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Preliminary Unconventional Reservoir Characterization of the Lower Cretaceous Skull Creek Shale, Wyoming


The large Cretaceous Western Interior Seaway in the northwestern part of the United States contains a thick section of mudrocks interbedded with numerous sandstones. Many of these mudrocks were deposited during marine transgressions and are source rocks. The Albian Skull Creek Shale is one of these units and appears to have not been characterized relative to its candidacy as a potential shale-gas system. A characterization is presented based on a description of the Cities Service #1 Federal CM core in Niobrara Co. Wyoming. The Skull Creek interval reaches 250 ft in the center of the seaway. The lack of thick sands and large scale hydrodynamic structures, rare bioturbation, and relatively higher TOC suggest a deeper water setting below storm-wavebase. An anaerobic to dysareobic bottom environment is interpreted on the basis of higher TOC, relative lack of burrow traces, extremely rare fauna, and abundant pyrite framboids. The anaerobic to dysareobic bottom environment implies a stratified water column below the oxygen minimum boundary. Silty, siliceous mudstone is the predominant rock type with various amounts of silt to fine-sand stringers interspersed. Only a few burrow traces were recognized in this core and these were Nereites, which is a common deeper water, low-energy burrower. Ar-ion milled samples were analyzed using a FSEM. Samples show low visual porosity of only several percent. The overall pore network is a mixed pore network. Interparticle pores appear to be the most common and consist of pores between grains and grain-edge pores. Intraparticle pores are relatively common and generally occur with within clay grains or pyrite framboids Organic matter pores are present in the smaller kerogen particles. The larger organic particles are woody material and do not contain nanopores. Mean TOC is 1.2% with a high of 3.2%. TOC appears to have a reverse relationship to sand and silt content. This reflects slower deposition of the muddier facies. Ro in the core ranges from 1.05 to 1.2%. A pseudo Van Krevelen chart shows all data points fall in the lower left corner of the chart suggesting a thermally mature mixture of Type II and III organic matter. Classification of kerogen quality using a Dembicki-type graph indicates that the weight percent TOC is fair to good but generation potential is poor. Core data in this study suggest marginal potential based on quality of organic material, but other areas may have better quality organic material.