MEASURED VERSUS CALCULATED MINERAL ASSEMBLAGES AND ABUNDANCES IN UNCONVENTIONAL MUDSTONES: A ROUND-ROBIN TEST OF DIFFERENT XRF TECHNIQUES

The description and quantification of mineral assemblages in hydrocarbon-bearing mudstones is an important component of completion strategy design. Accurate knowledge of minerals present and the relative proportions of minerals with contrasting rheological properties (e.g. brittle vs ductile) may be used to predict reservoir response to chemical and mechanical stimulation. However, there is tension between the economic demands of rapid well completion and the time required to accurately identify and quantify mineral assemblages.

Four geochemical analyses techniques: PXRF; conventional XRF by WDS; and, standardless WDS-XRF analysis of fused glasses and powders were applied to forty samples of side-wall core from unconventional reservoirs in the Permian Basin of Southeastern New Mexico. Normative mineral abundances were calculated using SEDMIN (Kackstaetter, 2014) and compared against mineral proportions calculated by Rietveld refinement of XRD patterns and qualitative interpretation of petrophysical log data. The goals of the study were to test the reliability, reproducibility, accuracy and precision of the techniques and their applicability to the mineralogical interpretation of well-log and core data, and identify the minimum work-flow required to make informed decisions in real-time concerning completion strategies.

Matrix Identification plots, based on petrophysical data, predict the dominant minerals in each reservoir, but are dependent on accurate bulk density values. Glass disc analyzes by conventional XRF is the most accurate and precise of the compositional analyses techniques, particularly for calcite-poor samples. Conventional WDS glass analysis of samples that contain >20% CaO or sulfur produced the least accurate predictions of mineral assemblage. Glass discs analyzed using a standardless XRF method game more accurate predictions for carbonate-rich samples, unless there was significant sulfur. PXRF analyses is necessary to identify sulfur, and should be used to improve predictions based on WDS glass and powder analyses. Powder XRD patterns should be collected in triplicate to promote reproducibility, especially for samples that contain less than 10% clay minerals.