A Photon-Counting Approach to Drill Core Chemostratigraphy: Working Without an Empirical Calibration in Mudstones

Abstract

While the development of matrix-matched energy-dispersive x-ray fluorescence (ED-XRF) empirical calibrations for a wide range of mudstones/shales continues with success, the technique of counting photons remains heavily underutilized. The photon-counting technique is centered on the deconvolution of raw ED-XRF spectra and the subsequent apportionment of photon counts to the various elements that constitute the rock samples. Photon counts are demonstrated to be proportional to elemental concentrations. Thus, while using the photon-counting technique fails to yield elemental concentrations in traditional weight %, it does indeed generate quantitative results that are easily plotted in graphical form, and are effortlessly manipulated in principal component and/or cluster/factor analysis exercises. Furthermore, analogous to the use of weight % and parts-per-million, the photon apportionment to each major element is interpretable in terms of mineralogical variability, and in the case of the trace element photon counts, is interpretable in the context of mineralogy and depositional/diagenetic conditions. Consequently, the application of the photon-counting technique to core- or cuttings-based studies provides a novel approach to developing a chemostratigraphy in the absence of reference materials. Results from the photon-counting and empirical calibration approaches will be presented for the same set of cores for the Bakken and Eagle Ford formations. In addition, both types of results will be processed through a hierarchical cluster analysis (HCA) routine with the outcome of developing models of chemofacies for core materials. A discussion of technique pitfalls, limitations, and advantages to the two techniques will be presented, as will their linkages with well log observations. The technical contributions of the photon-counting procedure are 1) developing paleodepositional/diagenetic context without an empirical calibration, and 2) enhancing the number of elements that can be inferred from photon counts but are not easily calibrated using traditional reference materials. The latter includes organophilic elements (e.g., bromine), and chalcophilic elements (e.g., selenium)—elements that, because of their affinity, may provide added insights not gained through traditional empirical calibration of raw XRF spectra.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016