Accumulation Mechanism Of Helium,a Strategic Resource,in Guanzhong And North Qaidam Basin

Helium,a strategic resource,is not only widely used in low temperature superconducting,industrial production and scientific research,but also has irreplaceable applications in nuclear industry and aerospace due to its chemical inertness,extremely low boiling point and low density.China has little domestic helium and depends on imports,making the security of helium resource grim.This study takes the Guanzhong geothermal field and the North Qaidam oil and gas field as research areas,uses the knowledge of rare gas geochemistry,oil and gas geochemistry,oil and gas geology and mineralogy and adopts the methods of rare gas isotope mass spectrometer,laser fluorometer,ultraviolet visible spectrophotometer,ICP-OES(inductively coupled plasma optical emission spectrometer),ICP-MS(inductively coupled plasma mass spectrometer)and EPMA(electron probe micro-analyzer).The enrichment and genetic types of helium in Guanzhong and North Qaidam Basin were studied.The accumulation mechanism of helium xvas discussed systematically from the three aspects of helium production,release and migration.The accumulation model and favorable enrichment conditions of helium were summarized.This research analysed the helium content and isotopic ratios of wellhead gas samples and determined tlie enrichment and genetic types of helium in the study area.The helium contents in associated gas collected from geothermal wells in Guanzhong Basin are 0.05-2.94%,which are higher than the helium industrial standard of 0.05-0.1%.In the North Qaidam area,the helium contents in the Mabei gas field(0.06-0.20%)are higher than the industrial standard,those in the Niudong gas field(0.01-0.02%)are lower than the standard,and those in Dongping gas field varies greatly(0.02-0.48%)and part of them reach tlie standard.The helium isotopic values suggest that helium in the two regions is both derived from the crust.On the base of the investigation of the geological settings of the global helium-rich gas reservoirs,the relationship between the crust-derived helium reservoirs and the granite bodies were revealed.Therefore,this study discussed the characteristics of helium generation in granite.The outcropped granite in the southern margin of the Guanzhong Basin and the underground granite in the Yin'e Basin are rich in uranium and thorium.The average contents of uranium and thorium are higher than the Clark values,but they are highly heterogeneous in granite.The electron probe shows that there are two occurrence states for U and Th in granite:U-Th independent minerals and U-Th isomorphic minerals.Granite can generate a large amount of helium.The amount of helium generated from granites related to the Guanzhong Basin is 14.276 billion square meters,which is much higher than the amount of helium gas estimated from the geothermal fluids in the basin(2.130 billion square meters).The release mechanism of helium in granite was studied and the release mode of helium in granite was summarized.Three kinds of gas release methods,namely vacuum crushing,melting and step-wise heating,were used to release noble gases in our studies,the results are as follows:1)Only a portion of the helium in the granite can be closed in the rock(1.5-17.4%).Combined with previous studies,the release ratio of helium in granite is as high as 80%.Argon is more difficult to release out of rocks than helium.2)Temperature is the primary factor controlling the release of helium in granite.At 250?,granite has no retention ability for 4He and the loss of 90%of 4He takes only 6.4 days.The 4He closure temperature of betafite is 125?.At this temperature,it takes 2.5 million years to lose 90%of the 4He.Combined with the 4He closure temperature of other secondary minerals,it is concluded that 27-250?(400-7800m),below 27?(shallow than 400m)and above 250?(deeper than 7800m)are the He complete,partial and none retention zone respectively.3)The fracture can further migrate the free He,which is not closed in minerals by temperature,into the fluid system of the shallow strata.The noble gas fractionation process in the Guanzhong Basin was discussed and the groundwater evolution and helium migration and accumulation mode in the geothermal field was established.The modelling results of air-derived noble gases(ADGs,e.g.,20Ne,36Ar,84Kr,130Xe)indicate that the heavy oil-water Rayleigh fractionation in open systems occurred in this region.There is a free gas reservoir near the Huaxian 2 well,where the 4He flux is high.The helium accumulation mode in the Guanzhong Basin is as follows.1)Groundwater were recharged and the dissolved ADGs were transported to the underground.2)Crust-derived gases(4He,40Ar*,etc.)were released from the crustal rocks and dissolved into the groundwater and the process lasts 0.30-1.98 Ma.3)After the hydrocarbon generation,groundwater underwent oil-water equilibrium.4)Major gas components(N2 and CH4)entered into the underground fluid system.With the increase of methane supplementary amount,there different helium reservoirs were formed in the Guanzhong Basin,namely,high helium-weak gas dissolved gas reservoirs,low helium-strong gas dissolved gas reservoirs,high helium free gas reservoir.The noble gas fractionation model in the North Qaidam Basin was discussed and the hydrocarbon charging and helium migration and accumulation model in oil and gas field was established.The North Qaidam area experienced a multi-stage phase fractionation process of oil-water and gas-water equilibrium.The Mabei area has the highest amount of oil and the Dongping area has the highest quantity of gas generally.The oil and gas characteristics of Niudong area are between Mabei and Dongping.According to the results of the fractionation model,the 4He content in the groundwater before oil-gas-Water fractionation was calculated,and the helium accumulation model in the Qaidam Basin was established.1)Groundwater recharge.2)Primary migration of helium.Helium generated by radiogenic decay of U and Th in the crust released and migrated into groundwater preserved in(micro)pores or(micro)fractures,which lasts 1.84-2.78,1.07-2.11 and 0.31-0.49Ma in Mabei,Dongping and Niudong respectively.3)Secondary migration of helium.When oil and gas are generated and migrated in a large scale,the groundwater would be in contact with the oil phase and the gas phase,resulting in the exsolution of helium,which has an extremely low solubility in water,out of groundwater.Helium would migrate along with the hydrocarbons and move into the gas reservoir.4)After the formation of oil and gas reservoirs,the supply of helium is negligible.Three favourable conditions for helium enrichment are proposed.1)The uranium-rich granitoid body and the neotectonic activity zone of the ancient platform can provide a rich source for helium.2)Free gas phase exists in the underground fluid system.Gas-water equilibrium makes helium gas continuously degassed into gas phase rather than migrate elsewhere,so the gas reservoir formed by the exsolution of water-soluble gas is very likely to be enriched in helium.3)The supplement of the major gas components in the gas reservoir is moderate.Otherwise,the gas formed in the later sage would dilute the gas with high helium content formed in the earlier stage.Therefore,it is more prone to find high helium content in a "low grade" gas reservoir,and comprehensive utilization of natural gas and helium gas can be carried out for this type of gas field.