The Ordos Basin is the home to China's largest grand tight sandstone gas field and the country's largest grand helium-rich gas field， Dongsheng Gas Field. It is also the first discovered great tight sandstone helium-rich gas field in China. This paper used geochemical methods to investigate the fluid inclusion， combined with the analysis of the gas isotopic composition of the present gas field， and revealed the ancient and modern natural gas geochemistry， accumulation evolution and geochemical characteristics of helium. The results showed that the current gas field was dominated by hydrocarbon gases， and the methane content was mostly from 90% to 95%. Meanwhile， the natural gas δ13C1， δ13C2， δ13C3， and δ13C4 in the current gas field were from -36.5‰ to -28.7‰，-25.3‰ to -22.1‰，-27.0‰ to -21.8‰，-25.6‰ to -20.7‰， respectively. And the distribution ranges of δ13C1， δ13C2 and δ13C3 in gas fluid inclusions were from -42.6‰ to -24.6‰，-32.7‰ to -18.0‰，-27.6‰ to -15.1‰， respectively. The contents of hydrocarbon gas and non-hydrocarbon gas in the fluid inclusions ranged widely， and mainly originated from kerogen thermal degradation in the source rock. The hydrocarbon gas in the inclusions contains natural gases captured from three stages： （1） natural gases with low carbon isotope generated from lower mature source rocks during shallow burial； （2） At the maximum deep burial stage， natural gases with higher carbon isotope are discharged from higher mature source rocks； （3）during the uplift stage， the natural gases with low carbon isotope are secondarily adjusted and captured by natural gases pre-accumulated in the early stages. This study comprehensively indicated that during the large-scale generation， discharge and charging process of natural gas， the Paleozoic coal-derived tight sandstone gas migrated mainly in the vertical direction with some lateral direction. When entering the tight sandstone and during the secondary migration and secondary adjustment of the later stage， the general trend of short distance migration was from south to north and west to east. Tight sandstone gas fields generally contain helium， and the overall resource scale is huge， ranking first in China. Helium gas has a typical crustal origination and shows diffusive distribution with local enrichment. The distribution and enrichment of helium gas are controlled by the combination of ancient U/Th-rich basement rock series and deep large faults.
Recent exploration has found that helium contents are higher in the Upper Paleozoic natural gas in Dongsheng Gas Field of Hangjinqi area in Ordos Basin， being more than 0.1% in many gas wells， which have good values for industrial development. Based on the systematic analysis of natural gas geochemical characteristics， it is found that hydrocarbon gases dominate the Upper Paleozoic natural gas in Dongsheng Gas Field， the helium contents vary from 0.045% to 0.487%， and helium occurs in a small fraction in a hydrocarbon- rich helium reservoir dominated by hydrocarbon gases. The contents of U and Th in the Upper Paleozoic coal measure source rocks are higher than those in the basement rocks， and the U and Th contents of quartz sandstone and quartzite are slightly higher than those in gneiss-granitic gneiss rocks in basement rocks in Hangjinqi area. It is found that the helium abundance of resolved gas in gneiss-granitic gneiss rock is higher than that in quartz sandstone-quartzite， and the helium abundance of resolved gas in coal-measure source rocks is much lower than that in basement rocks， which is mainly due to the dilution of large-scale hydrocarbon gas generated and the short geological time. The content of helium generated by the hydrocarbon source rocks is low， so it cannot form the industrial value of helium reservoirs. The 3He/4He ratios of the Upper Paleozoic natural gas of Dongsheng Gas Field vary from 1.83×10-8 to 6.25×10-8， and it is found that helium originated from the Archaean-Middle Proterozoic metamorphic rock-granite series with basement. The natural gas occurred with helium is typical coal-type gas， which comes from Carboniferous-Permian coal-measure source rocks， and has different sources from the associated helium gas. The hydrocarbon gas and the associated helium have the characteristics of different sources and the same reservoir. In the process of migration， accumulation and reservoir formation， the associated helium gas has a good space-time configuration with the reservoir， cap， trap and migration-accumulation combination of the Upper Paleozoic conventional natural gas reservoir. The helium enrichment in Dongsheng Gas Field is mainly controlled by the lithofacies development of basement metamorphic rock- granite series and the distribution of deep and large faults. Helium mainly distributes at the intersection of the second-order fault communicating helium source rock and the fourth-order fault being the transport system and the Archaean-Middle Proterozoic basement lithofacies development area. The distribution of the fault zone controls the spatial distribution of natural gas and its associated helium gas reservoirs.
