Characterisation of the Southern Negros Geothermal Field, Philippines

The Southern Negros Geothermal Production Field (SNGPF), situated in the municipality of Valencia, Negros Oriental, Negros Island, Philippines, belongs to The Energy Development Corporations's Negros Island Geothermal Business Unit. Covering approximately 64 000 hectares, it has a total installed capacity of 192.5 MW.

Geologically, the area lies within the tectonically active Philippine archipelago. The country is characterized by a complex of subduction zones, collision belts and marginal basins that define the Philippine Mobile Belt (PMB). This mobile belt is bordered by the east-dipping Manila, Negros, and Cotabato Trenches to the west and the Philippine and East Luzon Trough to the east. The geology of the area is dominanted by volcanic rocks with occasional sedimentary and intrusive units.

To date, a total of 80 geothermal wells have been drilled, including 51 production and 25 reinjection wells. Several geological investigations have been undertaken since exploration began in 1973. This includes a number of structural mapping campaigns from 1973 to 1996 by different teams of in-house geologists. However, many of these works were sector-specific (the field is divided into four sectors). Currently, no comprehensive structural analysis has been undertaken that encompasses the entire geothermal field.

The primary objective of this research is to improve fault targeting by increasing the chance that permeable structures will be intersected in future geothermal wells, using the SNGPF as a case study. Specifically, the project aims to achieve the following:

  1. characterise selected faults through their surface manifestations within the SNGPF
  2. identify then characterise these structures in the subsurface (0 – 2500 m depth) using the core samples of the drilled wells in the project
  3. compare and contrast the surface and subsurface characteristics of the selected structures and fracture fill histories to understand their geological relationships and filling histories, with emphasis on permeability
  4. construct a multiscale structural model for the area

The structural model will serve as the ultimate guide in determining which faults to intersect and which ones to avoid as drilling progresses.

The intention is that the methodology and results of this study will be applied in exploratory geothermal fields from equivalent geothermal settings, envisioning increased intersection of permeable structures during drilling, based entirely on surface and well core structural data.


 

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