Introduction
Helium is the second most abundant element in the universe after hydrogen, accounting for about 23% of the galaxy mass [1]. Although helium is widely present in cosmic space, the Earth interior has relatively fewer helium resources. This paper focuses on the large-scale, economically recoverable helium resources in natural gas reservoirs. Helium is a typical inorganic gas with a unique source and a complex, multi-circle reservoir formation mechanism, migration, and transport system. However, helium resources are often associated with natural gas and share the four essential factors of "reservoir, trap, caprock, and preservation" with natural gas. The migration and accumulation mechanisms, processes, and patterns of helium are similar to natural gas but also have some characteristic differences. Currently, as a scarce resource on Earth, helium is mainly extracted from natural gas separation, geothermal system extraction, air fractionation, ammonia synthesis (tail gas helium extraction), and uranium ore extraction. However, helium-bearing natural gas separation and purification is the only economical source of helium for industrial use [2]. China has limited helium-rich natural gas reservoirs with industrial exploitation value. Due to the limited abundance of helium resources and the high cost of helium exploitation and extraction, the annual domestic helium production is below 100×104 m3, while the annual demand is over 2 000×104 m3 and increasing annually by more than 10% [3]. The gap between supply and demand is significant, and the risk of resource shortages or even supply cuts in the helium market may persist for some time.
Previously, helium was applied as a technical tool to study the geochemical genesis and origin of natural gas [4]. Still, it has not been explored as a separate geological resource. However, in recent years, China has focused on helium as a specialized resource. Despite this, there is still a lack of systematic discussion and explanation of the characteristics, geological conditions, genesis, and enrichment patterns of helium, which are not only the fundamental issues of helium as a natural gas geology sub-discipline but also the primary issues that need to be solved and clarified in exploration. Helium exploration depends on natural gas exploration, and no systematic and targeted exploration research has been carried out independently yet. The characterization and evaluation of helium-rich favorable zones and targets are still in the exploratory stage, and there is a lack of qualitative and quantitative systematic characterization indexes. Additionally, the mechanism of helium migration and accumulation still requires in-depth research. For the discovered helium-rich natural gas reservoirs, the scope and flux of helium supply from possible helium source rocks, the geological evaluation of the transport channels and validity of the secondary migration, and the dynamics of the accumulation and formation of helium reservoirs need discussion.
This paper discusses and analyzes the common features and patterns related to helium resource formation and distribution. Three major geological problems are associated with helium resource formation: geological conditions, genesis, and enrichment patterns. We used the detailed exploration and production data of many typical helium-rich/bearing gas fields discovered and published to conduct the analysis. We performed a comprehensive statistical analysis to analyze the formation conditions and enrichment factors of the known helium-rich oil fields in China. Drawing on the methodology of petroleum systems and based on the study of helium "source-migration-accumulation" conditions, we revealed the main controlling factors of helium enrichment in natural gas. This will provide references for the subsequent exploration, development, evaluation and selection of favorable zones of helium, as well as for the technological development of the helium industry chain.
1. Helium resources and geological characteristics
Helium is currently a non-recyclable scarce mineral resource in China. Today, economically utilizable helium resources are mainly found in natural gas reservoirs, and associated helium exists in all discovered gas fields. Still, there is considerable variability in helium content and helium genesis in different regions. The helium accumulation is controlled by regional lithology (helium source conditions), fault systems, helium-accumulating natural gas carriers, and favorable tectonic settings, with larger helium content differences (Fig. 1 ) [4⇓-6]. The global helium-rich areas are mainly located within the active uplift zones of the ancient craton basins (with active basement faults and helium generation and release), the active zones with favorable trap and preservation conditions of the ancient craton margins, the slope crest of the foreland basins, and the zones of magmatism in the proximity of the faults of the rift basins.
(1) Regarding the source and origin of helium, different isotopes of the same element have different origins. There are two primary sources of helium, along with two stable isotopes: 3He and 4He. The primordial 3He (including crust-sourced helium and mantle-sourced helium) is produced by various nuclear processes within the solid Earth, such as heavy nuclear fission, n-reactions, α-reactions, and γ-ray reactions. A significant amount of primordial 3He is preserved in the mantle, then released into the shallow layers and the surface during the Earth’s degassing process [6]. The other source is radioactive 4He, mainly produced by the α-decay of the elements U and Th (Fig. 2 ) [7]. The 6Li(n, α)+3H(β-)→3He reaction can also produce 3He [8]. In addition, a tiny amount of cosmogenic 3He and 4He is produced by cosmic ray scattering. The isotopic abundance of 3He in the atmosphere is 0.000 13%, while the isotopic abundance of 4He is 99.998%. As a result, the abundance of helium is essentially determined by the abundance of 4He.
(2) In terms of physical characteristics of helium molecules, the long half-lives of U and Th α-decay in helium source rocks, the low rate of helium generation, and the very low concentration of helium in subsurface fluids and natural gas reservoirs determine helium unlikely to form a separate reservoir solely. Meanwhile, the small molecular weight of helium relative to other gases means more substantial permeability and diffusivity, making it easy to disperse. Therefore, the effectiveness of the trap, caprock, and preservation conditions becomes crucial for the accumulation of helium.
(3) Helium migration channels have the characteristics of cross-sphere, diversity, and complexity. Mantle-sourced helium migration requires channels that connect the crust and mantle, i.e., the deep-large faults that cross lithosphere or active magma channels. Crust-sourced helium migration channels are various, including basin basement faults, caprock faults, different levels and sizes of faults and fractures, unconformities, reservoirs, and more.
(4) The helium geological system shares critical similarities and differences with conventional natural gas systems (Table 1 ). The two systems have common points in the effective migration and accumulation and the formation of their reservoirs, considering the six essential geological factors: source, reservoir, caprock, trap, migration, and preservation. For helium, the focus should be on the three significant aspects of "source, migration, and accumulation". Some factors like trap and preservation conditions should be evaluated in conjunction with natural gas. Additionally, both systems share static geological elements and dynamic geological processes (Table 1 ).
