Sequence structure and non-structural traps of the Paleogene in the Weixi'nan Sag, Beibuwan Basin

Based on Vail sequence stratigraphic theory and analysis of seismic and logging information, the sequence in the Paleogene Liushagang and Weizhou formations in the Weixi'nan Sag is classified, the strata framework of sequence is defined and established, the characteristics of sequence structure are analyzed and the distribution of non-structural traps is revealed. One first-order sequence boundary, two second-order sequence boundaries, eight third-order sequence boundaries and ten third-order sequences are identified in the Liushagang and Weizhou Formations. According to the characteristics of sequence boundary structure, the third-order sequence boundaries can be divided into three types: type T1, type T2, and type T3. And according to the characteristics of sequence stratigraphic structure, the ten third-order sequences can be classified into four types: one-divided sequence, two-divided sequence, three-divided sequence, and four-divided sequence. The favorable areas for the development of stratigraphic overlap traps are above type T1 and type T3 sequence boundaries, while the unconformity barrier traps are developed below type T2 and type T3 sequence boundaries. The favorable development periods of lithologic traps are the early period of three-divided sequence and four-divided sequence and the late period of two-divided sequence and four-divided sequence.