AAPG Rocky Mountain Section Meeting

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Effective Source Rocks in Rocky Mountain Late Cretaceous Petroleum Systems

Abstract

Effective source rocks are defined as those source rocks that have generated or are generating hydrocarbons in a petroleum system. Unconventional targets in the Late Cretaceous sediments are important for many Rocky Mountain oil and gas developments. It is often convenient to assume that an organic-rich shale or marl that is most proximal to a chosen reservoir target is responsible for charging that reservoir. Complex structural timing, migration and thermal histories in these sedimentary basins are indicating that the effective source rock responsible for charging a specific organiclean reservoir interval may not be the juxtaposed organic-rich marl or shale. The Niobrara Group and Graneros-Greenhorn Formations are known to contain organic-rich intervals. The Niobrara intervals include the Niobrara A, B and C marls. In the Graneros and Greenhorn Formations the Bridge Creek, Hartland and Lincoln members are seen to contain organic-rich sequences. The objective of this study is to determine which source rocks are effective and contributing to regional petroleum systems. This study investigates Late-Cretaceous petroleum systems. Maturity modeling is employed to differentiate the level of organic maturity in the rocks. Organic richness and net source thickness are important parameters to consider when evaluating effective source rocks. Regional scale mapping is employed to determine the effective source rock fairways for the Denver Basin and the Powder River Basin. Many thousands of data points are used to control the vertical and lateral extent of these parameters. Those data include LECO TOC, pyrolysis (Rock-Eval, SRA, for example) and measured vitrinite reflectance for total organic carbon (TOC), net source thickness and maturity. Log-derived TOC in these intervals help to supplement richness and thickness determinations when rock data are sparse. Combining regional scale maturity, richness and thickness in specific horizons determines the regional extent of these effective source rocks. The products of these regional grids are presented as Effective Source Rock Fairway Maps for the Denver and Powder River Basins. Petroleum systems evaluations have been employed successfully for both conventional and unconventional systems. Having a complete understanding of all the elements of a petroleum system: source rock, reservoir rock, migration and trap reduces risk in an unconventional system and increases the chance of success in many Rocky Mountain, Late Cretaceous plays. Fluid prediction and drainage rock volumes are impacted so the engineering community often use these results for reservoir engineering and overall development decision support. The nature of the unconventional system is heavily influenced by the organic-rich shale rock. Reservoir characterization often includes the same tight rocks. Fluid prediction and drainage rock volumes are impacted so the engineering community often use these results for reservoir engineering and overall development decision support.