Table 1. Similarities and differences in the elements of the "helium-natural gas" geological systems |
| Similarities and differences | Systems/ elements | Conventional natural gas system | Helium system | Basis and rationale for geological assessment |
|---|---|---|---|---|
| Similarities (unity) | Petroleum system | Compliance with the common patterns of the "petroleum system", from source rock to trap, with the same/similar static geological elements and dynamic geological processes. | Addressing the common characteristics of helium and natural gas, establishing a comprehensive helium geological evaluation methodology and workflow, referring to evaluating petroleum systems and petroleum accumulation systems. | |
| Accumulation system | Sharing the six essential geological elements: source, reservoir, caprock, trap, migration, and preservation. | |||
| Migration and accumulation driving forces | Carrier natural gas and helium migrate in mixed phases; for conventional carrier gas, buoyancy plays the dominant role, and for unconventional carrier gas, hydrocarbon generation pressurization and diffusion lead; for water-soluble helium, hydrodynamic or diffusive migration occurs, and due to the Henry effect at the gas-water interface of a trap, the helium degasses into the natural gas phase and accumulates. | |||
| Accumulation characteristics | The same isotopic fractionation of gases exists during gas migration and accumulation. | |||
| Differences (specificity) | Source | Organic matter | 235U, 238U, 232Th α-decay in the crust | Given the unique characteristics of helium, develop targeted resource evaluation methods: helium-rich reservoir evaluation methods that specialize in helium migration and accumulation and helium resource evaluation methods. |
| Maturation and evolution | Thermal evolution of source rock burial, gas generated by organic matter. | Accumulation of helium released by prolonged decay of U- and Th-rich source rocks. | ||
| Molecule properties | Relatively larger molecules, higher permeability, higher concentration. | Smaller molecule, extremely permeable, very low concentration. | ||
| Primary migration | Hydrocarbon generation pressurization (phase change from solid kerogen to fluid petroleum resulting in volume increase). | Minerals heated above closing temperature, fracturing of rocks and minerals, and mineral dissolution. | ||
| Driving force for secondary migration | Buoyancy drive (conventional gas), gas-pressurized piston drive, or diffusion (unconventional tight gas, etc.). | Groundwater, buoyancy, differential pressure displacement, Henry effect. | ||
| Differentiation and fractionation | Relatively complex molecular structure, chemically diverse, experiencing relatively complex component differentiation and isotopic fractionation effects. | Relatively simple molecular structure, chemically stable, experiencing relatively simple or weak isotopic fractionation effects. | ||
| Accumulation (reservoir + trap) | Natural gas accumulates in the effective traps or sweet spots. | Append to carrier gas in traps, helium-saturated water degasses or desolubilizes into natural gas reservoirs in the presence of a gas phase. | ||
| Preservation | Capillary pressure sealing in caprock and lateral tight layer. | Effective sealing by the upper caprock and lateral tight layer. | ||
| Subsidiary adjustment | Seepage and diffusion, tectonic activity adjustment or destruction, re-migration, loss, or accumulation. | Helium accumulation is more sensitive to seepage and diffusion tectonic activity adjustment or destruction; re-migration, loss, or accumulation with carrier natural gas. | ||
Both the "petroleum system" and the "helium system" follow the standard geological patterns of the static formation elements and dynamic processes, organized as "source-trap". However, the properties and characteristics of the two systems are different in "source, migration, and accumulation", focusing on the co-existing "helium-rich/bearing hydrocarbon reservoirs." During helium migration and accumulation, the isotope fractionation of helium is relatively simple and weak, since helium has only two stable isotopes, 3He and 4He, with dominant 4He. Compared to the hydrocarbon natural gas, which has many components and isotopes, helium shows weaker and simpler differentiation and fractionation. This indicates that it experiences a weaker effect of geochromatography.
2. Helium sources and accumulation
The formation of helium-rich gas fields depends on three major factors: helium source conditions, transport conditions, and accumulation conditions (Table 2 and Table 3 ) [9⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-42]. These factors are determined by analyzing the geological characteristics and accumulation conditions of the discovered helium-rich gas fields. Reservoir, trap, caprock, and preservation conditions, etc., also play critical roles in forming helium-rich gas fields, but are generally considered part of the overall evaluation of the conditions required for carrier natural gas formation and reservoir formation.
Table 2. Division scheme and types of helium sources |
| Division scheme | Type | Subclasses/characteristics |
|---|---|---|
| According to geosphere | Mantle source | 3He/4He > 1.1×10-5 |
| Crust-mantle source | 3He/4He of 2.0×10-8-1.1×10-5 | |
| Crust source | 3He/4He < 2.0×10-8 | |
| Atmosphere source | 3He/4He = 1.4×10-6 | |
| According to rock type | Igneous rock | Granite, pegmatite, coal rock etc. |
| Sedimentary rock | Black shale, bauxite, coal rock etc. | |
| Metamorphic rock | Gneiss, granite gneiss, slate. | |
| According to natural gas system | Natural gas system | Source, reservoir, and caprock rocks. |
| Basement | Various rocks with rich U and Th in the ancient basement. | |
| Mantle | Primordial 3He during Earth's formation. |
Table 3. Formation conditions and main controlling factors for the enrichment of typical helium-rich gas fields in key basins |
| Basin | Tectonic unit/ region | Gas field/ region | Carrier gas types and features | Range and average of helium and non-hydrocarbon compositions | Formation conditions for helium-rich gas fields | Main controlling factors for helium enrichment | Ref. | |||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Main pay | Types and features | He/% | R/Ra | CO2/% | N2/% | Helium source conditions | Transport system conditions | Helium accumulation conditions | ||||||||||||||||||||||||||||||||||||||||||
| Anadarko Basin | Amarillo Uplift | Panhandle oil/gas field | K, P, C | Hydrocarbon gas from carbonate rocks | 0.01-2.20 (0.55/882) | 0.21 | 0-11.7 (0.26/882) | 1-98.2 (12.7/882) | Cambrian- Precambrian igneous basement | He-rich pore water migrates along basement faults | Large-scale alkane gas and anhydrite-shale whole seal in uplift zone | Large-scale alkane gas occurs in secondary migration for long distances. The whole region has low porosity, low permeability, low fluid potential, shallow burial, and is near the helium source; Cenozoic tectonic activity caused depressurization expansion, degassing, and accumulation with formation water migration | [9, 10] | |||||||||||||||||||||||||||||||||||||
| Hugoton Subsag | Hugoton gas field | P, C | 0.02-6.99 (0.52/687) | 0.18 | 0-11.60 (0.11/687) | 1.23-95.8 (16.60/687) | ||||||||||||||||||||||||||||||||||||||||||||
| Paradox Basin | Doe Canyon | Doe Canyon gas field | C | He-rich carbon dioxide reservoir | 0.19-2.62 (0.78/8) | 0.15 | 73.49- 97.24 (91.74/8) | 2.27-19.9 (6.11/20) | Regionally widespread basement magma | Carrier gas is partially dissolved in He-rich formation water, and degases into reservoirs after secondary migration along deep and large faults | Cenozoic magmatism provides large-scale carrier gas; salts and anhydrite rocks wholly seal | Helium-rich formation water with large-scale CO2 co-migrates, degases, and accumulates | [9, 11] | |||||||||||||||||||||||||||||||||||||
