Ultra-low permeability-tight sandstone （conglomerate rock） gas reservoir is an important exploration target of natural gas at home and abroad. As a crucial oil and gas exploration basin in China， Bohai Bay Basin has discovered tight sand （conglomerate rock） gas in many depressions. However， the current research only focuses within depressions， lacking of systematic study on the distribution， origin and reservoir pattern of tight sandstone （conglomerate rock） gas reservoirs from the whole basin perspective. The research conclusively demonstrates that ultra-low permeability-tight sandstone （conglomerate rock） gas reservoir is widely distributed in the Bohai Bay Basin， covering various depression， multiple strata of Paleozoic and Cenozoic， and Upper Paleozoic coal-type gas and Paleogene oil-type gas. The Upper Paleozoic ultra-low permeability and tight sandstone gas reservoirs are mainly distributed in residual Upper Paleozoic strata of Huanghua， Linqing and Jiyang depressions. Those reservoirs are of structural gas reservoir type and mainly located in the sag and uplift areas， and mostly belong to structural gas reservoirs. Because the strata were buried deeply in the past， most of the reservoirs have high physical density. The majority of natural gas is typical coal-type gas originating in Carboniferous and Permian measures， and other is Paleogene oil-type gas. Paleogene ultra-low permeability tight gas reservoirs are widely distributed in all depressions， including the 2nd， 3rd and 4th members of Shahejie Formation（Es2， Es3 and Es4） and multiple strata in Kongdian Formation； The gas reservoirs are located in the uplift， slope and steep slope zones of the sag， including a variety of trap types， such as lithology， lithology-structure， structure reservoir etc.； The Es2 and， Es3 are commonly sandstone reservoirs， distributed in gentle slope area. The Es4 and Kongdian Formation include sandstone and conglomerate reservoirs formations， and the glutenites are mainly distributed in steep slope area. Because the Paleogene stratum belongs to continuous deposit， the reservoir physical properties are primarily controlled by burial depth. Different depressions have different densification threshold depths， ranging from 3 200 m to 4 000 m. The natural gas is mostly Paleogene oil-type gas and part of it is Upper Paleozoic coal type gas. According to the relationship between gas sources and reservoir， four patterns of reservoir forming modes are developed in this area， including near source accumulation of new reservoir， distant source distribution of Paleozoic reservoir， fault distribution of Paleozoic new reservoir and section distribution of new Paleozoic reservoir， among which the former two modes are the main accumulation modes. Two sets of Carboniferous-Permian and Paleogene strata are developed， and the natural gas remaining resource has great potential in the Bohai Bay Basin. The next targeted exploration area of tight sandstone （conglomerate rock） gas would be situated in the beneficial zone of secondary hydrocarbon generation in Carboniferous-Permian System， structures adjacent to the edge of Paleogene gas generation center， and the sweet spot in lithology reservoirs would be considered as promising areas for further exploration. The research is of instructive significance for tight sandstone （conglomerate rock） gas exploration in Bohai Bay Basin.
Through a large number of rock thin section microscopic observations, scanning electron microscopy analysis and fluid inclusion homogenization temperature measurement, the lithological composition of tight reservoirs in the Shanxi Formation of Yan'an gas field in the southeast of Ordos Basin was studied, lithofacies types were classified, and the diagenetic evolution of different types of rocks is combined with the burial history, thermal history, and hydrocarbon charging process to analyze the time matching relationship between the key oil and gas charging period and reservoir densification. Studies have shown that pure quartz sandstone and quartz-rich low-plasticity lithic sandstone mainly develop mechanical compaction, secondary dissolution and kaolinite precipitation. Before the two key hydrocarbons are charged, the porosity is 15.8%-31.5%, which belongs to medium and high permeability reservoir rocks. The high tuffaceous heterogeneous quartz sandstone, plastic-rich granular lithic sandstone, and carbonate tight cemented sandstone have a porosity of 4.6% to 10.8% before the first key hydrocarbon charge, which belongs to ultra-low porosity-low porosity reservoirs. It is difficult to charge the hydrocarbons in the later stage. Therefore, the diagenesis of pure quartz sandstone and quartz-rich low-plastic granular lithic sandstone that maintained high porosity and permeability during early oil and gas charging has been inhibited, and the physical properties of the reservoir are relatively good. They are the dominant migration channels and accumulation spaces for late natural gas. This constitutes a sweet spot in tight sandstone gas reservoirs. The research results are of great significance for understanding the coupling relationship between reservoir densification process and accumulation, clarifying the formation mechanism of effective reservoir rocks and predicting the distribution of sweet spots.
