In the Ordos Basin,the Ordovician Majiagou Formation is an important replacement for natural gas exploration.Large-scale natural gas resources,such as those in the Jingbian gas field,have been discovered in the weathered crust at its top.However,the origins and sources of these resources remain highly controversial.In this study,we analyze the components and geochemical characteristics(e.g.,carbon and hydrogen isotopic compositions)of natural gas from the Daniudi gas field.Accordingly,the origins and sources of natural gas are clarified within the Ordovician weathered crust reservoirs.The results indicate that the natural gas exhibits dryness coefficient(C1/C1-5)values ranging from 0.911 to 0.979 and CO2 content from 1.68%to 14.39%.H2S is observed in some samples,with content varying from 0.07×10-6 to 3.37×10-6 and gas sweetness index(GSI)values of merely 10-7 to 10-6 order of magnitude.Additionally,the δ13C1,δ13C2,δ13C3,and δD1 values of natural gas range from-40.0‰ to-33.2‰,-35.6‰ to-24.9‰,-30.7‰ to-24.1‰,and-209‰ to-184‰,respectively.The natural gas in the Ordovician weathered crust reservoirs did not undergo significant thermochemical sulfate reduction(TSR).However,CO2 of inorganic origin,produced by the dissolution of carbonate reservoirs,was mixed into some samples during acid fracturing.Origin identification and gas-source correlation reveal that the natural gas in the weathered crust reservoirs is dominated by coal-derived gas generated from coal-measure source rocks in the Taiyuan Formation.Such coal-derived gas experienced significant fractionation during both vertical and lateral migration into the weathered crust reservoirs.Additionally,the gas reservoirs also contain a minor proportion of oil-associated gas formed by the secondary cracking of crude oil generated from the carbonate source rocks in the Majiagou Formation.

Helium, vital for advanced technologies, is generated in the crust via uranium (U) and thorium (Th) decay. The crystalline basement is a proven source of helium. In contrast, the role of sedimentary rocks, while often considered significant, has lacked definitive proof. This study provides the isotopic evidence to deconvolve these two crustal sources and verify the significant role of sediment. Our analysis of the Dongsheng Gas Field, Ordos Basin, China, reveals that air-derived noble gases reflect open-system water-gas equilibration, a process occurring within the reservoir. Whereas crust-derived noble gases likely pre-aggregated and migrated together with hydrocarbons, thereby preserving a clear signature of their original sources. Within this preserved signature, a depletion of Ne-21* (where * denotes a crustal origin) is observed. This depletion produces an unusual negative correlation between Ne-21*/Ar-36 and Ar-40*/Ar-36, in contrast to the expected positive trend. This pattern is interpreted as the mixing of a fluid from coal-bearing strata (Ar-40*/Ar-36 = 130 +/- 10) with a Ne-21*-depleted fluid from the basement (Ar-40*/Ar-36 = 2250 +/- 150). Modeling suggests that only similar to 0.26 % of basement-generated Ne-21* is retained. We propose a helium accumulation model in which initial preservation was inefficient owing to the absence of a seal, whereas the main accumulation was driven by hydrocarbon charging, which acted as a carrier fluid stripping helium from coal-bearing strata. These results demonstrate that U- and Th-rich sedimentary rocks can serve as a potential helium source in natural gas, which refines exploration strategies for helium in cratonic basins.

The Ordovician Majiagou Formation (O1m) in the Ordos Basin is a crucial exploration field for natural gas, and exploration of the Ordovician middle assemblage (O1m5 5-7) has recently yielded great breakthrough. The Daniudi gas field provides a good case study to determine the gas source for the strata. The O1m5 5-7 gas displays C1/C1-5 ratios of 0.932-0.985 and CO2 contents of 1.56%-11.75%, and the detectable H2S content ranges from 0.0002% to 1.8617%. The delta 13C1, delta 13C2, delta 13CCO2, and delta D1 values are -39.7 parts per thousand to -35.6 parts per thousand, -30.4 parts per thousand to -23.7 parts per thousand, -12.4 parts per thousand to -4.6 parts per thousand, and -204 parts per thousand to -185 parts per thousand, respectively. Identification of the gas origin and source indicates that the gaseous alkanes are commonly coal-derived gas. The gas was generated from the coal measures in the Taiyuan Formation (C3t) and subsequently migrated. A small amount of oil-associated gas, mainly from O1m carbonate source rocks, has been incorporated into the gas reservoir. The natural gas has experienced insignificant alteration by thermochemical sulfate reduction, and the relatively high levels of CO2 are probably associated with corrosion alteration of carbonate reservoirs by injected fluid during acid fracturing.

