2019 AAPG Annual Convention and Exhibition:

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A Comparison of XRD Mineralogical Variability and Techniques with Proxy Approaches to Defining Mineralogy and Rock Type: Examples from the Wolfcamp-Dean-Spraberry Succession of the Northern Midland Basin.


Geochemical and mineralogical data from cores, rotary sidewall cores, and drill cuttings from unconventional reservoirs provide valuable quantitative constraints for reservoir characterization studies, target optimization, and completions. However, the types of geochemical and mineralogical data available to users vary widely. For this reason, the quality of mineralogical data from x-ray diffraction (XRD) and geochemical data from energy-dispersive x-ray fluorescence (ED-XRF), plus derivatives of these data types (e.g., modeled mineralogy, and chemofacies), should be evaluated in the context of the specific analytical technique(s) utilized. A key concern is that the type of sample material used, e.g. cuttings vs core, dictates part of the uncertainty in the results. Additionally, the errors associated with XRD samples prepared and analyzed as bulk powders and interpreted using a whole-pattern fitting routine (Rietveld Refinement) are inherently larger than errors associated with XRD samples analyzed using the combination of spray-dried bulk with clay separation evaluated using the Reference Intensity Ratio (RIR) method. Here we present a comparison of “wellsite-grade” and “laboratory-grade” analyses of samples from two wells from the northern Midland Basin. The wellsite-grade XRD analysis approach is rapid, cost effective, and provides the screening process necessary for making better drilling decisions, but also making better decisions for subsequent laboratory-grade analyses. Thus, while the analytical quality of the data product is highly dependent on the technique applied and the type of sample material used, both wellsite- and laboratory-grade techniques are useful and possess key limitations/benefits. In the Permian-aged strata of the Midland Basin, the largest error between wellsite- and laboratory-grade XRD analyses tends to be associated with total clay mineral abundance and speciation. A healthy appreciation for sample type and XRD technique is a prerequisite for developing and fully implementing XRF-based, stoichiometry-underpinned mineral models.