It is considered based on the simulation tests and study of the hydrocarbon generating .sections related to oil generated from organic matter, gas from organic matter and gas from cracking of oil on the northern slope of the Tazhong Uplift in the Tarim Basin that the threshold of cracking gas from oil is deeper than the threshold of cracking gas from kerogen. The cracking gas from oil is very different from the cracking gas from kerogen in components and carbon isotopes. Some examples of gas fields, in which the gas was mainly derived from the secondary cracking of oil < show that the secondary cracking gas from oil is a special and important gas source.