Development of Noble Gases as Tracers of Subsurface Fluids - from Conventional and Unconventional Hydrocarbon to Geothermal Systems

Abstract

In order to counteract the ongoing global warming and to cope with the increasing demand for clean energy, advancements in our understanding of subsurface energy systems are required to guide future exploration and management strategies. Noble gases (He, Ne, Ar, Kr, Xe) are stable and chemically inert, making them excellent tracers to fingerprint various crustal fluids, e.g., groundwater, natural gas and oil. This dissertation includes a study in the conventional Panhandle and Hugoton Field (PHF) in Texas, Oklahoma and Kansas, a study in the unconventional reservoir of the Eagle Ford Shale in Texas, and a study in Mexican geothermal fields. In hydrocarbon systems, noble gases are analyzed together with major gas components (e.g., CH4, CO2) to trace the sources, migration and mixing of subsurface fluids, and to clarify the tectonic and magmatic evolution of these regions. In the geothermal systems, noble gases are analyzed together with heat, CO2, and stable isotopes (δ13C-CO2) to evaluate their sources and fractionation mechanisms. PHF gas samples show enrichment in terrigenic noble gases (4He*, 21Ne*, 40Ar*). A positive correlation between terrigenic noble gases and methane in west Panhandle suggests a common origin for both gases from adjacent basins. In east Panhandle, a positive correlation between terrigenic noble gases and depth points to a dominant upward noble gas flux from underlying rocks. The presence of a primordial mantle He-Ne component in east Panhandle and a mantle helium component of undetermined origin in west Panhandle suggest the presence of an open system. The primordial mantle component is likely associated with the presence of a mantle plume in the Wichita Igneous Province. The combined analysis of relative 40Ar ages and estimated

water/gas volume ratios suggests that groundwater plays a major role in the accumulation and distribution of terrigenic noble gases. Gas samples in the Eagle Ford Shale show a positive correlation between methane and crustal noble gases, suggesting a common origin for both. The noble gas dataset suggests also the presence of different hydrocarbon sources. Two distinct mantle components are observed: a primordial, solar-like component corresponding to an Ocean Island Basalts (OIBs) signature, and a component corresponding to a Mid-Ocean Ridge Basalt (MORB) signature. The MORB signature is likely representative of the Ouachita Rift during the breakup of Rodinia, while the OIB-like signature is consistent with an origin in the subcontinental lithospheric mantle, possibly from the shallow refractory reservoir beneath the Balcones Igneous Province. Relative 40Ar ages display significant variations in the Eagle Ford samples, suggesting a highly developed compartmentalization within the shale reservoir and the presence of different hydrocarbon sources. Fluid samples from Mexican geothermal fields display distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) in the Trans-Mexican Volcanic Belt (TMVB) and Baja California. High R/Ra ratios in the TMVB correspond to active magmatic heat sources while low R/Ra ratios in Baja California point to crustal contributions from the subducted Farallon plate. A combined analysis of heat/helium ratios (Q/3He) with 4He/36Ar ratios shows that convection is the main mechanism controlling the transport of heat and magmatic volatiles during magma degassing. Fractionation between heat and volatiles is caused by boiling and meteoric water dilution. The absence of significant fractionation in CO2/3He ratios suggests that helium degassing is controlled by the CO2 content in parental magma rather than helium diffusivity.