Abstract: There are numerous fault traps formed by consequent and antithetic faults in the Shahejie Formation of the southern slope of the Dongying Depression. The tectonics of these fault traps is favorable for reservoir formation but only part of the fault traps were formed into reservoir. Therefore, the geological properties of these faults restrict the effectiveness of their seal for oil and gas. This study examines various fault properties, such as activity, displacement, sandstone thickness to total formation thickness ratio, cross-sectional stress, interstitial fault tolerance, and shale gouge ratio to investigate the seal of the three main fault block types in the study area, namely consequent, antithetic, and composite consequent-antithetic fault blocks. The results demonstrated that the antithetic fault blocks of the systems with high sand content (Member 2 of the Shahejie Formation) is favorable for reservoir formation whereas reservoirs are less likely to form in the consequent fault blocks. Reservoirs can form in both the consequent fault blocks and the antithetic fault blocks of systems with medium and low sand content (Lower Member 4 of the Shahejie Formation and Kongdian Formation). In particular, formation in the antithetic fault blocks is more common. In addition, research on conjugate composite fault blocks with opposite dip directions shows that the seal of the oblique composite fault blocks is better than that of their collinear counterparts. The master and auxiliary faults with oblique composite fault blocks fulfill the empirical fault seal parameters and the corresponding collinear composite fault blocks is also favorable for reservoir formation.

Key words: fault seal, fault pattern, fault block type, fault block reservoir, southern slope of the Dongying Depression