Exploration and Development of the Cerro Prieto Geothermal Field

Abstract

Summary A multidisciplinary effort to locate, delineate, and characterize the geothermal system at Cerro Prieto field, Baja California, Mexico, began in the late 1950's. It led to the identification of an important high-temperature, liquid-dominated geothermal system which went into production in 1973. This paper summarizes and discusses the exploration and monitoring studies related to this field. Introduction The Cerro Prieto geothermal field is located in the Mexicali Valley, Baja California, Mexico, about 30 km [19 miles] south of the U.S. border (Fig. 1). It has been in production since 1973, being the first liquid-dominated geothermal system in North America to produce significant electrical power. The general geologic similarity between Cerro Prieto and the geothermal fields of the neighboring Imperial Valley (southern California), and the experience gained by the Comisión Federal de Electricidad of Mexico (CFE) in locating and developing the resource were the main factors that led to the signing, in 1977, of a 5-year agreement between CFE and the U.S. Energy R&D Admin., now the U.S. DOE, to conduct a cooperative study of Cerro Prieto.1 Lawrence Berkeley Laboratory (LBL) coordinated U.S. technical activities carried out under the agreement. Because of the success of this cooperative program, discussions are being held to sign a new DOE/CFE agreement to study Cerro Prieto and other geothermal areas in Mexico. Exploration for geothermal energy in the Cerro Prieto area began in the late 1950's, and the first deep exploration wells were drilled in 1960–61 over the thermal anomaly. By early 1983, about 120 deep wells (up to 3550 m [11,647 ft] in depth) had been drilled, delineating a good portion of the geothermal reservoir (Fig. 2). Presently, 180 MW of electricity are being generated; CFE, which manages and operates Cerro Prieto, is building two new power plants that will increase the output to 620 MWe (megawatts of electric power) before the end of 1985. A vast amount of data on the subsurface of the area and on the characteristics of the producing wells has been gathered. Cerro Prieto is one of the more thoroughly documented and best understood geothermal systems in the world. This paper reviews the exploration effort which first led to the discovery of the field, then to the delineation of the resource, and finally to the definition of the subsurface fluid and heat circulation. Other studies carried out on Cerro Prieto are summarized in Ref. 2. Geologic Setting of the Area Cerro Prieto is situated in the southern portion of the Salton trough, an actively developing structural depression filled with sediments. It is the landward continuation of the Gulf of California (Fig. 1) and a region of high seismicity and high heat flow. The Salton trough is the result of tectonic activity, which has created a series of spreading centers and transform faults that link the East Pacific Rise, an oceanic ridge, with the San Andreas fault system, a transform boundary (Fig. 3).3,4 The Cerro Prieto field is located on a tensional spreading center at the ends of the right-lateral strike-slip Imperial and Cerro Prieto faults.3,4 Although the mechanics for crustal deformation and heating and for magma generation and movement into the shallow crust are not completely known, it is believed that Cerro Prieto is an area where gabbroic magma rising from a deeper chamber is creating a volcanic oceanic-type crust.5,6 Rapid sedimentation into this pull-apart basin has provided the reservoir and caprock for the geothermal system, and the thick sediments obscure the presence of the heating and plutonic activity. An area of seismicity connects the Cerro Prieto and Imperial faults (Fig. 4), providing evidence for crustal extension and magma ascent via dikes. There probably exists a plexus of dikes and sills7,8 and perhaps some deeper plutons. Basaltic dikes have been intersected in wells drilled to depths of 3 km [2 miles] and more in the eastern part of the field.9,10 The only significant volume of young volcanics to breach the surface are the two rhyodacitic cones ( 0.11 million years before present) that comprise the Cerro Prieto volcano. The composition of these rocks indicates that crustal heating by the gabbroic intrusions has been sufficient to partially melt and mobilize the silicic crust.