Geology of the Cerro Prieto geothermal field

The Cerro Prieto geothermal field is located in the Mexicali Valley which is characterized morphologically by the presence of the elongated Cucapah range striking predominantly NW-SE. This range consists of Upper Cretaceous granite which has intruded and metamorphized the Cretaceous and/or Paleozoic sediments. Near the field is the rhyodacitic Cerro Prieto volcano which pierces through the Cenozoic deltaic sediments.We divided the deltaic sediments into two lithostratigraphic units: Unit A: Unconsolidated Quaternary deltaic sediments composed of clays, sands and gravels. Unit B: Consolidated Tertiary deltaic sediments composed of siltstone, shales and sandstones.Since the geothermal aquifer is intimately related to the structural behavior of the geological formations which are overlaid by unconsolidated deltaic sediments, it was necessary to apply different geophysical methods to understand the behavior of the consolidated sediments and the basement.Based upon geophysical surveys and the wells completed to date, a regional geologic model and several cross-sections of the Cerro Prieto field have been developed.Two fault systems have been defined. The principal one, which we called the Cerro Prieto system, has a predominantly NW-SE strike, parallel to important faults such as the San Andreas, San Jacinto, Cucapah, Imperial etc.Normal to the NW-SE system are faults with predominantly SW-NE strikes, which we designated the Volcano system. At depth, these two systems have apparently created a step-faulted horst and graben structure striking NW-SE with its eastern and western sides out.A number of cross-sections were made based on petrographic analyses of cuttings and core samples. These sections confirm the existence of two fault systems passing through the geothermal field, which complicates the structural interpretation of the field.The geologic sections based on the petrographic analysis of rock samples obtained from the drillings render very complicated a structural interpretation of the two fault systems which affect the geothermal field and which occur locally in great frequency.