Characteristics of growth fault architecture and its evolution in mudstone strata: Evidence from the core of Bohai Bay basin

Abstract

Mudstone strata are important components of extensional basins and play an important role in both hydrocarbon migration and preservation. However, studies on the architecture and evolution of faults across mudstone strata have not been conducted because of the influence of traditional views on mudstone sealing. Based on the core samples of the fault zone, we studied the architecture and evolution of growth faults across the mudstone strata using core sample scanning, petrology microscopes, and X-ray diffraction. During the plasticity–semi-plasticity, semi-consolidated, and consolidation stages, soft-sediment deformation structures (fossil body distortion, dike deformation, rearrangement of ooids), fracture and breccia successively formed. In the hanging wall damage zone and the fault core, fractures with a set of similar strikes are filled with calcites or dolomites, and they are then filled with clays or sands in the footwall damage zone with two groups of different strikes. In the fault core and the damage zone, breccias have different spatial distribution, mineral composition, and relative clay mineral content because of different causes. The number of breccia belts, which formed in the late stage, in the hanging wall damage zone is considerably less than that in the footwall damage zone primarily because of the uplift of salt gypsum in the footwall of the fault. As the displacement of growth faults gradually increases, the fault width rapidly increases in the early-middle stages, and it shrinks in the late stage, which may be that the activity intensity of the contemporaneous fault decreases rapidly after the Ed. The growth fault architecture that directly affects the permeability of mudstone strata emphasizes the need to re-evaluate fluid migration and sealing along faults in extensional basins.