The Ordovician natural gas resources are abundant in the middle and eastern of Ordos Basin. With the further exploration of deep natural gas， a mass of natural gas resources have been found in the Ordovician subsalt strata， but the genetic types and sources of subsalt natural gas are still controversial. Based on the data of natural gas composition and stable carbon isotope， combined with the actual geological background， this paper systematically analyzes the genetic types and sources of subsalt natural gas in Majiagou Formation， central and eastern Ordos Basin. The natural gas in the middle and east of Ordos Basin is mainly dry gas， and the gaseous non-hydrocarbon component content is less. The methane carbon isotopes composition （δ13C1） is chiefly between -40 ‰ and -32 ‰， and that of ethane （δ13C2） is between -40 ‰ and -28 ‰， which is characterized by sapropelic origin. According to the composition， carbon isotope and plane distribution characteristics of natural gas， the Ordovician subsalt natural gas can be divided into oil type gas， coal type gas and mixed gas. Oil type gas is widely distributed and comes from the subsalt carbonate source rock of the Ordovician Majiagou Formation， and some natural gas has TSR reaction. The distribution range of coal type gas and mixed gas is small， and they are concentrated near the edge of subsalt formation pinch out line. They mainly come from the Upper Paleozoic source rocks， and the Ordovician source rocks also make a small contribution.
The main burial depth of the Lower Cretaceous Qingshuihe Formation （K1q） in the western segment of the southern margin of the Junggar Basin is 3 600-5 600 m， although the burial depth is large， the high-yield oil and gas flow obtained inside it reveals that its physical conditions should be superior. However， there is still a lack of systematic research on deep reservoir characteristics and physical property preservation mechanism of Qingshuihe Formation， which seriously restricts the later oil and gas exploration process. Accordingly， this study comprehensively utilized various analytical and testing methods such as rock casting thin section， scanning electron microscope， physical property， X-ray diffraction analysis and fluid inclusion， and combined with formation test data such as drilling and logging and regional geological data， the petrology， physical properties and diagenesis of deep reservoirs of Qingshuihe Formation in the western segment of southern margin are systematically analyzed， and the preservation mechanism of physical properties is clarified on this basis. The results show that the Qingshuihe Formation reservoir is mainly composed of braided river delta distributary channel sandstone and glutenite with low compositional maturity， high tuff lithic content and medium structural maturity； the reservoir space is dominated by intergranular pores， and characterized by mesopore-macroporous and coarse throat. The average porosity is 6.15%， and the average permeability is 4.25×10-3 μm2； the diagenesis of the reservoir is mainly compaction， cementation and dissolution， but the diagenetic strength is weak， the cement content is generally low， and the calcite cementation is the main， and the reservoir is still in the early diagenetic stage B-middle diagenetic stage A； the braided river delta distributary channel sandstone with rich quartz， feldspar and poor cuttings and good sorting is the basis for the formation of deep high-quality reservoirs， and diagenetic compaction is the main factor of physical property loss， but the early carbonate cementation， the anti-compaction effect of chlorite crust and the late acid dissolution delay the physical property loss of the reservoir； the burial mode， overpressure and decreasing paleogeothermal gradient of early long-term shallow burial – late short-term rapid deep burial are the keys to delay the diagenesis process of Qingshuihe Formation reservoir and broaden the depth and time of dissolution under deep burial conditions， thus effectively promoting the preservation of physical properties.
The deep buried sandstone reservoirs in the upper member of Es3 in Niuju-Changtan subsag of Eastern Sag, Liaohe Depression are characterized by complex sedimentary system, large lateral variation of reservoir and few deep exploration wells, but there is little research on reservoir diagenesis. Based on casting thin-sections, SEM, X-ray diffraction, fluid inclusions and other technical means, this paper analyzed the properties, diagenesis characteristics and evolution of deep sandstone reservoirs in the upper third member of Shahejie Formation in Niuju-Changtan subsag, divided the reservoir diagenetic facies types, and compared the differential diagenetic evolution process of different types of reservoirs. The results show that the lithology of the upper third member of Shahejie Formation is mainly medium-fine grained lithic sandstones in the study area with low porosity and ultra-low permeability, and the reservoir pore type is mainly secondary dissolution pores. The diagenetic evolution of reservoir mainly experienced four stages including mechanical compaction for porosity reduction, early cementation for porosity reduction, acidic dissolution for porosity increase and late cementation for porosity reduction. The diagenesis and pore evolution of different types of sandstone reservoirs are significantly different.The densification time of sandstones from tight compaction facies and tight calcite cementation facies is the earliest, followed by sandstones from moderately diagenetic facies, and sandstones from strong dissolution facies are the latest. The sandstones of tight compaction facies and tight calcite cementation facies are affected by high matrix content and high carbonate cementation content respectively, and the porosity decreases rapidly in the early stage of diagenesis, resulting in the limited range and intensity of organic acid dissolution in the late stage. Due to the relatively weak compaction degree in the early stage, the sandstones of strong dissolution facies retain some intergranular pores, which are greatly affected by the dissolution of organic acids, and the reservoir physical properties are improved.
