Abstract: Based on well log and logging data, using experimental test method, the sealing condition and mechanism of three sets of Ordovician cap rocks in the northern slope of the central Tarim Basin were studied. Thick mudstones of the Upper Ordovician Sangtamu Formation cover the whole study area and provide good regional sealing for oil and gas. The difference of shale content of carbonate cap rocks in the Lower Ordovician Lianglitage 3rd-5th members is obvious, but the distribution of oil and gas is different, illustrating that shale content is an important factor affecting the sealing capacity of cap rocks in the Lianglitage 3rd-5th members. When GR value reaches 20 API, the Lianglitage 3rd-5th members work as effective cap rocks, sealing oil and gas in the underlying Yingshan Formation. Due to the influence of filling, cementation and selective corrosion of atmospheric precipitation, dense and high-resistivity layers were formed in the Yingshan Formation, blocking oil and gas mainly by the mechanism of displacement pressure difference. When the displacement pressure difference between high-resistance layer and underlying reservoir bed is above 1.5 MPa, the high-resistivity layer is considered as a local cap rock, providing certain sealing for oil and gas inside the Yingshan Formation.