| Paradox-Unita- Piceance Basin | Uncom- pahgre Uplift | Harley Dome gas field | J | He-rich nitrogen reservoir | 0.11-7.31 (3.86/20) | 0.11 | 0-1.1 (0.47/20) | 10.7-91.7 (62.1/20) | Precambrian basement granite, metamorphic rocks | Migrate along basement faults vertically in a short distance | Favorable trap conditions; basin-grade intact shale seals regionally | Helium accumulates in tectonic highs, sandstone reservoirs are only 300 m above the Precambrian basement (shallow burial with an average well depth of 236 m), and low-pressure zones | [9, 12] | |||||||||||||||||||||||||||||||||||||
| Persian Gulf Basin | Qatar Uplift | North Dome- South Par gas field | P, T | Hydrocarbon gas from carbonate rocks | 0.04% | 2.23 | 3.37 | Precambrian granite, metamorphic basement | Faults cut reservoirs and diagenesis modifies reservoirs. | Interbedded dolomite, limestone, and anhydrite provide good reservoir and cap rocks. | Low resource abundance and the world’s largest natural gas field, good conditions for trapping and preservation | [13, 14] | ||||||||||||||||||||||||||||||||||||||
| East Siberian Basin | East Siberian Basin | Kovykta and Chayanda gas fields | Riphean, Vendian, —C | Hydrocarbon gas from carbonate rocks | 0.13%- 0.67% | 0.05- 1.29 | 0.1-0.3 | 1.5-25.26 | Archaean gneiss, crystalline schist, granite, rhyolite | Deep-large basement faults and unconformities | Carrier gas in high of the ancient uplift | Vertical and horizontal unobstructed transport system, carrier gas from ancient uplift, and lower Paleozoic regional thick salt layer as caprock | [15] | |||||||||||||||||||||||||||||||||||||
| Ordos Basin | Yimeng Uplift | Dongsheng gas field | P | Tight sandstone gas | 0.045- 0.487 (0.111 8/ 166) | 0.022- 0.025 | 0.03-0.20 (0.115/6) | 0.81-1.34 (1.18/6) | Proterozoic-Archean metamorphic-granitic rocks in the basement; U and Th content of Hangjinqi area is (1.49-19.40)×10-3 mg/g, followed by Upper Paleozoic U- and Th-rich hydrocarbon source rocks | The secondary large faults of Borjianghaizi-Ulanjilinmiao and quaternary faults effectively connect the basement helium source rocks and reservoirs | Carrier gas accumulates on a large scale but in low resource abundance; favorable spatial-temporal match of reservoir, caprock, trap, and preservation conditions | Helium is generated from the ancient metamorphic- granitic basement and ancient U-/ Th-rich hydrocarbon source rocks of the Upper Paleozoic, where the helium source connecting faults of the secondary faults intersect with the transport system of the quaternary faults, and where the helium content decreases from bottom to top vertically | [16-21] | |||||||||||||||||||||||||||||||||||||
| Yishaan Slope | Sulige gas field | P | Tight sandstone gas | 0.054- 0.091 (0.072/11) | 0.01- 0.03 | 0.65-2.64 (1.304/12) | 0.8-16.94 (3.168/12) | Lower Paleozoic basin basement with strongly magnetic deep metamorphic rocks and Upper Paleozoic U-/Th-rich hydrocarbon source rocks as helium sources | Basement faults, natural gas accumulation faults, and conduit layers | Carrier gas accumulates in large areas and scales but in low resource abundance; favorable spatial-temporal match of reservoir, seal, trap, and preservation conditions | Helium-rich basin basements and faults supply helium; low resource abundance, low pressure coefficients, and good sealing and preservation conditions in favorable areas | [20, 21] | ||||||||||||||||||||||||||||||||||||||
| Qingyang gas field | P | Tight sandstone gas | 0.073- 0.237 (0.11/12) | 2.86-3.72 | 0.88-0.89 | U-/Th-rich ancient basement granite-metamorphic rocks and Upper Paleozoic U-/Th-rich hydrocarbon source rocks as helium source | Basement faults connect lower bedrock and upper natural gas accumulation system | Moderate scale, low charging degree, low fluid potential areas, local uplift areas | U-/Th-rich ancient basement helium sources and Upper Paleozoic hydrocarbon source rock as helium source, transport system, local uplift zone, and low-pressure carrier gas | [21, 22] | ||||||||||||||||||||||||||||||||||||||||
| Middle of Jinxi Flexure Zone | Shixi block | P | Tight gas in the west, coalbed methane in the east | 0.02-0.23 (0.089/81) | 0.01- 0.08 (0.025) | 0-1.22 (0.33/81) | 0.04-19.46 (1.63/81) | The helium source of Jianjiagou-Zijinshan basement magmatic body and Upper Paleozoic hydrocarbon source rocks as helium source | Deep supracrustal/intracrustal faults in Lishi area; basin seal, favorable fault development zones | Moderate carrier gas abundance and size, multiple seals overlap | Two helium sources: basement and hydrocarbon source rock, favorable transport system, multiple reservoir- caprock combinations, and good preservation conditions | [23] | ||||||||||||||||||||||||||||||||||||||
| Weihe Basin | Xianyang | Xianyang | N | Geothermal water- associated gas | 0.61-2.31 (1.666/7) | 0.037- 0.068 (0.055/7) | 0.05-11.05 (4.47/7) | 70.29- 98.00 (82.309/7) | Proterozoic-Archean granites and gneisses of the basin basement, and peripheral Yanshanian and Indosinian granites. U and Th content are (1.9-11.8)× 10-3 mg/g and (10.6- 27.5)×10-3 mg/g | Xingping-Xianyang section of the Weihe fault, Doumenzhen- Lintong fault, and Sangzhen-Qinduzhen fault | Geothermal water, hydrocarbon gases, and favorable occurrence and preservation conditions for carrier gas | Granite basement and high-quality source rocks of carboniferous-Permian coal strata, inherited anticline and dome-like trap on the basement uplift of the Weihe Basin, and dominant channels of the fault system that cut the granite basement | [24] | |||||||||||||||||||||||||||||||||||||
| Xi’an | Xi’an | 0.42-3.23 (0.953/13) | 0.023- 0.060 (0.039 2/ 13) | 0.02-0.46 (0.153/13) | 79.62- 98.79 (88.525/ 13) | |||||||||||||||||||||||||||||||||||||||||||||
| Weinan | Weinan | 0-0.86 (0.236/9) | 0.054- 0.157 | 0.04-2.75 (1.403/9) | 0.26-96.05 (26.811/9) | |||||||||||||||||||||||||||||||||||||||||||||
| Tarim Basin | Southwestern Tarim Depression | Hotanhe | O, C | Condensate oil and gas | 0.300- 0.370 (0.316/10) | 0.06- 0.08 | 0.09- 1.36 | 10.39- 12.80 | Neoproterozoic granite helium source at the base of the platform-basin area, mainly crust-sourced helium and less mantle- sourced helium | Mazhatage deep-large fault and shallow structural seams | Favorable traps, crust, and mantle sourced helium and organic gases accumulate together | Basement helium sources; faults; Helium accumulates and enriches in the tectonic highs during the readjustment of Himalayan tectonic activities | [25, 26] | |||||||||||||||||||||||||||||||||||||
| Akmomu (Ak 1) | K | Mixing of organic pyrolysis gas with less deep gas | 0.038- 0.093 | 0.549 | 13.33 | 8.97 | Crust-mantle sourced helium in the basin- mountain coupling area, dominated by Paleoproterozoic granite | A fault system formed by the interaction between the fault fold belt of the South Tianshan Mt. and the Kunlun Mt | Favorable trap conditions for carrier gas formed by source and fault connection | Crust-sourced helium in the basement matches the fault transport system, and part mantle-sourced components migrate to the crust through the fault-fold zone and accumulate | [26, 27] | |||||||||||||||||||||||||||||||||||||||
| Northern Tarim Uplift | Hade, Yingmaili | C, E | Hade: Oil-type gas; Yingmaili: Coal-type gas | 0.000 2- 0.290 0 | 0.024- 0.062 | 0.12- 5.89 (1.99/7) | 2.58- 52.45 (15/7) | Helium source of basement granite and metamorphic rock in the platform-basin area | Basement faults and seal-fault systems of the basin | Crust-sourced helium from the Yingmaili gas field migrates to trap along with organic gases and accumulates | Favorable basement helium sources; fault systems; Himalayan tectonic activity; favorable traps and preservation conditions | |||||||||||||||||||||||||||||||||||||||
| Sichuan Basin | Southern Sichuan Basin | Weiyuan | Z, —C, P | Cracking gas of crude oil | 0.003- 0.342 (0.192/21) | 0.013- 0.022 | 1.23-5.07 (4.064/10) | 0.4-26.7 (6.872/22) | Pre-Sinian basement granite, Cambrian Qiongzhusi Formation shale, and Silurian Longmaxi Formation shale | Faults, fractures, pores, and other groundwater migration channels | Ancient uplift; long- distance migration, accumulation and adjustment along slope; extraction of helium in formation fluids | The helium source of the pre-Sinian granite basement; the U-/Th-rich mud shale of the Jiulaodong Formation; the ancient uplift with favorable trap conditions for helium accumulation in paleo-uplift | [28-31] | |||||||||||||||||||||||||||||||||||||
