Petrological Characteristics, Pore Structures, and Diagenetic Models of Slump-Type Gravity-Flow Deposits in the Jiufotang Formation, Naiman Sag, China

Slump-type gravity-flow deposits are extensively developed in the Jiufotang Formation of the Naiman Sag, representing a core frontier for deep-water subtle hydrocarbon reservoir exploration. However, these deposits exhibit strong internal reservoir heterogeneity, while their diagenetic mechanisms are complex and their development pattern remains unclear. Integrating macroscopic and microscopic investigation of cores, scanning electron microscopy (SEM), micro-CT, and high-pressure mercury injection capillary pressure (MICP) data, a systematic study was conducted on the petrological characteristics and diagenesis of the gravity-flow reservoirs. The results indicate that the lithology is dominated by feldspathic lithic sandstones with low compositional maturity. The present-day reservoir quality is governed by the high spatiotemporal coupling between deposition and burial diagenesis. Compaction is the absolute dominant diagenetic factor driving the densification of these reservoirs. The strong compaction resistance, derived from a low argillaceous matrix content and a well-developed grain-supported framework, is the key to the formation of high-quality reservoirs. Furthermore, three distinct diagenetic pathways are revealed: the “high-energy freezing—rigid pore preservation” pathway controls the development of high-quality exploration sweet spots; the “shear mixing—plastic pore reduction” pathway forms low-permeability transitional reservoirs; and the “viscous suspension—compaction densification” pathway indicates widespread tight sandstone exploration targets.