143-20 Mineralogical and Elemental Characterization of Niagaran Group Reef-Flank Facies for Lateral Confining Potential, Kalkaska Co., MI

Abstract:

The Pinnacle Reefs of the Silurian Niagaran Group have been the focus of numerous petroleum and carbon capture, utilization, and storage (CCUS) studies throughout the Michigan Basin. The reef structures have been prolific oil and gas producers and are actively used for CO₂ enhanced oil recovery (EOR). Despite the attention received, few studies have focused on evaluating lateral characteristics of the reefs, as most work has been focused on the central reef reservoirs and their vertical confining units. Understanding the lateral flanks of the reefs is essential information for drill permitting and subsurface management.

This study utilizes a high-resolution approach to evaluate lateral mineralogical and elemental variability of two cores along the Northern Reef Trend: State Kalkaska 1-15, located on the windward distal reef flank, and the Kalkaska 1-23 positioned in the regional inter-reef zone. Carbonate mudstones, wackestones, and conglomerate facies comprise both cores. Bulk-rock elemental analysis using portable X-Ray Fluorescence (PXRF) show higher magnesium (Mg) concentrations in the reef flank facies, and lower Mg concentrations in the more distal inter-reef facies. X-Ray Diffraction (XRD) identified relatively high abundance of dolomite [CaMg(CO₃)₂] in the reef flank core facies, which corresponds with the increase in Mg concentrations compared to the inter-reef region. Preliminary results show that intervals with more dolomite correlate with higher observed porosity.

This study highlights the advantage of a high-resolution elemental and mineralogical characterization in core work. Identification of patterns and trends in compositional data provides a framework that, when paired with thin section petrography, porosity, and permeability measurements, allows for a data-driven evaluation of the potential utility of these facies as lateral confining units in the Northern Reef trend of the Niagaran Group.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.