Geological CO2 and H2S storage is a mitigation measure to reduce the increasing level of green house and toxic gases in the atmosphere. Thus natural CO2 and H2S accumulations serve as natural analogues and are ideal study sites to investigate the long-term consequences of the slow gas water rock reactions acting on geological timescales. This thesis aims to improve the quantification of some of these chemical and physical processes occurring in the CO2 and CO2-H2S reservoirs by focusing on two separate studies in Bravo Dome and LaBarge, western USA. The first study in this thesis, at Bravo Dome, addressed CO2 dissolution into the formation water at Bravo Dome with the construction of a diffusion model based on carbon, oxygen and noble gas stable isotopic data from gas samples. This resulted in the estimation of the CO2 residence time with a more direct method and facilitated the calculation of a new dissolution rate of between 29.9 +11.8 -10.7 kt/a and 35.9 +/-12.3 kt/a implying that 28% of the originally emplaced CO2 dissolved. The second study in this thesis used different geochemical techniques to examine drill core samples from the CO2-H2S reservoir at LaBarge. Stable isotopes, rare earth element (REE) and fluid inclusion data demonstrated that the CO2-H2S reservoir has a complicated fluid history. This second study confirmed that the H2S in the reservoir is produced through thermochemical sulphate reduction (TSR) and showed that the reduction process was likely the rate limiting step, with the exhaustion of hydrocarbons being the limiting factor of the reaction. Mass balance calculations estimated a reaction duration of 80 ka. Reaction path modelling was also conducted to study the consequences of the magmatic CO2 influx and confirms that the hot CO2 influx (â¥200 Â°C) did not influence the TSR redox reaction. Additionally, the migration distance of the CO2 from the source to the trap was investigated. Nd isotopes suggest Leucite Hill as a volcanic source for the CO2. This would equate to a lat- eral travel path of >80 km for the CO2 and imply high lateral mobility of the gas before being trapped.