Sub-meter scale lithological heterogeneity in the form of intraformational baffles is known to play an important role in fluid flow and geochemical reactions. The processes governing fluid flow at these scales, the development of fluid flow upscaling laws and their integration in reservoir-scale models are well established. However, there exists a knowledge gap in the development of upscaling laws for geochemical reactions arising from the presence of sub-meter scale lithological heterogeneity. This study explores geochemical reaction rates in intraformational baffles and the development of respective upscaling laws for integration in reservoir models in order to better understand carbon mineralisation in geological CO2 storage reservoirs. Ten 2-D model realizations at sub-meter scale resolution are set-up to capture the lithological heterogeneity of the Paaratte Formation, Otway Basin. Reactive transport simulations are run on different realizations to develop an understanding of the processes controlling CO2-rock reactions. Three different approaches are developed to upscale the impact of baffles on reaction rates from cm-scale to reservoir scale based on the process understanding. The development of geochemical upscaling laws alone is not useful unless the laws are integrated in reservoir scale simulations. For this purpose, a workflow is developed where the upscaled properties are classified for composite rock types and their depth logs are derived. The composite rock type logs are subsequently populated in the reservoir scale model. Reactive transport simulations are then run for two reservoir scale models, one comprising the upscaled properties while the other without the upscaled properties. The results from two simulations are then compared to establish the importance of incorporating upscaled properties in reservoir scale geochemical simulations. The workflow for the development of upscaled rate laws and their implementation in reactive-transport models is presented in the context of CO2 storage reservoirs but it is applicable to any groundwater system as well.