陇东地区长7段夹层型页岩油储层分形特征及其与孔隙结构和含油性的关系

Fractal characteristics of interbedded shale oil reservoirs in Chang 7 member of Longdong area and their relationship with pore structure and oil-bearing property

页岩油储层的孔隙结构和非均质性对原油的赋存状态和渗流规律具有重要的影响,陇东地区长7段夹层型页岩油储层具有孔隙类型多样、尺寸跨度大、非均质性强的特征,增加了深入研究多尺度孔隙结构和非均质性的难度。结合铸体薄片、场发射扫描电镜、高压压汞及抽提实验,对陇东地区长7段夹层型页岩油储层的孔隙结构特征、分形特征、含油性进行了分析,并进一步探讨了分形特征对孔隙结构和含油性的影响。结果表明:整体上研究区长7段夹层型页岩油储层以溶蚀孔和黏土矿物晶间孔为主,其次发育部分残余粒间孔和微裂缝;微观结构参数表现出极大的非均质性,随着物性变差,最大进汞饱和度逐渐降低,排替压力逐渐升高,退汞效率均较低(平均为27.28%),表现出较差的孔喉连通性;分形结果表现出三段式分形特征,不同样品的分形拐点是有差异的,据此将储层孔喉划分为大孔喉、中孔喉和小孔喉;微观孔喉结构参数和含油性均与中孔喉的分形维数(D2)呈较好的相关性,D2与渗透率、最大进汞饱和度、残余油含量呈负相关关系,而与排替压力、退汞效率呈正相关关系。上述认识对评价研究区夹层型页岩油储层的孔隙结构及含油性提供了依据。

The pore structure and heterogeneity of the shale oil reservoirs is of great significance to the occurrence state and seepage law of crude oil. The interbedded shale oil reservoirs in Chang 7 member of Longdong area have the characteristics of diverse pore types, large size span and strong heterogeneity, which increases the difficulty of in-depth study of multi-scale pore structure and heterogeneity.The casting thin sections, field emission scanning electron microscopy, high-pressure mercury injection and extraction experiments were applied in this paper to analyze the pore structure characteristics, fractal characteristics and oil-bearing properties of Chang 7 interbedded shale oil reservoirs in Longdong area.Furthermore, the effect of fractal features on pore structure and oil content was further discussed.The results show that the interbedded shale oil reservoirs of Chang 7 member in the study area are dominated by dissolution pores and intercrystalline pores of clay minerals, followed by some residual intergranular pores and micro-fractures.The microstructure parameters show great heterogeneity.With the deterioration of physical properties, the maximum mercury saturation decreases gradually, the displacement pressure increases gradually, and the mercury removal efficiency is low(27.28% on average), showing poor pore-throat connectivity.The fractal results show three-stage fractal characteristics, and the fractal inflection points of different samples are different, according to which the reservoir pore throats are classified into large, medium and small pore throats. The microscopic pore-throat structure parameters and oil content have a good correlation with the fractal dimension(D2) of the medium pore throats.D2is negatively correlated with permeability, maximum mercury saturation and residual oil content, but positively correlated with displacement pressure and mercury removal efficiency.The above knowledge can provide a basis for evaluating the pore structure and oil content of interbedded shale oil reservoirs in the study area.