Helium and natural hydrogen resources in the subsurface have attracted substantial attention due to their essential roles in contemporary society. Globally, accumulations of these gases were predominantly found in the shallow crust; however, the impact of the multi-spherical interactions within the Earth, particularly crust-mantle interaction, on their generation-accumulation in tectonomagmatically active sedimentary basins remains unexplored. In this study, we have compiled 806 global gas data coupled with volcanic rock degassing experiments for the purpose of evaluating how the crust-mantle interaction influences the generation-accumulation of helium and hydrogen in the Bohai Bay Basin, China. In the Bohai Bay Basin, helium is primarily sourced from the decay of radioactive elements in the basement rock, with mantle-derived helium providing an additional contribution that averages at 18.85%. The formation of helium-rich reservoirs in natural gas systems, primarily driven by groundwater degassing dynamics, exhibits critical dependence on the gas-to-water ratio. Natural hydrogen is predominantly generated through low-temperature water-rock reactions in iron-rich rocks, radiolysis of water, and mantle-derived hydrogen-rich fluids. The subduction of the west Pacific Plate has induced the upwelling of asthenospheric materials, which transport deep mantle-derived volatiles (He, H2, CO2, N2, etc.) into the Bohai Bay Basin via deep-seated faults. As a result, helium and hydrogen resources are widely distributed yet locally concentrated in the Bohai Bay Basin. Our calculation reveals that the crust of the study area has cumulatively produced 68.0 & times; 109 m3 STP of helium and 23.1 & times; 1012 m3 STP of hydrogen, while the mantle has contributed 16.0 & times; 109 m3 STP of helium and 10.3 & times; 1012 m3 STP of hydrogen. The natural gas system in the Bohai Bay Basin has trapping efficiencies of 2.9% and 0.07% for these helium and hydrogen resources, respectively. A calculated maximum abiotic CH4 resource of 3.74 & times; 1012 m3 STP may be generated by the potential Fischer-Tropsch Type reactions. Such a large potential abiotic CH4 resource explains the very low trapping efficiency of hydrogen.

The origins of natural hydrogen in natural gas systems of sedimentary basins and the capacity of these systems to store hydrogen remain inadequately understood, posing crucial questions for the large-scale exploration of natural hydrogen. This study reports on the natural gas composition, stable carbon and hydrogen isotopic values, and helium isotopic values of gas samples collected from the Qingshen gas deposit within volcanic rocks of the Songliao Basin. Natural hydrogen primarily originates from water radiolysis, water-rock interactions (WRI), and mantle. The Qingshen gas deposit contains 95.23 × 10 9 cubic meters of abiotic CH 4 , of which 15.24 × 10 9 cubic meters was generated through hydrogen conversion via Fischer-Tropsch synthesis, with the maximum original hydrogen reserves calculated to be approximately 61.9 × 10 9 cubic meters. We estimated that the study area has generated a maximum total of 572 × 10 9 cubic meters of radiolytic hydrogen, 248 × 10 9 cubic meters of WRI hydrogen, and 127 × 10 9 cubic meters of mantle-derived hydrogen.

Natural gas from ancient strata in sedimentary basins tends to be enriched with helium (He). Lower Paleozoic gas in the Daniudi field (DNDF) of the Ordos Basin in central China provides a good case study to reveal He generation and accumulation mechanisms in deep strata. Understanding He accumulation in the Daniudi field is crucial because the increasing global demand for He requires the identification of new sources in existing natural gas reservoirs. Here, the geochemical characteristics of Lower Paleozoic gas from the DNDF are investigated, aiming to reveal the concentration, origin, and controlling factors of He accumulation. Natural gas in Lower Ordovician Majiagou Formation (O1m) reservoirs is He-depleted (0.0126 %-0.0187 %, mean 0.0153 %). The 3He/4He and corresponding R/Ra ratios range from 3.12 to 3.95 x 10- 8 and 0.022 to 0.028, respectively, indicative of a crustal origin of He. The He concentration was diluted by alkane gases, as indicated by a negative correlation between He content and the CH4 content, as well as the dryness coefficient; however, it was not affected by the degree of thermal maturity or the type of organic matter in the source rocks, as indicated by a lack of correlation between He content and the carbon isotopic ratios of CH4 and C2H6. The N2/He ratio of O1m gas is generally in a range from 1.14 to 7.65, which is markedly lower than the ratio (19-50) in natural gas with He commercial production value (He% >= 0.1 %). The He content in O1m gas is negatively correlated with the content of total dissolved solids (TDS) in formation water; the low He content may be attributable to the high TDS content. The pressure in Paleozoic strata of the DNDF has decreased since the Late Cretaceous due to continuous regional uplift, resulting in decreased He solubility in the formation water and the exsolution of previously dissolved He into gas pools.

