Petroleum System and Production Characteristics of the Muddy ("J") Sandstone

(Lower Cretaceous) Wattenberg Continuous Gas Field, Denver Basin, Colorado

By

Debra Higley and David Cox

U.S. Geological Survey, Lakewood, CO

Robert J. Weimer

Colorado School of Mines, Golden, CO

The Wattenberg field is a basin-centered, continuous-type gas accumulation with an estimated ultimate recovery of 1.27 trillion cubic feet of gas. The Cretaceous D and Muddy (“J”) Sandstones in the field were assessed as part of the U.S. Geological Survey National Assessment of Oil and Gas Resources project. The Wattenberg field possesses many unique geologic and geochemical features that make it a useful analog for continuous-gas accumulations in other foreland basins. Gas production from the Muddy (“J”) Sandstone in Wattenberg field occurs: 1) from within the most permeable and thickest intervals of Fort Collins Member delta-front, shoreface, and marine sandstones, 2) from the Horsetooth Member valley-fill channel sandstones, 3) in association with a large thermal anomaly that is delineated by well temperatures data and by vitrinite isoreflectance contours of 0.9% and greater, 4) in proximity to the bounding Mowry, Graneros, and Skull Creek Shales that are the hydrocarbon source rocks and the primary reservoir seals, and 5) between the Lafayette and Longmont right-lateral wrench fault zones with secondary faults that act as conduits. The greatest production in the field is from the Fort Collins Member.

The axis of greatest gas production is north 25^o to 35^o northeast, which parallels the basin axis. Recurrent movement of the five right-lateral wrench fault zones that crosscut Wattenberg field shifted the Denver basin axis to the northeast and influenced depositional and erosional patterns of the reservoir and seal intervals. Levels of thermal maturity within the field are anomalously high when compared to other areas of the Denver basin. The Wattenberg field thermal anomaly may be due to upward movement of fluids along faults associated with probable igneous intrusions. Areas of anomalous high heat flow within the field are also correlative with increased and variable gas-oil ratio.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming