| Title | Assessing Fracture Connectivity Using Stable and Clumped Isotope Geochemistry of Calcite Cements |
|---|---|
| Authors | Kristina K. SUMNER, Erin R. CAMP, Katharine W. HUNTINGTON, Trenton C. CLADOUHOS, Matt UDDENBERG |
| Year | 2015 |
| Conference | Stanford Geothermal Workshop |
| Keywords | stable isotopes, clumped isotope thermometry, geochemistry, cathodoluminescence microscopy, flow path connectivity, fracture connectivity |
| Abstract | Understanding flow path connectivity within a geothermal reservoir is a critical component for efficiently producing sustained flow rates of hot fluids from the subsurface. We present a new approach for characterizing subsurface fracture connectivity that combines petrographic and cold cathodoluminescence (CL) microscopy with stable isotope analysis (δ18O and δ13C) and clumped isotope (∆47) thermometry of fracture-filling calcite cements from a geothermal reservoir in northern Nevada. Calcite cement samples were derived from both drill cuttings and core samples taken at various depths from wells within the geothermal field. CL microscopy of some fracture filling cements shows banding parallel to the fracture walls as well as brecciation, indicating that the cements are related to fracture opening and fault slip. Variations in trace element composition indicated by the luminescence patterns reflect variations in the composition and source of fluids moving through the fractures as they opened episodically. Calcite δ13C and δ18O results also show significant variation among the sampled cements, reflecting multiple generations of fluids and fracture connectivity. Clumped isotope analyses performed on a subset of the cements analyzed for conventional δ18O and δ13C show calcite growth temperatures greater than 150°C—above the current ambient rock temperature. These temperature estimates from clumped isotope thermometry, combined with independent constraints on calcite δ18O values, enable the δ18O value of the waters from which the cements precipitated to be calculated. The elevated cement temperatures and elevated δ18O of water values demonstrate a deep mixing signal for the geothermal source fluids from which the calcite precipitated. Overall, the information gleaned from these analyses will aid in characterizing connectivity of fluid pathways at depth. This tool will allow geothermal resource managers and developers to determine optimal well stimulation targets, optimize production and injection programs, and better design step-out drilling for production and injection enhancement. |