Abstract

Interest in unconventional petroleum potential of the Late Cretaceous Graneros and Greenhorn formations has increased due to success of many producers in the stratigraphically higher, but mineralogically similar, Niobrara Formation. Previous research in the Graneros and Greenhorn formations has mainly focused on depositional cyclicity and stratigraphic controls, rather than describing hydrocarbon potential within these two intervals. The focus of this study is to examine the petroleum potential of these two organic-rich formations within the greater Wattenberg area of the Denver Basin.

The Graneros and Greenhorn formations represent regional transgression in the Cretaceous Western Interior Seaway where large amounts of organic-rich, calcareous sediment was deposited throughout much of the North American mid-continent. The ratio of carbonate to siliciclastic sedimentation increased as the transgression took place and water depths deepened, resulting in the lower Graneros Formation predominantly consisting of shales and marls and the higher Greenhorn Formation mainly consisting of marls and chalks. Geochemical analysis of organic-rich shales and marls within the two source units (Graneros Formation and Hartland Member of the Greenhorn Formation) indicate that significant amounts of hydrocarbons have been generated within the study area. Several unconventional chalk reservoirs of the Greenhorn Formation (Lincoln and Bridge Creek members) have been influenced by this oil generation, resulting in hydrocarbon accumulations in these as well.

Data from log-derived total organic carbon, oil and gas shows from formation drill stem tests, geochemical analyses, and geological characterization of the units within the study interval indicate that source rock maturation and hydrocarbon accumulation have taken place within the Graneros and Greenhorn formations in the study area. The Lincoln Member of the Greenhorn Formation is assumed to have the highest potential to yield hydrocarbons within the study interval, and the Bridge Creek Member also shows a strong possibility for oil and gas production. Low-angle shear fractures (~45°) provide wellbore deliverability as well as a migration pathway between source and reservoir rocks.

Current horizontal drilling and multi-stage hydraulic stimulation technologies allow the Lincoln and Bridge Creek members of the Greenhorn Formation to be a viable and likely reservoir target in and around the greater Wattenberg area. A major unknown variable at this point is whether or not the thin, tight reservoir facies of these two members can yield sufficient wellbore deliverability for sustained economic hydrocarbon production.