Gas Chemical and Carbon Isotope Composition as a Diagnostic Tool for Energy

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Abstract As the world is moving toward a low-carbon energy structure, natural gas has become critical in the supply of global energy. The genetic identification of different types of natural gas is important for the assessment of their source and exploration potential. Chemical and carbon isotopic compositions of produced gas are useful tools to assess their origin, thermal maturity, migration and to monitor gas production. In this study, we present different geochemical approaches to assess the origin of the gases and thermal maturity as well as to evaluate the effect of adsorption on shale gas during production. Thirteen gas samples from different conventional reservoirs were analyzed for their isotopic compositions to assess gas origin and thermal maturity. To assess gas production, seven gas samples were collected periodically from the same unconventional shale interval and were analyzed for their isotopic signatures. The results demonstrate that carbon isotopic composition of methane to pentane can be used as fingerprinting tool to identify different sources of gas. The studied gases are all of a thermogenic origin and were generated from Type-II marine source rocks. Mixing of gases generated from different source rocks were observed. The mixed gases are characterized by similar isotopic profiles in methane and ethane and they differ in C3+ hydrocarbons. Thermal maturity trends were identified on the studied samples using the relationship between methane, ethane, and propane isotopes versus the wetness index (total HC/methane concentration). To study the effect of gas adsorption during production, samples produced from the same hydrocarbon source rock were assessed at different time intervals (one-year period of production). The results show small variations between isotope compositions; thus, the impact of adsorption is still uncertain and a longer-term monitoring plan is designed. The results of this study shed insight into using isotope geochemistry as an important diagnostic tool for natural gas exploration and production.