| Eastern Sichuan Basin | Fuling | O—S | Shale gas | 0.034- 0.062 (0.045/44) | 0.05-0.37 (0.22/18) | 0.80-2.19 (0.89/18) | The contents of radioactive U and Th in shale are generally higher. The U content is (5.97-38.12)× 10-3 mg/g, averaging 24× 10-3 mg/g; The U content is (6.61-22.81)×10-3 mg/g, averaging 20×10-3 mg/g. There is a contribution of the basement helium source | Basement faults and caprock serve gas accumulation | The self-generated and self-stored carrier gas coexists with the self-generated and self-stored helium; U-/Th-rich basement helium source; the top and bottom plates seal; and the preservation conditions are good | The Wufeng Formation- Longmaxi Formation shale and U-/Th-rich basement are helium sources; and the shale gas layer has thick top and bottom plates with tight lithology, high breakthrough pressure, good sealing and preservation performance | [32, 33] | |||||||||||||||||||||||||||||||||||||||
| Middle Sichuan Basin | Ziyang | Z2d | Hydrocarbon gas | 0.01- 0.32 (0.115/6) | 0.01-6.59 (3.925/6) | 0.97- 11.88 (4.835/6) | Pre-Sinian granite | Basement faults and fault systems in the caprock of the basin | Favorable carrier gas reservoir and trap conditions; Weiyuan gas field was uplifted by 4 000 m during the Himalayan period | The basement granite helium source; fault system; favorable natural gas carrier conditions; and late adjustment led to low helium content | [34, 35] | |||||||||||||||||||||||||||||||||||||||
| Qaidam Basin | Dongping Slope | Dongping gas field | Bedrock | Cracking gas of crude oil | 0.045- 1.069 (0.331/31) | 0.007- 0.090 (0.031/4) | 0.01-2.02 (0.293/31) | 2.359- 30.490 (12.624/40) | Proterozoic, Caledonian and Indosinian granite and granodiorite and metamorphic basements | SN-trending deep-large faults and regional unconformities in the slope zone | Dongping Slope is between multiple hydrocarbon generation sags and has good carrier gases | The granite and granitic gneiss basement are widely distributed, and have the conditions for forming high helium content in the weathered crust of the bedrock and its highs | [36-38] | |||||||||||||||||||||||||||||||||||||
| Jiandingshan Structure | Jianbei gas field | Bedrock, E | Crude oil cracking gas | 0.160- 0.569 (0.277/4) | 0.01-0.504 (0.139/4) | 15.54- 22.34 (17.473/4) | Proterozoic and Paleozoic basement granite or granitic gneiss | Basement faults and regional unconformities | Tectonic highs, with favorable conditions for the accumulation and preservation of carrier gases | The basement helium source and transport system are matched with favorable migration, accumulation, and preservation conditions | ||||||||||||||||||||||||||||||||||||||||
| Songliao Basin | Changling Rift | Changling Sag | K | High CO2 content | 0-0.03 (0.008/7) | 1.9-2.3 (2.071/7) | 0.02-97.18 (17.95/15) | 0-6.73 (2.368/15) | Mantle source, basement granite, acidic volcanic rocks, and some sedimentary rocks | Deep-large faults, basin caprock faults and transport layer | Favorable carrier gas trap and preservation conditions; tectonic highs | The crust-mantle-sourced helium and the transport system match well with favorable traps | [39] | |||||||||||||||||||||||||||||||||||||
| Xujiaweizi Rift | Qingshen gas field | C, P, J, K | Mainly hydrocarbon gas | 0.001 6- 0.046 0 (0.018/20) | 0.771- 5.843 (1.601/20) | 0.000 3- 95.830 0 (10.528/20) | 0.33-10.24 (2.503/20) | Mantle source; basement granite; from Yingcheng Formation to Huoshiling Formation, the U content of volcanic rocks is (2.42- 2.90)×10-3 mg/g, and the Th content is (4.14- 7.80)×10-3 mg/g | Xushen fault, secondary faults, and transport layer | Tectonic highs, favorable carrier gas trap, and preservation conditions | Deep-large faults, basement and mantle-sourced helium, and tectonic highs match well with favorable traps | [40] | ||||||||||||||||||||||||||||||||||||||
| Bohai Bay Basin | Jiyang Depression | Pingfangwang gas field, Pingnan, Hua 17, Gaoqing, Yangxin | E, N | CO2 reservoir | 0.008 4- 0.084 7 (0.030/10) | 2.00-3.73 (2.84/13) | 68.85- 98.59 (84.635/19) | 0.06-5.43 (1.155/17) | Basement acid intrusive rocks, volcanic rocks, and organic-rich mud shale in the basin | Tanlu fault and branch system, natural gas transport system | Deep-large faults connecting favorable trap and preservation conditions | Tanlu (or branch) faults, basement, and deep helium source match well with favorable traps | [41] | |||||||||||||||||||||||||||||||||||||
| Huanghua Depression | Gangxi fault, Gangdong fault, Banqiao Sag | E, N | CO2 reservoir | 0.001- 0.048 (0.012/14) | 0.24-3.62 (1.66/20) | 0.26-93.61 (11.250/20) | 0.19-1.91 (1.001/17) | Basement basic- intermediate to acidic magmatic rocks | Gangxi/Gangdong faults and magmas are migration carriers of mantle- sourced helium | Favorable trapping and preservation conditions for carrier gas near faults | Areas with favorable trap and preservation conditions near deep faults and active magma zones | [42] | ||||||||||||||||||||||||||||||||||||||
Note: The data after “/” means the sampling counts. The formation conditions and main enrichment factors of helium-rich gas fields in this table are based on references [9⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-42]. The preliminary conclusions are extracted from exploration and production performance analysis combined with regional geological conditions. |
2.1. Helium source conditions and genesis identification
Helium has a variety of sources (Table 2 ). It can be divided as found in the different geospheres: the mantle, the mixture of the crust and mantle, the atmosphere, or the crust. It can also be sourced from different parts of a petroleum system, such as the hydrocarbon source rock, reservoir rock, caprock, basement, and mantle. In addition, the type of reservoir lithology can classify the source of helium as magmatic (granite, pegmatites, etc.), metamorphic (gneisses, granodiorite gneisses, etc.), and sedimentary (black shale, bauxite, coal rock, etc.) rocks all potentially containing helium. The presence of relatively high uranium and thorium radioactive minerals in the rocks mentioned above contributes to the potential ability of helium formation. Most helium-rich gas fields discovered so far have resulted from the decay of uranium and thorium elements in the basement, with a lesser amount sourced from hydrocarbon source rocks, gas reservoirs, and mantle sources with rich uranium and thorium.
It is important to note that crust-sourced helium is the main source of helium resources throughout the world's helium resource genesis. There are distinct differences in the isotopic geochemical compositional characteristics of helium from different sources. For instance, the helium found in natural gas in central and western China is mainly from the crust, with lighter isotopic compositional characteristics and R/Ra values less than 0.5 [43]. Additionally, there is an obvious regularity in the distributional trend of the data related to the R/Ra values and its host carrier gases, CH4 and CO2. Crust-sourced helium is in the upper left region of the envelope in Fig. 3 and is characterized by lower 3He/4He ratio and lower 3He content [37,39,44⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓ -55]. The main helium sources controlling the macroscopic distribution of helium-rich areas in China are the currently discovered helium-rich gas fields with their underlying ancient basement granites and metamorphic rocks. The helium in China’s eastern rift basins is characterized by crust and mantle mixing [44⇓-46], with R/Ra values ranging from 0.1 to 6.0, higher 3He/4He ratio, and higher 3He content (Fig. 3 ). The mantle-sourced helium content can be as high as 60% or more, in which the mantle-sourced helium is released along the tectonically magmatic zones and is preserved by accumulating in the favorable traps with carrier gases.
