Sedimentary system and depositional model of the Second Member of the Liushagang Formation in Haizhong Sag in Beibuwan Basin

The Weixinan Depression in the northern part of the Beibuwan Basin has achieved serial breakthroughs with high-yield oil and gas flows in many wells; the main source rock series is the Eocene Second Member of Liushagang Formation (the Liu 2 Member for short). However, there is no commercial oil and gas discovery in Haizhong Sag that is adjacent to the south of the Weixinan Depression. Not only deep and semi-deep lacustrine source rocks but also sub-lacustrine fan reservoirs were developed in the main source rock series of the Liu 2 Member. Therefore, it is of guiding significance for future exploration and development to study the development law of the sedimentary system on the potential hydrocarbon generation series (source rocks and sub-lacustrine fan reservoirs) in the Haizhong Depression. Based on the high-precision 3D seismic data and logging data, we discussed the development law of sedimentary system and established sedimentary model in the Liu 2 Member through comprehensive analysis on core facies, logging facies, seismic facies, and seismic attributes, and then established a depositional model. The results show that four types of sedimentary systems were developed in the Liu 2 Member, which are shore-shallow lake, deep and semi-deep lacustrine, sub-lacustrine fan, and braided river delta. Under the control of the half-graben structural pattern, which is steep in the north and gentle in the south, the deep and semi-deep lacustrine facies were developed in the northern steep slope zone and the sag zone, where sub-lacustrine fans were also developed at the western and eastern sub-sags with the provenances in the north and east directions. Also, the shore-shallow lacustrine facies and the braided river delta front facies were developed in the southern gentle slope shallow water zone. The good source rocks of the Liu 2 Member in the Haizhong Ssg were mainly developed at the first transgressive surface (TS), and the sub-lacustrine fan facies was mainly developed in the transgressive systems tract (TST), which reflects that the development degree of source rocks controlled by tectonic subsidence factor is stronger than that of lake-level change factor at the background of faulted lake basin. This study provided research ideas for establishing sequence position and symbiotic model between deep and semi-deep lacustrine source rocks and sub-lacustrine fan reservoirs in half-graben faulted lake basins.