Fault characteristics and their control on accumulation in block LS13-2 of Lingshui Sag, Qiongdongnan Basin

In the northern part of the Qiongdongnan Basin, the block LS13-2 of the Lingshui Sag is the only exploration area in the Meishan Formation known to have natural productivity, representing an accumulation model of source in the lower section and reservoir in the upper section. Source-connected faults serve as a bridge connecting source rocks and reservoirs, playing an important role in the vertical migration and accumulation of oil and gas. To clarify the fault characteristics and their controlling mechanisms on oil and gas accumulation in this area, the study focused on the F2, Fs, and flower-shaped faults above the paleotectonic ridge. The fault patterns, levels, periods, activity intensity, evolution models, and formation mechanisms were systematically analyzed based on the transtensional fault-related fold theory and detailed structural interpretation techniques. The accumulation controlling effects of faults through oil and gas transport and sealing were comprehensively evaluated in combination with production data from drilled wells. The block LS13-2A developed a fault system of three types, three grades, and three periods. The F2, Fs, and flower-shaped faults above the paleotectonic ridge were classified as second-, third-, and fourth-level faults, which were active during the fault-depression period, the Ledong Formation and the Sanya Formation sedimentary periods, respectively. Tectonic evolution were divided into four stages: Early Eocene, Oligocene, Early Miocene, and Pliocene. The formation mechanisms of the faults were obviously controlled by paleo-tectonic ridges and other preexisting structures. Faults in the area were divided into three types: continuous transport, late transport, and composite transport. Among them, the continuous transport F2 fault and the western part of the late transport Fs fault constituted a favorable transport system. Regarding sealing capacity, sand bodies were well developed in the Meishan Formation. The sealing capacity of the middle section of the F2 fault was relatively good, while the western and eastern sections were poor. In contrast, the Sanya Formation overall showed a mudstone-encased sandstone assemblage, and its sealing capacity was significantly better than that of the Meishan Formation. Traps which possess favorable preservation conditions in the Meishan Formation and large-scale reservoir development in the Sanya Formation are expected to become favorable targets for the next step of oil and gas exploration in the study area.