The Oxygen and Carbon Isotope Systematics of Carbonate-Hosted Geothermal Reservoirs at Cove Fort and Thermo, Utah

摘要

Cuttings from eleven geothermal wells in the Thermo (5) and Cove Fort (6) geothermal fields provide a rare window into the hot (<188°C), deep (<2500 m) portions of carbonate-hosted geothermal reservoirs. O and C isotopic analyses of carbonate minerals from 373 samples of geothermal well cuttings have been analyzed. Petrographic observations of 257 samples, and XRD data of 70 samples generated as part of this study (as well as legacy XRD data) have been compared to the isotopic data set in order to provide geologic context, such as rock textures, secondary mineralogy and zones of brittle deformation. In addition, drilling logs, well logs and spinner surveys have been used to characterize sub-surface reservoir geometries, modern temperature distributions and to identify modern zones of permeability intersected by the wells. Large variations in both δ~(18)O from +2.4 to +28.7 ‰ (VSMOW) and δ~(13)C from -10 to +7.5 ‰ (VPDB) are recorded in the carbonate reservoir rocks. The observed isotopic depletions are interpreted to result from multiple episodes of water-rock interaction occurring during diagenesis, weathering, hydrothermal circulation, igneous intrusions with associated contact metamorphism and faulting. Differences in the extent of ~(18)O/~(16)O and ~(13)C/~(12)C depletion between wells inside and outside of the producing geothermal field at Cove Fort, and between more- and less-productive wells at Thermo, suggest that the modern permeability structures of these geothermal systems play an important role in determining the isotopic signature of the carbonate reservoir rock. Modern water-rock interactions between meteoric fluids and the carbonate reservoir rocks result in ~(18)O/~(16)O depletions without corresponding depletions in ~(13)C/~(12)C due to the low concentration of HCO3" in these fluids. Decarbonation reactions and magmatic water-rock interaction during contact metamorphism have locally produced coupled ~(13)C/~(12)C and ~(18)O/~(16)O depletions with increasing depth and proximity to intrusive rocks where recrystallization of the carbonates and high-temperature alteration minerals are observed. Weathered horizons at the top of carbonate lithologies show coupled ~(18)O/~(16)O and ~(13)C/~(12)C depletions as a result of interaction with meteoric waters and atmospheric carbon.