Based on the relative content and isotopic composition characteristics of helium in natural gas of multiple structural units in Tarim Basin， combined with geological background and typical helium rich gas reservoirs， the geochemical characteristics and favorable exploration areas of helium in the basin are analyzed. The results show that helium rich natural gas in the Tarim Basin is well displayed， and the helium rich natural gas in platform area is better than that in foreland area. The helium in the Tarim Basin is mainly of crust origin. There is a positive correlation between the relative content of helium and nitrogen in the natural gas of the Tarim Basin， to a certain extent. When the relative content of nitrogen in the gas reservoir is high， the relative content of helium is also high. The general survey of helium exploration in Tarim Basin can focus on the gas reservoirs with high relative content of nitrogen. Helium may migrate mainly in the form of water-soluble in geological bodies， and its accumulation， possibly， are closely related to formation water. Influenced by the difference of partial pressure of different substances in formation water， combined with the Henry's law analysis， formation water can play the role of “helium pump” in the process of helium-rich natural gas accumulation. Good transport system and high-quality helium source are the basis for the formation of helium rich gas reservoirs. Many blocks in Tarim Basin have good helium rich natural gas display， especially in structural units such as the Shaya Uplift， Katake Uplift， Maigaiti slope and Bachu Uplift. This series of structural units has good helium source and natural gas accumulation conditions， which is conducive to the accumulation of helium rich natural gas. This series of structural units is the preferred block for helium exploration in the Tarim Basin. The Shunbei area has a good prospect for natural gas exploration， and this structural unit is a key block for marine natural gas exploration during the “14th Five-Year Plan”. Therefore， helium exploration and general survey in this area should be strengthened.
Helium is a kind of rare resource， which is related to national security and the development of high-tech industries. Based on summarizing the previous achievements and the analysis of helium content and isotopes of various gas fields in the northern margin of Qaidam Basin， the paper tries to explore the helium accumulation conditions in conventional natural gas reservoirs. The results showed that the helium content in Dongping Gas Field ranges from 0.012% to 1.07%， with an average of 0.24%. The contribution ratio of mantle helium in natural gas is calculated by using the crust-mantle binary mixing model. It is found that the contribution ratio of mantle helium is between 0.01% and 0.84%， with an average of 0.3%； the R/Ra value is between 0.003 5 and 0.059 2， with an average of 0.022 6， showing a crust-derived helium. The helium content in this area is negatively correlated with the CH4 content and positively correlated with the N2 content. The internal relationship between the two needs further study. Abnormal high temperature disturbance is the key to the primary migration of helium in bedrock in this area. Formation water is an effective carrier for helium enrichment in Mabei Gas Field and Niudong Gas Field， and the carrier for helium enrichment in Dongping Gas Field needs further research. The crust-derived helium was sourced from the granite and gneiss with rich uranium and thorium on the basin basement. The deep faults of the northern Qaidam Basin and the unconformity have provided good conditions for the migration of helium-bearing fluid. There are two types of caprocks， one of which is the anhydrite-bearing mudstone cover and mudstone cover above the basement， and the other is a top-sealed partial cover which is mainly formed by salty water seepage. These favorable reservoir forming conditions provide a good geological basis for helium enrichment in this area. According to the volume method， the helium resource in this area is about 27×108 m3， and the average of 4He content produced by per gram of rock for every year is about from （12.61~121.95）×10-20 m3， with an average of 48.81×10-20 m3。
Dongping granite in the northwest margin of Qaidam Basin is of great value in regional geological evolution research and resource development. Therefore， the geochemical characteristics and zircon U-Pb chronology of Dongping granite are analyzed. Dongping granite can be divided into two types. The first type is Adakitic granite with high Sr and low Yb， which has the geochemical characteristics of Adakitic rock. There are garnet， rutile and amphibolite residues in the source area， and no or only a small amount of plagioclase residue. It is speculated that the rock may be formed by partial melting of rutile-bearing eclogite in the thickened lower crust， with melting depth greater than 50 km. The other type is Himalayan type granite with low Sr and low Yb， and the residual facies in the source area are garnet， amphibolite and plagioclase， which may be formed by partial melting of granulite in the thickened lower crust， and the melting depth is about 40-50 km. LA-ICP-MS zircon U-Pb dating results show that the crystallization age of the two types of granites is about 418 Ma， which is the result of the same magmatic activity， and the tectonic discrimination diagram shows that both types of granites belong to the same collision granite. Based on previous research results， this paper believes that at about 418 Ma， the “Southern Altyn Ocean” has been closed， and turned to the stage of intracontinental subduction collision orogeny， resulting in the mutual overlap and thickening of the crust. As the thickened lower crust is heated by the underplating of mantle materials， partial melting occurs at different depths， forming two different types of granites in Dongping. The origin and content of helium in Dongping and its adjacent Jianbei and Niudong gas fields are closely related to the basement granite. Specifically， helium is mainly produced by the decay of U， Th and other radioactive elements in the basement granite and granite gneiss. The longer the basement granitoid body is formed， the higher the helium content in the gas field. Under the background of collision orogenic structure， the granite formed by partial melting of pure crustal materials has higher U and Th contents and stronger helium generation potential.
