Applied Models of Rare Gas Geochemistry in the Research of Natural Gases

Abstract: A large number of valuable analytic data of the chemical compositions and isotopes of natural gases have been obtained from 532 natural gas samples of 19 oil - gas - bearing basins in China. On the basis of the comprehensive research of the characteristics of natural gases it has been considered that the space distribution of the content and isotopic composition of rare gases in natural gases were controlled by tectonic settings. According to the ³He/ ⁴He values the distribution of the He isotopes in the oil - bearing basins can be divided into four areas: (1) eastern active area, where the natural gases of the basins near the Tancheng - Lujiang Fault have an average ³He/ ⁴He value of 10 ^-6, suggesting the addition of the mantle - derived volatile components, and the natural gases of the basins far from the Tancheng - Lujiang Fault have a mean ³He/ ⁴He value of 10 ^-7; (2) middle stable area, where the natural gases of the basins have an average ³He/ ⁴He value of 4× 10 ^-8, minimum of 5 × 10 ^-9, which means that He was monoradioactive; (3) northwest substable area,where the natural gases of the basins have a range of 3He/4He values from 10 ^-8 to 10 ^-7; and (4) southwest area of Hainal Island, along both sides of Honghe River Fault, where the natural gases of the basins have a range of ³He/ ⁴He ratio from 10 ^-8 to 10 ^-7 and averages 10 ^-7, indicating that He was crust - derived. A reverse Y- shaped origin model of rare gases has been proposed and two-origin types have been provided:(a) Crust - and mantte - derived type, distributed in the eastern active area, where the ⁴⁰Ar/ ³⁶Ar and ³He/ ⁴He values of rare gases increase with the addition of mantle - derived gases and (b) crust - derived type, distributed in the stable and substable area of Centrol and West China, cbaractermng that the ⁴⁰Ar/ ³⁶Ar values of rare gases increase but the 3He/4He values decrease with geological ages. Meanwhile, the time - accumulating - effect of Ar and He isotopes in natural gases is also researched. The formula which can be used to calculate the ages of crust -derived gas source rocks using He and Ar isotopic methods is suggested.The formula can be applied to tracing and comparating gas source rocks, which is of practical signficance for gas exploration. In addition, the rare gas geochemistry, hydrocarbon gas geochemistry, and the related geological backgrounds of the central north gas area of Erdos Basin, the Chuandong gas area of Sichuan Basin,the Ya 13 -1 gas fields and the Dengfang- Ledong gas zone have comprehensively been researched and some new views on the three large - or middle - sized gas areas have been suggested.