Passive margins can be divided into ramp type and S type from the perspective of morphology. Previous studies mainly focus on their influences on the delta development and sediment budget into deepwater systems. However， the individual impacts on submarine canyons and submarine fans as well as the role of shelf breaks and slope breaks for shaping deepwater depositional systems remain unsettled. In this study， stratigraphic forward modeling is applied to investigate the depositional systems on S type and ramp type of passive margins， including the spatial distribution， sediment budget， sequence stratigraphic frame， temporal evolution， and flow dynamics. The study reveals that an S type is featured by poorly-developed and progradation-dominated delta， asymmetrical erosion in the canyon， well-developed submarine fan， less affected by sea-level change. In contrast， a ramp type is characterized by well-developed and aggradation-dominated delta， the upper to middle canyon covered by delta， less distinct asymmetrical erosion， poorly-developed submarine fan， significantly influenced by sea-level change. In conclusion， shelf breaks result in less sedimentation on the shelf and more transferred into deepwater whereas slope breaks lead to less in the canyon and more unloaded at canyon mouths.
The tight reservoir of the Lower Ganchaigou Formation in the Yingxi area of Qaidam Basin has strong heterogeneity and complex migration and accumulation characteristics， which seriously restricts the scale exploration and development of tight oil. In this study， the tight reservoir samples of the upper member of Xiaganchaigou Formation in the Yingxi area were selected. Based on the observation of rock slices， X-ray diffraction tests and high-pressure mercury intrusion （HPMI） tests， the physical simulation experiment of petroleum accumulation was carried out to clarify the characteristics of tight mixed rock reservoirs and explore the characteristics of tight oil filling and reservoir formation. The results show that the average porosity and average permeability of reservoirs in Yingxi area are 4.12% and 0.007 1×10-3 μm2， respectively. The permeability is mainly affected by the pore structure. The physical properties and pore structure parameters of limestone reservoirs are better than those of siltstone reservoirs and gypsum reservoirs. The migration and accumulation ability of limestone reservoirs is better than that of siltstone reservoirs and gypsum reservoirs. The oil filling and migration in tight reservoirs are characterized by non-Darcy flow， with starting pressure， and the oil saturation growth curve shows the characteristics of rapid increase and gradual stability. The migration and accumulation of oil in tight reservoirs are controlled by the coupling of permeability and displacement pressure. The migration and accumulation modes of the tight reservoir can be divided into three types： Type I is the rapid speed growth-high saturation type； Type II is the medium speed growth-medium saturation type； Type III is the slow speed growth-low saturation type. The threshold conditions for oil migration in tight reservoirs are： Y=5.841 6 e-318.6X， and the threshold conditions for forming stable migration channels are： Y=9.848 1e-269.9X （x represents the reservoir permeability， Y is displacement pressure gradient）.
In order to study the kinetic characteristics of methane adsorption under supercritical state， the anthracite of Anyang Longshan Mine was taken as the research object. The isothermal adsorption experiment and adsorption kinetic experiment were carried out with the magnetic suspension balance high-pressure isothermal adsorption instrument. Based on the commonly used adsorption kinetic equation and unipore diffusion model， the kinetic characteristics of methane in the process of diffusion and adsorption to equilibrium in anthracite were discussed. The results showed that the “peak” isothermal adsorption line measured in the laboratory was the curve of excess adsorption capacity， and the absolute adsorption capacity corrected by intercept method accords with Langmuir adsorption model. The adsorption equilibrium time was negatively correlated with temperature and pressure， and the adsorption equilibrium time tended to be stable with the increase of pressure； Bangham kinetic equation is the most suitable to describe the kinetic process of methane adsorption in coal. The adsorption rate constant first increases and then decreases with pressure， and is positively correlated with temperature； The diffusion coefficient of methane in coal shows three stages with the pressure gauge： a sharp rise - a slow rise - a sharp decline.