Although the productivity of modern volcanic soils is well established, the fertilization effects of ancient volcanic ash on aqueous ecosystems remains contentious. Here we demonstrate volcanic fertilization effects on a Late Triassic lacustrine ecosystem based on micropaleontological and geochemical records from the Yanchang Formation of North China. Frequent eruptions of a regional volcanic arc system increased cyanobacterial populations and organic carbon sinking fluxes, as recorded by extreme total organic carbon content (>30 wt.%) and positive organic carbon and negative nitrogen isotopic excursions. In turn, high levels of primary productivity induced intense water-column anoxia, facilitating preservation of organic matter. These findings underscore the potential influences of volcanism on ecological conditions, primary productivity, and carbon sequestration throughout geological history.

The Lower Cambrian organic-rich mudstones preserve extensive records of microbial life, potentially contributing to the 'Cambrian explosion' (similar to 540 to 520 Ma) and the formation of ancient petroleum resources. However, the link of organic delta C-13 variations to microbial communities in the Lower Cambrian Formation within the Tarim Craton, part of the Central Asian Orogenic Belt, remains incompletely understood. Here, we conducted a comprehensive biomarker and delta C-13 analysis of mudstone extracts. The prevalence of eukaryotic organisms relative to prokaryotes, together with increased primary productivity, results in substantial C-12 enrichment in organic matter, with the converse relationship being equally valid. The Lower Cambrian mudstones formed in more reducing yet confined stratified environments. The presence of cholestane, ergostane, stigmasterane, and 4-methylsterane (e.g., dinosterane) indicates the occurrence of red algae, prasinophytes, green algae, and dinoflagellates, respectively. Notably, the predominance of ergosterane and stigmasterane over cholestane suggests a greater contribution from green algae over red algae. The even-carbon preference across the C-14-C-20 range is indicative of limited contributions from Gloeocapsomorpha prisca. The presence of 3 beta-methylhopane, 7-+8-monomethylheptadecane and 2 alpha-methylhopane points to contributions from methanotrophs and cyanobacteria. Since methanotrophs are active in an oxygen- and sulfate-depleted environment, we speculate that C-13-rich organic matter with a high value of 3-methylhopane index (3-MHI) may be deposited in waters with expanded anoxic zones and a scarcity of photoautotrophic sulfur bacteria (e.g., Chlorobiaceae and Chromatiaceae). This study highlights the constraints of microbial communities on organic delta C-13 variations and enhances our understanding of the evolution of ecology and primary productivity during the Precambrian-Cambrian transition.

Occurrence and abundance of molecular hydrogen in natural geologic reservoirs are enigmatic, due to its various sources, diverse migration pathways and complicated biological and chemical reactions. Natural gas samples containing hydrogen from producing wells in several sedimentary basins in China were collected in this study, and gas abundances and isotopic compositions of these gases were compared with those in global petroliferous basins and deep intrusive rocks. Several geochemical indicators were suggested for identifying sources, migration and accumulation mechanisms of hydrogen in the subsurface environment. Hydrogen contents in natural gas deposits have contributions from various sources with the following high-to-low order: microbial degradation > serpentinization > deep mantle volatile release > radiation-induced water decomposition > thermal cracking of organic matter. A hydrogen-rich reservoir in Kansas, USA, is specifically analyzed to determine its formation mechanism. This study suggests that future exploration of geological hydrogen resources may focus on the igneous rock bodies with overlying dense sedimentary rocks in the continental rift systems.

