Field Observations And Stable Isotopic Analysis Of Laterally Continuous Calcite Veins Associated With Fault Zones: Insight into Ancient Fluid Travel

Elizabeth Horne

The capability to store CO2 in subsurface geologic reservoirs is a proposed technique that will reduce the amount of human-generated CO2, a greenhouse gas, from accumulating in the atmosphere. By examining bedrock sites where evidence for ancient carbonate deposition, and where we see natural CO2 leaks, we can understand how fluid flow in the subsurface behaves in relation to faults and fractures. Understanding fluid flow in a natural system improves our understanding of risks involved in the storage of CO2 in subsurface reservoirs. We focus on two different outcrops of Mesozoic rocks associated with active CO2 leaks. The field locality for this work is in the Salt Wash Graben SE Utah, adjacent to the Ten-Mile fault, a normal fault with hundreds of meters of offset. Field observations at this location allow an understanding of crosscutting relationships of laterally extensive carbonate veins and travertine deposits associated with the Salt Wash fault zone. Maps of cross-cutting relationships in outcrop are used to understand timing of mineralization along with petrographic analysis of host rock and vein mineralization of calcite veins to understand relationships between host rock and mineralization. Both stable carbon and oxygen isotope analysis are used to understand changes in the fluid reservoir composition. Preliminary stable carbon isotope analyses give ?13C values between 3.9-6.0 per mil; variations may indicate change in fluid source, relative timing of mineralization, and depth of mineralization. Stable carbon isotope analysis is important because they serve as geochemical markers related to source fluids.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013