Chemical and Isotopic Characteristics of Sedimentary Basin Formation Waters for Evaluating Cross-Formational Mixing, Tracers of Contamination, and Li Resources

Abstract

Understanding the origin, diagenetic history, chemical composition, and migration pathways of saline fluids in sedimentary basins is important for extraction of subsurface resources (e.g., critical elements, such as Li), storage of alternative energy (e.g., H2) and saline produced waters, and long-term sequestration of anthropogenic CO2 and spent nuclear fuel. The presence of confining units, such as evaporites and shale, may impede basinal-scale fluid flow and can be a source of highly saline fluids to adjacent formations. Upward migration of saline fluids may contaminate surface waters or shallow aquifers. Identifying the source of saline fluids (e.g., fluids sourced from specific oil/gas reservoirs) through unique fluid isotopic signatures may help to remediate contamination. This issue is complicated by the fact that many subsurface reservoirs have been altered by extensive fluid injection activities (e.g., water flooding or disposal of produced, saline waters), which may modify original formation water chemistry. This dissertation aims to evaluate Sr isotopes as an adequate tracer for fingerprinting distinct sources of produced waters from overlapping oil/gas fields, influence of fluid surface storage and subsurface injection activities, and cross-formational fluid migration through evaporite confining units, combined with major ion, Br, and water stable isotope chemistry. Furthermore, this dissertation aims to explore the potential of Li production from sedimentary basin brines.To test the utility of Sr isotopes as a tracer of subsurface, saline fluid sources, Sr isotopes (87Sr/86Sr) of formation waters were evaluated from hydrocarbon reservoirs within three major oil/gas producing regions: the Williston, Appalachian, and Permian basins in North America. Based on a non-parametric statistical test, Sr isotope ratios of formation waters from multiple stacked oil and gas reservoirs within each basin have overlapping (i.e., non-unique) values. In regions where Sr isotope ratios of formation waters overlaps, Sr isotopes alone may not be a sensitive tracer of saline, produced water contamination in near surface environments, as previously proposed. Sr isotopes, along with major ion chemistry, Br, and water stable isotopes, were further applied to investigate the potential of fluid migration across a thick evaporite confining unit (Gotnia Formation) within the Kuwait Basin, as proposed by previous studies. Results indicate that the Pre-Gotnia (below the evaporites) and Post-Gotnia (above the evaporites) sections have distinct Sr isotopes signatures suggesting the Gotnia Formation impedes vertical fluid migration. These results are supported by major ion chemistry, Br, and water stable isotopes. The upward migration or downward leakage of saline water can deteriorate the quality of drinking water and modifies the formation waters chemical and isotopic signature. Interestingly, several of the subsurface reservoirs show extensive mixing with seawater, likely from water flooding activities for enhanced oil recovery. Replacement of formation waters by injection of different fluids may cause difficulties in fingerprinting sources of potential contamination, deteriorate the reservoir quality, lead to borehole scaling, and alter subsurface microbial activity. Li (and other critical elements) extraction from oil-field brines has been proposed, yet relatively little is known about the concentration of Li in various sedimentary basin fluids and potential extraction rates based on fluid fluxes. To address this issue, I investigated Li concentrations and potential Li extraction rates of formation waters from stacked oil/gas reservoirs in sedimentary basins across North America. Six of the basins contained [Li] above 65 mg/L, with the highest Li contents in the Smackover Formation in the Gulf Coast, E. Texas and Arkla basins (up to 1700 mg/L), which has been the focus of previous studies. The Paradox, Appalachian, Williston, Gulf Coast, East Texas, and Arkla basins also contained [Li] above 65 mg/L. In general, the highest Li concentrations are found in basinal brines sourced from highly evaporated seawater (halite to potash salt precipitation stage) from the geologic past. Li was then enriched by interaction with Li-rich rocks and minerals in the subsurface (e.g., detrital sediments, shales, and volcanic ash). Lithium concentrations and potential production rates are most promising in the Paradox Formation in the Paradox Basin, Clinton/Medina Groups in the Appalachian Basin, and Devonian section and Charles Formation in the Williston Basin. Results of this study show that basinal brines may be a competitive resource compared to more traditional continental brines for Li (and possibly other critical elements) production, worthy of further investigation.