北部湾盆地复杂孔隙结构含砾砂岩低阻油层综合评价方法

Comprehensive Evaluation Method of Low-resistivityReservoirs in Gravelly Sandstone with Complex Pore Structure in Beibuwan Basin

南海西部海域北部湾盆地存在大量含砾砂岩低阻油层,其孔隙结构和流体分布复杂,由常规方法计算的地层含水饱和度偏高,导致束缚水、可动水和可动油难以有效区分,地层流体性质判别和储层含油饱和度评价存在困难。鉴于此,根据压汞、薄片等岩心化验分析资料,结合测井、录井特征,从内因和外因等多方面因素对含砾砂岩储层低阻成因展开分析。在储层分类的基础上,对低阻和高阻油层的岩电参数分别进行精细刻画,得到适用于不同微观孔隙结构的评价参数。同时,结合核磁共振测井开展基于“形态法”的变T 2截止值处理,与传统固定T 2截止值相比,所获成果能够有效评价束缚流体和可动流体含量,显著提高了地层流体分析的精度和低阻油层的测井解释符合率,进而形成一套含砾砂岩复杂孔隙结构低阻油层综合流体识别和储层评价方法。应用该评价方法,在北部湾盆地老井复查后发现大量低阻油层,证实该方法具有较强实用性和操作性。该研究对低渗、低阻等复杂孔隙结构储层流体识别和评价具有重要意义。

Within the Beibuwan Basin in the western part of the South China Sea,there are a large number of low-resistivity reservoirs in gravelly sandstones,with complex pore structure and fluid distribution,and the water saturation of the formation calculated by conventional methods is on the high side,which makes it difficult to distinguish between irreducible water,movable water and movable oil,and makes it difficult to discriminate between the fluid properties of the formation and to evaluate the oil saturation of the reservoir.In view of this,the cause of low resistivity in gravelly sandstone reservoirs was analyzed from internal and external factors based on core assay data such as mercuric pressure and thin section,and combined with logging and mud-logging characteristics.On the basis of the reservoir classification,the petroelectric parameters of the low-resistivity and high-resistivity reservoirs were finely etched to obtain the evaluation parameters applicable to different microporous structures.At the same time,the variable T 2 cutoff value processing was performed based on the"morphology method"and in conjunction with nuclear magnetic resonance(NMR)logging.Compared with the traditional fixed T 2 cutoff value,the obtained results can effectively evaluate the content of irreducible and movable fluids,which significantly improves the accuracy of formation fluid analysis and the compliance rate of logging interpretation of low-resistivity reservoirs,and then forms a set of comprehensive fluid identification and reservoir evaluation method for low-resistivity reservoirs with complex pore structure in gravelly sandstone.By applying this evaluation method,a large number of low-resistivity reservoirs were found in the old wells,which confirms that this method is highly practical and operable.This study is of great significance to the fluid identification and evaluation of the low-permeability and low-resistivity reservoirs with complex pore structures.