Abstract: Based on the analysis of the L451 block,we know that the basement of the block is the typical structural slope-break zone located at the upward-gentle slope margin,and the cap rock overlapping on the slope of the basement shows the monoclines style.So the structure in the L451 block is simple and the faults controlled the characteristic of oil/gas.There are six faults trending in two main directions in the block,one is NNE,developed early and contemporaneously,and the other is NWW-EW,developed later.The corner-like block was formed by intersecting of the two groups of the faults.According to the analysis of geometric characteristic,the sealing properties of the faults are revealed by the several factors,such as the matching relationship between the fracture activity and the oil/gas migration stage,the extent of the continuous distribution of the mudstone-smearing zone.Then,using the fuzzy mathematics,we evaluate the sealing properties of the sandstone bodies,and the results are coinciding with the data of some wells in the oil filed.