A Comparison of Handheld Energy-Dispersive X-Ray Fluorescence (HH-ED-XRF) Techniques for the Evaluation of Core and Cuttings Chemostratigraphy: Examples From Late Cretaceous Strata, South Texas
Energy dispersive x-ray fluorescence (ED-XRF) has become a mainstay geochemical technique in the petroleum industry, especially in the analysis of mudstone-dominated successions. In the last few years ED-XRF has been utilized to generate supporting data sets, with applications ranging from directional drilling to reservoir characterization and optimization in unconventional plays. It is also used to generate data sets for correlation, elementally-defined mineralogical variability, and facies discrimination. With all of the emphasis on generating geochemical/chemostratigraphic data sets, it is important to define the limitations, errors, and best-practice strategies for generating the most useful results. In order to optimize the quality and usability of ED-XRF data sets, a rigorous approach to sample preparation and measurement techniques should be taken. If done properly, an ED-XRF analysis of a slabbed drill core face has the potential to yield quantitative geochemical results that help quantify individual facies and the degree of facies variability—at a range of scales, from fractions of an inch to feet, depending upon the requirements of the study. The spatial resolution of a well cuttings geochemical data set may be on the order of ten to thirty feet, and the retrieved samples are much more susceptible to contamination with respect to samples taken from drill core. The positive attribute of well cuttings is that they are far more accessible. Scanning of unconsolidated cuttings in sample cups is the easiest and fastest method for undertaking XRF analysis; however, pulverizing and pelletizing the cuttings before scanning ensures a more homogeneous matrix and increases the sensitivity to (and accuracy of) lighter major element concentrations (e.g., Na through Ca). In essence, the pelletized powder from a well cuttings sample mimics the slabbed face of a core, providing the optimal form of sample. Geochemical results from a Cretaceous Eagle Ford shale drill core and the accompanying set of well cuttings are evaluated in order to demonstrate the limits, pitfalls, and benefits of the various methods of generating XRF data sets.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015