Faulting and natural fracturing across the DJ Basin: impacts on production from Hereford field, northern Colorado

Abstract

The tectonic history and structural style of faulting and natural fracturing in the Denver-Julesburg (DJ) Basin have been studied extensively -- and with good reason. The natural fractures in the Niobrara formation, specifically in the Niobrara B Chalk, have been the driving mechanism behind the production from this low-porosity, low-permeability reservoir; without which, it may not be a viable play. Therefore, understanding the controls on fracture distribution and how fractures contribute to production is essential for conducting economic operations across the region. The B Chalk is not the only target reservoir in this area, however. The Codell Sandstone is also a viable reservoir target characterized by low porosity and low permeability. These characteristics, coupled with the fact that the Codell is much less naturally fractured than the Niobrara Chalk benches, make induced fractures during hydraulic fracturing operations a critical component of reservoir effectiveness and fluid deliverability. This project focuses on the structural characterization of the Hereford Field in the Northern DJ Basin with the main objective of identifying the controls on geologic heterogeneity that impact well performance; the understanding of the geologic controls can then be leveraged to optimize subsequent drilling and completions across the area. The complexity introduced by structural heterogeneity is further complicated by the fact that wells from a previous phase of development have been producing from the Niobrara B Chalk reservoir for over 10 years. Though it complicates the picture of optimizing subsequent drilling and completions, data from the legacy phase provides a unique opportunity to characterize in-situ reservoir conditions and how these contributed to the variability observed in the legacy phase of production. Various datasets were provided in order to inform the project objectives. This work integrates seismic and well data from the legacy phase of development, fracture log and microseismic data from the recent phase of development, and results from analyses performed by the geology team member on the project to form a more complete picture of how the initial structural context influenced the development of the petroleum system, and how this contributed to fluid and reservoir quality heterogeneity. It was found that controls on present-day expression of fractures were different for the two target reservoirs; the Niobrara was much more structurally-influenced (fracture reactivation by flexure) while fracturing was predominately controlled by stratigraphic heterogeneity in the Codell (mechanical properties and distribution of the higher quality, brittle pay zone). These observations made at the well-scale were extended across the region through the integration of seismic attributes with a vertical well study, providing the basis to make geologically-informed recommendations for future drilling and completions of wells in Hereford Field.