Accurate identification of natural gas origin is fundamental to the theoretical research on natural gas geosciences and the exploration deployment and resource potential assessment of oil and gas. Since the 1970s, Academician Dai Jinxing has developed a comprehensive system for natural gas origin determination, grounded in geochemical theory and practice, and based on the integrated analysis of stable isotopic compositions, molecular composition, light hydrocarbon fingerprints, and geological context. This paper systematically reviews the core framework established by him and his team according to related references and application results, focusing on the conceptual design and technical pathways of key diagnostic diagrams such as delta C-13(1)-C-1/(C-2+C-3), delta C-13(1)-delta C-13(2)-delta C-13(3), delta C-13(CO2) versus CO2 content, and the C-7 light hydrocarbon ternary plot. We evaluate the applicability and innovation of these tools in distinguishing between oil-type gas, coal-derived gas, microbial gas, and abiogenic gas, as well as in identifying mixed-source gases and multi-stage charging systems. The findings suggest that this identification system has significantly advanced natural gas geochemical interpretation in China, shifting from single-indicator analyses to multi-parameter integration and from qualitative assessments to systematic graphical identification, and has also exerted considerable influence on international research in natural gas geochemistry. The structured overview of the development trajectory of natural gas origin discrimination methodologies provides a technical support for natural gas geological theory and practice and offer a scientific foundation for the academic evaluation and application of related achievements.

Helium, a scarce strategic resource, and hydrogen, a zero-carbon clean energy source, are abundantly stored in sedimentary basins and are essential components of underground energy systems. However, comprehensive studies on their occurrence, resources, and exploration prospects in tectonically active sedimentary basins remain limited. Herein, helium and natural hydrogen resources in the Bohai Bay Basin in China, a tectonically active sedimentary basin, were investigated through comparisons with tectonically stable basins such as the Hugoton-Panhandle gas field and Mali. An average helium content of 372 ppm and an average natural hydrogen content of 0.34 % were identified in the natural gas reservoirs of the Bohai Bay Basin. Radioactive element decay, water-rock interactions, and water radiolysis within Earth's crust, along with mantle-derived gas, contribute helium and natural hydrogen to the natural gas reservoirs of the Bohai Bay Basin. Our assessment revealed 2.46 x 109 m3 of helium and 22.4 x 109 m3 of natural hydrogen in the Bohai Bay Basin, of which the natural hydrogen resources are equivalent to 67.12 TWh. The Boxing subsag in the Bohai Bay Basin was identified as a promising exploration area for helium and natural hydrogen due to sufficient helium and natural hydrogen production and favorable reservoir-cap rock assemblages. Moreover, the Boxing subsag with developed iron-rich intrusive rocks can produce orange hydrogen through water injection, and this area is also considered as an ideal site for carbon capture and storage and underground hydrogen storage.

CO2 is one of the important non-hydrocarbon components in natural gas, and it is relatively stable in deep underground high-temperature and high-pressure environment. High contents of CO2 are found in many gas reservoirs around the world and there is more research on how this CO2 is formed and what are the typical geochemical characteristics, but they need to be further combed and summarized. Therefore, based on literature research and data statistics, this paper analyzes the variation characteristics of CO2 content and delta 13 C CO2 value in global gases. It is concluded that CO2 in natural gas reservoir mainly includes mantle-derived, thermal decomposition of carbonate rock, thermal decomposition of organic matter, microbial, hydrocarbon TSR secondary transformation and organic acid corrosion. According to the statistical results, CO2 contents associated with TSR in both shallow and deep reservoirs generally do not exceed 20%, whereas CO2 contents of mantle-derived origin are typically above 15%. Combined with the case studies in Songliao basin, China, it is proposed that inorganic CH4 is produced by Fischer-Tropsch synthesis of CO2 gas in deep fluid, which increases the contribution of inorganic CH4 to gas reservoirs. Combined with the case studies in Sichuan basin, China, the hydrocarbon is oxidized to non-hydrocarbon gases H2S and CO2 by TSR under the action of sulfate, and the precipitation of CO2 to calcite will also reduce the porosity of the reservoir. Future research could focus on analyzing the favorable conditions for the enrichment of high CO2 reservoirs, investigating the potential for CO2 storage in depleted oil and gas fields with similar geological settings, and leveraging evidence of CO2 reservoir formation to infer volcanic episodes or cycles. This could help enhance our understanding of paleoclimate changes and provide insights into the potential impact on contemporary climate patterns. Therefore, the in-depth understanding of underground natural CO2 behavior can provide a basis for the study of underground fluid processes, carbon cycling and greenhouse gas carbon sequestration.

