Formation mechanism of fault accommodation zones under combined stress in graben basin: Implications from geomechanical modeling

A fault accommodation zone is a type of structure that is defined as regulating displacement and strain between faults structure. Increasing numbers of fault accommodation zones are being identified in graben basins, indicating the potential exploration target and petroleum accumulation areas. This study aims to analyze the formation mechanism and development of fault accommodation zones under combined stress by a numerical simulation method considering geomechanical modeling. Using three-dimensional (3-D) seismic interpretation and fractal dimension method, exampled with the Dongxin fault zone, the fault activity and fault combination pattern were conducted to quantitatively characterize the activity difference in fault accommodation zones. Combined with mechanical experiment test, a geomehcanical model was established for fault accommodation zones in a graben basin. Integrating the paleostress numerical simulations and structural physical simulation experiment, the developmental characteristics and genetic mechanism of fault accommodation zones were summarized. Influenced by multi movements and combined stresses, three significant tectonic evolution stages of the Dongxing Fault Zone (DXFZ) were distinguished: During the E_s³ sedimentary period, the large difference in the stress, strain, and rupture distribution in various faults were significant, and this stage was the key generation period for the prototype of the DXFZ, including the FAZ between large-scale faults. During the E_s² sedimentary period, the EW-trending symmetric with opposite dipping normal faults and the NE-SW trending faults with large scale were furtherly developed. The junction area of two secondary normal faults were prone to be ruptured, performing significant period for inheriting and developing characteristics of fault accommodation zones. During the E_s¹ sedimentary period, the high-order faults in the DXFZ exhibited the obvious fault depressions and strike-slip activity, and the fault accommodation zones were furtherly inherited and developed. This stage was the molded and formative period of the FAZ, the low-order faults, and the depression in the DXFZ.