The Geothermal Potential of Saipan, CNMI

Abstract

Saipan is the largest of the fourteen islands of the Commonwealth of the Northern Mariana Islands (CNMI) and is located in the Western Pacific Ocean. To the south is the island of Guam, which is part of the same chain of islands. Volcanics form the core of Saipan and are of Eocene Age (approximately 41 Ma) so the island has not been previously considered to have geothermal potential. Preliminary structural mapping of Saipan indicates major faults in a NE/ SW trend along both the western and eastern highlands paralleling the axis of the island. The faults splay-out creating a complex set of fault intersections that are possible zones of high permeability and deep circulation. Mapping and drilling of water wells on the western side of the island resulted in elevated temperatures, so a geothermal investigation was requested by the government of the CMNI, with field work completed in March 2008. Temperature logging of wells in Saipan recorded a bottom-hole temperature of 33.8°C at 125 m and a thermal gradient of 50 °C/km. The general Saipan Island background temperature is approximately 28.5°C. Surface water samples were collected from the lake and springs with waters varying from brackish EC values of 3680 ms/cm with high chloride, sodium, sulfate and magnesium levels to fresh water with EC values between 465 and 740 ms/cm. An elevated silicate concentration of 46.3 mg/l could be related to a larger flow path or deeper circulation within the volcanic sequence.