Geothermal hydrology of Valles Caldera and the southwestern Jemez Mountains, New Mexico

The Jemez Mountains in north-central New Mexico are volcanic in
origin and have a large central caldera known as Valles Caldera.
The mountains contain the Valles geothermal system, which was
investigated during 1970-82 as a source of geothermal energy. This
report describes the geothermal hydrology of the Jemez Mountains
and presents results of an earlier 1972-75 U.S. Geological Survey
study of the area in light of more recent information. Several
distinct types of thermal and nonthermal ground water are
recognized in the Jemez Mountains. Two types of near-surface thermal
water are in the caldera: thermal meteoric water and acid sulfate
water. The principal reservoir of geothermal fluids is at depth
under the central and western parts of the caldera. Nonthermal
ground water in Valles Caldera occurs in diverse perched aquifers
and deeper valley-fill aquifers.

The geothermal reservoir is recharged by meteorically derived
water that moves downward from the aquifers in the caldera fill to
depths of 6,500 feet or more and at temperatures reaching about 330
degrees Celsius. The heated geothermal water rises
convectively to depths of 2,000 feet or less and mixes with other
ground water as it flows away from the geothermal reservoir. A vapor
zone containing steam, carbon dioxide, and other gases exists
above parts of the liquid-dominated geothermal zone.

Two subsystems are generally recognized within the larger
geothermal system: the Redondo Creek subsystem and the Sulphur
Creek subsystem. The permeability in the Redondo Creek subsystem is
controlled by stratigraphy and fault-related structures. Most of
the permeability is in the high-angle, normal faults and
associated fractures that form the Redondo Creek Graben. Faults and
related fractures control the flow of thermal fluids in the
subsystem, which is bounded by high-angle faults. The Redondo
Creek subsystem has been more extensively studied than other
parts of the system. The Sulphur Springs subsystem is not as well
defined. The upper vapor-dominated zone in the Sulphur Creek
subsystem is separated from the liquid-dominated zone by about 800
feet of sealed caldera-fill rock. Acid springs occur at the top of
the vapor zone in the Sulphur Springs area. Some more highly
permeable zones within the geothermal reservoir are
interconnected, but the lack of interference effects among some
wells during production tests suggests effective hydraulic
separation along some subsystem boundaries. Chemical and thermal
evidence suggests that the Sulphur Springs subsystem may be isolated
from the Redondo Creek subsystem and each may have its own zone of
upflow and lateral outflow.

The area of the entire geothermal reservoir is estimated
to be about 12 to 15 square miles; its western limit generally is
thought to be at the ring-fracture zone of the caldera. The top of the
reservoir is generally considered to be the bottom of a small-
permeability 'caprock' that is about 2,000 to 3,000 feet below
land surface. Estimated thicknesses to the bottom of the
reservoir range from 2,000 to 6,000 feet. Reservoir temperatures
measured in exploration wells range from 225 degrees Celsius
just below the caprock to about 330 degrees Celsius in deeper
drill holes. Pressures measured in exploration wells in the Redondo
Creek area ranged from 450 to 1,850 pounds per square inch.
Steam-producing zones have been encountered above the liquid-
dominated zones in wells, but the extent of steam zones is not well
defined.

The reservoir contains a near-neutral, chloride-type water
containing about 7,000 milligrams per liter dissolved solids. No
thermal springs in the caldera have geochemical characteristics
similar to those of the geothermal reservoir fluids sampled in wells.

Oxygen-18 and deuterium isotope concentrations of
geothermal reservoir fluid indicate a meteoric origin. The
moat valleys in