Kinetic Isotope Fractionation Modeling in Natural Gas Systems: A Novel Tool for the Quantitative Assessment of Gas Generation and Gas Typing

Ellis, Geoffrey S.¹, Yongchun Tang², Ronald J. Hill¹ (1) US Geological Survey, Denver, CO (2) California Institute of Technology, Pasadena, CA

In contrast to the large amount of geochemical information that can be derived from crude oil, natural gas, which is compositionally simple, requires creative strategies for maximizing the value of the limited data that is available. Traditional approaches to the interpretation of natural gas isotope geochemistry are primarily dominated by empirical models of gas formation that result in geochemical descriptions that are specific to a particular basin or play and are not forward predictive. Laboratory closed-system pyrolysis measurements of quantities and isotopic compositions of gases, generated from specific source rocks or from secondary hydrocarbon cracking, can be extrapolated to gas generation conditions that occur in natural settings using well-established kinetic modeling techniques. Model results allow for the prediction of the most critical properties for gas generation and accumulation, including isotope fractionation patterns, temperature and maturity of the source rock, gas quality (e.g., wetness), and the gas to oil ratio (GOR). Additionally, integration of gas isotope model results with those of basin models allows for prediction of the geologic conditions related to gas generation within a basin (e.g., timing of gas formation, depth of gas kitchen, amount of gas formed in target areas). Several case studies (from areas such as the Piceance and Ft. Worth Basins) will be presented to illustrate how these results can provide critical information for mapping gas migration pathways, determining reservoir filling history, characterizing reservoir connectivity, and identifying multiple-source gases.