Chemostratigraphy of unconventional shale reservoirs: A case study of the Niobrara Formation within the Denver-Julesburg basin

Abstract

The Upper Cretaceous carbonate-rich Niobrara Formation is a prominent petroliferous unconventional reservoir that is generally characterized by fine-scale heterogeneities that complicate local and regional correlation of target zones, well placement and completion strategies. Within rhythmically interbedded reservoirs like the Niobrara Formation, the occurrence of thin marl and chalk beds within chalk- or marl-dominated intervals respectively, further impedes identification of marl or chalk benches. This study uses a combination of petrophysical and geochemical data from 6 oil and/or gas wells within the Niobrara interval of the DJ basin to identify and correlate eight (8) chemozones (C1, C2, C3, C4, M1, M2, M3 and M4) and four (4) chemostratigraphic packages (Sharon Springs, Smoky Hill, Fort Hays and Codell Sandstone) that are essential for constructing a robust and dynamic stratigraphic framework.The results showed synchronous variation in petrophysical trends, elemental, mineralogical and organic composition at contacts, ubiquitous cyclicity within, and across the Niobrara Formation in the northern DJ basin. The stratigraphic units are geochemically separated based on TOC, total clay, total carbonate, and quartz + feldspars, and Ca, Sr, Si, Al, Ti, Rb, Zr, Nb, K, and Cr, while the internal stratigraphic units within the Niobrara Formation are defined by integrating gamma-ray (GR), resistivity, carbonate proxy, marl proxy, carbonate/marl proxy and terrigenous input. The chalk and limestone units (C1–C4) have higher Ca, Sr, total carbonate concentrations and relative brittleness index relative to the marl units (M1-M4). This multiproxy approach reveals variable calcareous and terrigenous/detrital input, and paleoredox conditions as the main controlling factors behind the changing lithologies/cyclicity within the Niobrara Formation. Hence, an adequate characterization and correlation of inherent heterogeneities within marls and chalks of the Niobrara Formation may require incorporation of a multiproxy approach at various scales. By integrating color scale, geochemistry, TOC and wireline well log curves responses, higher resolution identification of chalk reservoirs and marl source beds can be achieved. Spatial distribution of chalks and marls consistently show a trend of total carbonate and Ca increasing southwards towards the Tethys Sea with highest average concentrations recorded in Fort Hays Limestone while quartz + feldspars, and total clay concentrations increasing northwards, suggesting a southern and northern provenance, respectively.