Abstract: There are significant differences between accumulations of crude oil and accumulations of natural gas in their distribution, hydrocarbon source, accumulation time and caprock thickness in Zhongguai, 5th and 8th districts of Karamay Oilfield,Junggar basin, Northwest China. The natural gas accumulations occur in the localities far away from the Ke-Bai thrust fault and are mainly sourced from over-matured humic kerogen in the Jiamuhe Formation of the Permian. However, the crude oil reservoirsare distributed in the localities near the fault, and are chiefly derived from the matured sapropelic kerogen in the Fengcheng Formation of the Permian. The charging time of the natural gas was later as compared with that of the crude oil, although both of them were accumulated in the Late Triassic to Jurassic. The cap rocks less than 10 m thick could hardly seal natural gas, but could effectively seal small scale crude oil reservoirs in this area.