Integrated analysis, reservoir characterization, and resource potential of the Niobrara Formation: Lowry Bombing Range, Arapahoe and Adams County, CO

Abstract

The Cretaceous Niobrara Formation is a productive, unconventional petroleum exploration target in the Denver Basin, Colorado. It is a self-sourced oil and gas play composed of alternating chalk and marl units. Ductile marls can serve as major source rocks with TOC values ranging 2-8 wt. %. These marly intervals can also serve as seals for the underlying brittle chalk reservoirs. Chalk intervals are comprised of carbonate-rich pellets, coccoliths, pelagic foraminifera, inoceramids, and oyster shells, and tend to have higher porosity and permeability values. Porosity distribution is controlled by the abundance of pellets, degree of bioturbation, and mineralogy within the chalk-marl matrix. Characterizing the pellet abundance and the depositional fabric provides a foundation for predicting the occurrence and distribution of reservoir intervals of the Niobrara Formation. This study involves a comprehensive evaluation and integrated approach for characterizing the reservoir potential in three wells from the Lowry Bombing Range in Arapahoe and Adams counties, Colorado. Goals of the project include: (1) a complete core description and identification of facies with an understanding of lateral and vertical heterogeneities; (2) reservoir characterization using petrographic thin sections, SEM photomicrographs, XRD bulk mineralogy, XRF analysis, GRI porosity/permeability data, and Source Rock Analysis; (3) a comprehensive description of the pore system and storage capacity; (4) a geomechanical evaluation of fracture-prone benches and fabrics that enhance brittleness; (5) identification of potential reservoir targets within the Niobrara Formation; and (6) an evaluation of the influence of regional paleo-high structures on thermal maturity and the total petroleum system. Ultimately, this study aims to identify the geologic parameters that contribute to productive wells throughout the Lowry Bombing Range. Nine chalk-marl facies were identified by describing the cores at high resolution for lithology, mineralogy, degree of bioturbation, sedimentary structures, fossil presence, contacts, and pellet abundance. Primary reservoir facies include pellet-rich marly chalks and bioturbated chalks to marly chalks. Carbonate-rich pellets appear to experience less compaction than the surrounding matrix and can maintain storage capacity for Niobrara rocks. Based on GRI data, average porosity and permeability values in reservoir facies were about 8% and 6.3E-4 mD, respectively. SEM photomicrographs indicate that storage capacity in pellets is dominated by intraparticle and interparticle porosity. The A, B2, and C Chalk intervals were defined as primary reservoir targets based on high carbonate concentration, low clay material, high porosity and permeability, favorable geomechanical properties, and high gas saturation. Based on subsurface correlations and mapping, the B Chalk is the thickest reservoir with the highest resistivity responses suggesting the best potential for hydrocarbon production.