Research Of Architecture And Its Evolution Of Delta Composite Point Bar Of A Large Meandering River

During the formation of a meandering river,the river channel experienced multiple lateral deposition and migration,leading to complex sand overlapping within the meander belts.Composite point bars with high heterogeneity commonly exist in meander belts.Some researchers have investigated the architecture of the single sand body,but previous studies simplified composite point bars.Therefore,the formation,evolution,and precise characterization of composite point bars are still unclear.In addition,the investigations of the formation and evolution of a meandering river fail to consider climate conditions during its deposition process.Currently,a novel method for precise characterization of reservoirs in the composite point bars is urgently required as terrigenous reservoirs with high production gradually undergo the late stage of development.This study investigates the evolution of the typical meandering river formation in major reservoirs in PI Member of B1-D block,D oilfield under warm and humid environments.According to the study on the dissection of configuration for composite point bars in meandering river formations,we developed a dissection of configuration technique based on the evolution process.In addition,the restoration of single point bars within composite point bars is also investigated in this study.The high-resolution sequence stratigraphic of 862 wells(19826 layers)in target formations of Pu I 1-4 Member was established using sequence stratigraphic theory,including 3 medium-term and 23 short-term baselevel cycles in Sapu reservoirs.The target formations are included in 5 short-term baselevel cycles.Depositional microfacies and energy facies were studied under various depositional environments.This study identifies 2 facies,3 subfacies,and 19 microfacies to establish corresponding logging facies.Furthermore,channel facies and logging facies of sand sheets are distinguished based on their energy facies by considering rock electrical response features in the target zone.Totally,51 energy facies under three depositional environments(flood plain with highly sinuous meandering rivers,upper distributary delta plain,and delta front)are identified.The distribution and precise characterization of depositional microfacies for 5 short-term baselevel cycles are achieved under the control of baselevel cycles.By considering viewpoints on deposition evolution,we analyzed the deposition evolution of target formation and characteristics of A/S changes in short-term baselevel cycles.The delta depositional patterns of five rivers were established according to features of depositional patterns,revealing the formation,geometry,scale,and energy unit changes and contact relationships of intra-and inter-facies of a single sand body within the stratigraphic sequence in a short period.Meanwhile,this study indicates the maturity and distribution of river channels and reservoirs within the stratigraphic sequence.Under the control of baselevel cycles,the identification,overlapping patterns,and evolution of single-period channel sand and 5 levels of configuration units for composite sand are achieved based on precise dissection of outcrops and cores.Moreover,we develop 5 methods of identifying a single channel sand body in composite sand and 3 vertical distributions of single-period river sand.Based on the distributions of single-period river sand,the 5 configuration units can be distinguished into 2 patterns of distributions of single-period river sand: dispersion and overlapping.Further,these configuration units are precisely divided into 7 overlapping patterns of single-period river sand and analyzed their genesis evolution.Dynamic evolution of the meandering river is developed using the information of multiple samples in the current river under the warm and humid environment,paleo deposition,and physical and numerical simulations.A logarithm decline relationship is observed between composite degree and evolutionary stages by projecting them on2 D coordinates.By fitting projecting composite point bar samples of the meandering river on 2D coordinates,the lower limiting value of the composite degree of the meandering river under warm and humid environments is 0.4,indicating that 40% of the point bar is preserved under the environment.The conclusion provides insights into describing reservoir features of composite point bars and supplements theory for existing fluvial sedimentology.Under the warm and humid environment created by the dense well network,we employed the formation-evolution analysis technique and finally obtained a total of41 point bars of meandering rivers in two time units,including 40 single point bars and 1 composite point bar.We extracted 335 configuration parameters and successfully restored the formation process of the composite point bar in the reservoir,thereby determining its evolution stages.The lower limiting value(0.4)of the composite degree curve was used to examine the rationality of point bar restoration in a single stage.According to the extraction results of configuration parameters,we deciphered the architecture of the large river-delta composite sand controlled by each geological unit.The quantitative manifestation results of the composite point bar are confirmed by tracer experiments,characteristics of multi-stage well net submergence,and numerical simulations.Moreover,static geological understandings accord well with dynamic production,indicating the correctness of our results.Overall,this study provides significant guidance for similar-type reservoir prediction and the investigation of late-stage residual oil during development.