Search for: All records

Abstract While it has been known for some time that reducing fluids have bleached red beds adjacent to fault zones and regionally across the Colorado Plateau, the volumes of fluids expelled along faults have never been quantified. We have developed and applied a suite of one-dimensional hydrologic models to test the hypothesis that internally generated, reducing fluids migrated up sub-basin bounding faults across the Paradox Basin and bleached overlying red beds. The internal fluid driving mechanisms included are mechanical compaction, petroleum and natural gas generation, aquathermal expansion of water, and clay dewatering. The model was calibrated using pressure, temperature, porosity, permeability, and vitrinite reflectance data. Model results indicate that sediment compaction was the most important pressure generation mechanism, producing the majority of internal fluids sourced during basin evolution. Peak fluid migration occurred during the Pennsylvanian–Permian (325–300 Ma) and Cretaceous (95–65 Ma) periods, the latter being concurrent with simulated peak oil/gas generation (87–74 Ma), which likely played a role in the bleaching of red beds. Batch geochemical advection models and mass balance calculations were utilized to estimate the volume of bleaching in an idealized reservoir having a thickness (~100 m) and porosity (0.2) corresponding to bleached reservoirs observed in the Paradox Basin. Bleaching volume calculations show that internal fluid driving mechanisms were likely responsible for fault-related alteration observed within the Wingate, Morrison, and Navajo Formations in four localities across the Paradox Basin in the Colorado Plateau, Utah and Colorado, USA. The volume calculation required that 33%–55% of the total basinal fluids, composed of hydrogen-sulfide and paleo-seawater, migrated into an overlying red bed reservoir (0.5 wt% Fe2O3). 

The Paradox Basin in the Colorado Plateau (USA) has some of the most iconic records of paleofluid flow, including sandstone bleaching and ore mineralization, and hydrocarbon, CO2, and He reservoirs, yet the sources of fluids responsible for these extensive fluid-rock reactions are highly debated. This study, for the first time, characterizes fluids within the basin to constrain the sources and emergent behavior of paleofluid flow resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34SH2S; 87Sr/86Sr) signatures of formation waters were used to evaluate the distribution and sources of fluids and water-rock interactions by comparison with the rock record. There are two sources of salinity in basinal fluids: (1) diagenetically altered highly evaporated paleo-seawater-derived brines associated with the Pennsylvanian Paradox Formation evaporites; and (2) dissolution of evaporites by topographically driven meteoric circulation. Fresh to brackish groundwater in the shallow Cretaceous Burro Canyon Formation contains low Cu and high SO4 concentrations and shows oxidation of sulfides by meteoric water, while U concentrations are higher than within other formation waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were derived from evaporated paleo-seawater mixed with meteoric water that oxidized sulfides and dissolved gypsum and have high 87Sr/86Sr indicating interaction with radiogenic siliciclastic minerals. Upward migration of reduced (hydrocarbon- and H2S-bearing) saline fluids from the Pennsylvanian Paradox Formation along faults likely bleached sandstones in shallower sediments and provided a reduced trap for later Cu and U deposition. The distribution of existing fluids in the Paradox Basin provides important constraints to understand the rock record over geological time. 

Abstract How subsurface microbial life changed at the bottom of the kilometers‐deep (hypo) Critical Zone in response to evolving surface conditions over geologic time is an open question. This study investigates the burial and exhumation, biodegradation, and fluid circulation history of hydrocarbon reservoirs across the Colorado Plateau as a window into the hypo‐Critical Zone. Hydrocarbon reservoirs, in the Paradox and Uinta basins, were deeply buried starting ca. 100 to 60 Ma, reaching temperatures >80–140°C, likely sterilizing microbial communities present since the deposition of sediments. High salinities associated with evaporites may have further limited microbial activity. Upward migration of hydrocarbons from shale source rocks into shallower reservoirs during maximum burial set the stage for microbial re‐introduction by creating organic‐rich “hot spots.” Denudation related to the incision of the Colorado River over the past few million years brought reservoirs closer to the surface under cooler temperatures, enhanced deep meteoric water circulation and flushing of saline fluids, and likely re‐inoculated more permeable sediments up to several km depth. Modern‐ to paleo‐hydrocarbon reservoirs show molecular and isotopic evidence of anaerobic oxidation of hydrocarbons coupled to bacterial sulfate reduction in areas with relatively high SO₄‐fluxes. Anaerobic oil biodegradation rates are high enough to explain the removal of at least some portion of postulated “supergiant oil fields” across the Colorado Plateau by microbial activity over the past several million years. Results from this study help constrain the lower limits of the hypo‐Critical Zone and how it evolved over geologic time, in response to changing geologic, hydrologic, and biologic forcings.