Different helium source rocks are characterized by varying characteristics， precursor element （U， Th） contents and occurrence states. U and Th in sediments primarily exist in the forms of adsorption and/or complexation with organic matter and clay minerals. The primary migration of helium generated in sediments is more likely to occur due to the absence of mineral crystal restraints. Therefore， the source rocks and reservoir rocks of gas pools act as the primary effective helium source rocks in sediments， while other sediments are not effective helium source rocks due to the fact that high porosity causes long saturation time of helium dissolution， thereby restraining the desolubilization and secondary migration of helium. Isomorphous U and Th were mainly enriched in silicate and phosphate minerals in magmatic rocks， and temperature acts as the main controlling factor affecting their primary migration. Granite is characterized by low porosity and low dissolution of helium， large-scale release of helium can happen under uplift movement and abnormal high temperature， acting as the helium source rock of helium-rich natural gases. Various forms of U and Th can exist in metamorphic rocks， which have higher porosity and higher soluble helium contents than granite， but this results in greater difficulty in helium release. Although the direct source rocks and reservoirs of natural gas reservoirs are effective helium source rocks， it is difficult to form He-rich natural gas due to the influence of hydrocarbon dilution. Sufficient He supply from basin basement or mantle-derived sources is a key condition for natural gas reservoirs to be rich in He.
At present， all the helium used in industrial development comes from the crustal-derived helium in the helium-rich natural gas reservoir. Natural gas is the carrier of crustal-derived helium， and its generation， accumulation， and helium release are closely related to the tectonic thermal evolution of the basin. It is important to systematically evaluate the influence of tectonic thermal evolution on the helium release in a basin to clarify the enrichment of natural gas and helium. The Weihe Basin， as the first sedimentary basin with helium mining rights in China， is rich in helium gas resources. This article takes the Weihe Basin as an example to systematically simulate the tectonic and thermal evolution history of the basin. At the same time， it deeply analyzes the occurrence characteristics of hydrocarbon source rocks and helium source minerals， estimates the amount of helium resources generated and released by the main helium source minerals in the Huashan rock mass， and explores the impact of basin tectonic and thermal evolution on the enrichment of helium rich natural gas reservoirs. The aim is to provide new ideas for the establishment and improvement of a helium resource investigation and evaluation system. The results show that： （1） The crustal-derived helium gas in the Weihe Basin mainly comes from helium source minerals rich in U and Th elements such as zircon， apatite， etc.， which are relatively scattered in rocks. The temperature range （>180 ℃） where natural gas is generated in large quantities and the main helium source minerals release helium gas has a high degree of overlap. （2） After the formation of the basement， the Weihe Basin underwent Paleozoic sedimentation and was subsequently strongly uplifted and eroded. A large number of Indosinian and Yanshanian granite bodies were formed on the surface， in which helium source minerals rich in uranium and thorium elements （mainly calcite， zircon， and apatite） continuously decayed to generate helium gas and partially enclosed the helium gas in the mineral lattice. The faulting of the Cenozoic era led to rapid subsidence of the basin since approximately 40 Ma， followed by accelerated subsidence around 5 Ma， resulting in rapid warming of the strata. Natural gas was generated from Paleozoic source rocks， and helium gas generated from helium source minerals was released in a concentrated manner. The two have a spatiotemporal coupling relationship. During the migration process， natural gas continuously carries scattered helium gas into traps， thereby forming helium rich natural gas reservoirs. （3） According to the helium sealing temperature of the main helium source minerals and the characteristics of helium gas accumulation in many basins with helium rich natural gas， the helium sealing zone （<60 ℃）， partially sealing zone （60-220 ℃） and unsealing zone （>220 ℃） can be divided.
