Restoration Of The Original Sedimentary Extent Of Bozhong Depression In Bohai Bay Basin During The Sedimentary Period Of The Third Member Of Shahejie Formation In Paleogene

Oil and gas in continental rift basin is usually characterized as short migration distance. Large scale oil and gas fields are usually around those sags with high-quality hydrocarbon source rocks, that is, rich hydrocarbon generation depression provides an important material basis for the formation of large scale oil and gas fields. So it is the first problem to solve to judge whether a sag is rich for oil and gas exploration. Bohai Bay Basin, which contains abundant oil and gas resources, is a typical continental rift basin in the east of China. Major hydrocarbon source rocks are in the third member of Shahejie Formation in Bozhong Depression. So it is an important part of oil and gas exploration to evaluate the hydrocarbon-generation potential in every sags among Bozhong Depression.But on previous research the isolated sags were usually viewed as the key point, while the relationship of these sags was ignored. The transformation of the basin after the sedimentary period of the third member of Shahejie Formation was researched little and less than systematic. While previous researches indicated that Bozhong Depression may be broader and more connected. The tectonic movement after the sedimentary period of the third member of Shahejie Formation broke the depression. So it is very important to restore the original sedimentary appearance of Bozhong Depression and find the relationship of the isolated sags during the sedimentary period of the third member of Shahejie Formation, for it will contribute to evaluate the hydrocarbon potential objectively in current sags. Solving this problem has practical significance in oil and gas exploration.Employing the research thought for reformed basins, based on a large amount of seismic sections, loggings and fission track data, the original sedimentary extent of the Bozhong Depression during the sedimentary period of the third member of Shahejie Formation is restored in this paper. The research shows that Bozhong Depression was broader and these sags in it were connected with each other. It was sedimentary on the main part of Shijiutuo Rise, Qinnan Sag and Bozhong Sag was connected during the sedimentary period of the third member of Shahejie Formation. There was an ancient rise only on the east of the area. Shaleitian and Chengbei Rise have existed as provenance. Shana and Chengbei Sag were connected with east Bozhong Sag. Part of Bonan Rise existed as provenance on the east, while it is sedimentary on the west of Bonan Rise. That is, Bozhong Sag and Huanghekou Sag were connected on the west. Miaoxi Sag had been strongly reformed since the end of the Eocene epoch, nearly one quarter of the third member of Shahejie Formation were denudated on the east of the sag. The Northern Miaoxi Rise and the Southern Miaoxi Rise were synsedimentary faulted blocks, which played the role of the provenance area for Miaoxi Sag in the sedimentary period of the third member of Shahejie Formation. It was connected with Bodong Sag on the central and north of Miaoxi Sag, and bordered on Huanghekou and Laizhouwan Sag to the south of the sag. Bodong Rise did not exist as well. Bodong Sag and Bozhong Sag were deposited as a whole. Bozhong Depression has formed under the mutual effects of the various factors instead of single taphrogenesis. According to the original appearance of Bozhong Depression, current isolated sags such as Qinnan, Bodong, and Miaoxi Sag may be hydrocarbon-enriched, and host good exploration prospects as well.