Scientific Drilling into the San Andreas Fault: Rare Gas 'Ages' of Matrix Fluids in Drill Core and Fluid Dynamics

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This project analyzes matrix fluid samples collected during SAFOD Phase III core runs to determine the flow of fluids into/from the fault zone using established noble gas methodologies pioneered by Osenbrueck (1996) and demonstrated by this group [Stute et al.,2005, 2007, Ali et al., 2007, AGU and EarthScope abstracts]. The 17 matrix fluid samples obtained from drillcore collected with previous NSF support during the SAFOD Phase III coring program (2007) will be evaluated with regard to pore fluid ages, age gradients and 3He/4He isotopic profiles of matrix fluids. These results will be interpreted to indicate the magnitude and direction of fluid flow into/from the SAFZ. The 4He and 40Ar "age" of the matrix fluid is a function of the water/rock ratio, production rates in the solid phase and the fraction of produced gases released to the pore fluid. The suite of noble gas isotopes to be measured in this study will be used to separate non-atmospheric (produced and accumulated) and atmospheric noble gas components. Essentially, the outer rind of the recovered drillcore was removed on site immediately after core recovery and a small subsample from the center was placed in a vacuum container. Atmospheric gases were then removed from the container and the container was sealed. Noble gases and matrix fluids are currently diffusing out of the recovered sample. During the course of this project, the noble gases released to the headspace will be determined mass spectrometrically and the total volume of matrix fluid will be determined by weight. Noble gas concentrations in solids are also used to determine the source function for excess noble gas abundances in matrix pore fluids and the release of noble gases from the rock. Step heating experiments will be used to determine the diffusive loss from the solid phase. The calculation of the matrix fluid "age" and "age" gradient will be used to estimate the time scale and direction of fluid transport in/near the fault zone on the time scale of the transport (10s to 100s of years).