Hydrodynamic and Stratigraphic Controls on Wyodak-Anderson Coalbed Natural Gas Reservoirs in the Fort Union Formation, Powder River Basin, Wyoming

McGarry, Dwain E.¹, Romeo M. Flores^2
1 U.S. Bureau of Land Management, Casper, WY
² U.S. Geological Survey, MS 939, Denver, CO

The Fort Union Formation (Paleocene) in Wyoming's Powder River Basin contains numerous shallow, thick coal beds that contain extensive coalbed natural gas (CBNG) resources, which are late biogenic in origin as indicated by their light delta C13 isotopic composition. In-place CBNG recoverable resources are estimated to range from 12 to 37 trillion cubic feet (TCF) with a mean of 23 TCF (Crockett and others, 2001). Most of the development and production to date has taken place from coal beds of the Wyodak-Anderson coal zone (that is, Anderson, Canyon, Wyodak, and Big George) at depths of 250 to 1,200 ft. Drilling and development are presently progressing to higher gas-volume coals in the deeper (2,500 ft) parts of the basin.

Joint studies by the U.S Geological Survey Central Region Energy Resources Team and the Bureau of Land Management's (BLM) Reservoir Management Group in the Wyoming State Office are beginning to define reservoir controls and relation among the various coal beds. Geologic analyses imply that discrete CBNG reservoirs result from coal-bed discontinuities (for example, splitting, merging, pinch-outs, and wants) and hydrodynamic isolation of the coals caused by these discontinuities.

Detailed analyses underway in the vicinity of the Triangle and Wormwood coalbed natural gas Units reveal apparent discrete CBNG reservoirs in the Wyodak-Anderson play. Structural mapping and stratigraphic analyses confirm development of multiple coal beds. Piezometric mapping, based on data from a limited number of groundwater monitoring wells operated by BLM, indicates distinct hydrodynamic regimes and vertical separation between CBNG reservoirs. Lateral reservoir confinement is also implied by stratigraphic analysis. Analysis of these reservoir controls might yield a better understanding of reservoir performance and compartmentalization, production characteristics, and exploitation of gas resources across the basin.