Petrophysical rock typing of unconventional shale plays: a case study for the Niobrara Formation of the Denver-Julesburg (DJ) Basin

Abstract

A petrophysical rock typing has been performed based on a methodology that integrated depositional and petrophysical rock analysis to identify distinct rock types in the Niobrara Formation of the Denver-Julesburg (DJ) Basin. The depositional rock analysis incorporated a literature study, interpretations of the geologic depositional setting and sequence stratigraphy, core descriptions, and analysis of well-log data. The petrophysical rock analysis was carried out using experimental core data on mineralogy, pore characterization, source rock evaluation, micro-textural image analysis, and ultrasonic acoustic velocity and anisotropy interpretations. Six Niobrara Formation lithofacies and three similar lithofacies-groups (All Marls, Middle Chalks, and Basal Chalk) were identied in the target well interval through the depositional rock analysis within the context of the large-scale geologic framework. The pore-scale petrophysical rock analysis was carried out to investigate whether these lithofacies-groups followed distinct rock types in the Niobrara Formation based on the analysis of experimental core data. The conclusions reached in this study included that, based on the mineralogy, pore structure, and acoustic velocity interpretations, the Basal "Chalk" may be a Basal Marl - at least locally in the studied target well. However, the Basal Chalk rocks contain lowest source rock potential and are not likely to contribute to economic hydrocarbon production for the target well. Both the All Marls and the Basal Chalk rock-groups have higher porosity, smaller pores, and smaller pore throats than the Middle Chalks. The rocks in the Middle Chalks have higher stiffness and pressure compliant pores. The Niobrara Formation is an unconventional petroleum system - its hydrocarbon-rich mudrock units were deposited in a shallow marine environment and have evolved as oil- and gas-prone source and reservoir rocks. Significant pore-scale variability in mineralogy, pore characterization, organic matter distribution, and acoustic velocity and anisotropy properties exist in these rocks. Hydrocarbon production from its low-porosity, nanodarcy permeability, and interbedded chalk-marl reservoir interval is very challenging. The petrophysical rock typing methodology adopted in this case study can be a useful formation evaluation tool for the Niobrara Formation. It can provide valuable information about its rock matrix, such as, the lithofacies identification and descriptions through depositional rock analysis, identifying reservoir and sourcing intervals, mineralogy analysis, organic matter evaluation, pore characterization, and acoustic velocity and anisotropy properties.