The main drawbacks in using the stable isotope ratios of organic elements (hydrogen, nitrogen, carbon, and oxygen) are their susceptibility to being changed by chemical processes in the environment (isotope fractionating processes) and the large number of different sources for many of the non-combustible gases and hydrocarbons. These factors introduce a significant amount of additional complexity in the interpretation of these tracers. One of the most promising tools capable of reducing some of this complexity is the noble gases.
The noble gases are completely un-reactive and are only generated by a limited number of sources in the subsurface. Their ‘inert’ reactivity limits the number of potential isotope fractionating processes to only the physical processes (equilibria, phase-transfer, migration) that have occurred in the subsurface. Subsurface fluids generally contain different mixtures of noble gases that were produced from the chemical components of the rock in the stratigraphy by various ‘radiogenic’ processes (4He, 21Ne, 40Ar), noble gases that have been introduced as a dissolved component of water that was in contact with the atmosphere (20Ne, 36Ar), and in some cases noble gases that have originated from a magmatic fluid component (3He).
Interpreting the different noble gas components in a gas, or dissolved in oil and/or water can provide advanced information for characterizing gas/oil/water source information. In many cases, including the noble gases into a geoforensic investigation can provide advanced insight into gas/oil/water migration pathways, reservoir stability/connectivity, and constrain local and regional transport mechanisms for subsurface injection programs (Enhanced oil recovery, carbon storage, natural gas storage).