Elemental composition quantified by Elemental Gamma Ray Spectroscopy Tools and the interpreted mineralogy provide important geological data for petrophysical analysis of bore-hole formations. Petrophysicists rely on the mineralogy measured by various analytical techniques on down-hole samples (core plugs or drill cuttings) for a confirmation of log-derived mineralogy. Disagreement in the core and log derived mineralogy is often considered as an error in the log-derived mineralogy caused by inaccurate tool measurement or by incorrect models used for mineralogy interpretation. However, several sources of errors can produce inaccurate core-based mineralogy measurements and subsequently incorrect conclusions regarding the validity of log-derived mineralogy. This phenomenon is more commonly observed in the shale formations characterized by high degrees of lithological and mineralogical variations.
An operator logged an elemental gamma ray spectroscopy tool in an organic mudstone formation in North America, acquired cores from the well, and analyzed a set of core samples by XRD analysis for mineralogy. They reported differences in the log-derived and core-derived mineralogy and assumed a discrepancy in the log-derived mineralogy. Since the core mineralogy results did not match with the expected mineralogy of the formation, the operator was requested to validate the core mineralogy results. The same set of samples were analyzed by another service provider and the XRD mineralogy results were consistent with the log-derived mineralogy and the generalized mineralogy of the formation. In another example, an operator stated the differences in the log-derived and core XRD-derived concentration of sulfur-bearing minerals. XRF elemental concentration of the same samples showed good agreement with the log-derived elemental concentration and did not support the higher concentration of sulfur-bearing minerals in XRD analysis.
The validation of core-derived data is vital to avoid errors caused by non-representative samples, differences in analytical procedures, and incorrect calibration of the equipment. Quantitative mineralogy validation between core and log-derived mineralogy is also generally difficult because of the differences in the log and core samples resolutions; however, errors in core data can be minimized by following standard procedures for the representative samples selection and assessing the analysis procedures and techniques using standard reference materials.
