CHARACTERIZATION OF PORE TYPES AND MORPHOLOGY IN A POTENTIAL CO2 SEQUESTRATION RESERVOIR: A CASE STUDY OF THE SMOKY HILL MEMBER OF THE NIOBRARA FORMATION, DENVER-JULESBURG BASIN

Carbon rich mudrocks, including the Niobrara Formation are proposed for underground storage of CO2. These mudrocks are self-sealing, have an affinity to adsorb gas and have wide spatial distribution in deep sedimentary basins. The Smoky Hill Member of the Niobrara Formation is a candidate for carbon capture and underground storage in the DJ basin, an area of CO2 generation along the Front Range (Rocky Mountains). In addition, >3000 horizontal wells are completed in the Niobrara Formation providing infrastructure for injection and storage of CO2. Effective evaluation of mudrock pore systems, requires a characterization of rock composition and pore structure. This is critical to evaluating porosity generation, evolution and preservation. The Smoky Hill Member of the Niobrara Formation is highly cyclic, alternating between carbonate- rich chalk intervals and a clay-rich marl interval. Chalks (including marly chalk) have mineralogical compositions averaging >70 wt.% total carbonate, clay content of <20 wt.% and <10 wt.% quartz and feldspars compositions combined whilst marls (including chalky marl) typically consist of total carbonate concentrations <60 wt.%, <30 wt.% clay content and <10 wt.% quartz and feldspars combined. The marls are generally richer in organic content with TOC values averaging 3.36 wt.% as compared to chalks (2.4 wt.%). Generally, the average porosity in chalks is higher than porosity in marls. Pores observed within the Niobrara Formation classify as intra-organic matter, interparticle, and intraparticle pores and occasional microfractures. Elongate and slot-like pores occur within clay particles, and circular pores are mostly intraparticle pores that occur within calcareous shells such as coccoliths and organic matter. Intercrystalline/interparticle pores are generally irregularly shaped pores that occur between pyrite framboids, grain rim/edge, organic matter rims, coccolith fragments, clay minerals and micrite.