Using Micro-XRF to Characterize Shales and Natural Fracture Systems

Abstract

The geochemical characterization of unconventional reservoir lithologies remains critical to predict well performance, the mechanical properties of reservoir rocks, and or other production properties. Micro-X-ray fluorescence (XRF) analysis, a relatively new analytical technique, constitutes a valuable analytical method for characterization of shales and natural fracture systems (in thin section or drill core slabs) through the generation of elemental maps. Currently, portable XRF units are widely used throughout the petroleum industry for geochemical analysis of well cuttings. However, unlike portable units, micro-XRF analysis allows for the generation of high-resolution (a 50–100 micron spot size) elemental maps of thin sections and polished rock samples, which can be assessed visually to resolve textural and chemical features (ranging from 0.0001 to 1 meters in size). Micro-XRF maps are quicker and cheaper to generate than maps generated using a scanning electron microscope (SEM). The resolution afforded by micro-XRF elemental maps makes this technique well suited for the characterization of shales and mudstones because rocks of this type show micron-scale chemical and textural variability. Elemental maps were collected on a laminated siltstone and sandstone from the Mowry Shale (Bighorn Basin, Wyoming). The sample contains a complex, extensional, natural fracture in filled with calcite. The distribution and concentration of Ca, K, Si, Fe, Al, S, Mn, and Ti are presented as individual maps, where the variability in color intensity on each map indicates the concentration of that element. An element map of Ca show the matrix if generally Ca-poor, and therefore deficient of Ca cements. A vertical fracture cemented by calcite contains variable Mn and low Fe content. Invasion of Ca-fluids into the host lithology during fracture infill is minimal, but can be observed in laminae of higher permeability. Maps of Ca and S clearly identify regions where gypsum filled secondary fractures. Results of this study suggest elemental maps created using Micro-XRF allow for the visual integration of textural and chemical data, and constitutes a promising new method to characterize shales and natural fracture systems. Furthermore, the distribution of elements within a section can be used to infer the mineralogy, highlight geochemical heterogeneities, and enrich petrographic descriptions.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015