Sequence stratigraphic model of Bozhong subbasin during a syn-rift to post-rift transition and its controls on regionally extensive sublacustrine fans
GONG Chenglin1,2, XU Changgui3, GUAN Dayong4, WANG Qiming4, LI Dongwei1,2
1 National Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum(Beijing),Beijing 102249,China; 2 College of Geosciences,China University of Petroleum(Beijing),Beijing 102249,China; 3 China National Offshore Oil Corporation,Beijing 100010,China; 4 Bohai Petroleum Research Institute,Tianjin Branch of CNOOC Ltd.,Tianjin 300459,China
Abstract：The marked difference in sequence stratigraphy between lacustrine and marine basins requires the establishment of sequence stratigraphic model with function of predicting sublacustrine fans,aiming to a better prediction of areally extensive sublacustrine fans in the Oligocene Dongying Formation of the Bozhong subbasin. Five unconformities and four maximum flooding surfaces are recognized,based on seismic and borehole datasets. They divide the Oligocene Dongying Formation into four third-order sequences,each of which contains an expanding systems tract(retrogradational to aggradational parasequences)and regressive systems tract(progradational parasequences),resulting in an expanding-regressive sequence model. Regionally extensive channelized sublacustrine fans(i.e.,channel-lobe complexes)are composed of feeder channels and terminational fans,which consist of lobe axes,lobe fringes,and lobe distal fringes. They are mainly seen to occur in expanding systems tracts immediately above the sequence boundaries,and mainly occur on the gentle slope in front of syndepositional slope-break zones. They represent comprehensive responses to high sediment supply and vigorous sediment-gravity flows under humid climatic conditions,and to syndepositional slope-break zones. Regionally extensive channelized sublacustrine fans,coupled to favorable hydrocarbon-accumulation conditions,can form big subtle traps,which are the most favorable exploration targets of Oligocene Dongying Formation of the Bozhong subbasin. Regionally extensive non-channelized sublacustrine fans(i.e.,mass-transport complexes)contain mass-transport deposits and sandy debrites. They mainly seen to occur in regressive systems tracts immediately above the maximum flooding surfaces,and mainly occur on the steep slope in front of syndepositional slope-break zones or the hanging wall of boundary faults. They represent comprehensive responses to low sediment supply and diluted sediment-gravity flows under arid climatic conditions and to the mass-wasting processes induced by high relief gradients and other triggers. Regionally extensive non-channelized sublacustrine fans,coupled to other favorable hydrocarbon-accumulation conditions,can form big subtle traps,which are the favorable exploration targets of Oligocene Dongying Formation of the Bozhong subbasin.
GONG Chenglin,XU Changgui,GUAN Dayong et al. Sequence stratigraphic model of Bozhong subbasin during a syn-rift to post-rift transition and its controls on regionally extensive sublacustrine fans[J]. JOPC, 2023, 25(5): 992-1010.
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