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Structural and sequence stratigraphic frameworks and palaeogeography of the Paleogene in central-southern Liaoxi Sag, Liaodongwan Bay area

Xu Changgui~(1,2) Xu Xiaosong~3 Qiu Dongzhou~3 Lai Weicheng~2 Zhou Xinhuai~2 1 China University of Geosciences (Beijing),Beijing 100083 2 Technical Department of China National Offshore Oilfield Corporation Limited - Tianjin, Tanggu 300452, Tianjin 3 Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, Sichuan  
Liaoxi Sag, located in the western Liaodongwan Bay area of the Bohai Bay Basin, was a half-graben in the Paleogene. The exploration maturity of the central-southern Liaoxi Sag is relatively low in Liaodongwan Bay area. In this paper, the structural and sequence stratigraphic frameworks, sedimentary systems and palaeogeography of the Paleogene in the study area are respectively analyzed. The topography in the cross section of the central-southern Liaoxi Sag was irregular in the Paleogene.An inherited inter-sag isolated-island was developed.The distribution of the island was chain-like in shape, namely inter-sag chain-like island. The central-southern Liaoxi Sag in the Paleogene is divided into two sub-sags by the island, eastern and western sub-sags. The island controlled the distribution of the sedimentary systems of the Paleogene. The sedimentary systems on the two sides of the island are obviously different. The structural framework in the axial section is more complex than that in the cross section.According to the well and seismic data, six sequence boundaries and five sequences can be recognized in the Paleogene. The geneses of each sequence boundary are analyzed, and the seismic features, logging response features and the composing features of system tract and sedimentary systems in each sequence are also described. The palaeogeographic characteristics of different sequences are analyzed in the structural and sequence stratigraphic frameworks. The sedimentary systems distribution of the Paleogene in the study area shows regularity. In the western part of the sag, the major sedimentary systems are alluvial fan, fluvial and delta facies, while towards the eastern part, the major systems are delta and lacustrine facies.The palaeogeographic evolution of the Paleogene in the central-southern Liaoxi Sag is controlled by factors such as structural transition boundaries, strike-slip structure, palaeo-geomorphology, and paleoclimate.The four factors above controlled the sedimentation of the lacustrine basin not in an isolated way but in an interactional one. The major factor was different in each stage. The periodic paleoclimatic changes controlled the expanding and shrinking of the lacustrine basin, the tectonic activities controlled the efficient accommodation, while the base level and lake level controlled both the supplying rate of clastic sediments and the depositional rate of the basin.
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