Petrophysics, geochemistry, mineralogy, and storage capacity of the Niobrara Formation in the Aristocrat PC H11-07 core, Wattenberg field, Denver Basin, Colorado

Abstract

The Niobrara Formation in the Denver Basin is an unconventional oil and gas play composed of highly cyclic, alternating chalk and marl intervals. These intervals act as the source, seal and trap for hydrocarbons generated in the organic-rich marl beds of the Niobrara Formation. The organic-rich marls act as source rocks and represent periods of varying anoxia and organic preservation. The calcareous, carbonate-rich chalks act as reservoirs and represent periods of carbonate production under varying oxygen paleo water conditions. Understanding the reservoir quality and the petrophysical, mineralogical, geological, and geochemical characteristic of the different members of the Niobrara Formation is key for future exploration and development. XRD, ICP-MS, XRF, TOC, nitrogen adsorption, WIP porosity, and well log data on the Aristocrat PC H11-07 core was used in this study. Petrophysical evaluation of the well logs shows that the chalks possess higher porosity values that average between 11-13% than the marls that have average porosities less than 11%. The density porosity method provided the best estimate of log derived porosity as opposed to the commonly used neutron-density porosity method. Water saturation is highest in the marls and was best estimated using the dual water saturation petrophysical model. A petrophysical workflow was developed that allowed generating rock types from physical core data and predicting the same rock types from petrophysical logs. This facilitates the integration of core-scale geologic elements to petrophysical analysis. The hydrocarbon potential of the Niobrara Formation is highly dependent on its organic content and maturity. Passey Delta Log R, Schmoker, and the Uranium method are widely used empirical relationships from logs for quantitative TOC estimation. The Schmoker and Passey Delta Log R methods present inconsistent results in Niobrara Formation. The Schmoker method over estimates TOC in the Niobrara Formation, while the Delta log R method underestimates TOC. The GR log and the uranium spectral log are the best proxies of organic richness in the Niobrara Formation. The uranium method provides the best estimate of log derived TOC. XRF, XRD and ICP-MS data was utilized to model the mineralogy in the Niobrara Formation using the concept of linear programming/optimization. The developed Matlab computer code “El Com” was successful at calculating mineralogy from available ICP-MS data. To study the storage capacity of the Niobrara Formation, WIP, nitrogen adsorption and FESEM work was performed on selected chalk samples. Results show that WIP method for porosity measurement is repeatable, yet not suitable for the Niobrara Formation due to the presence of swelling clays such as smectite. This method overpredicts porosity in clay-rich samples and underestimates porosity in tight samples with small pores. Nitrogen adsorption and FESEM analysis revealed a significant portion of both meso and macropores in the Niobrara Formation. Nitrogen adsorption data shows a consistent trimodal micro and meso pore size distribution with distinct peaks between 50-100nm (e.g. intraparticle pores between clay aggregates), at 3nm (e.g. intraparticle pores within clay platelets), and between 1.7-2nm (e.g. organic matter associated pores). Organic matter and clays contribute to the microporosity of the Niobrara Formation and provide the highest specific surface area for gas adsorption. FESEM analysis confirmed the presence of organic pores, interparticle and intraparticle pores that vary in morphology. Analysis suggests interparticle and intraparticle macro sized pores that range in size from 3 microns to less than a micron. The Niobrara Formation exhibits a mixed pore system with the pores being mainly interparticle confined between crushed coccolith grains and micrites. This is followed by intraparticle pores between clay aggregates and organic pores. Fracture porosity was not documented in the core and is not a main contributor to porosity in the Niobrara Formation.