| The sequence stratigraphic frame of Dongying formation in Chengdong area of Chengbei sag(Jiyang depression,Bohai Bay Basin)has been built based on the theories of Sedimentology,Sequence Stratigraphy.Within the Sequence stratigraphic frame,geologyical characteristics of each sequence and the corresponding depositional system have been studied with the use of 3D seismic data,well logs,cores,cumulative grain-size distribution curve and C-M graphic.Conclusions of the research as follow: 1)there are 3 sequences in the research area.2)The lower part of Dongying formation mainly contains dark mudstone with the sand of gravity flow deposit.The middle part of the Dongying formation transforms from the lacustrine mudstone up to the delta front sandstone;in the upper part of Dongying formation developed meandering river with interbedding of sandstone and mudstone.3)In the matter of sedimentary structure,there is not only sandy debris flow,but also the turbidity current in gravity flow.While,there are wedge Cross-bedding of medium-coarse sandstone in the delta front and the meandering river paleoenvironment,as well as the erodsion surface of the bottom of Channel and branch Channel in meandering river and the delta front respectively.4)The cumulative grain-size distribution curve of Dongying formation can be divided into 2 groups as the basical and the transiens type with 6 sub-group,the basical type including “simple line type”,“arch up type”,typical “three-phase type” and “two-phase type” as well as “jump-transit-suspend” three-phase type.While the transiens type including “low-rate two-phase type” and the “low-rate three-phase type”.It shows that the C-M graphic of gravity flow in the Dongying formation of the research area not only looks like the section paralleling the C=M line,but also has the form of transiens type from turbidity current to the tractive current.5)The source in the research area comes from SW according to the anaylsis of seismic data;there are 4 Channels and 2 ruptures in the study area.Deepwater slump fans in the research area were always developing along the bottom the the second rupture,which is near the center of the lake.6)The depositional systems of Dongying formation in the research area are as follow: from bottom to top,deep-semi deep lake,deepwater slump fan without Channel,deepwater sublacustrine fan with Channel,delta front of the braided river delta and the meandering stream.The deepwater sublacustrine fan including 3 subfacies: inner fan,mid fan and the outer fan.The inner fan has the microfacies of main Channel and lateral margin of main Channel;while the mid fan has branch Channel,the lateral margin of brach Channel and the inter-branch Channel;the outer fan contains sheet sand of turbidite and the lake mud.The deep-water slump fan has the near-end and the far-end.7)The gravity flow developed in the deep-semi deep lake and can be divided into 4 types by causes,including slide,slump,sandy debris flow and the turbidity current.Surfaces of slide and the deformation of the slump were the marks of the slide and slump rock.While the container of dark gray characterizes the sandy debris flow ripped up mud and the coulder-clay with the colour of dark gray or the brownish red.Sometimes appearing the overlay structure of the block sandstone with erosion surfaces;the turbidity current often has the normal graded bed sequence,with the flamy structure,sandball structure and other syn-deformation structure,and at the bottom,there often has the sole structure like glove cast and the Channel mould.8)The gravity sands,which depositing along the Channel to the center of the lake in SQ1 and early SQ2,while the delta front of braided river delta in the late period of SQ2 are the potential subtle reservoir and the main targets of exploration. |
PDF Full Text Request