Sedimentary Facies Analysis of the Neogene Along the Northern Margin of the Qiongdongnan Basin in the Northern South China Sea

ABSTRACT In marginal‐marine nearshore–shelf systems, sedimentary environments constrain sand–mud zonation and ichnofabric development, both of which are associated with spatial variations in reservoir quality. This study focuses on the Neogene strata of the Songtao Uplift on the northern margin of the Qiongdongnan Basin. By integrating cores, thin section, electrical micro‐resistivity imaging (ERMI) logs, conventional logs, and porosity–permeability data, a multi‐proxy facies indicator system was established to delineate sedimentary microfacies and construct a sedimentary facies model. On this basis, reservoir‐quality variations were evaluated. Six sedimentary microfacies were identified: upper nearshore, middle nearshore, lower nearshore, inner shelf, outer shelf, and turbidite depositional facies. The study area is dominated by a nearshore–shelf sedimentary system, with episodic turbidite deposits developed locally. Reservoir quality shows a systematic association with facies‐dependent sand–mud architecture and ichnofabric geometry. Sand‐rich nearshore intervals generally exhibit higher porosity and permeability than mud‐rich shelf intervals, with the upper nearshore representing the most favorable reservoir belt. The lower nearshore can locally retain relatively favorable properties where sandy beds coincide with Planolites – Thalassinoides ichnofabrics characterized by combined linear and reticulate burrow geometries, whereas the inner and outer shelf microfacies are generally characterized by low porosity and permeability. Accordingly, the “facies‐belt framework + burrow‐connectivity potential” approach is proposed as a hierarchical geological screening framework for identifying potentially favorable bioturbated intervals in analogous marginal‐marine nearshore–shelf systems.