Abstract:

Based on the analyses of chemical components and carbon isotopic composition of 35 natural gas samples from the Tertiary and Cretaceous reservoirs in 9 fields or trap structures in the northern edge of Qaidam Basin,we defined indexes for identifing genetic types of gas,determined the thermal evolutionary stages,and identified the sources of natural gases.The result shows that the natural gases in the north margin of Qaidam Basin can be divided into three genetic types:coal-type,oil-type,and hybrid-type.Some of the gas samples from Yikeyawuru,Nanbaxian,Lenghu-5,Eboliang and Mabei are of coal-type gas;several gas samples from Mabei and Lenghu-3,Lenghu-4 and Lenghu-5 are of oil-type,and some gas samples from Hulushan and Mabei are of hybrid-type.Samples of coal-type from Yikeyawuru,Nanbaxian,Lenghu-5,and Eboliang,are mostly at high maturation stage;those of oil-type from Lenghu-3 and Lenghu-4 are mostly at maturation stage,those from Hulushan are at over-mature stage,while samples from Mabei have highly-varying maturities.Gases in Lenghu-3,-4,and-5,Eboliang,and Hulushan,were mainly sourced from the Kunteyi depression,among which gas in Lenghu-3 was generated by sapropel organic matter and those from Lenghu-4 and-5,Eboliang,and Hulushan,were sourced from humic organic material.Gases in Nanbaxian were derived from humic organic material in the Yibei depression;and those in Mabei was originated from such depressions like Yibei,Saishiteng,Yuqia,and Gaxi.

Key words: natural gas, carbon isotope, genetic type, gas source discrimination, Qaidam Basin