Quantitative characterization and vertical evolution of fan delta sand bodies: A case study of the Liushagang Formation in the Weixi'nan Sag, China

The study on sand body connectivity and distribution patterns is of great significance for well emplacement and injection-production pattern analysis in the A oilfield of the Weixi'nan Sag currently at a pre-development stage. Based on the current drilling data, seismic data, and fault development characteristics, this study investigates the connectivity, geometric morphology, planar distribution, and vertical evolution of composite sand bodies (multi-stage superimposed channel sand bodies) within the fault block using seismic forward and inversion modeling. The El₃Ⅰ oil layer group in the third member of the Liushagang Formation is developed in the fan delta-front sub-facies, which mainly consists of subaqueous distributary channels. The thickness of single-stage subaqueous distributary channel sand bodies ranges from 2 to 6 m, and the width of composite channel sand bodies varies from 50 to 100 m. Under the long-term transgression background, the subaqueous distributary channels in the El₃Ⅰ oil layer group are relatively narrow, forming superimposed and continuous composite channel sand bodies through lateral migration and vertical stacking. The long-term base-level cycles control the width of subaqueous distributary channels, while the mid-term base-level cycles control the thickness of these channels. The subaqueous distributary channels developed during the late stage of mid-term base-level fall are thicker than those formed during the early stage. Accordingly, quantitative relationships between channel thickness and width are established for the early and late stages of mid-term base-level fall, to finely depict the evolution patterns of channel sand body geometry and stacking styles across different stages. These findings provide important guidance for accurately predicting the planar distribution and channel width of composite subaqueous distributary channels at different stages of the mid-term base-level cycles.