羌塘盆地南坳陷布曲组白云岩储层孔喉结构及其分形特征研究

Pore-throat structure and fractal characteristics of dolomite reservoir in the Buqu Formation in the southern depression, Qiangtang Basin

关键词:  孔喉结构  分形特征  常规压汞  低渗透白云岩储层  布曲组  羌塘盆地

Key words:  pore-throat structure  fractal characteristics  high pressure mercury  low permeability dolomite reservoir  Buqu Formation  Qiangtang Basin

摘要:

羌塘盆地中侏罗统布曲组白云岩为非均质性强的低孔、低渗储层，孔喉结构评价难度较大。选取南羌塘坳陷布曲组20件白云岩岩心样品，通过铸体薄片、扫描电镜观察，结合物性分析和常规压汞实验，研究其孔喉分布和分形特征，探索分形维数与白云岩储层物性参数、孔喉结构参数、孔隙成因之间的关系。结果表明：（1）研究区布曲组白云岩储集空间类型包括组构选择性孔隙、非组构选择性孔隙、裂缝3类，依据毛管压力曲线特征将其孔喉结构分为5种类型，其分形曲线具有明显的转折点，转折点为孔喉分布峰值，代表连通性好的大尺度孔喉向连通性差的小尺度孔喉的转变；（2）小尺度孔喉的层内非均质性较弱、层间非均质性强，大尺度孔喉的分形维数比小尺度孔喉小，但分布区间较宽，样品间差异明显，表明大尺度孔喉复杂多样，层间（整体）非均质性比小尺度强；（3）大尺度孔喉的分形维数与储层参数、孔喉结构参数相关性较好，可用于布曲组白云岩储层的孔喉结构定量表征，大尺度孔喉分形维数越小，储层物性越好，大尺度孔喉占比越多对储层整体渗透能力的贡献越大；（4）该套储层的储集空间与孔喉结构以沉积形成的碳酸盐沉积物孔隙为基础，白云石化作用和溶蚀作用共同控制了孔喉结构的差异性。

Abstract:

Pore-throat structure of dolomite reservoir with strong heterogeneity, and with low porosity and permeability is very complex, so it is difficult to evaluate the pore-throat structure of the Mid-Jurassic Buqu Formation in Qiangtang Basin. Based on thin section observation, SEM, porosity and permeability test and mercury injection capillary pressure test of twenty dolomite core samples, to study the pore throat distribution and fractal characteristics, and to exploring the relationship between fractal dimension and physical parameters and pore throat structure parameters. The results show that:(1) The types of dolomite reservoir space include fabric-selective porosity, non-fabric-selective porosity and fracture, and according to the characteristics of capillary pressure curves, the pore throat structures were divided into 5 types, and the fractal curve has obvious turning point, a turning point for the distribution of pore throat peak, on behalf of the connectivity of good large-scale pore connectivity to the small scale of the pore throat of poor change. (2) The heterogeneity of small-scale pore throat is weak in layer and strong in interlayer. The fractal dimension of large-scale pore throat is smaller than that of small-scale pore throat, but the distribution range is wider and the difference between samples is obvious, which indicates that large-scale Pore throat is complex and diverse, interlayer heterogeneity is stronger than small-scale pore throat. (3) The fractal dimension of large-scale pore-throat has a good correlation with reservoir parameters and pore-throat structure parameters, which can be used to quantitatively characterize the pore-throat structure of Buqu Formation dolomite reservoir. The smaller the fractal dimension of large-scale pore-throat, the better the reservoir physical property, and the larger the proportion of large-scale pore-throat, the greater the contribution to the overall permeability of reservoir. (4) The reservoir space and pore-throat structure of Buqu Formation dolomite reservoir were based on the carbonate sediment pores formed by deposition. Dolomitization and dissolution jointly control the difference of pore-throat structure.