Pore structure of unconventional reservoir is fundamental for understanding hydrocarbon storage, fluid transport, and geomechanics. The pore structure of shale gas reservoirs has been studied extensively, while investigation regarding the pore structure of shale oil reservoirs remains limited. The Bakken formation is one of the largest contributors to the growth of unconventional oil in the U.S. In this study, 13 core samples collected from the Bakken formation were examined through a series of experiments to investigate the geochemical properties and mineralogy, especially pore structure. Mineralogy analysis through the X-ray diffraction (XRD) test showed that quartz and illite are the major components for the upper and lower shale members, while quartz, feldspar, and dolomite dominate the middle member of the Bakken formation. Rock-Eval source rock analysis illustrated that all of the shale samples contain a significant percentage of organic matter. Nitrogen and carbon dioxide adsorption results showed that isotherm curves obtained from nitrogen adsorption are reserved S-shaped (typical type II curve), indicating that pores are mainly micro- and mesopores. Linear regression analysis of pore structure parameters with respect to total organic carbon (TOC) and mineral composition reveals that the TOC content has a positive relationship with micropore volume, while meso- and macropores are controlled by clay content. Development of micropores in organic matter is thermal-maturity-related. Shale samples with vitrinite reflectance higher than 1.0% have a higher surface area, suggesting that more micropores were developed in the organic matter after maturities of shales reached oil window level. In addition, results of the fractal analysis showed that samples with higher fractal dimension values are featured by more micropore volume, smaller pore diameter, and larger specific surface area.