Recent studies have shown that shale reservoirs can be an attractive candidate as a CO2 storage reservoir or a caprock for geologic CO2 sequestration. Shales are source rocks, and thus, there is a continuous diagenetic process that can alter its properties during thermal maturity before it reaches maturity. However, there is a significant knowledge gap in the geochemical and other alterations in shales due to the interaction with supercritical CO2 (ScCO2) stored in the underlying reservoir unit during the diagenetic process. This study investigates the changes in mineralogical properties and Total Organic Carbon (TOC) of shales during thermal maturation when it is exposed to ScCO2 at in-situ conditions and assesses its implications for CO2 utilization and sequestration. Here, we used Bakken shale and Green River shale samples, and exposed the samples to ScCO2 for a specific period. This was followed by inducing the samples to thermal maturity for the pyrolysis process. Subsequently, we evaluated the mineralogical properties and TOC of the pre- and post-CO2 treatment of mature shale samples. Our results from this study provide novel insights for CO2 utilization and sequestration in the mature Bakken and Green River shale formations.