Chemostratigraphy of the Late Cretaceous Western Interior (Greenhorn, Carlile, and Niobrara formations), Denver Basin, CO, U.S.A
|Institution:||Colorado School of Mines|
|Keywords:||Chemostratigraphy; Niobrara Formation; ThermoScientific Niton XL3t ED-XRF Analyzer; Greenhorn Formation; Carbon Isotope Stratigraphy; Sequence Stratigraphy|
|Full text PDF:||http://hdl.handle.net/11124/20247|
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… Advisors/Committee Members: Humphrey, John D. (advisor), Sonnenberg, Stephen A. (committee member), Sarg, J. F. (J. Frederick) (committee member), Canter, Karen (committee member), Graves, Ramona M. (committee member).