GEOLOGICAL CHARACTERISTICS AND ENRICHMENT FACTORS OF HELIUM RESOURCES IN CHINA

The exploitable helium (He) resources in China are mainly associated in natural gases. Based on the geochemical anatomy of typical He-rich/bearing gas fields(reservoirs), this work systematically elucidates the geological characteristics, enrichment rules and geological evaluation techniques of He in natural gases and provides theoretical and technical support for exploring He-rich gas resources and selecting extractable He gas fields.

The degree of He enrichment in natural gas varies in different regions of the world. In China, He is widely dispersed, but most of them are He-poor with content ~ 0.1%, and more than 80% of gas wells have He content < 0.1%. The main reason for the existence of He-rich reservoirs in the U.S. is the development of large and ancient U/Th-rich cratons, and less tectonic stability loss and late damage, with relatively simple migration systems. The geotectonic background and He accumulation conditions in China are opposite, as developing small to medium-sized cratons, and complicated multi-rotation tectonic evolution with relatively complex migration systems.

Based on the analysis of the formation conditions and enrichment characteristics of He-rich gas fields found in China, He enrichment is controlled by three major factors: ①High-quality He source: According to the source lithology, it can fall into granite, pegmatite, gneiss, granite gneiss, black shale, bauxite, sandstone, etc. The He in natural gas in central and western China is mainly crustal with R/Ra<0.5, since ancient basement and granite control the macroscopic distribution of He-rich areas. The He in the eastern rift basins is characterized as mixed crustal-mantle source with R/Ra 0.1 - 5.8, while mantle-sourced He is mainly migrated along the tectonic magmatic activity zone. ②Effective migration system: The basin basement fractures, cover fractures, unconformities and transporting layers in China’s central and western regions are effective migration systems. In China’s eastern region, the basins are in an active tectonic environment of extensional rifting, and several sets of deep major fractures are developed in the basin, with mixed He genesis. ③Favorable accumulation carriers: Favorable natural gas traps and geological environment with proper carrier gas, and less tectonic activity in later stages are the basis for He accumulation and stable preservation.