Ordos Basin is the largest natural gas producing area in China. The discovery of two helium-rich natural gas fields， Dongsheng and Qingyang， shows a good helium resource prospect. Sulige Gas Field is the largest natural gas field discovered in China. In order to evaluate the helium resource prospect of the gas field， geochemical analysis of natural gas components， alkane gas， carbon isotopes of carbon dioxide， helium components and isotopes of the gas field is conducted. The geochemical characteristics of gas and helium in the Paleozoic in Sulige Gas Field have been preliminarily identified， and the main controlling factors of helium reservoir formation have been discussed. The results show that the composition of natural gas in the Upper Paleozoic is obviously different. The Upper Paleozoic natural gas has typical wet gas at mature stage and dry gas at higher than mature stage. The Lower Paleozoic natural gas is mainly dry gas with partial contribution of wet gas. The Paleozoic is dominated by thermogenic natural gas， the Upper Paleozoic is dominated by middle-late humic gas， which is coal-derived gas， mainly from Carboniferous and Permian coal measure source rocks， and the lower Paleozoic is dominated by late sapropelic dry gas and oil cracking gas. The helium content of Paleozoic natural gas is higher than that of conventional natural gas （0.03%）， which belongs to middle helium gas， and the Upper Paleozoic is higher than the Lower Paleozoic. The helium accumulation in Sulige Gas field is mainly influenced by the ancient and modern structural location， the high helium generation intensity and relatively low hydrocarbon generation intensity of helium source rocks such as U-Th rich basement granite and granite gneiss， the development of basement fault and the complex gas-water relationship， which is favorable for the helium to dissolve out of the water and enter into the natural gas reservoirs.
Helium source rocks are the material basis for the formation of helium rich gas reservoirs， and are one of the key research contents in helium resource exploration. At present， the evaluation of helium source rocks is relatively simple， and it is necessary to conduct intensive and systematic research on helium source rocks. Based on the systematic analysis of the potential helium source rocks in Qingyang Gas Field， Ordos Basin， including uranium and thorium content， uranium and thorium occurrence state， types and sizes of minerals enriched in uranium and thorium， helium release mode and geological process， and combined with the comparative study of different helium source rocks， this paper evaluates the effectiveness of the potential helium source rocks in this area. The results show that the U and Th contents of potential helium source rocks in the study area are the highest in bauxite and the lowest in siliceous rock. In mudstone， coal， dolomite and siliceous rock， uranium and thorium are mainly adsorbed or stored in small heavy minerals， and the helium produced is released in the form of decay recoil， which has the characteristics of continuous release. While uranium and thorium in basement granite gneiss， bauxite and sandstone are mainly stored in large heavy minerals. When the formation temperature does not exceed the sealing temperature， helium is accumulated in heavy minerals. When the abnormal high temperature breaks through the sealing temperature of helium in heavy minerals， the accumulated helium is released centrally， which has the characteristics of “episodic release”. Combined with the scale and U and Th content of helium source rocks， it is considered that the basement granite gneiss， mudstone， bauxite and sandstone are the main helium source rocks in this area， and dolomite， siliceous rock and coal are the secondary helium source rocks. In the early Cretaceous， natural gas was accumulated on a large scale. At this time， helium gas was released under the action of abnormal high temperature， which was separated from the source rock and coupled with natural gas.
The Ordos Basin is rich in natural gas resources， and its helium resource potential has been confirmed in the Yimeng Uplift， Weihe Basin and other regions. However， the characteristics of natural gas helium content and the main controlling factors of enrichment in other areas of the basin need to be clarified through further geological exploration and scientific research. Through the collection of natural gas samples from typical wells in the southwest of Ordos Basin， and the analysis of natural gas composition， helium isotope， major and trace elements of rocks， combined with the simulation of single well geological history， temperature and pressure evolution history， the distribution characteristics， sources and main controlling factors of helium in the Upper Paleozoic of Qingyang Gas Field in Ordos Basin were analyzed， and the helium enrichment model was established. The results show that the helium content of the Upper Paleozoic in Qingyang Gas Field is 0.068%-0.310%， and the average helium content is 0.154%， which is a medium-high helium gas reservoir. The helium distribution shows a trend of low in the north and high in the south. The helium gas in Qingyang Gas Field is a typical crust source， which mainly comes from Archean-Proterozoic basement metamorphic rock-granite series， supplemented by sedimentary helium source rock. Helium enrichment is mainly controlled by central paleo-uplift， formation temperature and formation pressure， basement fracture and tectonic movement. The central paleo-uplift makes the basement shallowly buried. The basement-type helium source rock is the main and the sedimentary-type helium source rock is supplemented to provide sufficient helium. The basement fracture provides a channel for the vertical migration of helium. Low formation pressure and high formation temperature are conducive to helium dissolution. The “seesaw” tectonic movement controls the direction of natural gas migration and accumulation， forming a multi-source helium enrichment model under the background of paleo-uplift.
