Abstract: The Fengxi Area in the Qaidam Basin is characterized by lacustrine carbonate interbedded with thin terrigenous clastic rocks, belonging to complex mixed carbonate reservoirs. Due to the characteristics of thin interbedded deposition, variable lithology, strong heterogeneity and complex pore structure, evaluating effective reservoirs is challenging. For this purpose, using high-resolution resistivity imaging (FMI), lithoscopic logging (LS) and drilling core data, based on the identification of sedimentary structures and rock bedding, the rock structure was classified into five types: algal limestone, dark spotted, massive, weakly layered and strongly layered; using nuclear magnetic resonance logging (CMR) and mercury injection experiments, the pore structure was classified into four types; combining the rock structure classification and pore structure classification results, a set of logging identification methods suitable for effective mixed carbonate reservoirs in this area was developed. The study shows that the dominant reservoirs in the Fengxi Area are widely distributed and laterally continuous and stable, but the vertical thickness of dominant reservoirs varies among oil units. Among them, the dominant reservoirs of unit N₂¹Ⅳ₃ are mainly concentrated on the western flank of the main structure, with reservoir thickness gradually thinning eastward; the dominant reservoirs of unit N₂¹Ⅴ₅ are mainly concentrated in the central part of the main structure, with reservoir thickness gradually thinning toward both ends. The research results effectively support well placement and reserves submission studies in the Fengxi Area.

Key words: rock structure classification, pore structure, reservoir classification, mixed carbonate reservoir, logging identification, Qaidam Basin