Various pathways to accelerate the recovery of hydrocarbons from unconventional subsurface environments have been proposed, including the use of high-temperature fluids. However, the influence of temperature on the structural and microstructural changes in complex subsurface materials characterized by compositional and morphological heterogeneity are not very well understood. In this study, in operando ultrasmall angle, small angle, and wide-angle X-ray scattering (USAXS/SAXS/WAXS) measurements were performed to link the structural and morphological changes in shale when heated from 30 to 1150 degrees C. The structural changes observed from the wide-angle X-ray scattering measurements were complemented by the changes in the porosity, particle size, and surface morphology. Overall, the combined USAXS/SAXS measurements showed enhanced smoothness in the pore-solid interface on heating. The dehydroxylation of illite and calcination of calcium carbonate were noted prior to the formation of denser and sintered silicate and alumino-silicate phases such as Mg2SiO4 (forsterite), Fe(0.3)Mgo(0.7)(SiO3) (enstatite), and Al2.35Si0.64O4.82 (mullite).