The helium content in the Carboniferous oil-associated gases in the Hadexun area of Tarim Basin are as high as 0.15%-0.70%， with an average of 0.32%， much higher than the lower limit of current economic grade for helium extraction （0.05%）， and the remaining geological reserves of helium are 523.65×104 m3， which is a medium-sized helium-rich field. There are significant differences in helium content among the three blocks. On plane， different zones in this oilfield show diverse helium contents. Zone HD4 has the highest helium contents， from 0.62% to 0.69% （average 0.66%， N=2）； followed by Zone HD113 and Zone HD1， from 0.20% to 0.44% （average 0.34%， N=4） and from 0.15% to 0.70% （average 0.27%， N=17）， respectively. The planar distributions of helium contents are primarily controlled by the distributions of deep-seated faults. Vertically， there are remarkable differences in helium content of the two suites of gas-bearing systems. Relative to thin sandstones from the Kalashayi Formation （0.15%-0.44%， average 0.24%， N=12）， the Donghe sandstones from the Bachu Formation have the greater helium content （0.15%-0.70%， average 0.40%， N=11）. The vertical distribution of helium contents is controlled by the source and reservoir configurations of helium systematics. According to gas composition characteristics， carbon isotope characteristics of alkane gas， genetic calculation results and seismic data interpretation， the helium in the Hadexun area is contributed by Yurtus Formation shale， intrusive rocks and ancient basement. The Kalashayi Formation mudstone with a thickness of more than 130 m provides good sealing conditions for the effective preservation of helium. In summary， helium accumulation in the Hadexun area follows the “two-near helium-capturing model”， namely， the closer to both deep-seated faults and underlying helium source rocks， the higher helium content in reservoirs.
In order to deepen the Jurassic hydrocarbon exploration of Turpan-Kumul Basin， based on lithology and lithofacies analysis， considering tectonic activity and paleoclimate change， according to the relative balance of rates of potential accommodation change with sediment + water supply， a process-based infilling model of Jurassic infilling was constructed. The results show that： （1） The Jurassic has undergone three types and five stages of sediment infilling， which are overfilled （Badaowan Formation）， balanced-fill （Sangonghe Formation）， overfilled （Xishanyao Formation）， balanced-fill （Sanjianfang-Qiketai formations） and underfilled （Qigu–Kalazha formations）； （2） The organic-rich sediments above flooding surfaces， especially around maximum flooding surfaces， in profundal strata， and within some intervals of lake-plain strata （source）， highstand clastic shoreline strata， lowstand incised valley fills and lake floor fans （reservoir） and distal transgressive and highstand prodelta strata （seal） combined good source-reservoir-cap assemblage in overfilled stage. The organic-rich sediments above the flooding surfaces in para sequence scale and the lower portion of highstand systems tracts （source）， lake-floor fans， incised-valley fills， and shoreline clastics deposited during transgressions and highstands （reservoir） and prodelta mudrocks in late transgressive and early highstand systems tracts （seal） combined favorable source-reservoir-cap assemblage during balanced-fill phase. The organic-rich sediments above the initial transgressive surface （source）， transgressive sheetflood clastics， early highstand fluvial channels （reservoir） and upper transgressive and basal-highstand systems tract strata （seal） combined efficient source-reservoir-cap assemblage in underfilled phase. （3） The process-based infilling model of lacustrine basin can extend its relevant hypotheses or attributes into predictive realms， which is mainly reflected in the validity of facies description and the predictability of source rocks， reservoirs， sequence stacking patterns and hydrocarbon accumulation. It is predicted that a certain of source rock mainly composed of dark mudstone is developed in underfilled of Jurassic， and the incised-valleys， lake floor fans within the overfilled Badaowan and Xishanyao formations and the large-scale nearshore fans developed in balanced-fill Sanjianfang Formation are potential favorable exploration domains. The process-based analysis of lakes infilling is significant for theoretical study on evolution and hydrocarbon exploration and development of lacustrine basins.
