Abstract: Previous research has concluded that the natural gas is mainly composed of coal-bed gas in Guxi fault zone, one of the richest areas of deep natural gas in Jiyang depression. Based on the analysis of source rock characteristics, many indicators, such as chemical component, carbon isotope, rare gas isotope and light hydrocarbon composition of natural gas and biomarkers of associated crude oil, combining with the study on geologic condition of gas pools, are used to clarify the genesis and source of Paleozoic natural gas in Guxi fault zone. It is proposed that there are three types of source-mixed gas in Guxi fault zone rather than coal-bed gas. Natural gas in Gubei buried hill is mainly composed of coal-bed gas sourced by C-P source rocks, mixed with part of oil-type gas. Gas from Bogu 4 and Boshen 6 buried hill are both mainly consisted of high-mature oil-type gas generated by upper Es₄ source rocks in Bonan Sag. The former is mixed with coal-bed gas, and the latter is mixed with oil-associated gas from lower Es₃ source rocks.

Key words: coal-bed gas, source-mixed gas, genesis of natural gas, Guxi fault zone, Zhanhua Sag