Rock physics and fracture characterization of the Deadwood Formation, Williston Basin: Insights into geothermal resource development

Abstract

Sedimentary basin geothermal systems face critical characterization challenges from complex reservoir heterogeneity that traditional assessment methods inadequately address. This study develops an integrated petrophysical-structural framework for the Deadwood Formation in North Dakota's Williston Basin using advanced rock physics modeling and statistical fracture analysis. We employed Differential Effective Medium theory for bimodal pore structures (macropores 10–100 micrometers, micropores <1 micrometer), Kuster-Toksöz analysis for fracture-induced anisotropy with aspect ratios 0.001–1.0, and Gassmann fluid substitution with empirically constrained parameters. Formation Micro-Imager logs at 5 millimeter resolution enabled statistical characterization of 847 fractures across 450 feet, with uncertainty quantification through Monte Carlo simulation. Results demonstrate exceptional geothermal potential with a validated gradient of 34.6°C/km, significantly exceeding typical sedimentary basin values of 25–30°C/km, achieving 160–162°C at economically viable depths of 3.0–3.1 kilometers. Fracture networks follow log-normal distributions with volumetric intensities of 0.07–2.82 fractures/ft³ and a coefficient of variation of 79 %, requiring stochastic modeling approaches. Rock physics modeling successfully discriminates reservoir zones with correlation coefficients exceeding 0.87, identifying Members B and A as optimal targets. Economic analysis demonstrates commercial viability with levelized electricity costs of 8.7 cents per kilowatt-hour (confidence interval: 6.1–12.4), competitive with renewable alternatives. The superior depth-to-temperature ratio of 18.9–19.4 m per degree Celsius provides 25–45 % cost advantages over typical sedimentary prospects. Parameter bounds were constrained by core and log data (φ = 0.08 – 0.18; K_s = 37 – 43 GPa; K-f = 0.02 – 2.3 GPa across steam-brine scenarios), with dry-frame moduli from DEM directly feeding Gassman substitution. This integrated framework advances sedimentary geothermal assessment while establishing replicable protocols for global application, contributing to sustainable energy transition goals.