New Mexico Geological Society Annual Spring Meeting — Abstracts

Understanding New Mexico’s geothermal potential has been a high priority over the last decade due to increasing demand for renewable energy. Subsurface temperatures and groundwater flow in the southern San Luis Basin in the vicinity of Taos are evaluated in this study using 18 new and 19 published temperature profiles; chalcedony geothermometer estimates and published groundwater discharge temperatures are also included in this assessment. Thermal manifestations include well-known hot springs (33-41°C) in the center and along the southern margin of the basin and boreholes with high geothermal gradients (50-75°C/km) and high discharge temperatures (27-31 °C) on the western margin of the basin adjacent to the Tusas Mountains near Tres Piedras. This zone of elevated gradients extends eastward from Tres Piedras toward the Rio Grande, paralleling U.S. Highway 64. Geothermal gradients in the southern part of the basin south of the Rio Pueblo are generally low to average (24 to 32°C/km) and the thermal profiles are commonly disturbed by subsurface groundwater flow. Similarly, gradients north of the Rio Pueblo and east of the Rio Grande are 28 to 36°C/km, which are average values for the Rio Grande rift. In general, temperatures in wells north of the Rio Pueblo are warmer than those to the south. Geothermal gradients in the Miranda graben southwest of Ponce de Leon hot springs are elevated (32 to 58°C/km); again the gradients are disturbed by flowing groundwater. Many of the deep monitoring wells that are close to the Sangre de Cristo Mountain front on the east side of the basin are characterized by thermal profiles that suggest downward groundwater flow (i.e. recharge); many of these same wells also have indications of lateral flow of cold water, particularly in the basalt flows in Servilleta Formation. Two deep wells toward the center of the basin east of the Rio Grande that are proximal to faults have thermal profiles that are indicative of weak groundwater upflow. The cause of the elevated gradients on the western margin of the basin is uncertain and requires further investigation. Youthful basaltic cinder cones that erupted between 191±4 ka and 232±8 ka (Matthew Zimmerer, personal communication, 2017) are located about 30 km to the west of Tres Piedras, but the heat associated with these small-volume eruptions has long since dissipated. The wells with elevated geothermal gradients near Tres Piedras lie in a structural setting that is similar to that of the Ojo Caliente thermal springs to the south. Both areas are on the east margin of the Tusas Mountains. Although the fault that acts as the conduit for warm water at Ojo Caliente is obvious, similar faults near Tres Piedras have not yet been identified, likely because such structures are buried. Reservoir temperatures calculated using the silica concentrations of groundwater in the southern San Luis Basin are generally low (30-80°C). The chalcedony geothermometer estimates derived from warm wells near Tres Piedras are 90 to 110°C. Overall, the geothermal potential of this region is seemingly greatest along the western margin of the basin near Tres Piedras.