Geologic reservoir characterization of the Niobrara Formation in the Trabing field, western Powder River Basin, Wyoming

Abstract

The Niobrara play in the Powder River Basin (PRB) and Trabing Field reflects a continuous-type accumulation with structural deformation and complex stratigraphy reflected at all scales. The field was previously studied in order to understand the petroleum potential of the Frontier Formation. The work done prior to this study have failed to focus on understanding the field-scale structural and stratigraphic architecture, and petroleum potential of the Niobrara Formation in the Trabing Field. Based on visual core analysis, the Niobrara B chalky marl interval consists of four lithofacies including three ichnofacies, whereas the Niobrara C chalky marl interval contains five lithofacies with four ichnofacies delineated. The lithofacies follows a progressive decrease in terms of the quality of preservation of the laminations. The extent to the preservation of the laminations has implications on the oxygen levels and organic matter preservation quality. Both intervals are slightly to intensely bioturbated while the C chalky marl interval contains more bioturbation. XRD analysis revealed marlstone lithology for the cored intervals. Based on the micro scale facies, the main diagenetic features include clay alteration and pyritization. SEM displayed interparticle and intraparticle pore types. Sequence stratigraphic aspects of the Niobrara Formation and cored intervals were studied using gamma-ray logs, gray-scale analysis and XRF trends. The sequence stratigraphic framework supported the observations in the literature. Based on XRF analyses, quartz content is determined to be terrestrially sourced, whereas, calcite formed authigenically and biogenically. S, V, Cr, and Mo enrichment took place under reducing conditions. Based on SRA, the B chalky marl interval contains TOC in the range of 2.37-2.88 wt.%. The C chalky marl interval has 2.04 to 4.78 wt.% TOC. Siliciclastic dilution was minimal and sufficient organic carbon was preserved to generate a good source rock. Both intervals contains good to excellent source rock potential. Van Krevelen diagrams indicated type II-III kerogen. The intervals corresponded to the early oil generation window. The organic matter is found mature enough to generate oil and associated gas based on Tmax and measured Ro values. Water saturation models including Archie’s and Indonesian methods matched the measurements taken from shale rock properties. Dual-water and Simandoux methods underestimated the water saturation. Organic richness models included the Passey and Uranium methods and these models displayed good correlation with TOC values obtained from SRA. Brittleness models based on elastic moduli and ECS logs displayed variations from the lithology and shear-strain character of the rocks. Brittleness models were found useful for capturing the reservoir intervals with improved geomechanical character. Overall, the reservoir facies were found continuous throughout the Trabing Field based on well-log correlations. The B and C chalky marl intervals were defined as good reservoir and source units. Bioturbation was found to have positive influence on the brittleness. The C chalky marl interval contains slightly better reservoir and source quality based on the SRA, petrophysical models, and lithofacies distribution.