Helium is a scarce strategic resource widely used in military， aerospace， medical and other high-tech industries； however， the world's helium resources are extremely unevenly distributed， and helium-poor countries such as China are facing serious helium supply security problems. Through a combination of research literature and experiments， we analyzed the world's most mature helium producing gas field （Panhandle-Hugoton Gas Field in the United States） and the Dongping Gas Field， which has a high potential for helium production in China， and explored the reservoir formation elements and helium enrichment mechanisms of both. The results show that both are helium-rich reservoirs with natural gas as the carrier gas and good reservoir properties， both are capped by low-permeability evaporites， and both have undergone multiple phases of tectonic alteration. Both reservoirs have experienced multiple phases of tectonic alteration. The differences between the two types of reservoirs lie in the fact that the former is older in granite formation， especially the granitic material rich in carbonate formations， which is concentrated in the Proterozoic. The former granites are older in age， especially the granitic material in the carbonate strata， which is concentrated in the Proterozoic， and is on average 400 Ma earlier than the granites in Dongping Gas Field. In the former， the helium transport pattern is biased toward a single saturated groundwater desolvation release， while in the latter， there is an “extraction” release of natural gas from the ancient granite reservoirs that “passed through”. By contrast， it is proposed that the formation of ancient high U and Th helium source rocks， sufficient carrier gas， good reservoir and cap properties， and sufficient groundwater （marginal water） are involved in the helium release. The helium-rich bedrock reservoirs are required for the formation of helium-rich rocks with high U and Th source rocks， sufficient carrier gas， good reservoir and cap properties and sufficient groundwater （bottom water） as a mediator for helium transport.
Helium is a scarce strategic resource and plays an important role in national security and high-tech fields. Songliao Basin， as one of the large continental petroliferous basins in China， has a good display of helium resources. However， there is no clear understanding of the source of helium， especially lack of systematic summary of its enrichment rule. Hence， the characteristics of helium source and distribution in Songliao Basin are summarized in this paper. Combined with tectonic evolution and magmatism， the migration and enrichment rules are preliminarily analyzed， and the favorable exploration areas of helium-rich gas reservoirs are predicted. The results show that the Songliao Basin is mainly composed of mantle-derived helium and crust-derived helium， and the contribution of mantle-derived helium is different in the north and south. The basement and peripheral granite， deep and large faults and favorable cap beds are the most important factors affecting helium enrichment. The central and southern parts of the basin are favorable areas for mantle source helium accumulation， the southeast is favorable area for crust source helium accumulation， and the central depression and southeast uplift are favorable areas for crust mantle mixed source helium accumulation. The results have guiding significance for efficient exploration and development of helium gas in Songliao Basin.
Helium is listed as an important strategic resource due to its unique physical and chemical properties and scarcity. At present， China's helium resources are in great demand and highly dependent on foreign countries. It is urgent to strengthen the exploration and research of helium resources. This paper takes the shale （gas） of the Sinian Doushantuo Formation and the Cambrian Niutitang Formation in the Qiannan Depression as the research object， and studies the origin and source of helium in shale gas by means of gas composition and carbon isotope， noble gas composition and isotope ratio， major and trace element analysis of shale. The results show that the shale gas in Doushantuo Formation and Niutitang Formation is dominated by N2， followed by CH4， with He content ranging from 1 533.1×10-6 to 2 323.7×10-6， and 3He/4He ratio ranging from 0.009 Ra to 0.010 Ra（Ra=1.4×10-6）， indicating that crustal source helium is the main source. The ratios of 21Ne/22Ne and 40Ar/36Ar in shale gas samples are higher than the corresponding air values， indicating that there are different proportions of 21Ne and 40Ar added from crust sources. The measured value of crust derived radiogenic 4He/40Ar in the studied gas sample is close to the value of crust derived radiogenic 4He/40Ar （3.09-6.00）， but far less than the theoretical calculated value of rock derived in-situ radiogenic 4He/40Ar， which indicates that there may be rare gases from the upper crust or air during the rock resolving that dilute the 4He and 40Ar of rock derived in-situ radiogenic. Based on the theoretical calculation of in-situ helium generation rate and crustal helium release flux in shale intervals， the in-situ accumulation time and crustal recharge time of 4He in shale gas samples are far less than the sedimentary age of corresponding strata， indicating that tectonic movement caused the in-situ helium loss in the process of formation deposition.
