INFLUENCE OF INTERNAL FLUID GENERATION MECHANISMS ON REDBED ALTERATION IN THE PARADOX BASIN, UTAH

Conceptual models of Paradox Basin copper and uranium ore formation call upon an initial fluid flow event in which reducing basinal brines migrate from the Paradox Formation via faults, resulting in the bleaching of overlying red beds. The presence of spots of hydrocarbons within these bleached redbeds and peripheral to the ore deposits suggest the bleaching fluid is co-generated with hydrocarbons. Here, we developed and applied a suite of one-dimensional hydrologic models to investigate the viability of internal fluid sources driving reducing/ore forming fluids up mini-basin bounding faults across the Paradox Basin. The model was calibrated using pressure, temperature, porosity and vitrinite reflectance data. The internal fluid generation mechanisms we included are mechanical compaction, petroleum and natural gas generation, aquathermal expansion of water, and clay dewatering. We ruled out gypsum dehydration as this likely predated hydrocarbon generation and occurs at shallow depths. Our model suggests sediment compaction was responsible for the majority of anomalous pore pressure generation within the Paradox Basin. Peak fluid migration up the basin bounding faults occurred when the basin experienced maximum subsidence rate. When pore pressure exceeded 85% lithostatic pressure, we allowed hydraulic fracturing to occur which increased the permeability of all formations 100-fold. We saw two peak flow events, that occurred simultaneously with hydraulic fracturing, throughout basin evolution. The range in timing of peak flow events across the Paradox Basin varied between: 75-100 and 280-310 Ma and occurred concurrently with peak oil/gas generation. We investigated the impact of internal fluid generation mechanisms on the alteration of redbeds by allowing a fraction of the migrating fluid to flow into an overlying Jurassic reservoir. The reservoir was assumed to have 20% porosity and 0.5wt% Fe₂O₃. The alteration fluid was composed primarily of paleoseawater and reducing agents: acetate and H₂S. We found the percentage of fluid responsible for fault related bleaching within the Paradox Basin varied spatially. For instance, 30-40% of the fluid generated by internal mechanisms within the Slick Rock district was required to bleach the 6 km of altered Morrison Formation extending from the Dolores zone of faults.