Processes of hydrogen genesis during seismic cycles in active fault zones (ProHydroGen)

点击翻译按钮获取中文摘要

In this project, we plan to make use of a U-tube-KASMA device installed by Prof. Tullis Onstott (Princeton University) in a ~600 meter borehole intersecting an active fault zone in the Roodepoort Quartzite located 3.4 km below land surface in the Moab Khotsong gold mine. The borehole is part of the ICDP-funded DSeis project and used to monitor seismically triggered changes in in-situ geochemical and isotopic compositions along with microbial activity. We propose the operation of a gas monitoring system combined with the U-tube-KASMA installation, which will provide the unique opportunity to collect samples of minimal-altered geofluids and uncontaminated fracture gases from a deep active fault plane.During seismic unrest of the fault zone, we anticipate that there will be a geogas discharge including H2 which may serve as a nutrient source for deep microbial life. The geogas and especially H2 and O3 will be detected continuously using specific sensors of a portable gas analytical system directly mounted to the gas separator of the automated U-tube-KASMS. With the chemical and isotopic characterization of the individual fluids before and after seismic activity we hope to clarify the origin and the process generating the H2, which rely on cleavage of O-H bonds from water. In combination with data on the permeability and porosity of the fault zone, this research will help to understand different migration mechanisms of fluids from their source to the target depth. It will answer the question if low seismic events increase connectivity of isolated fluids and provide new pathways for migration of already existing H2 or expose fresh mineral surfaces for water-rock interactions, and release fresh mechano-chemically synthesized H2.Direct on-site analyses of the samples gained by the U-tube will determine just how rapidly changes in subsurface gas geochemistry are occurring in response to seismic events. The identification of seismic moment that provokes geogas peaks and the influence of distance and orientation of the focal point to the fault and the borehole should be identified.By collecting gas samples and analyzing them in the laboratory, we will be able to evaluate to what extent the H/D isotopic compositions of H2, and CH4 as well as 13CCO2 and 13CCH4 change and to verify if they derive from the same sources and if e.g. isotope exchange between these species is in thermodynamic equilibrium. Noble gas isotope measurements will allow for the calculation of residence times of the fracture fluids and will also help to answer the question if measured H2/He ratios match with a calculated H2/He radiolytic/radiogenic production yield. The data derived from gas chemical measurements are important input parameters for physico-chemical models describing the geochemical behavior of fluids, and when combined with seismic maps will better constrain the global abundance of the subsurface gas-chemical production processes in a fault zone.