To extract oil from oil sands reservoirs the techniques are required that reduce the viscosity from hundreds of thousands and several millions of cP to a point that it can flow. In Steam-Assisted Gravity Drainage (SAGD) process, injected steam releases its latent heat and reduces the viscosity of the oil. This oil, referred to as bitumen, encountered steam-oil reactions process leading to aquathermolysis and steam-rock reactions - geochemical reactions. Geochemical reactions are responsible for in situ gas production and alteration of produced water composition that can affect the reservoir pressure, porosity, permeability, bitumen viscosity and thus the recovery process. In this research it has been examined that how water composition changes during SAGD operation. A reaction model was defined expressing the geochemistry of SAGD within the formation and a reactive thermal reservoir simulation was built to represent reservoirs with different shale layer geometries at various distances from SAGD wellpair. For the first time, the results demonstrated that the produced water composition can be used to detect baffles and barriers and their distances from wellpair. The analysis of produced water can be used as a tool to monitor the process dynamics and understanding the reservoir heterogeneity.