鄂尔多斯盆地长7段致密砂岩二元孔隙结构及分形特征

Binary pore structure and fractal characteristics of tight sandstone:A case study of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin

致密砂岩的孔隙结构特征对致密油藏的勘探和开发具有重要意义。但对不同类型孔隙渗流特征及其影响因素认识不清,制约了对致密砂岩渗流能力和储集能力的认识,并影响了致密油藏有利区的评价和预测。为此,以鄂尔多斯盆地长7段致密砂岩为例,利用铸体薄片、扫描电镜、核磁共振、分形理论研究了致密砂岩的孔隙结构特征。结果表明:致密砂岩孔隙结构是一个由束缚孔隙和可动孔隙组成的二元孔隙系统,不同类型孔隙的形态特征和连通性不同。束缚孔隙主要由晶间孔组成,孔隙模型为黏土矿物堆积的包络面,孔隙半径小,连通性差,赋存在其中的流体流通性差。该孔隙形状规则,非均质性弱,分形维数低,平均值为1.3063。可动孔隙主要是联合孔隙,形状为刺球状,孔隙半径大,由溶蚀作用形成多个连接通道,因此连通性强。可动孔隙形状不规则,非均质性强,分形维数大,平均值为3.2416。不同类型孔隙对物性的影响也不相同。束缚孔隙对孔隙度贡献平均值为47.7%,可动孔隙对孔隙度贡献平均值为52.3%,两类型孔隙对孔隙度影响相近。但受连通性差异影响,束缚孔隙对渗透率贡献度平均值为0.2%,可动孔隙对渗透率贡献度平均值为99.8%。可动孔隙是影响致密砂岩渗透率的主要因素。利用分形维数可以很好地表征致密砂岩孔隙结构。以组合孔隙为主且位于烃源岩附近的致密砂岩是勘探开发的有利区域。

The pore structure characteristics of tight sandstone are of significance for the exploration and development of tight oil reservoirs.However,little attention is paid to the flow characteristics and influencing factors of different types of pores.This restricts the understanding of the flow and storage capacity of tight sandstone and affects the evaluation and prediction of favorable areas for tight oil reservoirs.Therefore,this paper focuses on the tight sandstone in the Chang 7 member of the Triassic Yanchang Formation in Ordos Basin,and casting thin sections,scanning electron microscopy,nuclear magnetic resonance,and the fractal method are adopted to research the pore structure characteristics of tight sandstone.The results indicate that the pore structure of tight sandstone is a binary pore system composed of bound pores and movable pores.The morphological characteristics and connectivity of different types of pores vary greatly.Bound pores are mainly composed of intergranular pores,and the pore model is the envelope surface of clay accumulation.The pore radius is small,the connectivity is poor,and the fluid flow is poor.The pore features a regular shape,weak heterogeneity,low fractal dimension,and an average value of 1.3063.The movable pores are mainly combined pores with a spiky-spherical shape.This type of pore has a large radius and multiple connecting channels formed by dissolution,resulting in strong connectivity.The shape of movable pores is irregular,with strong heterogeneity,a large fractal dimension,and an average value of 3.2416.The influence of different types of pores on physical properties is also different.The average contribution of bound pores to porosity is 47.7%,and that of movable pores to porosity is 52.3%.The two types of pores have similar content but are affected by differences in connectivity.The average contribution of bound pores to permeability is 0.2%,while that of movable pores to permeability is 99.8%.Movable pores are the main factor affecting the permeability of tight sandstone.The fractal dimension can characterize the pore structure of tight sandstone.Sandstone is mainly composed of composite pores and located near the source rocks is a favorable area for exploration and development of tight sandstone.