Deep-water Sequence Stratigraphy And Prediction Of Litho-traps Distribution In The Middle Sub-member Of 3^rd Member,Shahejie Formation,Dongying Depression

This study combines seismic profiles with well logging and core analysis data to characterise the sequence stratigraphy of the middle submember of the 3rd member,Shahejie formation?E2s3m?,Bohai Bay basin.The stratigraphic architecture,gravity-flow sandstone reservoirs,and litho-traps distribution have been investigated by application of sequence stratigraphy,petroleum geology and seismic sedimentology.The E₂s₃^m can be divided into one third-order sequence or ten para-sequence sets,based on the characteristics of sequence stratigraphic boundaries and sedimentary successions.Lowstand-transgressive and highstand systems tracts are separated by initial and maximum lake flooding surfaces.During the depositional process of E₂s₃^msequence,a series of syngenetic faults in nearly NE direction formed in Dongying lacustrine basin by regional extention.These normal faults make up the fault slope break zone as their ladder-like distribution in study area.The architecture of E₂s₃^m third-order sequence is controlled by the development of fault slope break zone and lake-level change under the condition of sufficient provenance supply.Activities of syngenetic faults lead to characteristics of trichotomy in systems tracts and control the distribution and thickness of lowstand systems tract?LST?.LST includes three para-sequence sets and its distribution is limited to the downdropped block of syngenetic faults.The thickness of transgressive systems tract?TST?is small,only one para-sequence set can be identified which corresponds to widely distributed shale in this area.Highstand systems tract?HST?developed anomaly as a result of adequate sediments supply.HST consists of six para-sequence sets and shows progressive progradation of Dongying delta.As a result,Dongying delta front,or shoreline position advanced more than 20 km to the NWW direction.HST is characterized by three different types of progradation in terms of seismic reflection configurations.Early sigmoidal progradation?relative rise of lake-level?,followed by oblique progradation?relative stillstand of lake-level?,which is then followed by oblique progradation associated with offlap?relative fall of lake-level?.These different progradation geometries indicate the succession developed from early ascending to nearly horizontal normal regression,followed by late forced lacustrine regression.Seismic sedimentology of delta-turbidite sedimentary system of E₂s₃^m was studied based on studies of sequence stratigraphy and sedimentology.According to a case study from parasequence sets 8,three delta-turbidite sedimentary systems from different provenances were confirmed within the 4th order sequence,and distribution of thin sandstone were identified by strata slices.Seismic velocity heterogeneity in delta-turbidite areas resulted from particular relationships of strstigraphic contact and sharply lateral changes in strata thickness and lithology.For seismic sedimentology in delta-turbidite system,phase adjustment after fine well-seismic calibrating for the target reservoir is the basis,and restriction from 4th order sequence?rather than systems tracts or 3rd sequence boundaries?is becoming the key for making isochronism certain of stratal slicing.Litho-traps related with gravity flow sandstones can be divided into two different parts in terms of the types of sandbody: sandbody isolated by source rock or sandbody laterally connected with faults.Hydrocarbon accumulation of the former sandbody is characterized by self-generation and self-accumulation,while the other is characterized by “lower generating-upper reservoiring”.Gravity flow depositional facies accompanied by source faults in structural high part with lateral sealing is more likely to form litho-traps.