The Difference Of Sedimentary Characteristics Between The Middle Submember And Upper Submember Of The Third Member Of Shahejie Formation Of Paleogene In The Eastern Dongying Depression

Large-scale deltas are developed in the middle-upper submember of the third member of Shahejie Formation(Es3)of Paleogene in Dongying Depression.However,seismic data show that the middle-upper submember of Shahejie Formation is a slope-break delta with high-angle progradation,while the upper submember of Shahejie Formation is a low-angle gentle slope delta with no obvious progradation.The sedimentary characteristics,differential evolution and main controlling factors of the two types of deltas remain unresolved.According to the analysis of cores,logging and seismic data,using the theories of sedimentology,sequence stratigraphy and ichnology,this paper has established the sedimentary system type of the middle submember and upper submember of the third member of Shahejie Formation(Es3)in the Eastern Dongying Depression,combined with the analysis of the main control factors,established its sedimentary model and evolution,and discusses its oil and gas geological significance.The results are as follows:There are 9 parasequence groups in the middle member of Shahejie Formation.In the early stage of each parasequence group,destructive sand bodies such as gravity flow and beach bar are developed,and in the late stage,slope-break delta and gravity flow deposits are developed.The delta front is dominated by underwater distributary channels and mouth bars.There are two parasequence groups in the upper submember of the third member of Shahejie Formation.In the late stage of each parasequence group,gentle slope deltas are developed and relatively undeveloped in the early stage.In the delta front,underwater distributary channels,mouth bars and distal bars are developed,mainly underwater distributary channels.Different types and combination patterns of sedimentary sand bodies in slope-break delta and gentle slope delta were identified.In the early stage of slope-break delta(SZ9-SZ8),the waves were strong during the lake transgression,and the scale of delta sand body was small.Only a small amount of underwater distributary channel and mouth bar sand bodies were developed.Beach bar was developed in the southern gentle slope shallow water area,and sliding-slide rock and debris flow were developed locally in the deep water area.In the middle of the slope-break delta(SZ7-SZ3),the lake level began to decline,the source supply was sufficient,and the scale of delta sand body was expanded.The sand body types were mainly underwater distributary channels and mouth bars.The beach bar transformation sand body was developed in the shallow water area,and the sliding-sliding rock,debris flow and turbidite were widely distributed in the deep water area.In the late slope-break delta(SZ2-SZ1),with the continuous decline of lake level,the scale of delta sand body continues to expand,mainly developing underwater distributary channel and mouth bar sand body,developing beach bar in shallow water area,and gravity flow deposition is significantly reduced.Slow slope delta(SS2-SS1)mainly developed underwater distributary channel,mouth bar and distal bar,and the scale of the delta was further expanded.It is proposed that the tectonic conditions play an important role in controlling the delta evolution,and the tri-terminal delta division scheme with tectonic conditions,river action and wave action as the main control factors is established.The early slope-break delta(SZ9-SZ8)was dominated by wave action,and mainly developed slope-break delta and beach bar deposits,but mainly beach bar sand bodies reformed by wind and waves.The mid-slope-break delta(SZ7-SZ3)is dominated by tectonic activities,mainly developing slope-break delta and gravity flow deposition,and the gravity flow sand body controlled by tectonic conditions increases significantly.The late slope-break delta(SZ2-SZ1)began to gradually transit to the gentle slope delta,and the underwater distributary channels increased significantly,and the gravity flow and beach bar sand bodies decreased significantly.During the gentle slope delta(SS2-SS1)period,river action was dominant,and a large area of underwater distributary channel was developed.The comprehensive study of lake level change,tectonic subsidence rate and sediment supply rate shows that ?/S ratio controls the sand body types and evolution laws of the two types of deltas.