The content of helium in geothermal water-associated gas in Weihe Basin is extremely high， which has attracted the attention of domestic scholars. The source and enrichment mechanism of helium becomes a focused scientific issues in recent years. The samples of geothermal water-associated gas from typical wells in Weihe Basin were collected， and the chemical composition， carbon and hydrogen isotopic compositions， helium and argon isotopic composition of these samples were systematically analyzed. The results show that most of the geothermal water-associated gases are coal-formed gas， the carbon and hydrogen isotopic compositions of the geothermal water-associated gas are similar to those of Carboniferous and Permian coal-formed gas in Ordos Basin， which were mainly from the contribution of source rock of coal series in Upper Paleozoic. The rest of the associated gas of geothermal wells is biogas， which is contributed by mudstone of Zhangjiapo Formation. The helium isotope ratios in geothermal water-associated gas show that the helium is mainly crustal source helium. It mainly originated from the U， Th radioactive decay of the basement granite. Although the helium content is high， the total content of natural gas and helium in geothermal water is very low. At present， the helium resource with geothermal water as the carrier still has no industrial value. According to the idea of natural gas exploration， looking for small and medium-sized natural gas reservoirs in Weihe Basin may be an effective means to break through the bottleneck of helium resource exploration in Weihe Basin.
Bauxite as a natural gas reservoir has not been reported publicly at home and abroad. In recent years， exploration has proved that the gas-bearing property of bauxite in Lower Carboniferous is good， and high-yield industrial gas flow has been found in some exploratory wells. Therefore， it is urgent to study the basic geology of bauxite reservoir. The characteristics and formation mechanism of bauxite reservoir in study area was determined by thin section， X-ray diffraction， scanning electron microscopy， constant-pressure mercury injection and nuclear magnetic resonance analyses. The results show that the sedimentary characteristics of bauxite in the study area are the same as those in the northern China area. They are all internal mechanic-chemical sedimentary bauxite with mechanical transport in some parts. The bedding structure is well developed， and the rock texture can be divided into sand-gravel texture， pisolitic-oolitic texture， clay crystal texture， grain （powder crystal） texture and so on. From the bottom to the top， it can be divided into five sections： Section A （Fe-bearing section）， section B （bauxitic mudstone section）， section C （bauxite section）， section D （siliceous bauxite section）， and section E （carbonaceous mudstone or coal line section）. The mineral composition of each section is obviously different. There are rich pyrite in section A， high content of clay mineral in section B， and over 90% content of diaspore in section C， the powder crystal authigenic siliceous rocks are developed in section D， and the organic matter is rich in section E. The reservoir is mainly distributed in the middle and upper part of C， namely the porous bauxite section. The formation of reservoir pores goes through three stages： the dissolved pores formed by the leaching of humic acid and atmospheric fresh water in penecontemporaneous period are the main stage of pore formation， accounting for 80% of the total visible pores； burial diagenesis stage is also an important period for pore formation， at this time， the layered diaspore with loose crystal structure is transformed into the dense plate-columnar diaspore， which forms intergranular pores accounting for more than 10% of the visible pores； the organic acids discharged from carbonaceous mudstone and coal seam can enlarge the dissolution pores and form intergranular dissolution pores， which can improve the reservoir conditions to some extent. The reservoir performance of the study area is good， and the average porosity is 14.67%， the average permeability is 5.57×10-3 μm2， which is a good reservoir of natural gas. The brittleness index is more than 90%， which is much higher than that of shale oil and shale gas in the basin. It has the geological characteristics of strong fracturing ability with high brittleness index （>90%）， high Young's modulus （36.4 GPa） and high Poisson's ratio （0.35） which means it is suitable for fracturing.
Taking the reservoir rocks of sandstone gas reservoirs in China as the research object， combined with high pressure mercury injection and outcrop reconnaissance and gas field data analysis， taking pore throat radius， core permeability， well test permeability and logging permeability as indexes， this paper established the microscopic and macroscopic heterogeneity characterization methods of sandstone gas reservoir， and studied the heterogeneity characteristics of core micro pore throat， outcrop profile， block and gas field. The results show that the microscopic pore throat structure of sandstone gas reservoir is extremely complex， and the flow channel is composed of a large number of pores， fractures and throats of different sizes， forming a complex flow network which shows strong heterogeneity both microscopically and macroscopically. Combined with reservoir heterogeneity， the physical simulation model and method of heterogeneous full-diameter long core are established， and the production effects of well distribution in high permeability area and dense area are compared and studied. Under the condition of 800 mL/min rationing production， the stable production period of well distribution in high permeability area is 60% longer than that in tight area， and the production decreases rapidly after the end of stable production period， and the low production period is short； the formation pressure of well distribution in high permeability area decreases faster than that in tight area， indicating that reserves can be used more quickly； the recovery percent in high permeability area rises faster than that in tight area， and the recovery percent at the end of stable production period is 51.2% higher while the recovery is 14.6% higher. The research results are of guiding significance for the scientific development and enhanced gas recovery of similar gas reservoirs.
