129-18 Characterizing Natural Gas Generation from the Illinois Basin Using Carbon and Hydrogen Stable Isotope Ratio

Abstract:

The Illinois Basin is a structural basin that covers Illinois, Indiana, and Kentucky. It was formed on a failed rift system from the Cambrian and deposited shallow marine sediments from Cambrian to Pennsylvanian. It has been producing oil and gas since the 19^th century, with more than 20,000 oil and gas wells in Illinois. Despite this rich hydrocarbon resource, natural gas in Illinois was viewed as a byproduct of oil and coal, not as a valuable resource and a potential reservoir driver to deliver more oil to the wellbore. This resulted in producers to ‘flare’ it or vent it to the atmosphere in the early days of hydrocarbon production. A consequence of natural gas being overlooked as a resource resulted in a dearth of research and compositional analyses within the basin.

In this study, we present a compositional, isotopic, and geospatial analysis of natural gas sampled in Illinois. The sample data were compiled from an older dataset published in 1981 and modern well data sampled predominantly in southern Illinois. Gas data from the 1981 dataset did not contain isotopic analyses, so the isotope analysis was only performed on the modern data. The data was then normalized to remove O₂ and N₂ due to air contamination. Data was then plotted using compositional and isotope metrics to determine gas source and composition. The compositional analysis yielded mostly methane-rich gas from shallow wells in northern Illinois, and heavier (longer chain alkanes), thermogenic gas from deeper wells in southern Illinois. The isotopic analyses revealed mostly mixed microbial and thermogenic gas from the dataset of modern wells. At least one well with characteristics of a coalbed methane source was documented from Pennsylvanian stratigraphy which contains abundant coal seams. The application of this data in research constraining basin evolution and hydrocarbon migration across the Illinois basin will assist resource development and storage opportunities in the future.

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