobacteria in the P2l1 is relatively high.（4）The sedimentary evolution of Lucaogou Formation can be divided into five stages： arid-brackish water stage→semi-arid-brackish water stage→arid-brackish water stage→humid stage→semi-arid-brackish water stage.The sedimentary and biological sources of the Lucaogou Formation are controlled by the paleoenvironment. Volcano-hydrothermal processes provide a large amount of nutrients and improve paleoproductivity. A stable reducing environment provides the necessary conditions for the preservation of organic matter.
The breakthrough of oil and gas exploration in the eastern Mahaidong structure of the northern margin of the Qaidam Basin reveals that the Jurassic source rocks in this area have strong oil and gas potential. This paper presents a detailed comparative analysis of geochemical indicators such as abundance， type and maturity of hydrocarbon source rocks developed in different depressions， and on this basis， the controlling factors of the differences in hydrocarbon generation potential are discussed. It is found that： （1）The Jurassic hydrocarbon source rocks in this region are mainly distributed in the fifth and seventh members of the Middle Jurassic Dameigou Formation， dominated by carbonaceous mudstone and dark mudstone or shale， respectively， among which the source rocks in the seventh member have higher abundance and are dominated by type Ⅰ-Ⅱ Kerogen， which is the key layer of source rock developed in this region； （2）The quality of source rocks in Hongshan depression is the best， and the thermal evolution has reached the mature stage， which is expected to be the key block for the next step of increasing storage and production； Yuka and Xiaochaidan depressions are followed by Hobson and Delingha depressions with poor quality； （3）The differences in the abundance and types of source rocks in the seventh member of different depressions are related to the sedimentary environment： the Hobson Depression developed lakeshore-swamp sediments in this period， while the Hongshan， Xiaochaidan and Yuka depressions were shallow-semi-deep lake， and the abundance and types of source rocks in the former are poor； （4）The difference of source rock maturity among different depressions is controlled by tectonic activities： Hobson， Hongshan and Delingha depressions are continuous subsidence depressions， while Yuka and Xiaochaidan depressions are tectonic reverse transition depressions. The former has high source rock maturity while the latter has relatively low maturity. This study has important guiding significance for the next oil and gas exploration deployment in Qaidam Basin.
Shale oil in saline lacustrine basins is a potential replacement target for unconventional oil and gas exploration， but the main controlling factors and models for the organic matter enrichment （OME） remain unclear. In order to reconstruct the palaeoenvironment during the organic-rich mudstone deposition and explore the OME mechanism in saline lacustrine basins， samples from the Lower Youshashan Formation in the Nanyishan area of Qaidam Basin were used to carry out experiments， such as TOC， major， trace， and rare earth elements. The results show that： The main controlling factor of the OME in the lower and upper parts of the Lower Youshashan Formation is preservation condition， and paleoproductivity， respectively. There are five OME peaks： Peak 1 is “mixed model”， which is controlled by preservation condition and paleoproductivity， peak 2 is “preservation model”， which is controlled by preservation condition， peaks 3 and 4 are “productivity model” which are controlled by paleoproductivity， and peak 5 is “mixed model”， which is controlled by multiple factors. Furthermore， dynamic changes of the driving factors for the OME may be the frequent fluctuations of paleoclimate.
The paleoenvironment and paleoclimate of the Paleogene Paleocene Lulehe Formation and Oligocene Xiaganchaigou Formation in Qaidam Basin have high research value. In this paper， taking the northern margin of Qaidam Basin as an example， the tectonic setting， paleoclimate and paleo-redox conditions of the depositional source areas of the Paleogene Lulehe Formation and Xiaganchaigou Formation are discussed by using the test data of rare earth elements， in order to reveal the changes of paleoenvironment and paleoclimate. The research shows that： the Paleogene Lulehe Formation and Xiaganchaigou Formation have obvious negative Eu anomaly and slight negative Ce anomaly. The REDOX indices δCe<0.95， Ceanom<0 and U/Th<0.75 consistently indicate oxidation conditions. The value of paleoclimate ΣREE （63.38-170.88）， Eu negative anomaly and Rb/Sr （mean value 0.44） indicate a warm and humid climate. The Sm/Nd values of 0.18-0.28 indicate that the sediments originated from the upper crust， and the corresponding relation of（La/Yb）N-ΣREE indicates that the sediments mainly originated from sedimentary rocks.
