Stratigraphic and geochemical implications on the petroleum potential of the lower Mancos Group (Niobrara), Piceance Basin, Colorado

Abstract

Subsurface stratigraphy, petrology, geochemistry and basin modeling are all essential to assess the petroleum potential of a formation. This study uses all four approaches to provide an overview of the Niobrara petroleum system in the Piceance Basin and correlate Denver Basin equivalent Niobrara subdivisions. Several crucial conclusions offer guidelines for future Niobrara exploration within the basin. The Niobrara Formation of the Piceance Basin differs from the Denver Basin in several key characteristics. Overall thickness of the formation increases greatly to the west. Thicknesses of 300 ft, common in the Denver Basin, become thicknesses of as much as 1800 ft in the Piceance Basin. To date, maximum TOC values from the Piceance Basin are ~3 wt.% whereas in the Denver Basin TOC values may be as high as 8 wt.%. Several factors may contribute to the lesser TOC but a significant factor is the increase in siliciclastics. Siliciclastics sourced from the Sevier Fold and Thrust Belt dilute the TOC as well as carbonate content. Unlike the Denver Basin scheme of organic-rich marls providing the bulk of sourcing to carbonate-rich benches of greater fracture porosity, the TOC and carbonate content share a positive trend in the Piceance Basin. Carbonate-rich bench equivalents have higher TOC content relative to the marl equivalents. The organic material is of mixed type II-III kerogen with significant type II input. At temperatures and pressures conducive to the production of liquid hydrocarbons the Niobrara Formation is likely liquids rich. Around the basin margin, specifically the northwestern and northeastern basin margins, high calibrated average log derived TOC trends indicate increased liquids potential where the Niobrara is at or near peak oil generation. Within a total Niobrara interval of as much as 2000 ft of thickness the upper carbonate rich members of the A Bench and B Bench equivalents provide well-defined targets for future exploration. The underlying lower Mancos Group, specifically the Storm King Mountain Shale member, also has significant petroleum generative potential, based on high TOC, across the basin. The Piceance Basin has great potential for further petroleum exploration and has great potential for further oil and gas production from the Niobrara and lower Mancos Group within underexplored producing fields to date. Recent investor headlines of Piceance Basin Niobrara discoveries from WPX Energy, Encana Corporation and Antero Resources support the remaining potential in the basin.