Heavy oil is widely distributed in the northwest edge of Junggar Basin, associated with which there is also any shallow natural gas. Compared with conventional natural gas, the carbon isotope of methane in shallow natural gas is lighter, larger in dry coefficient, and higher N2 contend of non-hydrocarbon. Based on analysis ,the shallow natural gas associated with heavy oil is crude oil biodegraded gas derived from microbial degradation of crude oil because of anaerobic bacteria (mainly methanogens ), at the same time there is any thermogenic gas associated. By comprehensive study, the shallow natural gas in northwest Junggar Basin is mainly bacterial decompositional gas from crude oil mixed with any thermogenic gas. Beside,geology environment and reservoir characteristics in northwest Junggar Basin are favorable to bacterial decompositional gas from crude oil ,and this type natural gas is great in exploration potential.