Effect of block rotation on fault sealing: An example in Dongpu sag, Bohai Bay basin, China

Pre-existing faults or early formed faults may be reactivated or locked in subsequent evolution in a rift basin. The reactivation or sealing of a fault affects sedimentation in the related fault block, and in turn controls the accumulation of oil and gas. Based on fault mechanics of reactivation, this paper established equations denoting the angle range of the reactivated faults or frictional sliding faults by using Mohr stress circles. The results were applied to explain the sealing property of pre-existing faults or early formed faults in Dongpu sag. In the Dongpu sag, reactivation or sealing of the faults in fault blocks is supposed due to the rotation of the fault block which was caused by the activity of high order faults. For quantitative calculation of dip range for activated faults, the sum of the maximum and minimum principal stresses were determined by acoustic emission experiment based on the Kaiser effect of rocks, and the cohesion and inner frictional angle of rocks were measured by rock physical experiment. The calculations show, when the dips of early formed faults are from 87° to 45°, the faults will remain activation in the subsequent deformation. Otherwise, the faults will be locked as sealing faults. The results of petroleum exploration show that the peak dips of sealing faults are from 36° to 40°, and the most of sealing faults are those with dip angles less than 46–50°. These results indicate that a shallower fault is of better sealing property in oil and gas reservoirs than a steeper fault in Dongpu sag.