Evaluation of Niobrara and Mowry Formation Petroleum Systems in the Powder River, Denver and Central Basins of the Rocky Mountains, Colorado and Wyoming, USA

Potential Niobrara and Mowry formation source rocks and produced oils from multiple Cretaceous reservoirs were characterized from seven Rocky Mountain basins to evaluate generation and migration of shale oil from these two petroleum systems. This is a subset of a comprehensive study of 14 Rocky Mountain basins.

Total organic carbon, Rock_EvalTM pyrolysis and vitrinite reflectance analyses of rock samples were coupled with a complete characterization of the produced oils, including saturate and aromatic carbon isotope compositions and sterane and terpane biomarkers.

Systematic variations in source rock quality and thermal maturity for both formations were noted and mapped in the seven basins. These variations are due to differences in depositional environments, water-column anoxia and Laramide evolution within the studied basins.

Determination of differences in depositional environments between the Niobrara and Mowry was based solely on biomarker interpretations. The Niobrara is an effective source rock due predominantly to enhanced productivity in the water column, in contrast to the Mowry, where enhanced preservation played the major role.

Three oil families were identified:

Mowry Family 1.1 member oils are mostly reservoired in the Lower Cretaceous Muddy Fm, stratigraphically just below the Mowry source. Conversely, Niobrara Family 1.2 member oils are mostly reservoired in the Upper Cretaceous Niobrara Fm. and Upper Cretaceous sands.

Many of the Cretaceous sub-family 1.3 oils occur in both Upper and Lower Cretaceous reservoirs and are slightly separated (statistically) from either the Mowry or Niobrara sub-families because many were generated at a higher level of thermal maturity. Some of the sub-family 1.3 oils that are of moderate maturity appear to have another marine shale source, perhaps the Upper Cretaceous Carlile/Greenhorn formations.

In the Powder River Basin, Mowry-sourced oils occur mostly in the eastern half while Niobrara oils are more central.

Light oils/condensates (with no biomarkers) produced from Cretaceous reservoirs in the Denver Basin correspond to the ‘Wattenberg Thermal Anomaly’. Just to the west and north, are low/moderate mature Niobrara oils. In the NW corner of the Denver Basin, where both Mowry and Niobrara-sourced oils are present, the deeper Mowry oils are somewhat more mature.

Low/moderate mature Mowry oils exist in both Laramie and North Park Basins while Niobrara oils only occur in the North Park Basin.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California