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Based on 3D seismic data, the polygonal fault system (PFS) discovered in the southern slope of the Qiongdongnan (QDN) Basin was studied through fine seismic interpretation and coherent cube analysis. The results show that: ① the PFS is mostly composed of small plate-type faults, with fault length of 1–3 km, maximum fault throw of 20–40 m and dip angle of 25°–45°; ② the PFS is mainly observed in the Huangliu Formation, deposited in the early stage of fast slope depression; ③ the PFS is layer-bound and can be separated into two tiers by T31′ interface with obvious channel incision features; ④ the PF of tier2 exhibit “y” structural style, including two parallel faults, master fault and associated fault, dipping in opposite directions. Their intersection relationship can be divided into three classes, and 14 sub-classes based on the fault intersection relationship of master fault and its associated fault; ⑤ the PFS is dominated by two strikes: NE60° and NW150°, indicating that it was controlled by tensile stress produced by differential settlement between the Xisha uplift and QDN basin; and ⑥ its genesis was jointly controlled by the syneresis of clay minerals and overpressure cyclical dehydration. [ABSTRACT FROM AUTHOR]

For unveiling coal-bearing source rocks in terrestrial-marine transitional sequences, the sequence stratigraphic framework and sedimentary facies of Lower Oligocene Yacheng Formation of Qiongdongnan Basin were investigated using seismic profiles, complemented by well bores and cores. Three third-order sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center, namely SQYC3, SQYC2 and SQYC1 from bottom to top. Coal measure in Yacheng Formation of Qiongdongnan Basin were deposited within a range of facies associations from delta plain/tidal zone to neritic sea, and three types of favourable sedimentary facies associations for coal measure were established within the sequence stratigraphic framework, including braided delta plain and alluvial fan, lagoon and tidal flat, and fan delta and coastal plain facies associations. Results shown that, in the third-order sequences, coal accumulation in landward areas (such as delta plain) of the study area predominantly correlates with the early transgressive systems tract (TST) to middle highstand systems tract (HST), while in seaward areas (such as tidal flat-lagoon) it correlates with the early TST and middle HST. The most potential coal-bearing source rocks formed where the accommodation creation rate ( Ra) and the peat-accumulation rate ( Rp) could reach a state of balance, which varied among different sedimentary settings. Furthermore, intense tectonic subsidence and frequent alternative marine-continental changes of Yacheng Formation during the middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. The proposed sedimentary facies associations may aid in predicting distribution of coal-bearing source rocks. This study also demonstrates that controlling factors analysis using sequence stratigraphy and sedimentology may serve as an effective approach for coal-bearing characteristics in the lower exploration deepwater area of South China Sea. [ABSTRACT FROM AUTHOR]