CHARACTERISTICS OF ISOTOPE COMPOSITION OF GASIFORM HYDROCARBON AND IDENTIFICATION OF COAL-TYPE GAS

Abstract

The natural gas which forms in the course of the formation of coal in the coal-series stra-ta is called the coal-type gas. It is an important genetic type of combustible gas. This papertakes the isotope composition of carbon and hydrogen as a key factor to make an approachto the identification of the coal-type gas and the oil-type gas. According to the isotope com-position of methane carbon and hydrogen, we have divided the natural gases in differentplaces into biogas, coal gas, oil-type gas and coal-type gas. There is a mixed gas area bet-ween the coal-type gas and the oil-type gas. The conditions for the formation of the areaare comparatively complex. The overmature oil-type gas overlies the coal-type gas in distri-bution. According to our studies on oil-gas areas in the Sichuan basin and Eerdos basin,we come to know that the coal-type gas in these basins has an isotope composition of highercarbon. Its mathematic expression of δ~(13)C_1-R_0 is δ~(13)C_1‰ = 8.641 lgR_0-32. 8, representing a mod-el of coal-type gas which forms in the continuously-subsiding basin. This paper also refersto the types of parent materials, organic facies and maturity of deep gas (more than 4000 m)which was discovered recently in the Zhongyuan (Central Plains) Oilfield. We have point-ed out that this deep gas is probably from the Carboniferous-Permian series. Its parent ma-terial is marine kerogen type Ⅱ. The source rock is at the thermal evolutional stage of liq-uid window.