Fluid Inclusion Stratigraphy (FIS) is a new technique
developed for the oil industry in order to map
borehole fluids. This method is being studied for
application to geothermal wells and is funded by the
California Energy Commission. Fluid inclusion gas
geochemistry is analyzed and plotted on well log
diagrams. The working hypothesis is that select
gaseous species and species ratios indicate areas of
groundwater and reservoir fluid flow and reservoir
seals. Analyses from multiple boreholes should show
the stratigraphy of subsurface fluids. Analyses are
performed by a commercial laboratory, Fluid
Inclusion Technologies.
The FIS method modified for use on the Coso
geothermal system has produced preliminary results
indicating that reservoir assessment is possible.
Approximately 1,700 samples from three producing
and one non-producing well have been analyzed.
Preliminary results show megascopic trends and
much fine scale detail when the logs are analyzed in
detail. Select species including carbon dioxide,
nitrogen, argon, methane, and hydrogen sulfide show
distinct differences between producing and non-producing
wells as well as indicating fluid flow and
locations of major fracture zones. Additional
interpretation of the preliminary results and method
will include consistency of results from one borehole
to another, agreement of FIS with well logs, and
appraisal of FIS analyses on well testing decisions,
well completion strategies, and in resource
calculations.

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