压汞法与数字图像分析技术在致密砂岩储层微观孔隙定量分析中的应用——以鄂尔多斯盆地吴起油田X区块为例

为了探究鄂尔多斯盆地吴起油田X区块三叠系延长组长4+5、长6和侏罗系延安组延9、延10等4个亚段致密砂岩储层的孔隙结构，对该区12块样品进行了储层物性分析、扫描电镜观察、全岩X衍射实验及高压压汞实验，并利用图像分析技术和分形几何学定量地表征了致密砂岩的孔隙参数与分形维数。此外，讨论了分形维数与样品物性(孔隙度、渗透率)、孔隙结构参数(平均孔喉半径、分选系数)、孔隙几何参数(主要孔径、周长面积比、孔体比)之间的关系，还量化分析了沉积相及成岩环境对孔隙结构的影响。分析结果表明，孔隙结构分形维数范围为2.164~2.895(平均2.395)。分形维数与渗透率、孔隙度、平均孔喉半径呈负相关关系，与分选系数呈强正相关关系。研究区致密砂岩主要孔径较小、周长面积比大、孔体比较小、分形维数高，且分形维数随着孔体比和周长面积比的增大而增大，随主要孔径的增大而减小。可见样品孔隙结构相对复杂、各向异性较强且沉积环境会影响储层岩石的成分成熟度和结构成熟度，压实、胶结、淋滤等成岩作用会对储层进行改造，二者对致密砂岩储层的孔隙结构有着至关重要的影响。 

Application of mercury intrusion method and digital image analysis in quantitative analysis of micro-scale pores in tight sandstone reservoirs: a case study of X block in Wuqi Oil Field,Ordos Basin

In order to investigate the pore structures of the tight sandstone reservoirs in the 4+5th and 6th members of the Triassic Yanchang Formation (Chang4+5 and Chang6, respectively), and the 9th and 10th members of the Jurassic Yan’an Formation (Yan9 and Yan10, respectively) in the X block of Wuqi Oil Field, Ordos Basin, 12 samples were collected to analyze reservoir properties with the approaches of scanning electron microscope observation, X-ray diffraction and high pressure mercury intrusion. We also quantitatively characterized the pore parameter and fractal dimension of the tight sandstones by the using of digital image analysis and fractal geometry. In addition, we discussed the relationship between fractal dimension and sample properties (porosity, permeability), pore structure parameter (average pore-throat radius, sorting coefficient), pore geometric parameters (dominant pore size, perimeter over area, and pore body-to-throat ratio). The influence of sedimentary facies and diagenetic environment on pore structures were also quantitatively analyzed. Results show that the pore structure fractal dimension ranges from 2.164 to 2.895, with an average value of 2.395. Fractal dimension is negatively correlated to permeability, porosity and average pore-throat radius, and positively related to sorting coefficient. Tight sandstones in the study area generally show properties of low dominant pore size, high perimeter over area, lower body-to-throat ratio, and high dimensions. The fractal dimension is positively related to body-to-throat and perimeter-to-area ratio, and negatively related to pore size. It is indicated that the pore structure of the samples is relatively complex and has strong heterogeneity. Depositional environment affects the compositional maturity and structural maturity of reservoir.