Abstract: Using an Integrated Database to Determine Regional Facies Variations and Petroleum Potential: Niobrara Formation, Rocky Mountain Region, USA

Barbara A. Luneau, Susan M. Landon, Mark W. Longman

The integration of core, log, stratigraphic, and geochemical data allows the subdivision of the Upper Cretaceous Niobrara Formation of the Western Interior seaway into three major facies tracts. Each has distinct reservoir and source rock characteristics. The distribution of chalk and shale-rich facies within these tracts reflects the influence of temperature, paleobathymetry, circulation patterns, and sea level fluctuations on deposition.

The spectrum of Niobrara lithologies includes chalks, marls, shales, and sandstones. A distinguishing feature is an abundance of chalk pellets in certain lithologies in each facies tract. The eastern tract, which covers the Denver, North Park, and Raton basin areas, contains the most chalk-rich and organic-rich facies. The lithologies in the northern tract, extending across the Powder River and Williston basin areas, are significantly more shale dominated, with chalk content decreasing northward and westward. High siliciclastic content in the western tract, extending across the Piceance, Sand Wash, and San Juan basin areas, reflects proximity to the tectonically active western thrust belt.

Source rock potential dramatically increases eastward across the seaway with measured TOC values as high as 7.6%. Type II kerogen dominates in the eastern and northern tract, and there is an increase in type III kerogen in the western tract. Thermal maturity varies regionally, and basin modeling can be used to define areas of peak oil generation through time. Organic-rich shale units within the Niobrara also serve as seals for conventional and fractured reservoirs. Productive lithologies include porous chalks, fractured chalks, fractured marls and shales, and sandstones.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994