The Ordovician ultra-deep carbonate reservoirs in the Tarim Basin are rich in oil and gas resources； however， affected by multiple periods of tectonic activity and strike-slip faults， their distribution shows strong heterogeneity. In regions developing fault-controlled fractures and caves， there are uncertainties in reservoir quality evaluation methods based on physical property parameters， and methods based on geomechanical parameters show advantages. In this study， quantitative characterization of geomechanical parameters including present-day in-situ stress， natural fracture and rock elastic modulus were carried out， the carbonate fracture-cavity geological model was established， the relationship between natural fracture density， elastic modulus， horizontal minimum principal stress and horizontal stress difference was built， reservoir quality evaluation indicator was defined and calculated， and finally， reservoir quality was quantitatively evaluated. The results indicate that： （1） In the ultra-deep fault-controlled fracture-cavity carbonate reservoirs， the spatial distribution of geomechanical parameters has strong heterogeneity and is significantly affected by faults. It is segmented along the fault extension direction. The elastic modulus and natural fracture density indicate high values near the fault zone， resulting in that the present-day in-situ stresses are low values around fault zones. （2） Reservoir geomechanical parameters have a significant response to fault-controlled fracture-cavity carbonate oil and gas reservoirs. The method proposed here is effective to evaluate reservoir quality， high reservoir quality evaluation indicator is distributed around strike-slip faults and adjacent regions， e.g.， Well X3 zones. The results can provide geological reference and support for efficient exploration and profitable development of fault-controlled fracture-cavity ultra-deep carbonate reservoirs.
In the Bozi-Dabei area of the Kuqa Depression in the Tarim Basin， the natural fractures of the Lower Cretaceous dense sandstone reservoir have developed. Since its deposition， the target layer has formed a complex fracture system with multiple causes and overlapping phases， providing effective enrichment and percolation spaces for the dense reservoir. Based on imaging logging data and drilling cores， characteristics of fractures with different attitudes and fillings in water-based and oil-based mud backgrounds were analyzed， leading to the creation of an identification atlas for natural fracture features from electrical and acoustic imaging data. Combined with field outcrop surveys， rock mechanics， and geochemical test results， the characteristics of complex natural cracks as well as distribution patterns were clarified. The target layer has undergone three major tectonic movements under a strong compressive stress environment. Upon detailed determination of the single well fracture development characteristics， five types of effective fracture combination patterns were identified. Based on the detailed determination of the fracture development characteristics of individual wells， five effective fracture combination patterns were identified. Coupled analysis of both dynamic and static production data from individual wells， such as unimpeded flow rates， gas production indices， and pump shutdown pressure drop rates with fracture features， indicated that the dominant natural fractures controlling production in the target layer are mainly governed by the natural fracture combination patterns， the development density of effective fractures， the extent of fracture distribution around the well， and the size of the angle between fractures and stress. These fractures have formed under the superimposed influence of multiple phases of tectonic activity， resulting in the current dominant fracture distribution system. Consequently， an evaluation criterion for dominant fracture-controlled enrichment and production of Lower Cretaceous ultra-deep tight sandstone reservoirs in the Bozi-Dabei area was established.
