Petrographic and biostratigraphic assessment of the Aristocrat PC H11-07, Niobrara, Wattenberg field, Weld County, Colorado

Abstract

The Niobrara Formation in the Wattenberg Field, Denver Basin, Colorado consists of interbedded chalks and marls. The Niobrara Formation is cyclic in nature resulting in chalky marls and marly chalks. The biostratigraphic data set has dated the Niobrara at the Coniacian to lower Campanian stages of the Upper Cretaceous. The Niobrara Formation is just over 300 feet in thickness in the Wattenberg Field. Petrographic and biostratigraphic framework characterization, Scanning Electron Microscopy (SEM), and Quantitative Evaluation of Minerals by scanning electron microscopy (QEMSCAN), and X-ray Diffraction (XRD) are essential accessory tools to the petroleum potential of any formation. This study uses all five approaches to provide an overview of the Niobrara Formation in the Wattenberg Field located in the Denver Basin. This study focuses on taking observations from macro to the micro-scale level. The high resolution petrographic, SEM, and QEMSCAN, and XRD methods have yielded seven microfacies that were identified in this study. The microfacies show distinctive characteristics and a great deal of variability found in the Niobrara Formation. The additional method for reservoir characterization in this study was the biostratigraphic analysis. The microfauna in the Wattenberg distinctly show how much the Wattenberg paleohigh has affected the diversity, abundance, and preservation. The low abundance, and diversity of the microfauna present in the Niobrara Formation suggest that the microfauna livelihood are highly dependent on food supply, nutrient supply, water temperature, and water salinity. The fact that not having any benthonic foraminifera in the Smoky Hill Member, and only a single species in the Fort Hays Member is evidence and suggestive that the food supply was low during the deposition of these members. There are twenty three planktonic species from 10 genera that have been identified in this study. Generally all samples were dominated by Heterohelix globulosa, Globigerinelloides ultramicrus and two Archaeoglobigerina species. Additionally, the significant nannofossils found are the Watznaueria, Prediscophaera, Zeugrhabdotus, and Tranolithus. The common planktonic foraminifera in the Niobrara Formation include Hedbergella and Heterohelix. Several nannofossil events approximate the top of the Coniacian, including the first appearance datum (FAD) of Micula concava and the last appearance datum (LAD) of Eprolithus floralis, and the LAD of Quadrum gartneri. The planktonic foraminifer Whiteinella archaeocretacea defines the top of the Coniacian. Other significant nannofossil events include the FAD of Lithastrinus grillii, and the LAD of Rhagodiscus achlyostaurion and the FADs of Micrantholithus quasihoschulzi and the Micula decassata. The latter two are within the low Coniacian Fort Hays Member.