2.2. Helium migration conditions and mechanisms
An effective transport system for migrating helium from source to reservoir is crucial for it to enter the natural gas accumulation system (Fig. 4 ). The composition and complexity of the transport systems for crustal-mantle-sourced helium are quite different. An effective complex transport system of multiple tectonic layers is required for mantle-sourced and crust-mantle sourced helium to effectively accumulate and form a reservoir. This includes "lithospheric faults-basement faults-sedimentary layer faults (i.e., fault of the natural gas accumulation system)-effective transport layers", which control the migration, transport, and accumulation of helium into the reservoir. The migration, transport, and accumulation of crust-sourced helium are mainly controlled by a three-layer composite transport system of "basement faults-sedimentary layer faults-effective transport layers (unconformity and reservoirs, etc.)".
Fig. 4. Schematic diagram of the "generation-migration-accumulation" process of helium. |
The transport conditions include effective transport channels and favorable transport carriers. There are various transport carriers, such as groundwater, natural gas, and other fluids (oil field brine, hydrothermal fluids, etc.). An efficient and smooth transport channel is crucial for effective helium migration as well, especially in a subsurface vacuum zone. In China’s east and west regions, the structures of the transport systems vary due to differences in the geotectonic environment. Effective transport systems for crust-sourced helium include basement faults, caprock faults, unconformities, and reservoirs of the central and western basins. In China’s eastern region, the basins are in the active tectonic environment of extensional rift, and many groups of deep-large faults have developed in the basins. Hence, the deep migration of crust-mantle-sourced helium is related to supracrustal deep-large faults and magmatic activities. The shallow migration is related to the basement faults and the transport channel of the natural gas accumulation system. Moderate tectonic activities (strike-slip, extension, uplift, magmatism, etc.) can facilitate the release and migration of helium and subsurface fluids. Moderate tectonic uplift is one of the favorable conditions, but not a necessary condition, and an efficient transport channel is the key. Rift basins are dominated by extensional rifting, but strong tectonic activities tend to cause adjustment and destruction.
According to the phase balance and phase-potential coupling relationship between "helium-natural gas-water" in fluids, based on the gas-bearing system of the subsurface helium source-natural gas trap, it is proposed that in the migration and accumulation process of helium, there are three main occurrence states and migration modes of helium, namely, water-soluble phase, gas-soluble phase, and free phase. Correspondingly, three migration mechanisms of helium gas, namely mass flow, seepage, and diffusion, are derived.
"Mass flow" refers to a group of helium molecules, such as helium-containing medium (water-soluble phase, gas-soluble phase) or free-phase helium, under the action of the pressure gradient (hydrodynamic force, tectonic stress, etc.) or buoyancy force, migrate in the form of independent phase as in clusters, or attaching to the carrier. In the process of helium migration, the first state is that the helium and its dependent carrier medium are relatively immobile, mainly manifested by the movement of the helium-carrying medium. For instance, the water-soluble helium migrates with the flowing water in the area with more active water dynamics (Weiyuan helium-rich field in the Sichuan Basin), and the gas-mixed phase helium migrates with the natural gas when extracted in the process of secondary migration of natural gas (Dongping helium-rich field in Qaidam). The second state is that the free-phase helium that exists in the area with less free pore water within the helium source rock, and the free-phase helium that is supersaturated with water-soluble helium and precipitates out of the helium in the shallower parts of the field during the process of secondary migration by buoyancy.
"Seepage" refers to the migration of helium-containing fluids or free-phase helium along porous medium, including rock pore throats, fractures, or water bodies. For example, the helium released from the decay of U-/Th-rich minerals in helium source rocks migrates along rock pore spaces and micro-fractures; the secondary migration process contains various helium-bearing fluids, including the supersaturated helium in fluids in a certain space/ time domain transforming to the free-phase helium by exsolution, migrating in porous medium, small throats, and micro-fractures.
"Diffusion" refers to the migration of helium in the molecular state (primary migration within the helium source rock—diffusion of helium released from radioactive decay), in the dissolved state (secondary diffusive migration of dissolved helium in pore water), or the free state as a result of a difference in concentration or osmotic pressure (primary migration in the micro-fractures or throats of the helium source rock, and secondary migration in the fractures or transport layers out of the helium source rock).
2.3. Accumulation and preservation
The accumulation conditions of helium include helium reservoir, caprock, trap, and preservation conditions, the filling degree of the carrier gas, the pressure coefficient, and the tectonic location of the gas reservoir, etc. In general, helium cannot accumulate independently. A small portion of helium can form the reservoir dependent on geothermal water but mostly relies on natural gas. Therefore, a favorable natural gas trap and carrier gas, a suitable geological environment, and minimal destruction from late tectonic activities are necessary for stable helium accumulation and preservation. The analysis of typical helium-rich gas fields and comparison of natural gas-helium accumulation reveals that there are three main mechanisms of helium accumulation: exsolution and convergence, buoyancy-driven, and differential pressure displacement [2]. These mechanisms will be discussed in the following section.
Helium requires more stringent seal and preservation conditions compared to hydrocarbon gases (Fig. 5 ) [56⇓-58]. The reason for this is the dynamic balance of natural gas migration and accumulation [59], along with the characteristics of late reservoir formation [60-61]. The natural gas reservoirs and their associated helium components that have been preserved today have better seal and preservation conditions and are in a state of dynamic balance between migration and accumulation. Caprock and its sealing mechanisms are diverse, such as capillary resistance or the "hydro-lock effect" of tight sandstone in deep basin gas, which forms an effective caprock for natural gas accumulation. This mechanism doesn't mean that deep basin gas does not require a seal.
Fig. 5. Helium dissipation in seal under different medium conditions. (a) Schematic diagram of reservoir-caprock assemblage and micro-fractures in a gas reservoir; (b) SEM photograph of a shale [55]; (c) Pore distribution of the shale in Silurian Longmaxi Formation [56]; (d) Distribution of full pore sizes of shale pore volume in Silurian Longmaxi Formation [57]; (e) Anhydrous and gas-containing state of micro-fractures; (f) Gas-water mixed state of micro-fractures; (g) High aqueous state of micro-fractures; (h) Anhydrous gas-containing state of wide fractures; (i) Gas-water mixed state of wide fractures; (j) High aqueous state of wide fractures. |
Late adjustment and destruction of gas reservoirs (layers) can lead to significant helium dissipation when compared to hydrocarbon gases. For instance, the Qinnan coalbed methane has an extremely low helium content, with an average of only 8×10−3 mg/g after dissipation [62]. The formation of micro-fractures caused by late basin uplift resulted in the dissipation of 40% methane and 90% helium, making it a helium-poor gas reservoir [63]. The necessity and importance of trap (seal) and preservation conditions during helium accumulation in natural gas are determined by harsh trap (seal) and preservation conditions, as well as the preferential dispersion of helium during later adjustment and destruction (Fig. 5 ). In shale, micro-, meso-, and macro-pores of different scales are developed, and the highest diffusion coefficient of dissolved helium is in aqueous solution when the pore throats are filled with water (Table 4 ) [64]. Usually, the pore throats of inorganic minerals are preferentially covered by water film, and the pore throats of organic matter are preferentially adsorbed by methane molecules. If there is any possible space, even if the remaining throat space is smaller than the diameter of one CH4 molecule (0.414 nm), as long as it is larger than the diameter of a helium molecule (0.26 nm), in the geological history at the scale of millions of years in the subsurface, due to the chaotic Brownian motion of gas molecules, helium has countless opportunities to break through the throat outlet to escape.
