Fine Characterization Of The Architecture Of The Composite Sand Body In The Meandering Belt

This paper takes the Gudong oilfield as an example to study the unique geological characteristics of meandering river reservoir.Due to the frequent migration of the channel,the early point bar and abandoned channel deposits are frequently eroded by the scour of later channels,which results in complex amalgamation of point bars in cross-section and the anomalous distribution of abandoned channels in the plan view.The current research of meander belts has mainly focused on the description and analysis of modern sediments and field outcrops,as well as on the description of the geometry and distribution of subsurface meander-belt deposits.Nevertheless,there still remains a lack of studies about the inner structures of subsurface meander-belt deposits.Taking the help of drilling data with high vertical resolution and seismic data with high lateral resolution,this paper describes the architecture and the distribution of internal structural units of the subsurface meander belt.Through these work,we hope to form a set of systematic interpretation methods for the dissection of the reservoir structure of the subsurface meander belt.We found that the proportion of the base-of-channel sandstone that infill abandonments distributed along the channel-belt margins is more than 65%of the boundary length of the meander belt,and that this proportion is more than 85%on the margin which is concave to the meander belt.Referring to the observation and researches of predecessors,we divide single point-bar sandbody into three parts based on the morphology and evolutionary sequence of point-bar sandstones:the channel sandstone under abandoned channel or base-of-channel sandstone that infill abandonments?PE sandstone?,earliest sandstone on the beginning of migration of the point bars?PI sandstone?and main part of point-bar sandstone.Then we discuss the seismic characteristics of the different position of the point dam by the forward model.With the evident response patterns in log and seismic sections,lateral stacking structures could be recognized:the stacking structure between earliest migrated sandstones of the point bars,stacking structure between base-of-channel sandstones that infill abandonments,and stacking structure between earliest migrated sandstone of the point bars and base-of-channel sandstone that infill abandonments.Then,the meander belt in the study area was studied through seismic sedimentary.The3D seismic horizon slices and attribute maps reveal the dimensions,distribution,and evolution of the N_g5²⁺³ fluvial architectural elements in the Gudong oilfield.Five channel stories were distinguished in the study area.The channel constantly migrated within the meander belt,and the initial channel story was mainly developed in the middle of the meander belt axis,while the last channel story was deposited near the margin of the meander belt.The distribution of channel was verified by the superposition relationship of different point bars and the tracer data.Finally,the distribution of the channels and the distribution of the point bars are determined.Through the analysis of empirical formula,it is found that in the period of channel sand deposit in the study area,mean annual flood discharge of the channel stories within the N_g5²⁺³ meander belt was 2002m³s^-1-5240m³s^-1,.The geometry of the channel story was affected by the discharge of the flow at the time when the channel story was deposited.The width and thickness of the channel stories are positively correlated with the discharge of the flow.However,the width variation of the later channel stories becomes progressively limited and it is not affected by the flow discharge.Through core observation and analysis,we divide the point bars into different units.On the basis of core analysis data,we have deduced the physical characteristics of each structural element in the point bars.The grain of the rest of the lateral-accretion beddings below the shale drape is relatively smaller than that of elements around the beddings and permeability of rest of the beddings below the shale drape shows exponential variation.While permeability of lateral-accretion elements also shows exponential variation.The permeability distribution functions of elements and beddings were used to establish the permeability distribution model of them.Meanwhile,the model,which was established by the relationship of permeability with space position,is in better agreement with the core characteristics.The model in which lateral-accretion beddings and units were modeled by using permeability-distribution function as constraint obviously increase in breakthrough time and reduce in recovery percentage of reserves compared to other models.Generally speaking,this paper puts forward a complete set of research methods for reservoir structure and physical properties of new complex meander belts for later study of remaining oil distribution characteristics.It not only provides a new understanding and research method for the study of the structure of meandering river reservoir in Gudong oilfield,but also has a good guiding significance for the overall adjustment and the tapping of potential blocks in the extra high water cut stage.