CHEMOFACIES, CORE-SCALE ELEMENT MAPPING, AND THE LINKAGES BETWEEN MINERALOGY, ROCK FABRIC, AND ESTIMATED ROCK STRENGTH IN TWO MUDSTONE SUCCESSIONS (Invited Presentation)

If a mudstone succession is heterogeneous at various scales, then the scale of analysis potentially dictates the scale of interpretation. However, significant effort has been placed on analyzing mudstone characteristics at one scale and subsequently scaling up/down the findings and associated interpretations. In an effort to explore the lower (but not lowest) limits of mudstone compositional heterogeneity and make comparisons to facies-scale characteristics, which often occur at the cm scale in typical mudstone successions, we present combined X-ray fluorescence (XRF) geochemical data sets that incorporate 1) cm- and mm-scale analyses undertaken with portable technology, and 2) “tens of micrometers”-scale analyses using semi-portable benchtop microbeam technology. Findings of this approach demonstrate both the utility and limitations of the facies-scale geochemical workflow, and place into context the limitations of studies that scale upward from ~10^-6-10^-3 m (e.g., SEM, micro-rebound hammer) to 10^-2-10⁰m (e.g., portable XRF, well logs).

Geochemical and micro-rebound hammer (MRH) characterization of mudstone-dominated strata within the Devonian-Mississippian-aged Bakken Formation (Williston Basin, MT) and the Early Permian-aged Wolfcamp Formation (Midland Basin, TX) are interpreted along with element-mineral model outputs that, underpinned by X-ray diffraction (XRD) results, help define the underlying mineralogical controls. For simplification purposes, it is initially assumed that element-mineral linkages made at one scale hold true at finer scales, and that MRH-based estimates of unconfined compressive strength (UCS) made at the mm scale are transferable to other scales. A chemofacies assignment approach based on grouping samples collected at the facies scale outlines key intervals for more detailed elemental mapping that is used to evaluate sub-facies-scale variability and characterize rock fabric. Ultimately, testing and refinement of the linkages between mineralogy, rock fabric, and rock strength of these strata sheds light on technique limitations and the scales of mudstone heterogeneity.