For a wide adoption of the Carbon Capture and Storage (CCS) technology, it is essential to provide a commercial operator with a reassurance of the predictability of their proposed site for CO2 storage through geochemical monitoring techniques. This is particular important for assessing residual and solubility trapping, which are more secure than structural trapping of free-phase CO2. It is difficult to quantify how much CO2 is stored by residual and solubility trapping across an entire storage site. Hence, there is a need to develop a test which can be performed at a single injection well during assessment of a potential site for CO2 injection. CO2CRC, one of the world-leading CCS research organisations, conducted the Otway Stage 2B Extension residual saturation test in December 2014 to determine residual trapping at their Otway test site in Victoria, Australia, using a single-well field setting. In direct collaboration with CO2CRC and other global research institutions (CSIRO Energy, University of Melbourne, Simon Fraser University, Lawrence Berkeley National Laboratory), we use water and gas geochemistry to establish the fate of CO2 injected into the Paaratte Formation at the Otway test site. More specifically, we study the application of oxygen isotopes and noble gases to reconstruct levels of residual trapping of CO2.