Helium is the inert gas with the lowest known melting and boiling points. It has extremely special physical properties and is widely used in many high-precision fields. However， with the increasing demand for helium in China， the rising price of imported helium， and the tightening of export policies of major foreign helium-producing countries， China is facing a serious situation of helium resource security. The helium-rich basins in China are widely distributed horizontally and have obvious zoning characteristics， with significant differences between the eastern and central-western helium-bearing basins. The eastern helium-bearing basins have crustal helium and mixed crust-mantle helium with the latter predominating， and is mainly controlled by tectonic factors and mantle-derived fluids， while the central and western helium-bearing basins are dominated by crustal helium. The helium is distributed in all stratigraphic ages vertically， and the helium content has the characteristics of “high at both ends and low in the middle”. The role of helium source rocks， tectonic factors， transport carriers， and capping conditions in helium reservoir formation is clearly identified， and the helium reservoir formation patterns in the eastern and central-western helium-bearing basins in China have been summarized accordingly. According to two groups of mutual constraints in helium accumulation： the mutual constraints of gas reservoir generation and tectonic activity， it is pointed out that “flux difference” is the key parameter for helium accumulation. The following suggestions are made for the exploration and development of helium resources in China： Resource assessment work should be carried out around the eastern basins with the mixing of crustal and mantle resources， the construction of helium separation and extraction equipment should be accelerated for the central and western helium-bearing fields to promote the industrial development of helium resources， and the importance of helium resource assessment work in unconventional gas reservoirs should be increased.
Recently， the exploration and research of helium resources have received much attention in China， but few researches about the accumulation conditions and mechanisms of crustal-derived helium-rich gas reservoirs have been reported. The physical and chemical properties and source conditions of helium have determined its strong uniqueness in generation， release， migration， accumulation and preservation stages. Helium is generated by radioactive decay of uranium and thorium in ancient rocks. Most of helium dissolves in subsurface fluids after releasing from the helium generating minerals， and then migrates as water-soluble or gas-soluble phase. Based on the migration and accumulation processes of helium and the characteristics of carrier fluids， three main accumulation models of helium were classified： （1） helium degases from groundwater， （2） independent gas phase extract helium， （3） gases degas from mixed fluids. Analyses and summaries of the accumulation characteristics of representative helium-rich gas reservoirs worldwide showed that two types of helium accumulation processes were developed， i.e.， episodic and continuous accumulations. The Panhandle-Huguton Gas Field in the United States has the characteristics of continuous accumulation. Helium was mainly dissolved and accumulated into the early hydrocarbon gas reservoirs over a long period of time. Whereas， the Rukwa Basin in Tanzania， the Weiyuan Gas Field in the Sichuan Basin and the Hetianhe Gas Field in the Tarim Basin show the characteristics of episodic accumulation. The formation of these helium-rich gas reservoirs were mainly controlled by tectonic activities. Helium and carrier gases simultaneously charged into these reservoir. Moreover， three key accumulation conditions for crustal-derived helium-rich natural gas reservoirs were proposed： （1） stable ancient basements and sufficient helium sources； （2） thermal events and tectonic activities that promote helium releasing， carrier fluids and tectonic activities that facilitate helium migration and accumulation； （3） carrier gas reservoirs that exist previously or formed together with helium.
Helium is a valuable strategic resource for the development of certain high-tech industries and national defense industries. In recent years， the research on helium in conventional natural gas reservoirs has attracted increasing attention. However， the research on the richness， distributions and origin of helium in unconventional reservoirs， such as shale gas and coalbed methane （CBM） is still in its infancy. This paper systematically sorts out the research literature on helium in unconventional reservoirs around the world， summarizes the contents and distributions of helium in shale gas and CBM across the world， and clarifies the isotopic characteristics of helium and other rare gases. In addition， the source and origin of helium in shale gas and CBM are resolved， and the main controlling factors of helium enrichment are uncovered. The research data shows that the helium contents contained in shale gas range from（0.12-3 100）×10-6， with an average of 378×10-6. The helium amounts of CBM range from （0.04-19 000）×10-6， with an average of 816×10-6. The helium concentrations in shale gas and CBM are generally lower than that in conventional gas reservoirs in the same area. This may be due to the long secondary migration distance of conventional natural gas enables more external helium to be captured. The 3He/4He values in shale gas and CBM are generally low， indicating that crustal radiogenic helium is the main source. The helium and neon isotope characteristics show that the contributions of mantle helium and atmospheric derived helium in shale gas and CBM are generally less than 5%. The calculation show that the supplement of external helium is essential for the formation of helium-rich shale gas and CBM plays. Superior helium source conditions （uranium （U） and thorium （Th） abundances， the sedimentary age， volumes and gas contents of shale and coal beds， etc.）， effective transport conditions， and good preservation conditions are important influencing factors for helium enrichment in shale gas and coalbed gas.
