Lobi and Mahagnao: Geothermal Prospects in an Ultramafic Setting Central Leyte, Philippines

Abstract

The Lobi and Mahagnao Geothermal Prospects are located in the center of Leyte Island, Philippines. Starting in 1980-1982, geological, geochemical, and geophysical surveys were conducted in central Leyte to evaluate viability of prospect areas found southeast of the successful Tongonan Geothermal Field.
Results of surface exploration studies indicated hotter reservoir temperatures in Mahagnao prospect in comparison to Lobi. Hence, in 1990-1991, two exploration wells (MH-1D and MH-2D) were drilled in Mahagnao to confirm the postulated upflow beneath Mahagnao solfatara and domes. Well MH-1D, targeted towards the center of the resource was non-commercial despite high temperatures of ~280?C because of poor permeability of the Leyte Ultramafics. Well MH-2D was drilled westward but intersected low temperatures of ~165?C. Drilling results showed that the only exploitable block in Mahagnao lies beneath the volcanic domes covering an area of ~5 km2.
In 2001-2002, the Lobi prospect was re-evaluated by conducting a magnetotelluric (MT) survey and structural geologic studies. Based on MT results, an exploration well (CL-1D) was drilled in 2003 to probe the MT anomaly southwest of Mt. Lobi. Located ~15 km north of well MH-1D, well CL-1D was likewise non-productive because it intersected the Leyte Ultramafics below -300 m MSL resulting in poor permeability and absence of circulating hydrothermal fluids.
Exploration drilling in both Mahagnao and Lobi areas proved that commercial geothermal production is not viable in ultramafic-hosted reservoirs. Recent magnetotelluric survey mapped out extent of the impermeable Leyte Ultramafics as well as the distribution of sediments that can serve as possible permeable reservoir for a low-temperature geothermal resource.