Lateral and vertical growth and linkage of normal faults in Bozhong8-4 structure of Western Bozhong Sag, Bohai Bay Basin, China

Abstract

Many previous studies have shown that normal faults are formed by growth and linkage of shorter fault segments through time and have suggested various fault growth models. Although these models have yielded useful information on the evolution of normal fault systems in map view, the three-dimensional (3D) evolution of normal faults is poorly understood. Here, we employ an integrated 3D seismic reflection and well dataset to investigate the 3D growth process of fault zone F1 in the Bozhong 8–4 (BZ8-4) structure of the Western Bozhong Sag (WBS). We i) constructed 3D structural models of individual fault surfaces, ii) calculated the dip angles and dip azimuth distributions on fault surfaces, iii) produced D-d profiles and throw contour maps of fault surfaces, and iv) investigated the 2D/3D segmentation characteristics of geometry and displacement distribution. We found that Fault F1 exhibits segmentation characteristics of geometry and displacement distribution both along strike and dip oriented directions. Vertically, fault surfaces can be divided into three areas (lower, middle and upper parts) that correspond to three stages of regional tectonic evolution (intense faulting, relatively quiescent faulting and post-rifting). The coalescence zone of the upper and lower fault surfaces shows as a sub-horizontal anomaly of geometry and displacement distribution that is sub-parallel to the traces of the horizons. The lower and upper parts of fault surfaces also show segmentation characteristics along strike, which are identified by vertically anomalous zones of geometry and displacement distribution. However, the origins of the segmentation in this case are different. The segmentation of the lower part of the fault developed during the rifting stage is caused by the intersection of faults with different strikes, whereas the upper part of the fault is the result of the lateral growth and linkage of overlapping fault segments. Therefore, not all fault segmentation characteristics along strike can be attributed to fault growth and linkage. Based on 3D fault segment identification, we therefore establish a 3D evolution model of fault zone F1. During the deposition of the Shahejie Fm up to the member (Mbr) 3 of the Dongying Fm in the Paleogene, the faults in this area were highly active, with the development of the NE-trending Fault F1, the NEE-trending Fault Fa and the NW-trending Fault F2. The intersection of Faults Fa and F2 with Fault F1 results in Fault F1 exhibiting segmentation in geometry and displacement distribution. From the deposition of the Mbr 2 of the Dongying Fm, these faults ceased to be active. Since the Pliocene (5.2 Ma), under the influence of transtensional stress field, an en-echelon fault zone was developed in the Neogene strata above fault zone F1 active in the rifting stage. While these faults grow and link laterally, their tip-lines propagate along dip-oriented directions and activate the lower faults and link vertically with them to form the present fault zone F1.