Nanopores in the shale play a vital role in methane adsorption, and their structural characteristics and origins are of great significance for revealing the mechanism of methane adsorption, desorption, and diffusion. In this paper, through low-temperature ashing and low-pressure gas adsorption experiments, the nanopore structure of original shales and ashed shales was quantitatively characterized, and the nanopore origins in the transitional shale of lower Permian in eastern Ordos Basin were analyzed. The results show that the pore volume (PV) and specific surface area (SSA) of nanopores in transitional shale reservoirs are 0.0217–0.0449 cm 3 /g and 13.91–51.20 m 2 /g, respectively. The average contribution rates of micropores (<2 nm), mesopores (2–50 nm), and macropores (50–100 nm) to PV are 18.78, 72.26, and 8.96%, respectively, and the average contribution rates to SSA are 66.19, 33.10, and 0.71%, respectively. In addition, it is found that the average contribution rates of inorganic minerals and organic matter to the SSA of micropores are 55.9 and 44.1%, respectively, and the average contribution rates to the SSA of mesopores are 92.3 and 7.7%, respectively. Combining the adsorption properties of the main clay minerals and kerogen in shale, it is concluded that organic pores control the adsorption of methane with an absolute advantage in transitional shales. It is of great significance to understand the mechanism of methane occurrence, desorption, and diffusion in shales by clarifying the origins of multiscale pores.