Natural fractures controlled by strike-slip faults in ultradeep carbonate reservoirs: A case study of the Middle and Lower Ordovician in the Tarim Basin, China

Abstract

Natural fractures controlled by faults in ultradeep carbonate strata play substantial roles as both fluid migration channels and storage spaces. However, characterizing the heterogeneous distribution of underground fractures within the complex three-dimensional geometry of strike-slip fault zones remains challenging. This study investigates the characteristics of natural fractures controlled by strike-slip faults in the fractured Middle and Lower Ordovician reservoirs of the central and northern Tarim Basin, China. Seismics, cores, and image logs were integrated to quantitatively analyze the intensity and dip angle of natural fractures and findings were verified using published sandbox simulations. The carbonate reservoir contains three main types of natural fractures: tectonic fractures, abnormal high-pressure-related fractures, and stylolites. Strike-slip faults control the distribution and characteristics of tectonic fractures across various scales. Generally, both fracture intensity and porosity exhibit a decreasing trend as the distance from the main fault surface increases. Compared with those in non-stepover zones along a strike-slip fault, natural fractures and faults in stepover zones are more developed along the fault strike, with significantly greater development intensity in central stepover regions than that at its two ends. Furthermore, strike-slip faults influence the dip angles of both natural fractures and secondary faults. The proportion of medium-to-low-dip angle fractures and faults in the stepover zone is greater than that in the non-stepover zone. Additionally, the proportion of medium-to low-dip angle fractures and faults in the middle of the stepover is greater than that at both ends. Therefore, strike-slip fault structures control the dip angle of natural fracture and the heterogeneity of secondary fault and fracture intensity. The linking damage zone in the stepover contains a larger volume of fractured rocks, making it a promising petroleum exploration target. The development of stepovers and the orientation of present-day in-situ stress substantially influence the productivity of fractured reservoirs controlled by strike-slip faults. The analysis in this study reveals that reservoir productivity increases as the angle between the strike-slip fault segment and the maximum horizontal principal stress decreases. This study provides valuable insights for quantitatively evaluating fracture heterogeneity in fractured reservoirs and establishing optimized selection criteria for favorable targets in fault-related fractured reservoirs.