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The ever-growing energy demand and recent discoveries of vast unconventional oil and gas reservoirs have brought significant attention to shale oil and gas resources as potential game-changers for the petroleum industry and energy markets worldwide. Although shale reservoirs are large in scale and offer the potential for long-lived production, extremely low matrix porosity and permeability, as well as complex heterogeneity, pose major challenges in obtaining economically viable oil and gas. A lack of predictive understanding of microstructure-based heterogeneity in shale rock limits the effectiveness of currently used exploration and production technologies. Hence, addressing the challenges of shale oil and gas exploration and production technology requires an in-depth understanding of microstructural features that control the oil and gas subsurface transport phenomena. A new holistic approach for characterization of multiscale structural heterogeneity in shale, presented in this thesis, couples micro- and nano-X-ray microscopy (micro- and nano-XRM) with focused ion beam scanning electron microscopy (FIB-SEM). This integrated approach provides a unique opportunity to characterize in great detail the complex three-dimensional (3D) microstructure of shale rock over multiple length scales, from the centimeter length scale to the single nanometers. To explore the practical significance and reach of this newly developed analytical framework, samples from the Woodford Shale and the Marcellus Shale were imaged several times with non-destructive XRM at successively higher resolutions, and then finally imaged with the high-resolution by destructive FIB-SEM serial-sectioning. Subsequently, in order to quantify the evolution of porosity associated with both organic and nonorganic (mineral) matter, the organic- and nonorganic-matter pore networks within both samples were extracted using the FIB-SEM models. The digital rock physics (DRP) 3D image-based characterization revealed the Woodford Shale and the Marcellus Shale samples to be primarily composed of varying amounts of organic and mineral matter. The findings also indicate complex pore systems, both within organic and nonorganic matrices. The pore network modeling (PNM) analysis suggested that pores and microfractures located at the interface between organic and mineral matter were the most abundant pore types in analyzed shale rock samples, and have the potential for better connectivity. Finally, representative pore/fracture networks, for continuum and non-continuum fluid flow studies, were separated and transformed into finite element models for future works.