Methodology for quantitative prediction of low-order faults in rift basins: Dongtai Depression, Subei Basin, China

Abstract

In eastern China, oil and gas reservoirs mainly have fault block traps. Low-order, small-scale faults are well developed, with small fault displacements and short extension lengths, but are difficult to identify by traditional seismic methods. Based on paleotectonic interpretations, rock mechanics experimental results and stratigraphic data, combined with seismic interpretation, structural evolution and fault activity analysis, a geomechanical model of the Majiazui area in the Dongtai Depression in the Subei Basin is established to construct a three-dimensional model of the paleostress field. Finally, the control of the principal stress, shear stress, and strain energy on the occurrence and development of low-order faults is quantitatively analyzed. This research shows that fault activity was weak during the Funing period (65-53 Ma), became strong during the Dainan period (53-46 Ma) and was strongest during the Sanduo period (46-37 Ma). The Sanduo period was the main period during which the low-order faults in the Majiazui area were formed. The minimum principal stress and strain energy controlled the development of low-order faults in the Majiazui area. The low-order faults are most developed in areas that experience high minimum principal stress and strain energy. In the Sanduo period, the minimum principal stress in the Majiazui area was tensile in the north–south direction, and the maximum principal stress was vertical. In an extensional stress environment, lateral shear stress controls the strike of faults, while vertical shear stress controls the inclination of faults. The faults in the northern Majiazui area are inclined mainly to the south, and the faults in the southern Majiazui area are inclined mainly to the north. These results are of practical importance for evaluating the oil–water relationship in oil-bearing fault blocks, solving the conflict between injection and production wells and improving oil recovery.