Fault growth and linkage: Implications for trap integrity in the Qi'nan area of the Huanghua Depression in Bohai Bay Basin, China

Abstract

Based on comparisons between fault growth histories and the distributions of paleo-oil and existing oil accumulations, the fault growth and linkage processes have been proposed as being critical in determining the location, timing, and oil preservation of “like-dipping” (with inclinations the same as those of the corresponding strata) and “oppositely-dipping” (with inclinations opposite to those of the corresponding strata) fault-bound traps in the Qi'nan area of the Huanghua Depression in Bohai Bay Basin. The critical fault throw is a key factor determining the leakage behavior and integrity of these traps. “Oppositely-dipping” fault-bound traps are developed at the throw maxima of the footwalls; these throw maxima are located at the trap crests. Once the throw exceeds the threshold, the oil columns in the traps become fully breached (emptied). “Oppositely-dipping” fault-bound traps with preserved oil are found in almost all single-layer systems (either fully preserved or fully breached). However, “like-dipping” fault-bound traps are found where interacting fault segments are hard-linked, and the throw minima are located at the trap crests. A closure bounded by a “like-dipping” reactivated fault might still retain hydrocarbons when the throw of crest is below the established threshold. “Like-dipping” fault-bound traps with preserved oil are found in single-layer and multilayer systems (either partially preserved or partially breached). Therefore, a trap integrity model is constructed on the basis of the aforementioned observations. The results of this model exhibit great consistency with the results derived from 31 drilled fault-bound traps containing petroleum fluid inclusions in the Qi'nan area. The model successfully explains the complex distributions of paleo-oil and preserved oil columns in the “like-dipping” and “oppositely-dipping” faults. In addition, the mechanisms proposed in the model may be applicable to other plays or basins.