~3He/~4He Ratios and Heat Flow in the Continental Rift-valley, Eastern China

Heat flow and the origin of helium in natural gases from fault basins of the continental rift-valley in eastern China are discussed in terms of heiium isotope geochemistry .^3He/^4He ratios in natural gases from the rift-valley range from 2.23×10^-7 to 7.21×10^-6,which are directly propor-tional to the concentration of helium and ΣNHC/ΣHC ration in natural tases.Geological and isotope geochemical data suggest that helium in natural gases consists predominantly of crustal radio-genic and upper mantle-derived helium.In a simple mixing pattern between crustal He and man-tle-derived He,mantle-derived helium in natural gases would account for 10-60%.Calculated values for heat flow (Q) range from 59.4 to 82.4mWm^-2,of which about 60 percent in the rift-valley is derived from the upper nantle.Natural helium reservoirs would be found in the areas where the up-per mantle uplifted greatly and heat flow is large in the continental rift-valley.

Tectonic and geochemical characteristics and reserved conditions of a mantle source gas accumulation zone in eastern China

Along both sides of the Tancheng-Lujiang Fracture Zone in eastern China, a series of mantle source gas pools constitute a massive-scale tectonic accumulation zone in NNE direction, with the mantle geochemical characteristics of high concentrations of CO2 and He, high 3He/4He-40Ar/36Ar ratio system and high δ13Coo2 ratios (the mainfrequency, -3.4‰-4.6‰), showing no difference from the tectonic framework of the area. In the area, thetectonic environment is a rift formed as a result of diapiric mantle injection and crust thinning to form graben-type basins and lithospheric fractures. The mantle-derived volcanic rocks and inclusions are well-developed and a high geothermal zone (mantlesource) exists in the area. The characteristics of the three components (solid, liquid and gas) of mantle, concentrated all over the same tectonic space zone, show that the rift system is of a good tectonic environment or passage for mantle degassing and gas migration. The main types of the gas pools are volcano, fault-block, anticline, buried hill and so on, but most of them are combination traps closely related with fracture. For the mantle source gas pools, rift is an optimum tectonic region, and nearby lithospheric fracture, mantle source volcanic rocks or basement uplifts are a favourable structural location when reservoir-caprock association develops.

Helium Isotope Evidence of Mantle Degassing in Rift Valley, Eastern China

Radiogenic 40Ar and 4He found in atmosphere are considered to be produced byradioactive elements（e. g. U, Th）in the inner earth. Isotope composition of no-ble gases and ratios of their components are important geochemical indicators for thetheory of earth degassing and geochemical evolution of the mantle. The continuousand 'catastrophic' models of earth degassing have been suggested. The mantledegassing would be actually 'catastrophic' during geohistory and heterogeneous in dif-

Characteristics of carbon and hydrogen isotopic compositions of light hydrocarbons

Light hydrocarbons named in the present paper refer to the natural gas-associated light oil and condensate 46 light oil and condensate samples from 11 oil-bearing basins of China were collected and their carbon and hydrogen isotopic compositions were analysed in terms of their total hydrocarbons, saturated hydrocarbons and a part of aromatic fractions, and gas-source materials and their sedimentary environments were discussed based on the above-mentioned data and the geological background of each area. From the view of carbon and hydrogen isotopic composition of total hydrocarbons and saturated hydrocarbons, it is revealed that the condensate related to coal-bearing strata is enriched in 13C and D while that related to the source material of type I-II is enriched in 12C. In general, the isotopic composition of carbon is mainly attributed to the inheriting effect of their source materials, whereas that of hydrogen principally reflects the correlationship between hydrogen isotopes and the sedimentary environment as well as the salinity of water medium. The author proposed, based on the data available, that the light hydrocarbons with δD value of less than -150%o might be considered as generated from the fresh to brackish water environment and those with δD value of higher than - 150‰ from the brackish to marine environment. It can be concluded that the features of organic materials are controlled by sedimentary facies and the different types of organic materials characterized by different compositions of stable isotopes. Additionally, the data of Jurassic condensate of Australia are listed, and its hydrocarbon-generating environment is deduced as fresh-water swamp based on the rich H and 13C

Distribution of the Helium Isotopes in Natural Gases From Oil-gas-bearing Basins in China

1 Distribution of Samples and the Measured Results Natural gases from 252 industrial gas wells of 16 oil-gas-bearing regions in China have been sampled in the past ten years.The samples were stored in 1 L double-valve, high-pressure cylinders.~3He/~4He value of the samples was analysed on a

Significance of the Isotopic Composition of He and Ar in Natural Gases

To identify the oil-gas source rocks is very important for the prospection of oil and gas, and it is a key method to detect the isotopic composition of He and Ar in natural gases for retracing source rocks. The detection of Ar isotopic composition was started in 1973 by Gavrigov et al., then the time-accumulating effect of radiogenic Ar was recognized by Chinese researchers after the discovery of the fact that the content of radiogenic Ar (i. e. 40Ar) increases with the increase of the age of source rocks, and the formula to estimate the age of source rocks was established with the consideration of the K