The detailed study of competitive adsorption of methane (CH4) and carbon dioxide (CO2) is essential for enhanced gas recovery and carbon dioxide sequestration operations. In this study, the adsorption of methane and carbon dioxide isotherms was measured at 25oC, 45oC, and 65oC at pressure up to 100 bar for overmature carbonaceous shales of Sungai Perlis beds, Peninsular Malaysia, with TOC ranges between 5.48 wt to 11.33 wt. There is no study reported on the competitive adsorption in these shales to date. Furthermore, these shales have never been analyzed for the estimation of carbon storage potential and pore size distribution. Therefore, the current study investigates the competitive adsorption behavior and carbon storage potential using a volumetric approach as well as the pore structure and its relationship with adsorption potential of shale. The mineralogy and organic contents were analyzed using X-ray diffraction (XRD) and TOC analyzer. The pore structure was analyzed using low pressure nitrogen adsorption mercury intrusion capillary pressure (MICP) and filed emission scanning electron microscopy (FESEM). The total carbon storage capacity of shale was estimated using the volumetric approach proposed by US-DOE on regional scale. The results indicated that the measured maximum excess adsorption potential of CH4 ranges between 0.019 mmol/g to 0.051 mmol/g, whereas the maximum excess adsorption potential of CO2 ranges between 0.026 mmol/g to 0.24 mmol/g. Both the supercritical Dubinin-Radushkevich (SDR) and Langmuir model best fit the experimental data for CH4. Whereas Dubinin-Langmuir k model (DLK) and Langmuir model best fit the experimental data for CO2. In comparison, the DLK model is considered best for subcritical CO2 adsorption. The accuracy of the isotherm models was evaluated using the error analysis technique, which indicates satisfactory performance. The studied samples indicate the presence of micropores, mesopores and macropores, the pores are open and permeable i.e., slit like and wedge shaped. The MICP indicate the presence of bimodal pore throat size distribution. The fractal properties of pore ranges between 2.29 to 2.69, which indicate the complex pore internal structure. Furthermore, there is a higher affinity to CO2 than CH4 under similar conditions of temperature and pressure. The adsorption ratio/selectivity ratio of CO2 over CH4 ranges between 0.56-11.24, which makes the shale the best target for carbon storage operations. The total carbon storage potential ranges between 0.48 to 1.75 Gt. The adsorption ratio increases with an increase in organic contents; however, clay minerals are not affecting significantly. The adsorption ratio decreased initially with increased pressure and then stable with increasing pressure, suggesting that reservoir pressure should be lower before the injection of CO2 into the reservoir. The adsorption volume increase with an increase in specific surface area and micropore volume. This study provides the basis for the estimation of carbon sequestration and enhances gas recovery. © 2022 Society of Exploration Geophysicists and the American Association of Petroleum Geologists.