Gas and Isotope Geochemistry of 81 Steam Samples from Wells in The Geysers Geothermal Field, Sonoma and Lake Counties, California

The Geysers geothermal field in northern California,
with about 2000-MW electrical capacity, is the largest geothermal
field in the world. Despite its importance as a resource
and as an example of a vapor-dominated reservoir,
very few complete geochemical analyses of the steam have
been published (Allen and Day, 1927; Truesdell and others,
1987). This report presents data from 90 steam, gas, and condensate
samples from wells in The Geysers geothermal field
in northern California. Samples were collected between 1978
and 1991. Well attributes include sampling date, well name,
location, total depth, and the wellhead temperature and pressure
at which the sample was collected. Geochemical characteristics
include the steam/gas ratio, composition of noncondensable
gas (relative proportions of CO2, H2S, He, H2,
O2, Ar, N2, CH4, and NH3), and isotopic values for deltaD and
delta18O of H2O, delta13C of CO2, and delta34S of H2S. The compilation
includes 81 analyses from 74 different production wells, 9
isotopic analyses of steam condensate pumped into injection
wells, and 5 complete geochemical analyses on gases from
surface fumaroles and bubbling pools.
Most samples were collected as saturated steam and
plot along the liquid-water/steam boiling curve. Steam-togas
ratios are highest in the southeastern part of the geothermal
field and lowest in the northwest, consistent with other
studies. Wells in the Northwest Geysers are also enriched in
N2/Ar, CO2 and CH4, deltaD, and delta18O. Well discharges from the
Southeast Geysers are high in steam/gas and have isotopic
compositions and N2/Ar ratios consistent with recharge by
local meteoric waters. Samples from the Central Geysers
show characteristics found in both the Southeast and Northwest
Geysers.
Gas and steam characteristics of well discharges from
the Northwest Geysers are consistent with input of components
from a high-temperature reservoir containing carbonrich
gases derived from the host Franciscan rocks. Throughout
the geothermal field, the carbon-isotopic composition of
CO2 is consistent with derivation of carbon from Franciscan
metasedimentary rocks. NH3 concentrations are high in most
Geysers well fluids, and are 2-3 orders of magnitude greater
than would be expected in a the gas phase exhibiting homogeneous
equilibrium at normal reservoir temperatures and
pressures. Evidently, NH3 is flushed from the Franciscan host
rocks at a rate that exceeds the reaction rate for NH3 breakdown.
Many wells show clear influence by fluids from reinjection
wells where steam condensate has been pumped
back into the geothermal reservoir. Six wells were resampled
over the time period of this study. One of these six wells was
strongly affected by a nearby injection well. Three of the six
resampled wells showed some signs of decreasing liquid/
steam within the geothermal reservoir, consistent with 'drying
out' of the reservoir due to steam withdrawal. However,
two wells exhibited little change.
Analyses of gases from five surface manifestations (fumaroles
and bubbling pools) are roughly similar to the deeper
geothermal samples in both chemical and isotopic composition,
but are lower in soluble gases that dissolve in groundwater
during transit toward the surface.