Chemostratigraphy of the Late Cretaceous Western Interior (Greenhorn, Carlile, and Niobrara formations), Denver Basin, CO, U.S.A.

Abstract

Late Cretaceous Western Interior mudrocks of the Denver basin, CO, U.S.A. have garnered renewed attention with recent advances in horizontal drilling technology. These unconventional resources are now considered to be viable hydrocarbon reservoirs. Current interests are focused on the Niobara Formation, which is the primary target for oil and gas operations in the Denver basin. The potential of the Greenhorn Formation has also begun to drawn attention in the last few years. With such an abundance of activity, there is a need to further development of our understanding of these sedimentary systems. The Greenhorn and Niobrara marine cycles define two 3rd order cycles, a stratigraphic interval almost entirely composed of carbonate mudrocks. The grain size and carbonate composition make the rocks prime candidates for applied chemostratigraphy. Advances in energy-dispersive x-ray fluorescence (ED-XRF) technology have also led to the development of handheld XRF analyzers, which provide rapid, efficient, inexpensive, and non-destructive sampling. With a variety of manufacturers (e.g. ThermoScientific, Bruker, Olympus) currently offering commercially available instruments, it is important to empirically evaluate the function and validity of an instrument. The purpose of the first part of this study is the evaluation of the ThermoScientific Niton XL3t GOLDD+ ED-XRF analyzer as applied to slabbed drill core of Late Cretaceous mudrocks from the Greenhorn, Carlile, and Niobrara formations of the Denver basin, CO. Forty samples were collected as thin-section billets consisting of the primary facies distribution of the Greenhorn, Carlile, and Niobrara formations from the Noble Energy Aristocrat PC H11-07 drill core. Each sample was measured for 1) elemental concentrations by ED-XRF (Niton), ICP-MS/ICP-OES and LECO method for sulfur concentration, 2) mineralogy by x-ray diffraction, and 3) total organic carbon by LECO method. Results show strong correlation between ICP-XRF for seventeen elements (R2 > 0.6; Ca, Zr, Si, Al, Mn, Rb, Sr, Ba, Fe, Nb, K, S, V, Ti, Mo, Th, and Zn) and are considered valid semi-quantitative measurements by the Niton. Moderate correlation was found for seven elements (0.6 > R2 > 0.25; As, Mg, Ni, Cu, Pb, Cr, and U) and are considered partially valid semi-quantitative measurement by the Niton. Elements exhibiting low correlation coefficients (R2 < 0.25; P, Cs, Bi, Sb, Sc, Sn, Ag, W, and Co) are considered invalid measurements by the Niton. The most appropriate Niton filter settings when using TestAll Geo mode for the measurement of GCN mudrocks are: Main – 30 seconds, Low – 30 seconds, High – 30 seconds, and Light – 90 seconds. XRD and TOC correlation to elemental data provide identification of elemental proxies for mineral phases and organic content. The instrument comparison shows comparable results for the two instruments, with the caveat that the mudrock calibration for the Tracer IV provides the benefit of fully quantitative measurements. The aim of the second part of this study is to build and test the application of a high-resolution elemental and carbon isotope chemostratigraphic framework for the Greenhorn, Carlile and Niobrara formations in the Denver basin, Colorado. The dataset consists of eight drill cores from the greater Denver basin with high-resolution handheld ED-XRF elemental measurements and high-resolution stable carbon and oxygen isotope measurements of the Greenhorn, Carlile, and Niobrara formations. Application of carbon isotope chemostratigraphy as a chronostratigraphic proxy has the potential to show relationships on the timing of stratigraphic units across the basin. Elemental data will further define the sequence stratigraphy and paleoenvironmental conditions by using the preexisting stratigraphic framework of the Late Cretaceous Western Interior as a guide for interpretations. The results show carbon isotope chemostratigraphy to be consistent with lithostratigraphy for closely spaced (<100 km) cores in the Denver basin. Application of carbon isotope chemostratigraphy as a chronostratigraphic proxy then identifies lithostratigraphy as consistent with chronostratigraphy at this scale. Eleven carbon isotope excursions are clearly identifiable and show correlation to the English Chalk Reference Curve of Jarvis et al. (2006). Furthermore, carbon isotope trends show broad correlation to eustasy. Elemental chemostratigraphy shows well-defined sequences at the 3rd and 4th order based on relative elemental concentrations and element ratios. Specifically, Ca (%), Si/Al, Mo (ppm), and Mn (ppm) define sequence stratigraphic surfaces that correspond to those from previous workers. Fourth order cycles in the Greenhorn cycle (inclusive of both the Greenhorn and Carlile formations) are notably more frequent and of shorter duration than those of the Niobrara Formation.