Clay minerals are abundant in sedimentary formations and are dominated by nanoscale pores that provide a large surface area for physical interactions with surrounding reservoir fluids. Among them, adsorption has direct relevance to many geo-energy applications, including geological CO2 storage and enhanced gas recovery, because the injected CO2 and recoverable reservoir fluids largely exist in a dense adsorbed phase. In this regard, experimental sorption data of CO2 and hydrocarbons on clay minerals under subsurface conditions allow us to estimate the storage capacity for CO2 and the amount of natural gas recoverable from enhanced production. In this thesis, we present a combined experimental and modeling approach used to systematically investigate the sorption of supercritical CO2 and CH4 on three illite-smectite clay minerals (dry and hydrated) over a wide range of temperatures (25 °C to 115 °C) and pressures (up to 300 bar). The presented workflow includes comprehensive characterization of the nanoscale pores of the clay minerals using multiple methods, including gas and vapor sorption, that sheds light on the differences in sorption uptake of supercritical CO2 and CH4 that originate from clay-specific pore size distributions. An adsorption model based on the lattice density functional theory (LDFT) was adopted to quantify such observations through simple parametrization of the clay nanopores. In particular, the model results showed that saturation of supercritical fluids inside nanopores is fluid- and temperature-dependent and quantified the CO2/CH4 interaction energies with clay surfaces that agreed well with the adsorption metrics obtained from the experimental measurements. The ability of the LDFT model to incorporate the clay nanopores was further exploited to investigate sorption on smectite clay minerals that expand upon hydration. The results successfully demonstrated that enhanced sorption of CO2 and CH4 on hydrated smectite is due to the expansion of smectite interlayers that introduces additional micropore space.