BEYOND BOUNDARIES: EXPLORING INTERPRETATIONS FROM CORRELATION OF RESISTIVITY WITH VARIOUS RESERVOIR DATA IN MT. APO GEOTHERMAL AREA

Abstract

Magnetotelluric (MT) method is one of the most used resistivity methods in geothermal industry. Due to its extensive investigation depth, it is commonly used for defining the reservoir boundaries. In 2017, an MT survey was conducted in Mt. Apo geothermal field (MGPF) with the aim of updating its resistivity model. A total of 170 MT soundings were subjected to 1D and 2D inversion modeling. The 1D approach presented a conservative interpretation at reliable depths while 2D inversion provided reservoir characterization at further depths using statistical methods. The typical resistivity signature exhibited by volcanic-hosted geothermal areas in the Philippines shows a three layered resistivity structure composed of a continuous updoming low resistivity layer (<10?m) in between higher resistivity layers with the intermediate layer commonly associated with the reservoir’s clay cap. The recent Mt. Apo survey revealed an atypical feature not previously identified. It is characterized as a NE-trending moderately resistive anomaly that appears to cut through the supposedly continuous conductive layer of the reservoir. The anomaly appears in both 1D and 2D inversion models. This paper discusses the various interpretations of this resistivity anomaly as correlated with local structural geology, regional tectonics and well data revealing spatial association with acid zones and structural trends and potentially indicating an unidentified regional structure.