Validation of a geochemical logging tool for in situ major element analysis in boreholes using inductively coupled plasma atomic emission spectrometry

Abstract

The geochemical logging tool (GLT) provides in situ measurement of the concentrations of 10 elements commonly found in sedimentary rocks: Si, Al, Fe, Ca, K, Ti, S, Th, U, and Gd. The element abundances thus determined can be used to derive lithology, sand class, mineralogy, porosity, permeability and cation-exchange capacity. The usefulness of this tool will depend upon the precision and bias of measurements. This paper describes the results of deploying a GLT at the Imperial College borehole test site in a sequence of finely layered siliciclastic and carbonate rocks. The bias of the GLT was quantified for six elements (Si, Al, Fe, Ca, K and Ti) by comparison with measurements on corresponding core samples made by inductively coupled plasma atomic emission spectrometry (ICP-AES). The high density of core measurements (approximately every 25 cm) allowed them to be averaged over a depth interval equivalent to the vertical resolution of the GLT thus reducing errors due to differences in sample size. A statistically significant bias for the GLT for all elements was calculated from a simple linear regression; values were between –55 and +5%. The detection limits and dependence of precision on concentration for Si, Al, Fe, Ca, K and S were determined using duplicate analyses from two logging runs in the same borehole. Values for the detection limits ranged from 0.26 for Fe to 2.70 mass-% for Si. When the precision varied with concentration, the high level precision was found to be less than 6.5% in most cases. It was found that the processing applied to the raw counts data obtained from the GLT played a major role in the determination of detection limits, precision and also the accuracy of measurements obtained using the GLT.