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Constraining Burial History and Fluid Pressures Through Combined Structural, Clumped-Isotope, and Fluid-Inclusion Analyses

Abstract

The advent of clumped-isotope (CI) analyses in stable isotope geochemistry has provided basin modelers another robust tool for better constraining a basin's history. In carbonates, a clumped-isotope analysis is based on the temperature-dependent affinity that occurs between the isotopically heavy isotopes of carbon and oxygen. This preferential clumping is especially pronounced for low temperatures that are important for basin modeling of hydrocarbon-rich systems. When the results of clumped isotopes analyses are combined with structural and fluid-inclusion analyses of carbonate veins, it is also possible to determine the fluid pressure(s) and composition(s) associated with vein(s) formation. Commonly, multiple generations of carbonate veins form during the evolution of a basin. It is general possible through detailed structural analysis to assign relative timing of different vein generations to each other and to the overall basin's evolution. A clumped-isotope analysis provides the temperature of vein formation. Fluid-inclusion analysis provides the chemistry of the fluid (i.e., salinity, presence of HC). And by combining the temperature from the clumped isotope analysis with the isochore data of the inclusions, it is possible to constrain the absolute pressure of the fluid at the time of vein formation. To date such determination of fluid pressure has not been possible except for a small suite of fluid inclusions that had just the right chemistry and conditions of formation. All of this information can then lead to a better model for the basin's history. Several examples and what we have learned about combining clumped-isotope and fluid-inclusion studies are discussed in this presentation. These include drill cored samples from the San Andreas fault (SAFOD project), surface collected samples from a CO2 charged dome in central Utah, and samples from the Eagle Ford play of south-central Texas.