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Chemical Speciation and Carbon Isotope Systematics During Kerogen Maturation: An Experimental Study


Carbon isotope measurements of alkanes have been used as an effective tool to constrain the origin of hydrocarbons. The pattern of carbon isotope compositions of ethane and propane, along with isotope value of methane, are important indicators of thermal maturity of source-rock kerogen. Recent laboratory experiments, however, suggested that pressure and the presence of water could be crucial factors affecting hydrocarbon formation and then carbon isotope fractionations. To assess reaction pathways and controlling factors of carbon isotope compositions of hydrocarbons, a series of hydrous pyrolysis experiments are performed for systematic identification of all organic compounds and corresponding carbon isotope measurement. Experiments are performed at 350°C and 300 bars of total pressure. Kerogen was isolated from Green River shale by a series of acid treatment. In each experiment, the reactor contains 1.5 grams of kerogen and 30 grams of deionized water. After 24 hours of reaction in the first experiment, gaseous products are removed under cryogenic conditions for chemical and carbon isotope analyses. The bitumen product is retrieved and separated into saturated hydrocarbons, aromatics, resins, and asphaltenes (SARA) by HPLC for subsequent analyses. The gaseous compounds from the experiment consisted of CO2, methane, ethane, propane, i-butane, and n-butane. Comparison of mass 44 peak of each compound in IRMS analysis indicates that the abundances of n-alkanes decreases with carbon number, with CO2 being more abundant than all alkanes. The dC values of alkanes are −46.69‰ (C1), −38.51‰ (C2), −35.33‰ (C3), −30.43‰ (n-C4), and −33.91‰ (i-C4), with CO2 having the highest value of −26.20‰. For bitumen, gravimetric analysis has shown that weight percentages of saturated, aromatics, resins and asphaltenes are 1.1, 3.4, 29.5, and 66.0, respectively. High resolution GC/MS and carbon isotope analyses are in progress to identify detailed chemical compositions within each group, and to measure their carbon isotope values as well. The second experiment with longer reaction time and others under different temperature and pressure conditions are also underway to observe changes, if any, in abundance and carbon isotope values of all organic compounds. That would help us get a better understanding of evolution of chemical compositions and carbon isotope systematics during kerogen maturation processes, facilitating reservoir characterization and effective exploration.