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Pore structures can significantly impact the mechanical and physical properties of the rock such as permeability, strength and durability. Understanding the microstructures of the rocks accurately and quantitatively is essential to petroleum engineering for evaluating and development of oil and gas, especially for the unconventional reserves with abundant interior nanoscale pores such as shale. In this paper, we studied the pore structures of rock samples from Middle Bakken Formation which is a typical unconventional reservoir in North America. High resolution SEM images of five samples were derived after sample preparation. After determining the threshold of the images, we extracted the pore spaces at various magnifications and determined the representative elementary area (REA). Then we analyzed the pore structures properties such as pore size distributions and pore shape distributions of the five samples at their representative elementary area and applied statistics analysis method to compare their distributions. After that, we analyzed their heterogeneity and isotropy properties which have been identified as an important factor affecting reservoir productivity.