Table 4. Diffusion coefficients of common natural gas components in water (25 °C) [64] |
| Natural gas components | Diffusion coefficients/(10−5 cm2·s−1) |
|---|---|
| Air | 2.00 |
| Ar | 2.00 |
| CO2 | 1.92 |
| C2H6 | 1.20 |
| He | 6.28 |
| H2 | 4.50 |
| H2S | 1.41 |
| CH4 | 1.49 |
| N2 | 1.88 |
| O2 | 2.10 |
Helium primarily occurs in natural gas, though it can also be present in water, forming water-soluble helium. The geochemical characteristics and distribution of water-soluble helium in groundwater are significantly different from those of helium in natural gas. However, they share some common characteristics, just different by the carriers. They both have the same “source-migration- accumulation” geochemical characteristics and factors that control enrichment.
In some provinces of China, water-soluble helium has been discovered with a high-volume fraction of up to 9.23% in the Huayin Well WR-76 in the Weinan region of the Weihe Basin [65⇓⇓-68]. The ratio of 3He/4He in some sampled wells in the Weihe Basin is between (2.06-9.39)×10−8, which is typical of crust-sourced helium [67]. According to the statistics of helium content in sampled wells in different regions of the Weihe Basin, the average water-soluble helium volume fraction is higher in the Weinan region (2.211%), followed by the Xi'an and Xianyang regions. In the Jinzhong Basin, the helium volume fraction of 20 wellhead gas samples from Well JH-1 ranges from 8.50% to 18.86%, with an average of 13.40% [68]. This high helium content is rare in the world. In the Sanshui Basin, the helium volume fraction of Well Bao-1 and Well SS-3 is up to 0.259%, and the ratio of 3He/4He in some sampled wells is (0.010 6-6.360 0)×10−6, which is crust-mantle mixed helium. Despite the high helium content in underground water-soluble gas, the total volume is relatively limited. The cost of water-gas separation and gas-helium separation and purification is much higher than that of helium extraction from natural gas. Therefore, water-soluble helium is a potential resource that is currently difficult to utilize economically.
3. Helium enrichment and distribution patterns
The geotectonic dynamics background and natural gas accumulation systems in different regions of the world have significant variability (Table 3 and Table 5 ). The degree and scale of helium enrichment also vary widely. In the United States, there are several high-helium hydrocarbon gas fields such as the Panhandle-Hugoton field (with an average helium content of over 0.5% and a maximum value of 7%) [69], the Harley Dome field (with an average helium content of over 3.86%, an average helium content in production wells of 7%, a maximum value of 7.31%, and an average N2 content of 85%) [70], and the Doe Canyon field (with an average helium content greater than 0.78% and a maximum value of 2.62%, and an average CO2 content of 91.7%) [11]. In China, helium is widely dispersed in various types of natural gas fields, but the overall grade of helium resources is low (most of them are helium-poor resources with a helium content of about 0.10% or less than 0.10%). However, there are helium-rich resources with richness ranging from poor to high, and with strong heterogeneity. The main reasons for the development of helium-rich/high gas reservoirs (fields) in the United States can be attributed to the development of large and ancient U-/Th-rich cratons, simpler transport systems, relatively stable tectonic background, and lesser late damage and dissipation (Table 5 ) [71]. However, the geotectonic background of China is opposed to the conditions for helium accumulation, with the development of small and medium-sized cratons, large span of the transport systems, complex tectonic evolution of multiple cycles, strong tectonic activity, and relatively poor preservation conditions.
Table 5. Comparison of formation conditions and geological characteristics of helium between China and North America [71] |
| Location | Geological background | Helium source rock | Reservoir characteristics | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tectonic background | Sedimentary background | Types of source rock and organic matter | U, Th contents | Basement helium source rock | Deep mantle helium source | Main lithology | Reservoir distribution | ||||||||
| North America | Stable tectonic background. | Mainly marine sediments. | Marine source rock, simple type of organic matter. | Higher content and stable distribution. | Ancient large craton releases much helium from U and Th decay. | Mainly in large tectonic magmatic activity zones (island arc and earthquake zones, etc.) | Carbonate rocks, clastic rocks. | Larger, stable, and continuous reservoirs. | |||||||
| China | Multiple cyclic tectonic evolutions, strong late tectonic activities. | Marine, paralic, and continental facies. | Source rocks in marine, lacustrine, and paralic facies, and complex types of organic matter. | Various contents and strong heterogeneity. | Medium-small cratons release various amounts of helium from U and Th decay. | Eastern extensional region. | Carbonate rocks, clastic rocks, lacustrine carbonate rocks. | Small reservoirs with strong heterogeneity and worse continuity. | |||||||
| Location | Transport system | Helium-rich/bearing reservoir characteristics | Preservation conditions | ||||||||||||
| Basin basement | Basin caprock | Carrier gas | Helium | Sealing and preservation | Late destruction and adjustment | ||||||||||
| North America | A relatively simple transport system, and a shorter distance. | Relatively simple types and structures of the transport system of marine geology bodies. | Large gas reservoirs/fields, complex and diverse components. | Generally higher content, wide distribution of helium- rich resources (>0.30%). | Relatively stable distribution of caprocks, better-sealing performance, and preservation conditions. | Relatively stable tectonic background, weaker late destruction and adjustment. | |||||||||
| China | Relatively complex transport system, larger distance differences between east and west regions. | Relatively complex types and structures of the transport system of continental geological bodies. | Small gas reservoirs/fields, mainly hydrocarbon components, and small CO2-rich and N2-rich gas reservoirs/fields. | Generally lower content, and most helium resources are lean with a volume fraction of ~0.10% or below. | Unstable distribution of caprocks, worse sealing performance and storage conditions. | Relatively strong tectonic activity, stronger late destruction and adjustment. | |||||||||
Nitrogen in natural gas comes from various sources such as the mantle, lithosphere, basement, sedimentary layers, groundwater, and atmosphere. Different sources have varying isotopic compositions (Fig. 6 ) [72]. N2 and helium contents generally show a positive correlation trend (Fig. 7a ), but the correlation coefficients vary from field to field. The distribution of bulk data in the same field/region is more discrete, and the correlation in local regions and layers is more complex. Whether there is a positive correlation or a local negative correlation between He and N2, and whether there is a genetic correlation between the two, needs to be confirmed using nitrogen isotope compositions and other evidence to discern their sources. The distribution of data points and the trend of the envelope in the He-N2-CO2 contents in natural gas show that the relationship between He-N2, and He-CO2 is similar and complex (Fig. 7b ). Each two is not monotonically positively or negatively correlated, which might be due to the complexity of their genesis and sources. Helium in natural gas mainly comes from the radioactive decay of U-/Th-rich minerals. The polygenic nature of N2 determines that He and N2 have different sources but the same reservoirs. The possibility of the same sources and coexistence for He and N2 needs to be confirmed by evidence such as the isotope geochemistry of N2. In some gas fields like Harley Dome mentioned above, He and N2 are non-positively correlated, and even partially negatively correlated.
In summary, the formation of helium-rich gas reservoirs requires certain geological conditions such as a favorable geotectonic background and the proper conditions for helium “source-migration-accumulation” (Table 3 and Fig. 4 ). Like the natural gas systems and accumulation systems, the generation, migration, accumulation and distribution of helium are also controlled by three crucial geological factors: an ancient large and efficient helium source, a smooth and effective transport system, and favorable accumulation carriers and preservation conditions.
