--> The Unconventional Resource Potential of the Pennsylvanian Leo/Minnelusa Formations of the Powder River and Denver Basins

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The Unconventional Resource Potential of the Pennsylvanian Leo/Minnelusa Formations of the Powder River and Denver Basins

Abstract

The Leo and Minnelusa Formations were deposited in a shallow epeiric seaway that covered much of what is now the Denver and Powder River Basins. During the Pennsylvanian (Desmoinesian), eustatically driven fluctuation in base level led to the deposition of alternate cycles of organically rich marine source rock and shallow marine, carbonate (generally dolomite), facies. Multiple source intervals, deposited in response to laterally continuous maximum flooding events, of 1 to 10 feet in thickness, occur over hundreds of feet. Within the Powder River Basin, these source rocks are mature and have generated significant volumes of high sulfur (wt%), low API gravity (<40) and low GOR oils (as evidenced by reservoir fluids and geochemical analysis). Total Organic Carbon within these rocks averaged between 5 to 10 wt%, with average Hydrogen Indexes of 300 to 500. Modelled Gas Liquid Ratios are low (100-3000), related to the initial facies and its current maturity; suggest fluids would likely be viscous. Although the cumulative thickness and maturity of the source rocks was attractive, individual source beds are volumetrically insignificant from the perspective of unconventional storage. Thus, the inter-bedded dolomitic carbonates were examined to investigate whether these could hold the charge and be susceptible to fracking. Porosity/Permeability relationships are an order of magnitude or higher times those observed within known unconventional systems (Eagleford). Pore-space was wet but some fractures were oil filled. Reservoir pressure with depth follows a hydrostatic trend, reflecting the post-Laramide uplift and denudation of the basin flanks. Only a few conventional accumulations have been discovered in the basin center. Most traps are stratigraphic or related to local structural closures around the basin margins. These have been plumbed to the kitchen through the interbedded dolomites which act as efficient carrier beds. Thus, although the Leo/Minnelusa is a high quality source rock interbedded with rocks with excellent storage potential, the high permeabilities and release of pressure have allowed the escape of its unconventional potential.