Geologic reservoir characterization of the Niobrara B interval in Redtail field: Weld County, Denver Basin, northeast Colorado

Abstract

The Late Cretaceous Niobrara B interval in Redtail Field is an important source rock and unconventional reservoir in the Denver Basin. In Redtail Field the B interval is split into the older B2 chalk and the younger B1 chalk. The B2 chalk is thicker than the B1 and has high resistivity zones. In Redtail Field, the Niobrara B interval consists of coccoliths, foraminifera, inoceramids, oysters, clay, siliciclastic silt, carbonate cement, vertical calcite filled fractures, and horizontal stylolites. Based on physical observations, four lithofacies were identified. Facies 1 is a homogeneous-chalk with few thin clay interbeds. Facies 2 consists of planar laminated chalks and marls that are interbedded with thin clay drapes. Facies 3 is a dark-marly chalk with planar laminations and sparse interbedded chalks. Facies 4 consists of structureless homogenous-marl with very few planar laminations. Facies 2 is the most common facies throughout the entire Niobrara B interval. Observational core description, supplemented by XRD data, indicate that the B1 chalk contains more clay than the B2 chalk. Dunham’s classification of the two intervals categorizes the B1 chalk as a pellet wackestone to a pellet packstone and the B2 chalk is classified as a pellet packstone. Analysis of petrographic thin sections impregnated with blue epoxy and observed under ultraviolet light indicated that most of the porosity is intraparticle within fecal pellets and the B2 chalk contains more pore space than the B1 chalk. FE-SEM imagery illustrated that the majority of the porosity within fecal pellets is interparticle between coccoliths, and calcite overgrowths on coccoliths. Between fecal pellets there is interparticle porosity between coccoliths, calcite overgrowths on coccoliths, foraminifera, and calcite crystals as well as intraparticle porosity within coccolith spines, foraminifera chambers and pyrite framboids. Organic matter exists in interparticle and intraparticle pore space and concentrated in horizontal stylolites. The B2 chalk contains larger and more frequent horizontal stylolites than the B1 chalk and there is a drastic decrease in fecal pellets and micro porosity in close proximity to the stylolites. The concentration of planktic foraminifera is greater in the B2 chalk and the characteristics of the foraminifera vary greatly. There are foraminifera chambers (that are empty or filled with different minerals, affecting the amount of effective pore space for hydrocarbon storage. XRF analysis shows that in Redtail Field the B1 chalk contains more siliciclastic and detrital elements (Si, Al, K) and less carbonates (Ca) than the B2 chalk. This supports observational core descriptions that the B2 chalk contains less clay than the B1 chalk. XRF cross plots indicate that the Al and Si elements are detritally sourced and that calcite formed authigenically and biogenically in the Niobrara B interval. XRF cross plots also indicate that S, V, and Mo enrichment took place under reducing conditions. High concentrations of Mo and high ratios of Al/Fe indicate that both the B1 and B2 chalk were deposited in anoxic to euxinic conditions. Cross plots of TOC versus XRF data indicate that anoxic to euxinic conditions favored the preservation of organic matter and higher concentrations of micronutrients correlate to more organic matter. The depositional environment of the Niobrara Formation has been previously interpreted as open marine which would suggest dominantly Type II kerogen with some contribution of Type III kerogen from nearby terrestrial environments during relative sea-level fall. Geochemical results from six wells in Redtail Field confirm a dominantly Type II kerogen with some Type III kerogen. The Tmax for the B1 chalk ranges from 427-445oC and the Tmax for the B2 chalk ranges from 431-445oC. The TOC content in the B1 chalk ranges from 1.97-6.41 wt.% and in the B2 chalk ranges from 0.599-14.1 wt.%. which indicates a very good source rock potential for both intervals. The production indices (S¬¬1/(S1+S2)) (PI) have a range of 0.078-0.516 for the B1 chalk and 0.0421-0.601 for the B2 chalk, which indicates thermal maturity. The B2 chalk contains thin beds with extremely high levels of TOC (up to 14 wt.%). These thin beds contain more siliciclastic and clay content and are termed kerogenites for this study. The clay is dominantly mixed illite/smectite and the intervals were deposited in strong anoxic to euxinic conditions indicated by high Mo and V concentrations. The kerogenites have interparticle pore space between clay platelets and intraparticle pore space in pyrite framboids and planktonic foraminifera. Although the TOC is high, the S2 value is also high indicating that the organic matter is not completely thermally mature. In Redtail Field, the Niobrara B1 and B2 chalks share similar characteristics; however, there are some key differences. The B2 chalk is thicker, has thicker intervals with high resistivity, contains less clay and higher carbonate content, and has higher TOC values. Both the B1 and B2 chalks should be targeted intervals for horizontal completions, but overall, the B2 chalk has better reservoir characteristics and is the superior chalk in the Niobrara B interval.