Magnetotelluric- and Gravity-defined Geophysical Structure of Ultramafic hosted Geothermal Prospects: Lobi and Mahagnao, Central Leyte, Philippines

David M. Rigor, Jr., Domingo B. Layugan and Sylvia G. Ramos

PNOC Energy Development Corporation, Energy Center, Merrit Road, Taguig City, Metro Manila, Philippines

Abstract

The resistivity anomaly delineated by magnetotelluric survey in Mahagnao reflects both the upflow region and the shallow ultramafic basement as confirmed by subsurface data obtained from well MH-1D. Based on the recent magnetotelluric (MT) data, the size of the potential geothermal resource was reduced from the previous estimate of 5 km² to <3 km². The resource is possibly confined within the shallow, ~500m-thick, relatively permeable sediments where temperatures between 180 and 200⁰C were measured. High bottomhole temperature of ~280⁰C was encountered in MH-1D, but was however recorded within the impermeable Leyte Ultramafics. All these characteristics downgraded the economic viability of the prospect.

Well CL-1D was drilled to target the postulated upflow zone beneath Mt. Lobi as indicated by the elevated resistive basement defined by MT. However, this resistive basement turned out to be the cold (~150⁰C at 1600m depth), impermeable Leyte Ultramafics. The absence of hot fluid circulation within the basement rocks led to the premature drilling termination of the well.

The resistivity and gravity structure of Lobi and Mahagnao clearly reflects the stratigraphic sequence underlying the Lobi-Mahagnao area. The moderately high resistivity surface layer coincides with the upper Plio-Pleistocene volcanic pile, the conductive layer represents the middle Miocene-Pliocene sedimentary sequence, while the resistive basement outlines the impermeable Leyte Ultramafics.

The mapped broad gravity high anomaly encompasses almost the entire area beneath Central Leyte where the Lobi and Mahagnao geothermal prospects are located. Drilling anywhere within the high resistivity anomaly beneath these prospects will most likely be non-productive due to the presence of shallow, dense and poorly permeable Leyte Ultramafic basement.