Abstract

This report presents an integrated study of the origin of natural gas in the coal beds of the Tongue River Member, Fort Union Formation, located in the Powder River Basin of Wyoming. The origin of natural gas in these coal beds is particularly important because they serve as a source of copious amounts of gas in the form of coalbed natural gas accumulations. To better understand the source of the coalbed natural gas in the Powder River Basin, we evaluated burial, thermal, and maturation histories and reaction kinetics of hydrocarbon generation in the Tertiary coals and organic-rich shales of the underlying Cretaceous stratigraphic section.

Our evaluations suggest the following conclusions:

• The isotopic composition of natural gas in the Tongue River Member coal beds demonstrates that it is a mixture of thermogenic and biogenic gas.

• The coal beds have had insufficient thermal exposure to generate notable amounts of thermogenic gas.

• During burial, the Tongue River Member coals generated huge quantities of short- chained carboxylic acid and acid anions, providing an ideal environment for methanogenesis (CH₃COOH + bacteria → CH₄+ CO₂).

• Acetate generation (methanogenesis) in the coals probably began approximately 50 million years ago (Ma) at a burial depth of 500 meters and continued until approximately 10 Ma at a burial depth of 1,000 meters.

• At present, the Tongue River Member coals have little or no additional capacity to generate acetate; any acetate available in the coals for methanogenesis was generated in the past but was not used by methanogenic bacteria.

• The extremely large surface area (combined matrix and fractures) within coal beds provides a multitude of sites for absorption of both biogenic and thermogenic natural gas.

• The thermogenic gas in the coals originated in the underlying organic-rich shales in the more deeply buried Cretaceous stratigraphic section. The source of the biogenic gas in the coals is the coals themselves; this biogenic gas resulted from methanogenesis driven by the in-situ interaction of microbes with a huge quantity of carboxylic acids and acid anions generated during relatively shallow burial (500-1,000 meter depth).