Logging identification and prediction of diagenetic facies in the first member of Dainan formation, Southern Gaoyou Sag, Subei Basin, China

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. Logging response templates were established to enable vertical prediction of diagenetic facies in uncored intervals. By integrating seismic data, the planar distribution of diagenetic facies was predicted, revealing that high-quality reservoirs (Weakly Compacted–Weakly Clay Cemented Dissolution Facies) are primarily distributed in the eastern and southern fan-delta front, while low-quality facies prevail in the central and western areas. This study provides a practical workflow for diagenetic facies prediction and highlights promising targets for future exploration. The study results have certain guiding significance for future exploration and development.