Accurate evaluation of the physical property limits is one of the key parts for efficient development of deep carbonate gas reservoirs. However， previous studies did not fully consider the influence of formation pressure and water saturation. For this reason， taking the deep carbonate gas reservoir of Longwangmiao Formation in Anyue gas field as the target， the influence of pore structure and irreducible water on gas flow was studied through core experiments under reservoir conditions. Then， a similarity transformation model considering the influence of start-up pressure gradient， stress sensitivity and pressure variation was established； and the physical property limits of dry and irreducible water reservoirs were evaluated. The results showed that pore structure， water saturation and production pressure difference are the main factors affecting reservoir productivity. Under the production pressure difference of 10-50 MPa， the permeability lower limits of reservoirs without water are between 0.420×10-3 μm2 and 0.049×10-3 μm2， the irreducible water greatly reduces the gas production rate， and the permeability lower limits of reservoirs with irreducible water is twice that of reservoirs without water. This study will provide a reference for reservoir evaluation， productivity prediction and scheme adjustment of deep carbonate gas reservoirs.
Rock hydraulic fracturing involves a complex fluid-structure interaction process. The regularity of hydraulic fracture propagation and the coupling mechanism between fracturing fluid and rock mass are the current research hotspots and difficulties. The extended finite element method （XFEM） based on the ABAQUS platform has the advantages of high calculation accuracy and small calculation volume， and its soil module can simulate the process of fluid-structure interaction， analyzing the basic parameters such as the propagation path， width， and stress of hydraulic fractures. Based on the XFEM， this paper studied the influence of different conditions （single-perforation fracturing， two- perforation asynchronous fracturing， two-perforation synchronous fracturing） on hydraulic fracturing propagation path， fracture width， fracture length， perforation pressure and other parameters. The simulation results show that： （1） Under the conditions of two-perforation asynchronous fracturing and synchronous fracturing， the magnitude and direction of the minimum horizontal principal stress and the tensile strength of reservoir are changed due to the interference of hydraulic fractures. These changes in turn alter the propagation path of hydraulic fractures. （2） Among the mentioned three conditions， the width of hydraulic fracture formed by single-perforation fracturing is the smallest， and its initial fracture pressure of reservoir rock mass is the lowest. The hydraulic fracture formed by two-perforation asynchronous fracturing has the largest width， the longest length， and the highest initial fracture pressure of reservoir rock due to the interference of existing fracture. The values of width， and fracturing pressure of the hydraulic fracture under the synchronous fracturing are between the above two conditions and its fracture length is the shortest. （3） The perforation spacing has significant influence on the initiation pressure of reservoir， the maximal fracture width and length. The simulation results can provide theoretical guidance for selecting suitable fracturing methods under different conditions and fracturing requirements.
At present， the research on transforming offshore abandoned gas reservoir into underground gas storage is still in its infancy. Taking the lithologic gas reservoir of H gas field in Qiongdongnan Basin as an example， based on the study of static characteristics such as gas reservoir structural characteristics， sealing evaluation， reservoir characteristics and gas reservoir characteristics， combined with productivity evaluation， dynamic reserve evaluation， dynamic reserve evaluation and so on in the process of gas reservoir development， connectivity analysis and research are performed to carry out the design and demonstration of gas storage operation parameters such as storage capacity parameters， injection production capacity， working gas volume， number of wells and well pattern. Finally， numerical simulation is used to simulate the operation indexes of gas storage according to the designed injection production scheme. The analysis and simulation results show that： （1）The block is a lithologic gas reservoir with good sealing performance， internal connectivity， high permeability and ultra-high permeability， full of gas and no movable water； in addition， the block has large gas storage scale， high productivity， no formation water and low condensate content. （2）The operation pressure of the gas storage is 23-34.5 MPa， and the storage capacity is 75.1×108 m3， working gas volume 26.1×108 m3， only 10 injection and production wells are needed， the minimum export pressure is 15.3 MPa， and the average daily gas production of the block is 2 094×104 m3， peak shaving daily gas production reached 3 200×104 m3. Combined with the understanding of offshore supporting process technology， it is concluded that h gas field is very suitable for the construction of underground gas storage compared with the geological conditions of domestic onshore gas storage.
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