REDUCING GEOLOGIC RISK UNCERTAINTY FOR CARBON SEQUESTRATION USING MODERN GRAVITY SURVEYS AND MODELING IN IRON COUNTY, UTAH

The Iron Springs mining district in southwest Utah is one of the largest sources of raw iron ore in the United States. Steel from this ore is currently manufactured overseas, resulting in significant business costs and an undesirable carbon footprint. To reduce waste and domesticate production, proposals exist to build a direct-reduced iron plant and develop a carbon capture and storage site to inject ≥500,000 metric tons of CO₂ per year into an adjacent basin. Highly porous and permeable strata of the Jurassic Navajo Sandstone present a favorable reservoir target that is overlain by several sealing units including anhydrite and carbonate strata in the Jurassic Temple Cap and Carmel Formations and the intrusive Miocene Three Peaks quartz monzonite. Considerable geologic risk is involved in site development since there are few subsurface control points and the targeted injection site includes elements of the Sevier orogeny, Iron Springs intrusive complex, and Basin and Range extensional features.

As part of a multidisciplinary site characterization funded by the U.S. Department of Energy, the Utah Geological Survey is conducting geophysical surveys and geologic data acquisition to characterize subsurface geology and reduce structural, seal, and reservoir risk uncertainty. Modern gravity data will complement existing legacy data made up of geophysical surveys, petrophysical logs from exploration wells, and 2D seismic profile data. Gravity surveying is a cost-effective and non-invasive technique used to quantify changes in material density at depth and delineate subsurface geometries, adding independent checks on data interpretation from other geophysical methods and rock property analyses. We designed the gravity survey to maximize the potential of defining subsurface basin geometry and subsurface unit architecture. Gravity measurements were made using a relative gravimeter with refined inter-station spacing focused along the axis of the Three Peaks laccolith to improve intrusion extent maps previously inferred from regional-scale geophysical surveys. Gravity surveys provided insightful 2D cross sections and a 3D geophysical basin model that will be key elements in our multidisciplinary workflow for future site assessments.