Publications

Globally there has been growing interest in gas exploration. A better understanding of the processes associated with the formation and alteration of gas can result in more efficient exploration. This paper examines these processes and how geochemistry can be used to decipher a gas accumulations history.Gas has multiple formation mechanisms. The mode of formation is reflected in a gas bulk and isotope geochemistry. Examples are used to show how geochemical attributes can establish the mode of formation and alteration history. For example, with some limitations the significance of biogenic gas contributions can be estimated using the isotopic discordance of methane and the wet gas components. When a biogenic contribution is present methane appears isotopically lighter than would be anticipated. The presence of a biogenic component is significant in that it addresses the timing of trap development. The importance of primary vs. secondary cracking can also be established using the differences in the isotopic composition and the relative abundance of ethane and propane. This information and the estimated thermal maturity of the gas based on its isotope composition is key to establishing the source of the gas.Just like oil, gas may undergo significant alteration through both water washing and biodegradation. Water washing is established through increases in gas wetness and in the iC4/nC4 ratio. Biodegradation decreases gas wetness and results in isotopically heavier C3, nC4, and nC5. These processes not only alter gas composition but may reduce the volume of gas present.Non-hydrocarbon components, principally CO2 and N2, can be significant in the region. Non-hydrocarbon content varies among basins and fields, and within individual fields. These components may also have multiple origins and their presence decreases the value of an individual accumulation. Their origin is established through the integration of isotope and geologic data.