3.1. Helium sources on an effective scale are the resource base for the formation of helium-rich fields
An efficient source of helium is a necessary condition for the formation of helium-rich fields. The effective scale for helium sources lacks a quantitative standard, but typically requires large thickness and volume, high U and Th content, and being relatively old, shallowly buried, and close to the trap. If the trap can be directly overlying on top of, or adjacent to, an ancient U-/Th-rich basement, this can ensure sufficient helium generation and accumulation. Favorable helium sources exist in the basement of helium-rich gas reservoirs or natural gas accumulation systems discovered so far (Table 3 ). The content of helium varies considerably from basin to basin and from gas area to gas area. The data in Table 3 shows the distribution of tested helium contents from the discovered gas fields. As more samples are tested and new gas fields are discovered, the distribution of the helium content data may change. Natural gas reservoirs contain helium from both the crust and mantle.
Crust-sourced helium is produced by the α-decay of U and Th in rocks stored in the basement or caprocks of a basin. Ancient sedimentary rocks (mud shale, bauxite, coal rock, etc.), magmatic rocks (granite, etc.), and metamorphic rocks (gneiss, etc.) rich in U and Th elements can all be effective helium source rocks. The size, age, U/Th content, depth, and proximity to natural gas reservoirs determine the magnitude, migration and accumulation efficiency, and accumulation coefficient of helium release from radioactive elements in helium source rocks. Generally, the larger the size and thickness of the helium source, the older the age, the higher the U/Th content, the shallower the burial, and the closer the distance to the natural gas reservoir, the more favorable it is for effective helium migration, accumulation, and enrichment.
Mantle-sourced helium is the primordial helium endowed in the deep Earth (mantle), exists inside the Earth during its formation, and is characterized by the relative enrichment of 3He. Mantle-sourced helium is usually released into shallow crustal fluids and natural gas reservoirs by degassing from deep-large faults or active magma zones. The effective supply of mantle-sourced helium mainly emphasizes the accessibility and effectiveness of deep-large faults.
Analysis of the discovered helium-rich gas fields shows that high-quality and sufficient helium sources are the foundation for forming helium-rich gas fields. Different types of rocks like U-/Th-rich basement granites, granitic metamorphic rocks, acidic volcanic rocks, organic-rich shales or coal seams, bauxites, etc., contribute to the formation of helium-rich gas fields to varying degrees. The efficient long-time release of helium from the ancient U-/Th-rich basement, as well as U-/Th-rich and organic-rich shale and coal seams, is necessary for the effective supply of helium to the gas fields. Although the rate of hydrocarbon generation in U-/Th-rich shale and coal seams is much higher than that of helium generation, helium generation is continuous and long-lasting, while hydrocarbon generation is mainly limited to a short-term peak period. The generated helium in shale or coal seams is retained in situ and self-preserved, which makes it unique compared to outer-generated helium-rich gas reservoirs. The shale gas and coalbed methane have rich helium, and thus should not be overlooked for their effect on helium generation. There are many examples of high-helium coalbed methane and helium-rich/-high shale gas (with helium content of more than 1%) in Australia, Poland, and the United States [75]. The helium content in coalbed methane in China varies in a large range. In 2022, we collected 8 samples of coalbed methane in the eastern edge of the Ordos Basin, of which 4 samples contain helium (0.10% to 0.16%).
3.2. Advantageous transport systems and favorable carrier media are necessary conditions for helium migration from the source to the carrier gas trap
Primary migration of U and Th elements in helium source rocks occurs along fractures and cracks after decay, and they are released outside the helium source rocks and into the transport medium (rock pores, fractures and vugs, and the gas, water, and other fluids in them) for secondary migration. Usually, the deep fault and fracture systems that connect the basement and caprock of a basin, especially the intersection of faults in different directions, are advantageous channels for efficient helium migration. After entering the natural gas accumulation system, helium is attached to the formation water or natural gas in the reservoir as a carrier, and under the effect of hydrodynamic force, buoyancy, or diffusion, the secondary migration occurs until it enters the trap or sweet spots and accumulates. For example, in Dongsheng helium-rich large gas field in the Ordos Basin, the secondary and fourth-level faults in Borjianghaizi and Ulanjilinmiao effectively communicate the basement helium source rocks and reservoirs, and the helium released from the basement helium source of ancient metamorphic rocks and granites migrated upward through the above faults and accumulated in the junction of the secondary faults that connect helium source rocks and the fourth-level faults as the transport system, and the helium content gradually decreases vertically from the bottom up [17].
For most helium-rich gas fields, in addition to the transport systems and carrier medium in the natural gas-bearing system themselves, they also need helium sources beyond the natural gas system, i.e., a smooth and efficient transport channel for the U-rich, Th-rich rocks, or even mantle helium sources in the deeper parts of the basement or lithosphere. Usually, there are three main sources of helium and argon rare gases in natural gas and geofluid systems with corresponding characteristic indicators [47,66]. Therefore, the determination of the existence of an effective mantle-sourced channel can be based on a combination of isotope indicator characteristics of mantle-sourced helium (e.g., heavier helium isotopes or higher R/Ra above 0.5 or 1.0, Fig. 8 ), mantle-sourced carbon dioxide and heavier argon isotope compositions (Figs. 8 and 9 ). In addition, CO2, the non-hydrocarbon carrier component of helium, can be used as a secondary indicator of the presence of a deep mantle-sourced channel, with heavy δ13CCO2 and high CO2 content (typically greater than 60%) being indicative of a mantle-sourced inorganic origin [52,76]. Effective input of mantle-sourced helium requires basic conditions such as faults that cut through the lithosphere to the mantle or active magma zones, and basement faults for crust-sourced helium transport systems. Current helium resources with industrial economic value are mainly contributed by crust-sourced helium.
3.3. Favorable helium-accumulating carriers and preservation conditions are key to the formation of helium-rich/containing gas fields
Helium is difficult to form reservoirs on its own due to its extremely low concentration and needs to accumulate with carrier gases [87-88]. As mentioned above, the formation of helium-rich gas reservoirs and helium enrichment are usually controlled by three main control elements, namely, "high-quality helium source, efficient transport, and suitable carrier", of which "suitable carrier" is the place of helium storage and accumulation. Helium-rich gas reservoirs have a distribution rule and geological pattern of "near helium source, adjacent to faults, low potential, and high position". The distribution of helium-rich fields in China depends on or is controlled by favorable helium-accumulating carriers, i.e., favorable tectonic backgrounds, natural gas formation, and preservation conditions, as well as appropriate abundance and large scale, etc. (Fig. 10 ) [87]. For helium to form a large-scale beneficial accumulation, it is necessary to have the favorable conditions of "connected source and trap, low potential and high position, matched natural gas and helium", that is, the helium source and trap are connected, carrier gas has relatively low-pressure coefficient and is in the low-abundance area and local tectonic highs, the scale of the natural gas and the helium supply flux are well matched to be conducive to the helium accumulation to form large-scale economic reserves. Too large or too small a natural gas scale is not conducive to large-scale economic helium accumulation. Helium does not favor areas with natural gas carriers of "high (abundance), large (size), or strong (hydrocarbon generation)". Instead, the relatively "low potential areas" (local areas with low hydrocarbon generation intensity, low charge, low abundance, low-pressure coefficients, etc.) are often favorable pointers for helium migration and accumulation.
