HELIUM ISOTOPE COMPOSITIONS IN SEDIMENTARY BASINS IN CHINA

The abundance and isotopic composition of He have been determined for a variety of natural gases (oil-field gases) in sedimentary basins in the continent of China. The He-3/He-4 and He-4/Ne-20 ratios range from 0.004 to 4.99 Ra (where Ra denotes atmospheric He-3/He-4 ratio = 1.4 x 10(-6)), and from 100-39,000, respectively Helium in the gas sample is composed of mantle-derived He and crustal radiogenic He. The high H e-3/He-4 ratios were observed in the sedimentary basins which underwen t Cenozoic extension in the eastern domain, such as the Songliao, Liao he, North China, Subei, Sanshui and South China Sea basins. The basins where mantle-derived He is recognized are characterized by the presen ce of surface and subsurface volcanic rocks, high terrestrial heat flo w, deep faults and thinned crust. On the other hand, regions which und erwent pre-Tertiary extension such as the Jungar, Tarim, Tuha, Caidam and Yumen basins in the NW and tectonic loading such as the Ordos and Sichuan basins in the central domain are marked by almost pure radioge nic He-4 of crustal origin and thus by an apparent absence of mantle-d erived He, which coincides with complete lack of Cenozoic volcanics, l ow terrestrial heat flow and thickened crust. Such a He-3/He-4 distrib ution pattern clearly indicates that tectonic environment control the distribution of He isotopes in the continent of China.