Classification Evaluation Of "Sweet Spot" In Low Permeability Reservoirs In The Middle Sub-Member Of The Third Member Of Shahejie Formation In Niu 20 Block Of Niuzhuang Sag

Low-permeability reservoirs contain abundant oil and gas resources and occupy an important position in petroleum exploration and development at this stage.Niu 20 block is a typical low permeability turbidite lithologic reservoir in Niuzhuang sag.After more than 30years of rolling exploration and development,the reservoir has a lower level of recovery.This indicates that Niu 20 block has good prospects for oil development and still has great development potential in the context of overall low permeability.Rolling exploration around the block is expanding outward,and good oil and gas production has been seen.However,due to the difficulty in identifying and heterogeneity of turbidite,the success rate of rolling exploration is still low.At the same time,there are large differences in oil and gas production between different development wells in the block.Relying on traditional reservoir macro-heterogeneity research can no longer meet the needs of current development.It is necessary to analyze the reservoir heterogeneity from the reservoir genesis and the remaining reserves mobilization,explore the“sweet spot”evaluation method in the development stage,and point out the“sweet spot”distribution area applicable to the development stage.This paper makes full use of core,analysis,earthquake,logging and other data to carry out classification evaluation of low-permeability turbidite reservoir“sweet spot”in Es₃^z of Niu 20 block.The main research results of the thesis are as follows:（1）According to the sedimentary characteristics of turbidite,it is proposed that the N20block mainly develops two types of gravity flow sand bodies:sandy debris flow deposition and turbid flow deposition.（2）The turbidite reservoir rock type in the study area is lithic feldspar fine sandstone.The reservoir space is dominated by primary pores,and the throat is mostly flaky.The overall physical properties of the reservoir are poor,and the mesoporous low permeability reservoir is developed.Reservoir heterogeneity is serious.（3）It is proposed that the sedimentary factor is the material basis for the development and distribution of"sweet spot".It is believed that the reservoir in the study area is sedimentary turbidite deposits.The single-stage turbidite sand body has a small scale of development,and the plane is flowery,potato-like or irregular.Due to the chronological development of the turbidite sand body,the illusion of“lamination and contiguous”of the sand body has aggravated the difficulty of reservoir heterogeneity.（4）It is proposed that the diagenesis provides reservoir pore conditions for the formation of“sweet spot”.It is believed that the thick layer turbidite sand body reservoirs have strong dissolution and transformation,and more broad-form throats are formed,which is conducive to oil and gas reservoir and seepage.（5）Based on the analysis of geological factors and development factors affecting the distribution of“sweet spot”,the evaluation parameters of“sweet spot”were determined.Considering the reservoir and seepage capacity of the reservoir itself,the mainstream throat radius,movable fluid saturation,pseudo-starting pressure gradient,effective porosity,and effective thickness are determined as geological evaluation parameters;considering remaining oil reserves and production,the permeability will be considered.The coefficient of variation,residual oil saturation,remaining oil reserves abundance,and yield were determined as development potential evaluation parameters.（6）Using the fuzzy mathematics evaluation method,the reservoirs in the development stage of the Niu 20 block are divided into four categories.Among them,the first two categories are“sweet spot”that can be developed at this stage.The evaluation results were verified by the N20-21well,and the evaluation results were consistent with the actual results.The method developed in this paper for the classification evaluation of“sweet spot”in the development stage of low permeability reservoirs has important theoretical value.At the same time,it has a guiding role in improving the development effect of low permeability reservoirs and improving oil recovery.