Report Abstract

GEOTHERMAL HYDROLOGY OF VALLES CALDERA AND THE SOUTHWESTERN JEMEZ MOUNTAINS, NEW MEXICO

By Frank W. Trainer, Robert J. Rogers, and Michael L. Sorey

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 the north and east areas of the caldera may be the principal recharge zones of the reservoir. Downward flow along fault zones and fractures probably is the primary mechanism of recharge. Recharge water probably enters the edges of the reservoir at depth, heats up, rises convectively within the reservoir, and discharges laterally to the west and southwest.

Outflowing mineral water appears to be limited to the western and southwestern parts of the Jemez Mountains. Hydrothermal features outside Valles Caldera are restricted largely to Cañon de San Diego. Subsurface escape of reservoir fluid from near and beneath Valles Caldera has formed a discharge plume of reservoir water mixed with dilute ground water, which extends down Cañon de San Diego. The Jemez Fault Zone transports a relatively large portion of this flow. Soda Dam and Jemez Springs are derivatives of geothermal outflow from the reservoir. Near Jemez Pueblo, subsurface mineral water merges with the regional aquifer in fill deposits of the Albuquerque Basin.

Total geothermal discharge from the caldera is difficult to estimate; all estimates based on chemical mass balance suggest a small fluid discharge. About 1.0 cubic foot per second of caldera-derived geothermal fluid is estimated to be carried by the Jemez River between Jemez Pueblo and San Ysidro, and about 0.4 cubic foot per second is estimated to be carried as underflow in the same reach. Estimates of total discharge from the geothermal reservoir to the Rio Grande at its confluence with the Jemez River range from 2.0 to 3.6 cubic feet per second.

Numerical models of the geothermal system range in complexity from one-dimensional, single-phase fluid-flow models to three-dimensional, multiphase-fluid and heat-flow models. The models have been developed primarily to assess reservoir productivity and longevity and potential effects of development on thermal-water discharge in the Jemez River. A period of actual geothermal development, in which the system is stressed and the hydrologic changes are measured, is needed to calibrate or test the models. Existing models must be considered preliminary and cannot provide accurate answers to questions involving long-term changes to the geothermal system. No existing models account for the presence of carbon dioxide in reservoir fluid, which appears to be sufficient to markedly extend the depth over which two-phase conditions occur naturally and possibly to influence reservoir drawdown during development. Development of geothermal energy in Valles Caldera would probably change the hydrochemical discharge from the southwestern Jemez Mountains. Quantifying such changes through systematic monitoring would be valuable in better understanding the geothermal system and in testing, refining, and calibrating numerical models of the system.

Abstract from WRIR 00-4067


 
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