关键词: 氦气, 页岩气, 煤层气, 富氦非常规天然气, 富集成藏机制

Abstract: At present, it is the most important way for industrial helium production by extracting helium from helium-containing and helium-rich natural gas reservoirs. It is of great significance to evaluate the helium content and formation mechanism of unconventional gas with huge reserves, which can enrich the source of helium resources. The geochemical characteristic analyses of 450 helium-containing shale gas and coalbed methane (CBM)samples taken from the world show that the helium content of shale gas ranges from 6×10^-8 to 0.011 4 (0.057 % on the average), that of CBM ranges from 5×10^-7 to 0.022 8 (0.115 % on the average), and that of more than 24 % of unconventional gas samples can reach 0.05 % . Unconventional gas generally has a low ³He/⁴He ratio (from 0.002 to 0.93, 0.071 on the average), indicating that crust-derived helium is the main source. The radioactive decay calculations of uranium and thorium contents indicate that the helium content generated in shale gas source is consistent with the distribution characteristics of the currently measured helium content. The ⁴⁰Ar/³⁶Ar ratio in helium-rich shale gas is the same as that the accumulative ratio of contemporaneous source rocks. Large faults connected with deep mantle source or basement were usually not developed in helium-rich shale gas reservoirs. All of those indicate that helium in shale gas is mainly resulted from the radioactive decay of uranium and Th-rich minerals in the shale source. Helium has a stronger adsorption capacity than methane, and the lower concentration of helium in shale gas reservoirs may lead to capillary resistance, which are the primary causes for the preservation of helium in shale gas. The content of helium generated in CBM source is usually lower (less than 20×10^-6)than that currently measured in gas reservoir, and the helium in helium-rich CBM is primarily originated from the radioactive decay of basement rocks. Helium-producing potential, reservoir permeability and hydrodynamics are the main reasons for the differences in helium enrichment mechanism of shale gas and CBM. The favorable zones for the enrichment of helium-rich unconventional gas include the pre-Carboniferous shale with certain gas content above 1 m³/t and uranium content above 10×10^-6, and the coal seam with late accumulation and sufficient helium source supply (including crust source helium and mantle source helium)at the bottom of the reservoir. In addition, the Cambrian shale in the Middle and Upper Yangtze areas and the Carboniferous-Permian coal seam in Ordos Basin are considered as the favorable exploration horizons for unconventional helium-rich gas in China.

Key words: helium, shale gas, coalbed methane, helium-rich unconventional gas, enrichment and accumulation mechanism