Cerro Prieto Case History: Use of Wireline Logs to Characterize a Geothermal Reservoir

The Cerro Prieto geothermal field is located in Baja California, Mexico, in the, Salton Trough-a rift basin filled mainly with Colorado River sediments. A comprehensive wireline log analysis was undertaken as part of a multidisciplinary study of this geothermal system. It establishedthe physical properties of the various sedimentary units;the depositional environment and hydrothermal alteration of the units;the location, attitude, and displacement of faults; andthe subsurface circulation of the geothermal fluids. Presented are the methodology that was used and the application of the results to further exploration and development of this high-temperature geothermal resource. Introduction The liquid-dominated Cerro Prieto geothermal field is located in the sediment-filled Mexicali Valley of Baja California, Mexico, about 20 miles [30 km] south of the U.S. border (Fig. 1). More than 100 deep exploration and development wells have been drilled in the area (Fig. 2), a few reaching crystalline basement. Analysis of the vast amount of data collected from these wells has given us a good understanding of the geologic characteristics of this high-temperature (up to 680F [360C]) geothermal resource. The exploration effort at Cerro Prieto is summarized in an earlier paper. paper. The purpose of this paper is to discuss the wireline log analysis that led tothe development of geologic and hydrogeologic models of the field,an understanding of the depositional environment of some of the sedimentary units identified in the subsurface, andthe identification of postdepositional changes in these units. These studies have postdepositional changes in these units. These studies have allowed us to determine the variations in porosity, permeability, thickness, and lateral continuity of the permeable (and less permeable) layers in the system-crucial parameters for the design permeable) layers in the system-crucial parameters for the design of drilling and completion of new wells and for the development of a reservoir management plan. Geologic Setting and Recent History of the Area The Mexicali Valley is part of the Salton Trough, an actively developing structural depression that resulted from tectonic activity that has created a series of spreading centers and transform faults that link the East Pacific Rise to the San Andreas fault system. The Cerro Prieto field is associated with one of these spreading centers, where the crust is being pulled apart by right-lateral strikeslip movement along the Cerro Prieto and Imperial faults (Fig. 3). During the early Pliocene, the current configuration of the Gulf of California began to develop by major crustal extension, which split Baja California from the Mexican mainland. At that time, the waters of the Gulf of California extended northward to about the Salton Sea area. The progradation of the Colorado River delta into the Cerro Prieto area began in the mid- to late Pliocene. The southwesterly advance of the delta was essentially complete by the late Pliocene. This resulted in the conversion of the Salton basin to a nonmarine depositional basin. By the mid-Pleistocene, the marine connection between the Gulf of California to the south and the Imperial Valley to the north was severed. Geologic and Hydrogeologic Models of Cerro Prieto The subsurface stratigraphy at Cerro Prieto is characterized by vertical and lateral variations in lithofacies. The lithologic column consists ofan upper part of unconsolidated and semiconsolidated sediments (Unit A) that is mainly sands, silts, and clays, anda lower part of consolidated sediments (Unit B) that is mainly sandstones and part of consolidated sediments (Unit B) that is mainly sandstones and shales. The hydrothermal alteration of the deeper layers and the existence of hydrothermal mineral zonation around the reservoir have been documented by careful mineralogic studies of well cuttings and cores and by analysis of wireline well logs. Following the general approach of Lyons and van de Kamp, Halfman et al. used wireline and lithologic log data to delineate and to classify the lithologic sequences penetrated by the wells into three lithofacies groups: sandstone, sandy shale, and shale (Figs. 4 and 5A). The sandstone beds basicallyare thick, permeable, and well-defined (with some interbedded shales) in the sandstone group,are thinner and less permeable (with a higher percentage of intercalated shales) in the sandy-shale group, andare even thinner ( less than 10 ft [ less than 3 m]) in the shale group (e.g., Fig. 4). The main geophysical logs used to develop this model include gamma ray (GR), spontaneous potential (SP), deep induction (ILD), and compensated formation density (RHOB). JPT P. 793