Hydrothermal Mineral Zones in the Geothermal Reservoir of Cerro Prieto

Abstract

Detailed petrologic studies completed to date on ditch cuttings and core from 23 wells in the Cerro Prieto field have led to recognition of regularly distributed prograde etamorphic mineral zones. The progressive changes in mineralogy exhibit a systematic relationship with reservoir temperature.
The Cerro Prieto reservoir consists of a series of sandstones, siltstones, and shales composing part of the Colorado River delta. The western part of the field contains relatively coarser sediments apparently derived from the Sierra de los Cucupahs, immediately to the west of Cerro Prieto. The most abundant detrital minerals in the ediments include quartz, feldspar, kaolinite, montmorillonite, illite, chlorite, mixed-layer clays, calcite, dolomite and iron hydroxides. Some of these minerals were also formed diagenetically.
The following progressive stages of postdepositional alteration in response to increasing temperature have been observed: (l) Diagenetic zone (low temperature), (2) illite--chlorite one (above ~ 150°C), (3) calc-aluminum silicate zone (above ~ 230°C) and the biotite zone above ~ 325°C). These zones are transitional to some degree and can be further subdivided ased on the appearance or disappearance of various minerals.
One immediate application of these studies is the ability, from a study of cuttings obtained during drilling of a well, to predict the temperatures which will be observed when the well is completed.