Precious gas (helium and molecular hydrogen) is a critically strategic resource. Using genetic identification methods and geological-geochemical analysis, its formation and enrichment are classified into two types: tectonomagmatic active basins (Songliao and Bohai Bay Basins) and stable cratonic basins (Ordos Basin). In the Songliao and Bohai Bay Basins, the origin of precious gas is associated with crust-mantle mixing, primarily linked to mantle degassing, water-rock reactions in mafic-ultramafic magmatic rocks, and water radiolysis. In the Ordos Basin, precious gas is derived from crustal degassing: helium originates through radioactive decay in sedimentary and basement rocks, while natural hydrogen is formed from reactions of water and rock in crystalline basements, radiolysis of water, and thermal evolution of hydrocarbon source rocks. The favorable conditions for precious gas enrichment include sufficient gas flux (radioactive element content and decay duration, water-rock reactions in olivine- and pyroxene-bearing mafic-ultramafic igneous rocks), favorable migration pathways and tectonic positions, and effective seals (e.g., saline aquifers and evaporites). The enrichment process of precious gas is primarily controlled by generation timing, geological temperatures, and accumulation-dispersion efficiency. Accordingly, we identify potential enrichment formations for precious gas: the Shahejie Formation of Boxing Subsag (Bohai Bay Basin), Huoshiling Formation of the Changling and Xujiaweizi Fault Depressions (Songliao Basin), and Lower Shihezi and Majiagou Formations (Ordos Basin).

Drimane-type sesquiterpanes exhibit exceptional thermal stability and ubiquity across organic-rich sedimentary systems, from hydrothermal vents to crude oils, positioning them as critical molecular intermediaries in biomarker transformation pathways. The systematic GC-MS analysis of multi-phase crude oils from the Tarim Basin's ultra-deep reservoirs (depth > 7 km, temperature > 140 degrees C) identifies thermally-controlled molecular transformations. Cheilanthanes, particularly C-19 similar to C-23 homologues, exhibit greater thermal stability than hopanes. Drimane-type sesquiterpanes display source-dependent initial abundances and maturation-driven transformations: 4,4,8,8,9-pentamethyldecane (RD1) accumulates progressively, 4,4,8,9,9-pentamethyldecane (RD2) shows transient accumulation followed by depletion, while drimane (D) and homodrimane (HD) decline continuously. The transmethylation reaction under high thermal stress leads to RD1 enrichment, making it an ultra-stable end-member in the thermal maturation of C-15 drimane-type sesquiterpanes. Maturity indicators derived from trimethylnaphthalenes and methylphenanthrenes exhibit good correlations with thermal gradient, whereas dibenzothiophene-derived parameters demonstrate the weak correlation. In contrast, sesquiterpane-derived ratios display superior strong correlation with thermal gradient compared to aromatic hydrocarbon indices. The RD1/(RD1 + D + HD) ratio is proposed as an optimized molecular proxy for assessing thermal effects and phase behaviors in sedimentary basins. This study provides a potential tool for investigating the thermal effects of diverse geological processes, such as plate movement, earthquake slip, magmatic intrusion, and hydrothermal activity.

氦气是一种不可替代的战略性矿产资源,全球的商业性富氦气藏（He>0.1%）都是在油气勘探过程中偶然发现的.通过对全球75个富氦气田和1048个天然气样品统计分析发现,天然气中氦气普遍具有“稀”“伴”和“杂”的特征,富氦气田的埋深通常小于4500m. He-CH 4 和He-CO 2 气田的氦气含量明显低于He-N 2 气田（He>1%）.然而,前两种气田的氦气地质储量主要为10～7～10 11 m 3 量级,而后者仅为10～5～10～7m 3 量级.氦气与烃类气体在成因和运移方式存在显著的差异.氦气聚集以及长距离运移必须借助载体（地层水、烃类流体、N 2 、幔源流体等）,氦气的运移通道不局限在沉积地层,可延伸至盆地基底、下地壳,甚至岩石圈地幔.然而,氦气与烃类气体的成藏条件几乎可视为等同.烃类气体的存在不仅促进含氦流体中氦气快速解析和高效聚集,而且降低了氦气扩散性能、减少逸散通量.膏盐岩和厚层泥页岩作为盖层,有利于氦气在地质时间尺度下长期保存.大型富氦气田集中分布在古老克拉通盆地的隆起区及其周缘,几乎全部为壳源型.根据He含量与He/N 2 比值图版,壳源型氦气藏可划分为基岩供氦性、基岩-沉积岩联合供氦型和沉积岩供氦型.综合考虑中国的氦气品位、氦气资源禀赋、天然气工业化进程,以及目前的氦气提纯工艺,中、西部鄂尔多斯、塔里木、四川和柴达木盆地的富氦气田是氦气工业提取优先部署区域,部分（特）大型含氦气田可作为重要接替区域.

