The basic principles of organic geochemistry and research methods are the foundations for recognizing and classifying petroleum system

and source rock is the most important factor among all the four factors of pool forming.The number of different sets of source rocks in a basin is the number of petroleum systems of it.If a basin contains two or more sets of superimposed source rocks

there would form multiple petroleum system.Such kind of petroleum system usually developed in the Eogene fault depressed basins in East China and multicycle basins in West China.The time of hydrocarbon generation

expulsion and accumulation could be determined through the analysis and correlation of biomarkers of the system.Multiphase of oil generation indicates that the system underwent multiphase of reformation.