Fig. 10. Geochemical distribution of helium tectonic geochemical zones in petroliferous basins in China (modified from Ref. [87]). |
As mentioned above, there are three main mechanisms for helium gas accumulation: exsolution and convergence, buoyancy-driven, and differential pressure displacement. Generally, natural gas carriers and associated helium in conventional reservoirs and traps accumulate by "buoyancy", while unconventional natural gas and helium authigenic from hydrocarbon source rocks accumulate in the sweet spots by "differential pressure", such as pressurization from hydrocarbon generation, capillary pressure difference, and diffusion. In regions with stronger tectonic activity and hydrodynamic force, due to late tectonic uplift, temperature and pressure drop, reduced helium solubility in water, exsolution of helium-bearing supersaturated water, helium migrates to natural gas traps and accumulates by buoyancy. If helium-bearing formation water migrates to the gas-water interface in the trap, due to the Henley effect, it occurs direct "exsolution and convergence" and accumulates. In regions with weaker tectonic activity and hydrodynamic force, the water body is usually immobile relatively. For instance, the natural gas and helium released from hydrocarbon source rock can migrate and accumulate in the gas-soluble phase in the trap. During long-distance migration and accumulation of natural gas, due to the Henry effect, the dissolved helium is continuously "ex-tracted" from the groundwater, or exsolved when there is a free gas phase, then migrates into the trap by buoyancy [87-88].
Natural gas requires higher seal and preservation conditions than crude oil and is therefore characterized by a dynamic balance of migration and accumulation and late accumulation. Although helium can coexist with other natural gas molecules by attaching to them and parasitizing them, the van der Waals force reduces dispersion and dissipation. However, the molecular diameter of helium is smaller than that of methane, and it is highly diffusive, so it has the priority of overflowing and dissipating compared with other gas components, especially when it is subjected to strong tectonic activities, adjustment, and destruction in the later stage, therefore, the requirements for the seal and the preservation conditions are higher, and it is of utmost importance.
During the long geological evolution, there is a dynamic balance of helium migration and accumulation (Brownian motion, dissolution, seepage, and diffusion) between the gas reservoir, caprock, and overlying strata. The directions of the "main force" and "resultant force" depend on the replenishment of the helium source, the helium abundance and pressure coefficient between different mediums, and the water content of the caprock. In the case of a relatively adequate helium and natural gas supply, a relatively worse caprock may be able to seal the natural gas and helium, and maintain the dynamic balance of the gas reservoir for a certain period. However, in general, and especially for selecting favorable helium zones and evaluating targets in new areas and wells, it is important to pay much attention to the seal and preservation conditions, which are important prerequisites for the accumulation and stable preservation of helium and even natural gas, as well as for ensuring drilling success.
In summary, based on the anatomy of typical helium-rich gas reservoirs at home and abroad, the phase balance of helium-gas-water in subsurface fluids, and the analysis of phase-potential coupling, it is clarified that helium in natural gas has three sources, namely, crustal radioactive decay, mantle degassing, and atmosphere genesis (trace amount). The study proposes that there are three types of helium migration states and modes: "water-soluble phase, gas-soluble phase, and free phase", and three types of migration mechanisms: "mass flow, seepage, and diffusion". It reveals that helium accumulation mainly has three mechanisms: exsolution and convergence, buoyancy-driven, and differential pressure displacement. It is proposed that helium enrichment is controlled by three major factors, namely, "high-quality helium source, efficient transport, and suitable carrier", and the distribution is controlled by four factors, namely, relative "near helium source, adjacent to faults, low potential, and high position". The formation of large and economic reserves needs the favorable conditions of "connected source and trap, low potential and high position, and matched natural gas and helium". Accordingly, a framework model of helium based on the system of "source-migration-accumulation" was established (Fig. 11 ) to reveal the geological conditions, genesis mechanism, and enrichment law for forming helium resources.
Fig. 11. Schematic diagram of the framework model of helium "source-migration-accumulation". |
In the above-mentioned "source-migration-accumulation" system of helium, several aspects worth paying attention to in terms of reservoir formation conditions and exploration evaluation are as follows: (1) Helium-rich factors with an integrated source-reservoir carrier gas (coalbed methane, shale gas): Coal seams and shale generate a large amount of gas during the relatively short term, but produce helium in long term. The degree of helium dilution does not depend on high TOC content and high gas production, but on limited retained gas volume, and the effective supply of helium from their own helium source. Moreover, there is a contribution from the basement helium source. (2) There are multiple types of structural combinations conducting helium, including extension, strike slip, compression, uplift, and other tectonic activities that can generate fault transport systems and migration and accumulation forces. Moderate tectonic activities are conducive to the migration and accumulation of underground fluids and helium, and structural uplift is one of the favorable conditions. Strong tectonic activities often lead to damage and adjustment. (3) There are various migration dynamics of helium: hydrodynamics, buoyancy, and pressure difference (hydrocarbon generation pressurization, helium concentration difference, etc.). Both the hydrodynamic active zone and stable zone can effectively migrate and accumulate helium in different ways. (4) The correlation between co-associated components: The relationship between the contents of He and CH4, CO2, N2, and H2 is not a genetic dependence, but a "different sources and same reservoir" associated relationship with apparent correlation in contents. (5) The importance of caprock and preservation conditions: Caprock is the constituent element of a trap and the prerequisite for the formation and stable preservation of helium-rich gas reservoirs. The strong permeability, high diffusion, and easy dissipation of helium determine that caprock and sealing conditions are crucial.
4. Conclusions
The mechanism of "generation-migration-accumulation" of helium is significantly different from that of natural gas. In addition to the contribution of helium sources from gas reservoirs and hydrocarbon source rocks in natural gas systems, most of the external main helium sources are the slow α-decay of basement U-/Th-rich elements, or the release of helium from the deep crust and mantle, which migrates along the composite conducting system with multi-tectonic layers of the exosphere and accumulates along with suitable carrier gas. The effective composite conducting system with multi-tectonic layers, such as "lithosphere faults-basement faults-sedimentary layer faults-effective transport layer", controls the migration and transport of mantle-crust sourced helium, and the migration and transport of crust-sourced helium mainly controlled by the latter three of the above-mentioned factors. Based on the analysis of the "helium-gas-water" phase balance in underground fluids, we propose three types of helium occurrence states and migration modes, namely, "water-soluble phase, gas-soluble phase, and free phase", as well as three types of helium migration mechanisms, namely, "mass flow, seepage, and diffusion".
The formation of helium-rich gas reservoirs is usually controlled by a "high-quality helium source, efficient transport, and suitable carrier", i.e., U-rich and Th-rich ancient basement and rocks in the basin, the composite transport system that communicates "source-reservoir" spanning various spheres, and the favorable tectonic background with a relatively low charge, low abundance, low partial pressure, carrier gas at local high, and other control factors. There are three main mechanisms for helium accumulation: exsolution and convergence, buoyancy-driven, and differential pressure displacement. The discovered helium-rich gas reservoirs have a distribution pattern and geological model with relatively "near helium source, adjacent to faults, low potential zone, and high position".
Helium accumulation is mainly controlled by favorable tectonic background, regional rock terrain (helium source conditions), fault system, and suitable natural gas carriers for helium accumulation. Helium content varies largely from region to region. Helium-rich areas are mainly located in the active uplift zones (basement fault activity, helium generation, and release) in ancient craton depressions, in the active zones of the ancient craton margins with favorable conditions of trap and preservation, in the slope relief zones of the foreland basins, and the active magma zones of the intracontinental rift basins near faults.
The exploration and evaluation of favorable helium- rich areas should rely on "natural gas-helium" concurrent/ parallel exploration, and under the favorable conditions for helium generation, migration, and accumulation, and based on evaluating and implementing the coupling and matching between "source-migration-accumulation" elements of helium and "generation-reservoir-caprock" conditions of natural gas. Based on the favorable trap-carrying gas areas in local regions with low potential and high position, we will comprehensively evaluate and optimize favorable helium-rich areas with "connected source and trap, low potential and high position, matched natural gas and helium”.
Nomenclature
R—3He/4He of samples, dimensionless;
Ra—3He/4He of air, dimensionless.