3-Dimensional Modeling of the Molecular and Isotopic Composition of Gases from Mamm Creek Field, Piceance Basin, Colorado

Geoffrey S. Ellis¹, Paul G. Lillis¹, Terrance Dewane², and Stephen P. Cumella³
¹U.S. Geological Survey, Denver, CO
²EnCana Oil and Gas (USA) Inc, Denver, CO
³Bill Barrett Corp., Denver, CO

The Mamm Creek field is one of the largest gas fields in the Piceance Basin, Colorado, with estimated reserves of more than one trillion cubic feet (Lillis et al., 2008). Gas production is predominately from low porosity/permeability sandstones of the Williams Fork Formation of the Upper Cretaceous Mesaverde Group. The Cameo coal zone within the lower part of the formation is generally thought to be the primary source of the gas (with possible contributions from minor intraformational organic-rich units). Gas compositional and isotopic data from field samples were combined with other geologic information from the field into a three-dimensional model using the Trinity* software package (Zetaware, Inc.). Additionally, source-specific kinetic isotope fractionation parameters were determined based on sealed-tube pyrolysis experiments, then were extrapolated to the natural time-temperature conditions of the Piceance Basin using the GOR Isotopes* software package (GeoIsoChem, Inc.). Integration of the kinetic model results with geologic and geochemical data provides new insights into the sources of the gas, the timing of gas generation, migration pathways, and reservoir compartmentalization. In particular, these results indicate that there may be an additional source of gas in Mamm Creek field. Figure 1 shows that the carbon isotopic composition of propane relative to ethane from this area is slightly depleted compared to the predicted values for the Cameo coal. This may indicate mixing of Cameo coal-sourced gases with a secondary, isotopically lighter source, most likely from the underlying Upper Cretaceous Mancos shale which contains type II kerogen (Johnson and Rice, 1990). Combining the kinetic isotope fractionation model with published burial history data for the Piceance Basin (Yurewicz et al., 2008) allows us to estimate that primary gas generation occurred from approximately 45 to 10 Ma (Figure 2). Moreover, spatial variations in the gas geochemistry throughout the field indicate reservoir compartmentalization, the existence of which helps in determining migration pathways and timing of localized gas emplacement. Overall, the integration of a source-specific kinetic isotope fractionation model with local geologic and geochemical data in a three-dimensional framework provides a powerful tool for improved understanding of the generation and accumulation of natural gas in Mamm Creek field.

References

Johnson, R. C., and Rice, D. D., 1990. Occurrence and geochemistry of natural gases, Piceance Basin, northwest Colorado. AAPG Bulletin v.74, no. 6, p 805-829.

Lillis, P.G., Ellis, G.S., Dempsey, M., and Cumella, S., 2008. Origin of gas in the Mamm Creek Field, Piceance Basin, Colorado. AAPG Rocky Mountain Section Annual Meeting, Denver Colorado. July 9-11, 2008.

Yurewicz, D. A., K. M. Bohacs, J. Kendall, R. E. Klimentidis, K. Kronmueller, M. E. Meurer, T. C. Ryan, and J. D. Yeakel, 2008, Controls on gas and water distribution, Mesaverde basin-centered gas play, Piceance Basin, Colorado, in S. P. Cumella, K. W. Shanley, and W. K. Camp, eds., Understanding, exploring, and developing tight-gas sands— 2005 Vail Hedberg Conference: AAPG Hedberg Series, no. 3, p. 105–136.

Figure 1. Stable carbon isotopic composition of propane vs. ethane for gases from Mamm Creek field and a kinetic isotopic fractionation model for gases from a sample of the Cameo coal in the Upper Cretaceous Williams Fork Formation. The model results indicate a second source of gas with depleted carbon isotopic composition (possibly from the Mancos shale).