Identification of submarine fan in the Meishan Formation of the Ledong-Lingshui Sag in the Qiongdongnan Basin and its depositional model

The Neogene lithologic traps in the Ledong-Lingshui Sag have become hotspots for exploration of oil and gas in the South China Sea in recent years. Some large gas fields are discovered closely related to submarine fans and turbidite deposits. In order to obtain more natural gas discoveries to enhance the exploration success rate in the lithologic traps, it has become an urgent task to researchers to identify the submarine fans and to establish their depositional model. With the support of drilling and 3D seismic data from the Ledong-Lingshui Sag and following the principles of sequence stratigraphy, we started researches on the cores from drilling holes. Lithofacies, logging facies and seismic facies as well as sedimentary structures of the Meishan Formation submarine fans in the Ledong-Lingshui sag are carefully studied using well logging, FMI imaging logging and chemical data as tools. The submarine fans of the Meishan Formation is dominated by massive siltstone, with small cross bedding and scouring surfaces, which is recognized as turbidity current deposits later modified by tractive current based on grain size distribution pattern such as C-M diagram. Three types of submarine fans, say, channel type, basin floor type, and underflow reformed type, are defined. The deposits may be further divided into three periods, and the formation and evolution of the submarine fans in each period as well as the main source direction are affected by paleotopograpic framework. With the fans moving from the Lingshui sag to the Ledong sag, the scale of fans increased gradually. Upon the consideration of the type and distribution of slope breaks, relative sea level changes, deltaic source supply, and paleogeomorphological control over sedimentation, a depositional model including source, channel, slope and fan is established for the Meishan Formation in the Ledong-Lingshui Sag.