The Depositional Model,3-D Architecture and Heterogeneous Model of Point Bar in Meandering Channels

Key words: 
• point bar of meadering channels /
• depositiomal model of lateral accretion body /
• lithofacies /
• heterogeneity

Abstract: This paper analyses the forming process of point bar,hydrodynamic regime and sc oure-and-fill mechanism of flood event,and combines the vertical sequence of p oint bar,the type of lateral accretion body(LAB),lithofacies,granularity, porosity and permeability,sets up the tranverse and vertical depositional model and the sedimentary stack of LAB in point bar,3-D architecture of point bar, and the heterogeneous model. At a stage in a flood event,the velocity of rich-sand helical bottom flow from concave bank to convex bank decreasing makes that depositional grain thins,beddi ng form changes and bedding scale becomes smaller,tranverse lithofacies variatio n (eg.pebbly Crs.Grn.sand→parallel-bedding Med.Grn.sand→trough cross-bedding fine Grn.sand→ripple cross-lamination silt)forms。At different stage in a flo odevent,different hydrodynamic regime forms different lithofacies in vertical,th ese can constitute 4 types of rhythm:I.Fining-upwards rhythm(FUR)of Rising-flo od Scouring(RS)and Falling-flood Depositing(FD);Ⅱ.Coarsening-upwards rhythm(C UR) of RD & FS;Ⅲ.CUR of RD & FUR of FD;Ⅳ.no or little sediments of RS & FS.The transverse and vertical models of LAB constitute the depositiomal model of LAB. It is the product in whole process of a flood event. Many wedge-form and lunate-shape LABs deposited in different flood events late rally stack in imbricated shape and constitute point bar.This is the sedimentary stack of LABs.and The vertical succession of PI2 point bar in well GJ-45 con sists of 8 LABs,there is 5-15cm thickness lateral accretion mudstone between LA Bs. According to above forming process and mechanism of point bar,fivefold hierarch y bounding surface(BS) and solid body between BSs are divided in a point bar san dstone body (PBSB),sets up 3-D architecture of point bar which reveals geometri c external form,the controlling frame,hierarchy,rule and spatial characteristic of inter nal structure in sandstone-body and heterogeneity of point bar.This me thod of heterogeneous study is better than mathematical method.5th BS controls r elatively independent oil-water moving unit;4th and 3th BS controls the frame o f very thin lateral accretion mudstone and sandstone,the sedimentary stack of L ABs is the key to study spatial distribution permeability of in PBSB;2th BS and depositional model of LAB are the key to study spatial distribution permeabili ty of in LAB.