Isotopic analysis of sulfur cycling and gypsum vein formation in a natural CO2 reservoir

In order to assess the long-term security of geologic carbon storage, it is crucial to study the geochemical behavior of sulfur in reservoirs that store CO2. Fossil fuel combustion may produce mixtures of carbon dioxide and sulfur gases, and the geochemical effects of sulfur–CO2 cosequestration are poorly understood. This study examines sulfur mineralization from a core drilled in a stacked sequence of natural CO2 reservoirs near the town of Green River, Utah. These reservoirs include the Entrada and Navajo Sandstone, which are separated by the Carmel Formation caprock and transected by a system of CO2-degassing normal faults, through which saline CO2-charged brines discharge. Our objective in this study is to evaluate the mechanisms and timing of secondary mineral formation, particularly gypsum formation, in the CO2 reservoirs and intervening caprock. The Carmel Formation contains beds of gypsum within a fault zone. Secondary veins of gypsum exist throughout the Entrada Sandstone and Carmel Formation. We report sulfur and oxygen isotope data (δ34SSO4 and δ18OSO4, respectively) measured in gypsum and δ34S measured in pyrite, and the oxygen and hydrogen isotope composition (δ18O and δD, respectively) of gypsum hydration water. The multiple isotope approach allows us to trace the sources of sulfur in the reservoirs and, when combined with structural and petrological evidence, the progress of fluid-rock reactions and relative timing of vein mineralization.The secondary gypsum veins in the Carmel Formation derive from mixing of fluids with two isotopically distinct sulfate sources: sulfate from the gypsum beds within the Carmel Formation and sulfate-rich brines that originate from evaporites in the underlying Carboniferous Paradox Formation. The gypsum veins in the Entrada Sandstone have a relatively wide range of δ18OSO4, both isotopically more enriched in 18O and more depleted in 18O than the primary gypsum sources. We suggest that some aqueous sulfate in the Entrada Sandstone may cycle through multiple valence states as it undergoes reduction and reoxidation, resulting in the replacement of its oxygen atoms and allowing the occasional formation of gypsum with anomalous low δ18OSO4. Our data from gypsum hydration water indicates that groups of gypsum veins formed at two different times. Gypsum veins in the Entrada Sandstone and some veins in the Carmel Formation likely formed during Quaternary CO2-charged brine discharge events, while other veins located close to the gypsum beds in the Carmel Formation formed earlier, likely during cycles of dehydration and rehydration associated with the Laramide-age (40 Mya) faulting. We conclude that calcium-sulfate mineral formation in brine-filled fractures may play an important role in inhibiting fluid migration in geologic reservoirs that contain CO2.