Origin of Gas in the Mamm Creek Field, Piceance Basin, Colorado

Paul G. Lillis¹, Geoffrey S. Ellis¹, Michael P. Dempsey², and Stephen P. Cumella^3
1U.S. Geological Survey, Denver, CO
²EnCana Oil and Gas (USA) Inc, Denver, CO
³Bill Barrett Corp, Denver, CO

The Mamm Creek field, one of the largest gas fields in the Piceance Basin, Colorado, produces mostly gas (estimated reserves more than one trillion cubic feet) and some liquids from sandstones of the Williams Fork Formation of the Upper Cretaceous Mesaverde Group. The source of the gas is widely regarded to be coal beds (including the Cameo coal zone) within the lower part of the formation. Recently acquired gas compositional and isotopic data, integrated with other geologic information from the field, provide new insights into the sources of the gas, the timing of gas generation, migration pathways, and reservoir compartmentalization. Specifically, these data indicate that the field has more than one major source of gas. The western portion of the field appears to contain gases derived predominantly from the intraformational coals; however, isotopically lighter and wetter gases in the northeastern part of the field point to a marine shale source, possibly the underlying Upper Cretaceous Mancos Shale. Comparison of Mamm Creek gas geochemistry data with the results of a kinetic isotope fractionation model for gas generation from the Cameo coal and a thermal history model for the Piceance Basin indicates that gas generation occurred from approximately 45-10 Ma. Additionally, spatial variations in gas geochemistry throughout the field clearly indicate the presence of multiple reservoir compartments. This demonstrates the utility of gas geochemistry for providing further insight into the reservoir geology when geophysical data are unavailable.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado