Reservoir characteristics and production of the Niobrara A interval at Redtail field: Weld County, Denver Basin, Colorado

Abstract

In the vicinity of Redtail Field, Weld County, Colorado, sufficient thickness of theNiobrara A interval allows for the commercial development of this bench. This development marks the only widespread drilling and production of Niobrara A wells in the Denver Basin. This study at Redtail Field discusses why a hydrocarbon accumulation is present in the Niobrara A interval and how the techniques used to identify and exploit the accumulation may be used in other areas prospective to the same interval. Three carbonate facies (chalk, chalky marl, and marl) and two non-carbonate facies(shale and bentonite) were defined for this study from macroscopic inspection of the Razor 25- 2514H core. The carbonate facies all act as reservoir rocks, but the chalky marl and marl facies also act as source rocks. The non-carbonate facies both act as seals, while the shale facies also acts as a source rock. Porosity in the Niobrara A interval has been shown to be the highest in the Niobrara Formation (Aydin, 2017). Porosity values range from 10.93% to 12.75% in the reservoir rocks, while the non-reservoir rocks range in value from 3.31% to 6.02%. Oil and gas saturations in the carbonate facies of the Niobrara A interval average 26.26%and 59.33%, respectively, while water saturations average 14.41%. The non-carbonate facies exhibit oil and gas saturations that average 4.7% and 13.0%, respectively, and water saturations of 82.4%. Total organic carbon values from 18 samples are classified as very good to excellentquality, with non-carbonate values ranging from 4.29 weight percent to 9.32 weight percent, and carbonate values ranging from 1.9 weight percent to 4.19 weight percent. TMAX values indicate mature organic matter generation for 14 of 18 samples analyzed, with no overmature values found within the Niobrara A interval. Additionally, calculated vitrinite reflectance values from 17 of the 18 samples indicate early oil generation, and one value indicates immature generation. Temperature mapping shows a hot spot under Redtail Field, named the Redtail Field HotSpot, that is likely responsible for the thermal maturity of kerogen in the area. Areas surrounding the Redtail Field Productive Fairway show lower projected temperatures at the reservoir depth, which has likely resulted in immature kerogen that may serve to occlude porosity and permeability from allowing hydrocarbon to migrate updip. Original oil-in-place calculations, based on data from the Razor 25-2514H andextrapolated to one land section of area, shows 15,891,260 barrels of oil. 69.01% of this hydrocarbon is found within the Niobrara A Chalk interval. Recovery factors based on cumulative oil production and EUR are 3.79% and 6.89%, respectively. The geologic constraints for the location of Redtail Field include a Niobrara A Reservoirinterval thickness of 40 feet to 50 feet, the underlying Redtail Field Hot Spot that has placed the reservoir interval at approximately 160°F, and thermally immature rock surrounding the Redtail Field Productive Fairway that has not allowed hydrocarbon to migrate away from Redtail Field.