In view of the unclear understanding of the reservoir distribution characteristics and the genetic mechanism of high-quality reservoirs in the Pinghu Formation of the N structural belt in the Xihu Sag of the East China Sea shelf basin， the reservoir characteristics and the main controlling factors of high-quality reservoirs are analyzed by using rock thin sections， scanning electron microscopy， XRD whole rock minerals and clay minerals， logging and testing data. The results show that the rock type of Pinghu Formation is mainly feldspar lithic sandstone. The lithology is mainly fine and medium sandstone. The buried depth of the main reservoir is more than 3 400 m. The primary pores are mainly intergranular pores， and the dissolution pores are mainly intergranular pores. The porosity is 5.0%-23.60%， the average porosity is 14.88%， and the median is 14.67%. The permeability is （0.10-1 377.52）×10-3 μm2， the average permeability is 71.19×10-3 μm2， and the median is 3.99×10-3 μm2. High-quality reservoirs are mainly medium-coarse sandstone. The thickness of the sand body is generally greater than 10 m， the quartz content is 60.0%-84.0%， the cement content is 1.0%-23.0%， the porosity is 15.0%-23.6%， and the permeability is （1.7-1 377.52）×10-3 μm2. High-quality reservoirs are affected by various factors such as sedimentary conditions， diagenesis and overpressure. Under the influence of tide， the tidal channel， tidal sand dam and other types of sand bodies developed under strong hydrodynamic conditions are mainly medium-coarse sandstone. The sandstone has good sorting， low shale and matrix content， and good physical conditions. The overall content of cements is low and the content of carbonate cements is 10 m， which provides a good material basis for high-quality reservoirs. The interbedded structure of coal seam-mudstone-sandstone is the most favorable lithologic combination condition for dissolution. Hydrocarbon generation pressurization promotes organic acid flow.
The overall exploration level of volcanic rocks in the Mao3 Member of the Permian in northern Fuling， Sichuan Basin is low. Controlled by the Permian volcanic eruption mechanism in eastern Sichuan， various types of volcanic rocks developed vertically. Among them， pyroclastic rocks are a new way to achieve large-scale reserve increase. Defining the distribution characteristics and exploration potential of Permian pyroclastic rocks in the northern Fuling area is the primary issue in the research. Based on the data of drilling， outcrop and seismic， the exploration potential of pyroclastic gas reservoir in the research area is discussed based on the petrological characteristics of the volcanic rocks， the reservoir characteristics of pyroclastic rock and the hydrocarbon accumulation characteristics of the Mao3 Member. The results show that： （1） The reservoir is controlled by lithofacies and tectono-diagenesis， and the reservoir space is a complex of pores and fractures. The stronger the karst landform and fracture transformation， the better the reservoir conditions. （2） The pyroclastic rocks of Mao3 Member are covered by stable lithologic association such as mudstone， limestone and tuff， which have the characteristics of low frequency， medium-weak wave peak at the bottom， intermittent and disorganized seismic reflection between layers， and the petrophysical characteristics of medium-high density and relatively high impedance. Combined with the controlling factors such as lithofacies， fractures， geomorphology and reservoir thickness， it is predicted that the favorable reservoir is mainly distributed in the northeast of the working area， with a thickness of 10-20 m and an area of about 73.7 km2. （3） The volcanic clastic reservoir in the Mao3 Member has the characteristics of “double source hydrocarbon control and near source accumulation”， high hydrocarbon generation intensity of natural gas， good spatio-temporal matching relationship of source cap， and great exploration potential， which can be used for old well retesting and exploration well deployment in the favorable reservoir area in the northeast of the study area.
Due to the thin thickness of the effective reservoir， strong heterogeneity and large lateral variation， it is difficult to predict the reservoir in fine detail， and the distribution pattern of the large-scale high-quality dolomite reservoir is unclear. Post-stack interpretation is often used in conventional prediction methods， but the seismic records covered by thin reservoirs cannot be identified due to resolution. To this end， a reservoir weak signal recovery method based on the elimination of strong interference effect is proposed. On the basis of high resolution inversion of seismic signal， sort the reflection coefficient， generate the reflection coefficient matrix and reverse the reflection coefficient； remove the interference influence of the strong reflection coefficient energy on adjacent strata to recover the weak reservoir signal， and the method is applied to the fine prediction of reservoir distribution in the northern slope of Sichuan Basin. This study shows that： （1） The seismic signal of the upper reservoir in Maokou Formation is affected by the interference of the overlying Longtan Formation， causing the reservoir signal to be blocked. The main frequency of seismic data， the thickness， average velocity and distance from the Longtan Formation are the “four elements” of the interference effect； （2） The weak signal recovery effect of the reservoir based on the reflection coefficient decomposition calculation is better than the matching tracing method. It can effectively reduce the influence of the strong wave peak formed by the overlying strata on the seismic signal imaging of the internal reservoir of Maokou Formation； （3） The reservoir prediction technology based on the reflection coefficient decomposition algorithm has certain advantages in the thin reservoir prediction， and the predicted results have a high coincidence rate. This research results can provide a reference for the prediction of Paleozoic carbonate reservoir in Sichuan Basin.
