Characterization of Micro-Nano Pore Structures in Shale Oil Reservoirs and FIB-SEM 3D Reconstruction: A Case Study

Abstract

Abstract Shale reservoirs in lacustrine sedimentary basins represent crucial sectors for shale oil exploration and development. Given their mixed sedimentation, complex origins, and pronounced heterogeneity, the stratigraphic reservoirs exhibit diverse spatial types. Identifying the characteristics of the shale reservoir, such as the distribution pattern of storage space, porosity volume distribution, reservoir properties, and three-dimensional structure of porosity, assists in pinpointing high-quality reservoirs, thus reducing exploration and development risks. This study examines representative shale samples from three formations of a lacustrine sedimentary basin, analyzing shale reservoir pore size and distribution characteristics, and characterizing the complexity of their porosity through low-temperature nitrogen adsorption and high-pressure mercury intrusion experiments. Argon ion polishing-scanning electron microscopy and focused ion beam-scanning electron microscopy techniques provide an intuitive characterization of the form and size of the shale reservoir space/porosity. In particular, FIB-SEM can reconstruct the spatial distribution of shale on a nanometer scale, authentically reproducing the three-dimensional structure features of shale nanopores, allowing for more precise results in analyzing porosity three-dimensional spatial components, pore size distribution, and connectivity. The study reveals that the quartz-rich Formation A exhibits developed mesopores and macropores in the shale, with large pore and small throat interwoven arrangements. Simple and well-connected pore structures render it easily exploitable, albeit with a smaller storage volume. Formations B and C, dominated by dolomitic shale and mixed shale properties respectively, show extremely developed shale mesopores and underdeveloped macropores. Their complex pore structures house larger storage spaces, indicating high extraction potential. Shale in Formation B, enriched with carbonate rock minerals, presents interwoven arrangements of microcracks and pore systems, featuring a higher coordination number and superior seepage capacity, despite a considerable number of unconnected closed pores. Formation C, having a higher clay content, contains numerous and dispersed isolated pores, incapable of interconnection. Connectivity can only be achieved through a small number of lamellar cracks.