Prompted by increased interest in understanding microporosity, recent efforts at describing and classifying pore types in mudstones have focused primarily on siliceous, gas producing unconventional reservoirs with little attention being paid to carbonate, mixed oil-and-gas producers. The Niobrara Formation in the Denver-Julesburg Basin is a self-sourced resource play producing oil and natural gas from low permeability chalks. Key reservoir lithologies consist of chalk, chalky marl and marl. These lithologies contain flattened chalk fecal pellets which play a significant role in providing porosity. Integration of depositional fabric with pore-type distribution emphasizes the unique textural and depositional nature of chalk and provides a starting point for evaluation of diagenetic porosity modification. Chalk depositional textures comprise two main subdivisions. The first, called rainstone, includes chalks that form largely from settling of planktonic skeletal remains and fecal pellets as marine snow. New terms related to pelagic chalk textures are pelagic mudstone, pelagic wackestone, and pelagic packstone. The second, called allochthonous chalk, consists of chalks formed from syndepositional tectonic disruption of the seafloor, resulting in mass-movement and redeposition of chalk as turbidites and slide sheets. New terms related to allochthonous chalk textures are allomudstone, allofloatstone, and allorudstone. A chalk porosity classification consisting of four major pore types is presented that can be used to quantify Niobrara chalk pores and relate them to depositional texture, porosity networks, diagenetic history, and pore distributions. Interparticle porosity occurs largely between coccoliths and coccolith fragments, and decreases with burial ranging from 27-38% to 5-17%. Intraparticle porosity occurs within chalk pellets, coccospheres, coccolith plates and foraminifera tests, and also decreases with burial. Organic matter pores are intraparticle pores located within organic matter and are related to hydrocarbon generation. Channel pores, where present, can have significant influence on hydrocarbon storage and permeability networks. In the Niobrara, burial diagenesis in the form of mechanical compaction, chemical compaction, and syntaxial cement overgrowths, modifies pore shape and abundance. Porosity distribution is controlled by the abundance of chalk pellets and the mineralogy of the matrix. Permeability is a function of matrix lithology (micrite-rich vs. silt- and clay-rich). Understanding chalk depositional and diagenetic processes, and how they relate to porosity formation and pore evolution provide a foundation for more accurately predicting the occurrence and distribution of hydrocarbon source and reservoir rocks within the Niobrara.



College and Department

Physical and Mathematical Sciences; Geological Sciences



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Niobrara, Denver-Julesburg Basin, chalk, rainstone, allochthonous chalk, marl, coccolith, chalk pellet, chalk porosity, chalk diagenesis

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Geology Commons