Magneto-telluric (MT) surveys in a challenging environment at Lihir Island, PNG

Abstract

Since an initial pilot power development of 6 MW was commissioned in 2003, the geothermal resource at Lihir Island in Papua New Guinea has been progressively developed to power gold mining operations with present capacity at about 56 MW. A desire to further expand power generation led to the execution of Magneto-Telluric (MT) surveys of the system in 2004 and 2007. These surveys were conducted under challenging conditions that included active mining operations centred on the geothermal resource and extreme terrain across much of the survey area. There was reason to also suspect an extension of the geothermal resource to sea and this was reinforced by a survey of shallow seafloor temperatures made in 2007.
After successfully using 3D inversion in 2004 to model MT data collected on coastal areas, the 2007 survey therefore added more coastal measurements to improve coverage of the resource that extends under the Luise harbour. The coastal part of the survey included measurements in shallow water on the coastal platform at low tide. Despite the high temperatures and very high salinity of the reservoir waters, the main reservoir of the Lihir geothermal system presents a high resistivity feature lying beneath a typical layer of high conductivity due to argillic alteration in the shallow part of the system – the “clay cap”. The extent of the clay cap has helped indicate the extent of the system beyond the area that was originally proven by drilling and used to support the initial power development. Subsequent delineation drilling guided by the MT survey results has significantly extended the area of proven high temperature resource.
The experience at Lihir strengthens the concept that the conductive hydrothermal clay alteration cap is a strong geothermal indicator even with high salinity reservoir fluids and demonstrates that MT surveys are an effective tool for guiding step-out delineation drilling of fields already under development.