The knowledge of the primary migration of helium is key for understanding the accumulation process of helium. In this review， the diffusion and release of helium in minerals and the controlling factors are systematically reviewed. Helium generated by the α decay of radioactive U and Th would undergo interstitial migration， combines with the vacancies， and accumulates to helium bubbles， which promotes the release of helium. Helium can also be released through fission tracks induced by α decays. The difference in the composition and crystal structure of the minerals leads to different ability for storing helium. High temperature facilitates the diffusion of helium and the growth of helium bubbles， benefiting helium release. Fracturing and the transformation of minerals can also cause the rapid release of helium. Elucidating the processes of the primary migration of helium and the controlling factors should be of significant importance for understanding the source， migration and accumulation of helium reservoir.
Solubility models of CH4， CO2 and noble gases are widely used in earth sciences. Their solubility models play important roles in studying of homogenization pressure of inclusions， temperature variations in the past， gas migration and accumulation laws， helium gas pool formation， and volumetric ratio of natural gas and groundwater. Here， solubility models of CH4， CO2 and noble gases in pure water and aqueous NaCl solution is reviewed. These models with high precision and wide applicability are emphatically introduced， including： （1） CH4 solubility model in 0-250 ℃， 0.1-200 MPa and 0-6 mol/kg NaCl solution； （2） CO2 solubility model in 0-450 ℃， 0.1-150 MPa and 0-4.5 mol/kg NaCl solution； （3） Models for calculating solubilities and Henry’s constants of atmospheric noble gases at 0-80 ℃； （4） Models for calculating Henry’s constants of noble gases in pure water； （5） Solubility models of noble gases in 0.1 MPa， 0-65 ℃ and 0-5.8 mol/kg NaCl solution. Some calculated values by solving model equations are presented. The solubility models of CH4 and CO2 are more complex but highly accurate， and their application range is wide. However， the solubility models of noble gases have relatively low accuracy and narrow applicability， and thus need to be corrected. In noble gas-CO2-H2O system， there was no difference between measured noble gas partitioning and that predicted in pure noble gas-water system at low CO2 density. However， at high CO2 density， partition coefficients express significant deviation from pure noble gas-water systems. Currently， it is difficult to accurately evaluate respective solubility of CH4， CO2 and noble gases in their mixture， the solubility model of CH4 -CO2-noble gases needs to be further studied.
Tarim Basin is an important natural gas exploitation basin in China and there are also many helium gas pools with industrial extraction value. However， the research on helium generation potential in this area is still in a blank stage. The helium production of rocks per unit time and per unit mass from the Ordovician in the Gucheng area are as follow： mudstone> muddy limestone> high-uranium dolomite > dolomite > limestone. Based on the natural gamma spectroscopy logging data， a set of helium resource evaluation procedure and method are proposed. We take 10 drilling wells from the Gucheng area as a case study. The results show that the helium generation rate of Ordovician strata in the Gucheng area is 1.5×10-10 cm3/g， and the helium generation is 13.91 km3. The area of high helium generation rate is located in the Well Gucheng 14 zone， followed by the Well Gucheng 12 zone. Wherein， the helium generation rate of mudstone in the Queerqueke Formation is the highest， accounting for 83.5% of the total helium generation， which indicate that Queerqueke Formation is the main source of helium. But it is estimated that the helium content in natural gas is about 0.033%， which is lower than the industrial mining standard. The reason is that the organic matter-rich mudstone produced a large number of hydrocarbons that dramatically diluted the concentration of helium. Therefore， caution should be exercised when evaluating helium resources on hydrocarbon source rocks. The establishment of helium evaluation method based on natural gamma spectroscopy data promotes the development of helium estimation in petroliferous basins and provides new ideas for the exploration and survey of helium resources in China.
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