Natural fractures and their effectiveness in deep tight sandstone reservoirs of foreland thrust belts in the southern Junggar Basin, China

Abstract

Strong tectonic activities and diagenetic evolution encourage the development of natural fractures as typical features in deep tight sandstone reservoirs of foreland thrust belts. This study focused on the Jurassic in the southern Junggar Basin to comprehensively analyze the fracture characteristics and differential distribution and, ultimately, addressed the controlling mechanisms of tectonism and diagenesis on fracture effectiveness. Results revealed that the intensity of tectonic activities determines the complexity of tectonic fracture systems to create various fracture orientations when they have been stronger. The intense tectonic deformation would impact the stratum occurrence, which results in a wide range of fracture dip angles. Moreover, as the intensity of tectonic activities and deformations weakens, the scale and degree of tectonic fractures would decrease continuously. The control of tectonism on fracture effectiveness is reflected in the notable variations in the filling of multiple group fractures developed during different tectonic activity periods. Fractures formed in the early stages are more likely to be filled with minerals, causing their effectiveness to deteriorate significantly. Additionally, the strong cementation in the diagenetic evolution can cause more fractures to be filled with minerals and become barriers to fluid flow, which is detrimental to fracture effectiveness. However, dissolution is beneficial in improving their effectiveness by increasing fracture aperture and their connectivity to the pores. These insights can refine the development pattern of natural fractures and contribute to revealing the evolutionary mechanisms of fracture effectiveness in deep tight sandstone reservoirs of foreland thrust belts.