Differential characteristics of internal structures of strike-slip faults and their multiscale interactive identification mode: a case study of Carboniferous volcanic rocks in Chepaizi Uplift, Junggar Basin

Multi-phase transpressive strike-slip faults are well developed in the Carboniferous strata of the Chepaizi Uplift, Junggar Basin. These faults are characterized by severe weathering at the top, strong heterogeneity, poor stratification, and indistinct marker beds, resulting in great difficulty in fault identification. To better understand the unit characteristics within the fault zone and establish a method for their identification, a detailed analysis was carried out based on field geological investigations. By integrating core data, well logging, seismic data, and analytical testing, the differential characteristics of internal structural units of transpressive strike-slip faults at different levels were clarified. On this basis, an innovative multiscale interactive calibration identification mode was established. The results showed that the strike-slip faults consist of three structural units: fault core, slip-fracture zone, and induced fracture zone. Faults of level 4 and above exhibit complete development of all three units and five zones, while level 5 faults do not develop fault core. Fault cores develop fault gouge with severe cementation, high compaction, and almost no permeability. In contrast, both the slip-fracture zones and induced fracture zones develop multiple sets of fractures, accompanied by dissolution pores. These zones have higher acoustic time (AC) and caliper (CAL) log values, and lower bulk density (DEN), among which the slip-fracture zones have better physical properties. For the same fault, the active side exhibits more extensive and larger-scale fracture development than the passive side. Across a fault zone, from protolith to fault zone and then back to protolith, the degree of core fragmentation increases first and then decreases; AC and CAL values also increase first and then decrease; DEN values decrease first and then increase; imaging logging images change from dark to bright. The proposed identification mode enables quantitative characterization of internal fault structures and provides a method for fracture prediction in areas lacking geological data. This approach holds important practical value for clarifying the controlling roles of strike-slip faults in hydrocarbon accumulation.