Simulated experiments and geochem ical analyses of different types of hydrocarbon sources（source rocks，solid bitumen，asphaltic sandstone，crude oil/oil seepages）inmarine sequences ofSouth China reveal the coexistence ofmultiple hydrocarbon sources and theirmutual transformation and continuous or overlapping hydrocarbon generation.Marine source rocks in South China commonly feature in a state conversion of gas-generating parentmaterials and a relay of hydrocarbon generation process，presenting a characteristic that can be described as“coexistence ofmultiple sources and continuity ofmultiple gas generating phases”.An index system for tracing is established in accordance with such effects as the parentmaterial inheritance，isotopic fractionation and accumulation.It is based on stable isotopic components and important tracingmeans such as composition，biomarker，light hydrocarbon，nonˉhydrocarbon gases as well as rare gas isotopes and trace elements，and can be used to trace the process of transformation of different sources and hydrocarbon generation.The ternary tracing system of gas generation and accumulation is further improved.This paper presents a gas source rock tim ing model，gasˉsource correlationmethod，identification index of gas origin and their tracing，aswell as a prelim inary model for source identification and quantification ofm ixed gas.