Abstract

Integration of lithologic and wireline log data allows subdivision of the Upper Cretaceous Niobrara Formation into stratigraphic units that provide a framework for predicting both hydrocarbon source and reservoir potential of the formation throughout the Rocky Mountain region of the western United States. Regional mapping of these Niobrara subdivisions also provides a basis for interpreting the nature of deposition in the Western Interior Seaway during a time of relatively high sea level. Water depth, climate, proximity to the thrust belt bordering the seaway on the west, paleobathymetry, current circulation patterns, and sea level fluctuations all played a role in shaping the nature and distribution of Niobrara lithologies.

The Niobrara and stratigraphically equivalent formations to the west contain a spectrum of lithologies including chalks, marls, shales, and sandstones. Coccolith-rich fecal pellets, probably formed by pelagic copepods that thrived during times of high sea level as water circulated through the seaway, provide a distinctive feature of the formation. These chalk pellets occur even in some siliciclastic-rich intervals in the western part of the seaway.

Hydrocarbon production comes from three major lithologies: microporous and fractured coccolith- and planktonic foraminifer-rich limestones present mainly in the eastern part of the seaway; fractured sand-rich facies, mainly in the western and southwestern parts of the seaway; and fractured marls and shales, mainly in the central part of the seaway. Black shales, some of which are quite rich in coccoliths and chalk fecal pellets, provide the major hydrocarbon source rocks. Similar shaly beds also provide sealing facies over more chalky or sandy reservoir intervals. Organic richness in the source intervals generally increases from less than 1% in the siliciclastic-rich western facies to more than 7% in the eastern clastic sediment-starved facies. Thermal maturity of these source beds varies regionally on the basis of burial depth and local heat flow.