Seismic Sendimentary Study Of The 1st Member Of Shahejie Formation Of The Paleogene In South Of Lixian Slope Of Raoyang Sag

The Lixian slope is located in the Raoyang Sag of Bozhong Depression in Bohai Bay Basin.It is a gently slope belt which inherited and developed in the Paleogene.In recent years,great breakthroughs of hydrocarbon exploration and development have been achieved in lithologic reservoirs in slope belts.The main reservoirs are the 1st member and 2st member of the Paleogene Shahejie Formation.Among these intervals,the research mainly focuses on the 1st member of the Paleogene Shahejie Formation.Based on the core,well logging,and 3D seismic data,a high-precision sequence stratigraphic framework was established in the study area.The 1st member was divided into a third-level sequence named SQ1 in which can be further divided into a lacustrine transgressive system tract(TST)and a lacustrine regression systems tract(RST).Under the control of the stratigraphic framework,using the method of composition maturity,heavy mineral type,ZTR index and zircon U-Pb dating to indicate the sediment provenance of the 1st member.Studies have shown that the 1st member develops three sediment sources that originated from southwest,west and northwest direction,and the southwestern sediment source is the main source for the Lixian Slope.Using core facies,well logging facies,and contour maps of sandstone thickness,two types of sedimentary facies,shoal-water delta facies and lacustrine facies were identified.In the shoal-water delta facies,it mainly develops delta inner front sub-facies and delta outer front sub-facies which be consisted by subaqueous distribute channels,inter distributary bays,and distal bars.Lacustrine facies mainly develops two sub-facies,beach-bar and shallow-lacustrine sub-facies.The beach-bar sub-facies can be further discriminated into sandy clastic beach-bar and bioclastic beach-bar,which mostly occur with up-coarsing rhythm and compound rhythm.The thickness of the bar is about 6 m and the thickness of the bar-margin is about 3 m.The shallow-lacustrine sub-facies is dominated by thick-layer horizontal bedding and massive bedding of gray-black mudstone.The related theory and methods of seismic sedimentology are used to precisely describe the distribution of sedimentary facies,and detailed analyses are made to the sedimentary characteristics of shoal-water delta facies and the lacustrine facies,and the evolution of the these facies in the 1st member of the Shahejie Formation.In the stratal slices,the amplitude boundaries can be seen of the different sub-facies in shoal-water delta.In the vertical direction,the sedimentary range of the shoal-water delta in the southwest of the study area is increasing gradually,which obviously develops the feature of "small delta-plain,big delta-front".The range of delta-plain is about 18 km to 22 km and the sedimentary range of the shoal-water delta in the northwest is narrowing gradually,showing the characteristics of "large delta-plain,small delta-front".The beach-bar facies shows a strip-shape along the lakeshore line in slices.Based on the characteristics of these sedimentary facies,the depositional model of the shoal-water delta and the beach-bar in lacustrine facies in the gentle slope belts of the fault depression basin has been established.What's more,shallow water delta front of the Lixian slope has been proposed as a favorable zone for oil and gas exploration.