Petroleum geology of the B chalk benches of the Niobrara Formation: Wattenberg, Silo, and East Pony fields, northern Denver Basin, CO/WY

Abstract

The Denver Basin has been explored and exploited for its immense petroleum potential for over 120 years. It is historically known for its conventional drilling success in the Permian Lyons Sandstone, Upper Cretaceous “D” and Lower Cretaceous Muddy “J” sandstones. With recent advancements in drilling and completions technology, exploration in the Denver Basin has transitioned to be focused on the unconventional Cretaceous stratigraphy present in the basin. The Niobrara is currently being developed extensively within Wattenberg Field located just north-northeast of the Denver, CO. At the same time, exploration has extended outside of the Wattenberg Field proper and many locations are now being targeted for future exploration and production with the potential of having higher liquids to gas ratios, as well as, better permeability and porosity. Both Silo Field, located directly north of Wattenberg, and the East Pony and Redtail fields located northeast of Wattenberg Field, lie within some of these promising hydrocarbon-rich regions. These fields are currently being explored, with the B chalk Member of the Niobrara is the best horizontal target in the area, with porosity values that reach as high as 13-16%, and contains greater than 70% of the hydrocarbon as liquids. This study provides an in-depth look at the B1 and B2 chalk and marl benches of the Smoky Hill Member of the Niobrara including, 1) detailed core description and identification of six main facies, their vertical and lateral heterogeneities, and their depositional models, 2) reservoir characterization using petrographic thin sections, FESEM imaging, XRD bulk minerology, Source Rock Analysis, and porosity data, and, 3) petrophysical well log analysis and subsurface mapping. Detailed core descriptions, petrographic thin section interpretation, and analysis of FESEM images of the B1 and B2 chalk and marl benches identified six main facies. These facies vary in location, thickness, structural features and major calcareous and terrigenous constituents. These facies vary from pure chalks to massive fossiliferous marly chalks, to pure marls. Location, thickness, and patterns in which these facies occur have an effect on overall effectiveness of the chalk benches as reservoirs. XRD bulk mineralogy analysis, geochemical interpretations, and porosity values taken from the B1 and B2 chalk and marl benches help differentiate the two chalk beds. Integration of results from this study show that the B2 chalk is the dominant target in the study area. Results show the B2 chalk is: 1) dominated by Facies 1,2, and 6,which have the greatest qualitative preserved porosity, 2) contains carbonate contents as high as 96% with only 3% siliciclastic input, increasing chances of greater amounts of preserved porosity, 3) it is comprised of the greatest amount of preserved nanno-fossils (containing high amounts of porosity), 4) it is organic rich with TOC values ranging from 1.5-4.5 wt. %, 5) it has very good to excellent free hydrocarbons present in the formation, and is in the mature oil window with Type II oil prone kerogen, 6) calculated porosity for the B2 chalk ranges from 5-12.5%, and 7) the B2 chalk occurs as the most widespread unit, and has the most homogenous stratigraphic thickness throughout the study area. The B1 chalk should be considered a secondary target where appropriate, as it has very similar qualities to the B2 chalk, but has slightly lower grade reservoir characteristics. Given the lack of detailed reservoir characterization of the B benches of the Niobrara, and the increasing interest in pursuing the B chalk benches throughout the promising hydrocarbon rich regions in this study area, additional research to help define and characterize the B1 and B2 chalk and marl benches was necessary.