Abstract

The Upper Cretaceous Niobrara Formation and its stratigraphic equivalents contain good to excellent source rocks capable of generating significant volumes of hydrocarbons where the formation has achieved adequate thermal maturity. The hydrocarbon source rock potential of the Niobrara varies stratigraphically and geographically. Wireline log data, lithologic descriptions, and analytical data including Rock-Eval pyrolysis analyses provide an understanding of the distribution of source potential within the Niobrara. Lithologies exhibited by the Niobrara and stratigraphically equivalent formations in the western portion of the Western Interior Seaway include chalks, marls, shales, and sandstones. Marly and shaly units are the major hydrocarbon source rocks, although the chalky lithologies also provide some source potential. Total organic carbon (TOC) content as high as 8% has been measured in the east-central portion of the Seaway in siliciclastic sediment-starved facies. Westward and northward, the influx of siliciclastic sediments dilutes organic matter. TOC values, averaging 1 to 2%, are significantly lower to the west and north than in the Denver Basin and Great Plains area. In the Niobrara, the most common kerogen is Type II, although in the Piceance Basin, a mix of Type II and III is present.

Laramide structural modification of the study area has been the primary control on thermal maturity in the formation. The Niobrara is immature in the shallow portion of basins and has begun to generate gas only in the deepest basinal locations with elevated heat flow (e.g., the Wattenberg Field area in the Denver Basin). Local heat flow variations resulting from volcanic activity have had an impact on source rock maturity in limited areas such as the Raton Basin. Timing of generation, as with maturation, is primarily controlled by Laramide sedimentation. One-dimensional maturity modeling calibrated with measured thermal maturity from R_O and T_max values can be used to predict timing, relative volumes, and composition for hydrocarbons generated in many of the Laramide basins.