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-S1l1-1； （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.
The evaluation of remaining reserves is particularly important for the evaluation of gas field development effect and the further improvement of gas field reserve utilization. Therefore， taking Changning shale gas field in southern Sichuan Basin as the research object， the reservoir distribution， remaining reserves evaluation and the improvement of reserves utilization are systematically analyzed. Methods included organic geochemical testing， triaxial rock mechanics experiments and numerical simulation. The results show that： The distribution of the average remaining reserves of the single well is clarified. The recovery degree of the well-controlled reserves of the platform in the study area is 45%-70%， and the average remaining reserves of the well are （0.5-1.5）×108 m3， there is potential in some areas. There are three types of unused reserves in Changning shale gas field： Blank area of well pattern deployment， insufficient fracturing area and vertical unused area. In the area of single well remaining reserves >1.0×108 m3， according to the principle of geology-engineering-development coupling well selection， repeated fracturing well optimization is carried out. The area with network fracture development and the spacing between the pre-deployed wells is greater than 500 m is selected， and the deployment and implementation of the same zone infill wells are carried out according to the surface well site conditions. According to the analysis of reservoir， rock mechanics and numerical simulation， sublayer⑤ is the optimal target of the upper gas layer， the vertical distance from the lower gas layer⁃sublayer① is more than 20 m， the network natural fracture development area is preferred， the pressure coefficient is greater than 1.2， and the continuous thickness of the upper gas layer I reservoir is more than 10 m. In the favorable area， the staggered tridimensional development is expected to increase the platform reserve utilization rate by 30%. The results of this study provide references for the well pattern deployment strategy of shale gas blocks.
A set of high-quality organic-rich shale is developed in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in northern Guizhou， which is expected to have a good resource prospect. However， shale reservoir characteristics， gas-bearing characteristics and their controlling factors of shale gas are still not clear. Based on the previous data and the latest drilling， this paper systematically analyzed organic matter characteristics， reservoir physical property and gas-bearing characteristics of the Wufeng-Longmaxi shales， and further discussed the factors of shale gas content. The results show that Wufeng-Longmaxi formations in northern Guizhou are shallow-deep water shelf facies， and high-quality shales are primarily distributed in the deep-water shelf facies in Daozhen-Xishui areas， with the maximum thickness of 80-100 m. Organic-rich shales in Wufeng Formation and the lower member of Longmaxi Formation have high TOC content， mostly higher than 2%， with good organic matter type and moderated maturity， and therefore they have good conditions for shale gas formation. High-quality shales have high content of brittle minerals， and therefore they are easy to be fractured. Shale pore types are various， dominated with organic matter nano-scale pores， and the studied shales have relatively low porosity and permeability. Methane adsorption capacity of the studied shales is strong， which is mainly controlled by TOC and thermal maturity. Shale gas content in different regions varies greatly. The shale gas content is relatively high in Xishui and Daozhen areas， whereas it is relatively low in Tongzi-Zhengan areas. Gas contents of the shales in the same structural unit is controlled by TOC content， nanometer-sized pore development degree and water saturation， however， shale gas content in different structural unit is determined by structural preservation condition. Shales with good preservation conditions， e.g.， wide and gentle structures， far from large faults and deep burial depth， generally have high gas content.
The western Hubei area， which is adjacent to the Sichuan Basin， have great shale gas resources potential in the Upper Permian marine shales， and have achieved a series of exploration discoveries in recent years， such as Enshi， Hongxing and other areas. However， there are many differences in reservoir performance， gas-bearing property and other enrichment conditions， hence the shale gas display effect in different regions are diverse. Taking the Wujiaping Formation II in Hongxing block and Dalong Formation in Enshi area as the research objects， the detailed characterization of shale gas enrichment differences and analysis of controlling factors have been carried out by comprehensive use of drilling， logging， seismic and laboratory analysis and testing data. The results show that： （1） The original quality of the Wujiaping Formation II is relatively similar to that of the Dalong Formation， with a slight difference in the thickness of organic shale； （2） The reservoir performance， source-reservoir coupling and gas-bearing property of the Wujiaping Formation II are better than that of the Dalong Formation； （3） The strong Meso-Cenozoic tectonic compression around the Yangtze led to earlier initial uplift of the Dalong Formation in Enshi area， resulting in greater denudation of the overlying strata， and relatively poor shale gas preservation conditions； （4） The burial depth and structural style can effectively indicate the current preservation conditions. The Wujiaping Formation II in Hongxing block that was deeply buried and developed in a wide -gentle anticline， is more conducive to shale gas enrichment than the Dalong Formation， which is shallowly buried in the complex syncline in Enshi area. There have different structural styles and preservation conditions for two sets of shale in the two areas， which bring different difficulties in exploration and development. Therefore， comparative research should be strengthened and classified practical methods should be taken from the perspective of accelerating exploration and development.
The reconstruction of underground gas storage of low permeability lithology gas reservoir in Ordos Basin has various reservoir features， complex sealing conditions， and hydrogen sulfide in the Ordos Basin， which makes geological evaluation and construction operation difficult. The key technologies for designing and operating gas storage in low-permeability lithologic reservoirs are summarized from three aspects： site selection evaluation， index design， and operation optimization. This research establishes the site selection evaluation technology and site selection index system of gas storage in low-permeability lithologic gas reservoirs， and ten favorable construction areas are determined. Besides， the formation of gas storage in low permeability lithologic reservoirs index design technology effectively improves the injection-production performance of gas wells， expands the operating pressure range， optimizes the injection-production well type and location deployment mode， and supports the optimization design of four gas storages. Moreover， the paper forms the evaluation technology of the whole life cycle operation of gas storage. The relevant findings help petroleum engineers continuously improve the peak shaving capacity of the in-service gas storage. Meanwhile， we optimize the hydrogen sulfide production gas elutriation mode， and rearrange the geology， wellbore， and ground integrated management and evaluation methods， thus realizing the long-term safe operation of the gas storage in low-permeability lithologic reservoirs. Three key technologies have been successfully applied to the site selection， design， and operation of SH， YU， SD， and LW gas storage in the Ordos Basin. The scheme design indicators are basically consistent with the actual dynamic evaluation， and the peak shaving capacity is 7.5×108 m3. In-service gas storages in the research areas have realized more than eight years of safe operation.
At present， there is no precedent for reconstruction of lithological gas reservoir into UGS （underground gas storage reservoir） in China. Taking S block in Ordos Basin as an example， the study starts from geological features like sealing condition， reservoir property， fluid feature and well productivity. Study results show that MW5 formation of the study area yields excellent reconstruction condition for its great sealing property， medium permeability，relatively high productivity，great communication and zero interlayers.Furthermore， study also indicates there’s no movable formation water and fluid there contain low hydrogen sulfide content. Based on reservoir characteristics， the study area is designed to be reconstructed with a mixed well pattern of 12 vertical and directional wells and three horizontal wells. The UGS will be operated in pressure range of 13.6-32.0 MPa， with a storage capacity of 2.23×109 m3. Under these operating indexes， the center area with relatively high permeability can be well developed and it is estimated that the maximum production rate may yield 12.8×106 m3 and the working gas may reach 1.08×109 m3， which is 48.4% of the total reserves. In conclusion， the study area shows a great potential of rebuilding UGS， which can yield high productivity with very limited wells.
When there is a low-velocity or high-velocity layer in the formation， a strong reflection interface usually occurs， and the weak reflection signal of the reservoir is shielded by the strong reflection signal， and the seismic profile shows a strong event， which reduces the accuracy of reservoir prediction. Therefore， it is of great significance to remove the shielding effect of strong reflection on weak reservoir signals. According to the sparse signal decomposition theory， this paper proposes a method of removing strong seismic reflection using soft threshold based on dynamic dictionary decomposition. Firstly， the dynamic dictionary is established by using the Ricker wavelet， and the seismic strong reflection waveform is decomposed into the sparse representation of the dynamic dictionary. Then soft threshold method is used to remove the components of the dynamic dictionary with large amplitude， so as to achieve the purpose of removing the strong reflection. Compared with the traditional strong seismic reflection removal method， this method can better improve the lateral discontinuity after the removal of strong reflections. It also reduces the dependence on strong reflection horizon， and no longer requires accurate strong reflection horizon data. The test of physical model data and the application of actual data show that this method can effectively remove the shielding effect of strong reflection and highlight the effective signal of the reservoir.
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