Sealing capacity evolution of trap-bounding faults in sand-clay sequences: Insights from present and paleo-oil entrapment in fault-bounded traps in the Qinan area, Bohai Bay Basin, China

Abstract

Fault seals have been proven to be a significant risk to exploration success, and methods of evaluating the sealing capacity have been developed in recent decades. However, less attention has been paid to the dynamic change characteristics of the sealing capacity during fault growth. In this paper, the relationship between Shale Gouge Ratio (SGR) and across-fault pressure difference (AFPD) is calibrated to define the seal failure envelope in the Qinan area, thus providing a method for estimating the fault sealing capacity. The sealing capacity of the YEZ fault is evaluated by the SGR-AFPD relationship, which shows that the oil column heights sealable by the YEZ fault (Hseal) are greater than the actual accumulated oil column heights (Hactual). The existence of paleo-oil column is consistent with upfault oil leakage during the reactivation phase, which explains why Hseal > Hactual. As faults undergo multiphase reactivation, the throw accumulates gradually, the minimum SGR of the fault increases and eventually converges to the average Vsh of the entire sequence, which increases the fault sealing capacity during the stable phase and Hseal. In the initial fault formation stage, the minimum SGR and Hseal is small because the reservoir is self-juxtaposed. After fault reactivation, the minimum SGR and Hseal increase compared with that of the initial formation stage because the reservoir slides against more layers of shale. However, if the fault reactivates after hydrocarbon charging, although the minimum SGR and Hseal may increase or remain stable, the Hactual value decreases because of post-charge reactivation-induced leakage. In this case, Hactual may be smaller than Hseal. Thus, although calibrating the SGR-AFPD relationship is effective for predicting the fault sealing capacity, the fault activation phases and hydrocarbon charging history should be clarified first to identify hydrocarbon reservoirs damaged by bounding fault reactivation to avoid overestimating the hydrocarbon column heights.