Deep-lacustrine Sediment Gravity-flow Sedimentary Systems In The Third Member Of Dongying Formation Draped On The Eastern Slope Of The Chengbei Low Uplift,Bohai Bay Basin

Deep-water sediment gravity-flow sedimentary system,formed across slopes and the base-of-slope,represent a major type of petroleum reservoir in both marine and lacustrine basins worldwide.Stepped slope as an important type of slope,over 75 percent of Paleogene deep-water petroleum discoveries throughout the world are concentrated in the sediment gravity-flow sedimentary system along these slopes.Sedimentologists have only a preliminary understanding of deep-water sediment gravity-flow sedimentary system on stepped slopes at present.In recent years,a few published studies have revealed the submarine sediment gravity-flow deposits on stepped slopes in continental margins.In contrast,sediment gravity-flow deposits on stepped slopes in deep-lacustrine basin margins remain poorly understood.In view of this research situation,this study analyses deep-water sediment gravity-flow deposits in middle and lower parts of the third member of Oligocene-aged Dongying Formation,which is draped on the eastern slope of the Chengbei Low Uplift,to investigate the facies types,fluid transformation characteristics during deposition,spatial and temporal evolution characteristics and controlling factors of deep-water sediment gravity-flo w deposits on stepped slopes in lacustrine basin margins.The paleotopographical reconstruction result indicates that the study area displayed stepped slope topography incised with canyons and gul ies before the deposition of the Ed3 Member.Fourteen lithofacies were recognized in the deep-water sediment gravity-flow deposits of the study interval,which were deposited by cohesionless debris flows,high density turbidity currents,low density turbidity currents,liquefied laminar flows and hybrid flows.These lithofacies were further grouped into six microfacies,distinguished on the basis of lithofacies composition and vertical facies successions observed in cores and well logs,namely fan valey fils,confined channel deposits,poorly-confined channel deposits,levee deposits,interchannel deposits and lobe deposits.These microfacies are grouped into four systems,namely fan valey fills,confined channel-levee,poorly confined channel-levee and lobe systems.Four systems have different development characteristics and spatial distribution characteristics,and as architectural elements constitute the deep-water channel-lobe complexes of the study area.There are three types of fluid transformation during the deposition process of the deep-water channel-lobe complexes in the study area,which are the transformatio ns from cohesionless debris flows to turbidity currents,from turbidity currents to liquefied laminar flows and from turbidity currents to hybrid flows.The internal structure,development scale and distribution characteristics of the deep-water channel-lobe complexes can evolve with time.The interaction between paleotopography,lake level fluctuation,hyperpycna l flood events and paleoclimate was the primary control on the development of deepwater stepped slope systems in lacustrine basin margins.Paleotopography directly controls the spatial heterogeneity and spatial distribution of facies.Lake level fluctuation directly causes the variability of accommodation space on stepped slopes across time,and thereby determined spatial and temporal variations in the architecture of the deep-water stepped slope systems.Hyperpycnal flood events,as trigger factors,caused the generation of the deep-water stepped slope systems in the study area.In addition,paleoclimate affects the sediment supply and lake level fluctuation.The deepwater stepped slope systems in lacustrine basin margins general y have good trap conditions,oil source conditions,hydrocarbon migration conditions and sourcereservoir-caprock associations.