Hydrogeochemical Evolution of Basinal Fluids in the Paradox Basin: Implications for Sources, Paleofluid Flow, Residence Time, and Water-Rock-Gas-Microbe Interactions
PublisherThe University of Arizona.
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EmbargoRelease after 11/06/2022
AbstractUnderstanding evolution of paleofluid flow through the Earth’s shallow crust is important for water, mineral, and energy resource management, including extraction of subsurface resources and storage of alternative energy and waste products. The migration of water, gas, and life between surface and deep subsurface systems and its consequence by water-rock-gas-microbe interactions can be evaluated by characterizing hydrogeochemical features of fluids (e.g., formation water and natural gas) in sedimentary basins. This study focuses on the Paradox Basin in the Colorado Plateau, which has iconic manifestations of multiple episodes of paleofluid flow, including widespread sandstone bleaching, ore mineralization (e.g., Cu; U; Fe; Mn), and hydrocarbons, CO2, and He accumulation. Based on molecular and isotopic signatures in formation water and natural gas samples, the origin, types, composition, distribution, and residence time of remnant fluids in the Paradox Basin have been evaluated to constrain the hydrochemical and geological histories responsible for paleofluid flow and solute transport. Highly evaporated paleo-seawater derived brines (i.e., connate brines), associated with the Pennsylvanian Paradox Formation evaporites, migrated into adjacent under- and/or over-lying formations through faults by compaction. These H2S- and hydrocarbon-bearing reduced, saline fluids were responsible for much of the sandstone bleaching in overlying Triassic-Cretaceous shallow sediments, forming reduced traps for later Cu and U deposition. Natural gas throughout the basin is primarily thermogenic in origin recording different thermal maturities of gas generation. Microbial methanogenesis may have been inhibited by the deep burial history of the Paradox Basin and abundance of sulfate. Salt dissolution above and below the evaporites, by topographically-driven meteoric recharge, provided a source of more oxic, sulfate-rich shallow brines. Deep meteoric circulation (up to 3 km depth in the last 1.1 Ma, based on 81Kr dating), in response to the recent denudation of the Colorado Plateau (<4-10 Ma), contributed to flushing of residual brines in aquifers above and below the evaporites and biodegradation of hydrocarbons in shallow reservoirs (based on molecular and isotopic signatures of hydrocarbon in natural gas, including clumped isotopes of methane). The origin, types, and distribution of existing fluids in the Paradox Basin provide important constraints to understand the evolution of paleofluid flow and subsequent water-rock-gas-microbe interactions recorded in sedimentary rocks over geological time.
Degree ProgramGraduate College