Architectural characteristics and evolution sequences of different types of faults in extensional basins—evidence collected from cores in the Dongying Depression

Abstract

Drilling cores taken from faults in the Bohai Bay extensional basin are investigated to identify the architectural characteristics and tectonic evolution of different types of faults in the clastic strata. Growth and normal faults divided by the diagenetic stage and fault throw exhibit varied structural characteristics. Regardless of the size of growth faults, the fault cores and damage zones are visible. Meanwhile, small normal faults primarily comprise damage zones with slip surfaces (including fractures and deformation bands). The breccias in the fault core of the growth fault rotate under the lubrication of an unconsolidated mudstone, resulting in a mudstone-wrapped state. However, the normal fault is formed in the consolidated stage of the mudstone layer, and the number of its structural breccias are mainly reduced by small faults or fractures lacking mudstone lubrication. The deformation mode directly determines the type of fault architecture and the propagation mode of the fault core. These normal faults (including vertical, flower, and conjugate fault zones) are the products of pure shear, wherein the σ1 principal stress axis is basically vertical, and they constitute a series of fractures or deformation bands without a unified main slip surface. These growth faults can form a complete fault architecture under simple shear, in which the σ1 principal stress axis is gradually inclined to a certain extent with an increase in the fault width. The difference between the two types of faults is essentially attributed to the fault core being easier to propagate under simple shear than under pure shear. In terms of structural evolution, the growth fault evolves from the original fault core with hanging-wall damage zone into a complete fault structure with a fault core and two damage zones. The normal fault is composed of the large conjugate fracture system or multiple single slip surfaces, which may form the fault core when pure shear changes to simple shear or when the displacement is large. The differences in the structural characteristics, deformation modes, and tectonic evolution of growth faults and normal faults would inevitably affect later fluid migration.