Differences of sedimentary characteristics of sublacustrine fans between conglomerate-rich and sand-rich types in the Weixinan Sag, Beibu Gulf Basin

Large oil and gas resources have been found in sublacustrine fans of the 1st member of Liushagang Formation in the Weixinan Sag, Beibu Gulf Basin. The sublacustrine fans in the study area were divided into conglomerate-rich and sand-rich types through the analyses of assay, core, logging and seismic data in order to clarify the reasons for different development effects of sublacustrine fans in different fields. The main reasons for different sedimentary characteristics include ancient landforms and provenance supply. Two kinds of sedimentary models of sublacustrine fans were summarized. Gravity slumping was the main trigger mechanism for sand-rich sublacustrine fans while tectonic movement was the main trigger mechanism for conglomerate-rich sublacustrine fans. Both had a good source-reservoir-caprock assemblage. However, due to poor physical properties, the favorable exploration direction for conglomerate-rich sublacustrine fans was to find secondary porosity development zones. Sand-rich sublacustrine fans were suitable to host effective traps due to better physical properties.