Abstract: The Ke-Bai fault is a large overthrust fault. Its sealing properties control the distribution of oil accumulation in the northwestern area of Junggar basin. By analysis of the tectonic stress, fault zone characteristics, logging response and formation water features, we come to a conclusion that the Ke-Bai fault is typical of sectional sealing. The fault seals well where the fault strike is vertical to the tectonic stress, the tectonic mudstone rock is developed, the thickness of the damage zone, change of AC and increment of SDT are relatively small, the formation water is of CaCl2 type near the fault zone, and the Na+ to Cl- ratio is less than 0.7. The fault seals badly where the intersection angle of fault strike with the tectonic stress is less than 45°, the thickness of the damage zone, change of AC and increment of SDT are relatively large, the formation water is of NaHCO3 or Na2SO4 type, and the Na+ to Cl- ratio is more than 0.85. The sector of the fault where it seals badly is the major migration pathway, while the foot wall part where the fault seals well is the preferential place for oil accumulation.