Characteristics of diagenetic facies and prediction of favorable areas in the 2nd sub-member of the first member of Dainan Formation reservoir in Gaoyou Sag, Subei Basin, Eastern China

Abstract Focusing on the reservoir in the 2nd sub-member of the first member of Dainan Formation (E₂d₁²) in the southern Gaoyou Sag, Subei Basin, this study systematically analyzed the E₂d₁² reservoir characteristics and diagenetic features on the basis of thin-section identification, scanning electron microscopy (SEM), X-ray diffraction (XRD), and petrophysical property data, and integrated analysis with well logging data was employed to investigate diagenetic facies characteristics. Results reveal that the reservoir predominantly comprises feldspathic lithic quartz sandstone. Besides residual intergranular pores, pore types include feldspar intragranular dissolution pores, lithic intergranular dissolution pores, and minor micropores (microfractures and moldic pores), and exhibit a pore-throat combination pattern characterized by the co-development of submicron- to micron-sized pores and submicron-sized throats. Combining core analysis data with diagenesis characteristics, the diagenetic facies in the study area were classified into four types: weakly compacted–weakly clay cemented dissolution facies, intensely compacted–dissolution diagenetic facies, moderately compacted–carbonate cemented diagenetic facies, and intensely compacted–clay mineral cemented diagenetic facies. Distinctive logging response templates were established for identifying these diagenetic facies types, enabling vertical identification and prediction within individual wells. Integrated seismic data analysis further enabled planar distribution prediction of diagenetic facies across the study area, revealing that favorable diagenetic facies predominantly concentrate in the eastern and southern sectors, while poor-quality facies cluster in central and western regions. The study results have certain guiding significance for future exploration and development.