Fracture Identification using Fluid Inclusion Stratigraphy

Abstract

Fluid inclusion stratigraphy (FIS) uses gas analyses of fluid inclusions to determine fluid types in geothermal systems. Peaks in FIS data are assumed to be related to location of fractures. The working hypothesis is that open fracture systems can be identified by their FIS chemical signature; that there are differences based on the mineral assemblages and geology of the system; and that there are chemical precursors in the wall rock above open, large fractures. Correlating fracture locations in cores to peaks in the FIS data indicate that select chemical species are useful in distinguishing large fracture zones from small fractures. FIS data for Steamboat Springs, Karaha, Glass Moutain, and Coso geothermal systems indicate that species such as CO2, H2S, and select organic species are useful in identifying fractures. Permeable zones are indicated by a large change in the relative concentrations of the ratio of CO2/N2. The large changes in the ratio of CO2/N2 when plotted with depth, correlate with lost circulation, alteration and vein locations on well logs and cores. Identifying dense fracture areas as well as large open fractures from small fracture systems would assist in evaluating the permeability of a well and in fracture stimulation selection for enhanced geothermal systems.