Helium is an irreplaceable strategic mineral resource, and commercial helium-rich gas fields (He>0.1

Based on the analysis of light hydrocarbon compositions of natural gas and regional comparison in combination with the chemical components and carbon isotopic compositions of methane, the indication of geochemical characteristics of light hydrocarbons on the migration features, dissolution and escape of natural gas from the Dongsheng gas field in the Ordos Basin is revealed, and the effect of migration on specific light hydrocarbon indexes is further discussed. The study indicates that, natural gas from the Lower Shihezi Formation (P1x) in the Dongsheng gas field displays higher iso-C5−7 contents than n-C5−7 contents, and the C6−7 light hydrocarbons are composed of paraffins with extremely low aromatic contents (<0.4%), whereas the C7 light hydrocarbons are dominated by methylcyclohexane, suggesting the characteristics of coal-derived gas with the influence by secondary alterations such as dissolution. The natural gas from the Dongsheng gas field has experienced free-phase migration from south to north and different degrees of dissolution after charging, and the gas in the Shiguhao area to the north of the Borjianghaizi fault has experienced apparent diffusion loss after accumulation. Long-distance migration in free phase results in the decrease of the relative contents of the methylcyclohexane in C7 light hydrocarbons and the toluene/n-heptane ratio, as well as the increase of the n-heptane/methylcyclohexane ratio and heptane values. The dissolution causes the increase of isoheptane values of the light hydrocarbons, whereas the diffusion loss of natural gas in the Shiguhao area results in the increase of n-C5−7 contents compared to the iso-C5−7 contents.

The combustion of hydrogen produces only water without releasing CO2, and thus hydrogen is considered to be the most environmentally friendly form of green energy. As a result, the potential of naturally-occurring hydrogen as a renewable carbon-free energy source has attracted a research boom amongst global communities in energy, science and technology. The "Hunt for natural hydrogen heats up" was selected as one of the "2023 breakthroughs of the year" in the journal Science. Direct extraction of natural hydrogen with economic values stored in economic quantities in geological bodies is probably the cheapest approach to produce hydrogen. Therefore, natural hydrogen is often called "gold hydrogen". The geographical and geological distribution of natural hydrogen is summarized in this study, and its mechanisms of formation are discussed, with a view to help assess the resource potential of natural hydrogen. The study indicates that natural hydrogen is widely distributed on Earth. Natural hydrogen seeps are mainly located at the structurally active belts such as mid-oceanic ridges, ophiolite belts in the convergent margins of the plates, magmatic active zones, hot spring areas, and structurally stable zones such as Precambrian continental basements. Hydrogen has been discovered in natural gas from Chinese petroliferous basins such as Qaidam, Subei, Songliao, Bohai Bay, Ordos, and Tarim. However, little attention has been paid to it in the past. Hydrogen in natural gas is typically present in low concentrations, and therefore there are limited studies on the geological formation mechanisms of hydrogen, which has aroused extensive attention and conjecture due to the diversification of the formation environment. The reaction of water with iron-bearing minerals, represented by serpentization, is thought to be the most important mechanism of hydrogen formation, and the decomposition of water by radiation during radioactive decay is also considered as a possible mechanism for significant hydrogen generation. The Earth's core and mantle contain both primordial hydrogen and hydrogen produced by different chemical reactions, and hydrogen generated by both these deep sources can reach the surface through degassing. Moreover, the decomposition of organic matter and biological activity are also believed to be potential approaches to generate hydrogen. These mechanisms have all existed since primitive Earth time and will continue in the future. The processes such as the water-rock reactions are reproducible in nature, therefore, natural hydrogen is considered as a renewable energy. The main concern of the natural hydrogen industry is the availability of sufficient natural hydrogen resource. The annual natural hydrogen fluxes from mid-oceanic ridges, water-rock reactions and decomposition of water by radiation are commonly in the magnitude of 10(10)-10(13)g according to different estimates. However, hydrogen diffuses easily due to its small molecular size and is chemically reactive. Therefore, natural hydrogen in geological bodies is commonly too dispersed to extract commercially. Several hydrogen research institutions and companies have been established abroad in recent years, and attempts have been made to explore for natural hydrogen. However, exploration is still in the infancy. Surface hydrogen seeps may indicate the potential for finding large-scale hydrogen gas pools underground. The exploration and exploitation of natural hydrogen resource have broad prospects, and it is necessary to further understand and evaluate the occurrence of natural hydrogen resources and develop hydrogen detection technology and equipment. This will not only help with understanding generation, migration and enrichment mechanisms of natural hydrogen, but also reduce the risk in hydrogen exploration and promote the development of a hydrogen energy economy.