To elucidate the mechanism of supercritical CO2 （ScCO2） on the micro-porous structure of shale， this study focuses on the Chang 73 sub-member of the Yanchang Formation in the Ordos Basin. Based on organic geochemical and mineral composition analyses， low-temperature gas（CO2， N2） adsorption experiments and nuclear magnetic resonance （NMR） scanning methods are employed， combined with multi-scale fractal theory， to comprehensively analyze the changes in shale micro-porous structure and its multi-scale fractal characteristics. The detailed mechanisms by which ScCO2 affects shale pore structure are clarified. The study results show that after ScCO2 treatment， the total organic carbon （TOC） content of the shale samples decreases， the quartz content increases， while the contents of clay minerals and feldspar decrease. Compared to temperature variations， TOC and mineral components are more sensitive to pressure changes. Additionally， shale pores are mainly distributed in the micropore range （0-2 nm） and mesopore range （2-50 nm）， which contribute significantly to the specific surface area， while macropores （>50 nm） are fewer but significantly contribute to the total pore volume. After ScCO2 treatment， the total specific surface area of the shale samples decreases， while the total pore volume， average pore diameter， and effective porosity all increase. The total specific surface area and average pore diameter are more sensitive to temperature changes， while the total pore volume and effective porosity are more significantly influenced by pressure. Moreover， shale pores exhibit multi-scale fractal characteristics： micropores have a higher fractal dimension， while meso- and macropores have lower fractal dimensions. After ScCO2 treatment， the fractal dimensions at all scales decrease， indicating an improvement in the complexity of the shale pore structure. The fractal dimension of micropores is significantly positively correlated with TOC content， while the fractal dimension of meso- and macropores has a stronger correlation with quartz and clay mineral content. This indicates that changes in shale mineral characteristics are intrinsic factors affecting micro-porous structure， while ScCO2 treatment conditions are important external factors. The interaction of both determines the evolution of the shale pore structure. The research findings provide important scientific basis and practical guidance for the optimal selection of carbon capture， utilization， and storage （CCUS） target layers.
Marine-terrestrial transitional shale gas is another important strategic replacement resource after the commercial development of marine shale gas. Marine-terrestrial transitional shale has the characteristics of strong heterogeneity， rapid depositional phase change， and complex lithology combination. Geological theories of marine shale gas can not be fully applied to marine-continental transitional facies， and the controlling factors of shale gas enrichment in the marine and continental transition facies are not well understood， which restricts efficient exploration and development. Taking the Shan2 3 sub-member shale in the Daqi area of the eastern margin of the Ordos Basin as an example， the geochemical characteristics and reservoir characteristics of shale were investigated through experiments such as microscopic analysis， gas adsorption， high-pressure mercury intrusion， breakthrough pressure， diffusion coefficient， and overburden pore permeability. This research elucidated the controlling factors of shale gas accumulation in marine-continental transitional facies. The research results indicate that the Shan2 3 sub-member shale in the marine-continental transitional facies exhibits high organic matter abundance， high maturity， and predominantly humic-type characteristics. The pore types are mainly dominated by inorganic mineral pores， with relatively fewer organic pores and microfractures. The conclusion suggests that the enrichment of marine-continental transitional facies shale gas is primarily controlled by a combination of organic matter abundance， pore size， lithological composition， and structural evolution. High organic matter abundance enhances the adsorption capacity of shale， providing more adsorption sites for methane gas molecules in micropores. The combination of shale-coal and shale-ash favors the in-situ enrichment of shale gas. Stable tectonics and appropriate burial depth facilitate the preservation of shale gas. Furthermore， an evolutionary model for the storage and sealing capacity of marine-continental transitional facies shale gas in the Dagang area has been established. The above findings can provide geological theoretical guidance for sweet spot prediction and rapid development of pilot test areas for marine-continental transitional facies shale gas.
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