Petrological and mineralogical characteristics of “red material layer” in Patuha geothermal field, Bandung, West Java

Abstract

Abstract Patuha Geothermal Field is located in Ciwidey, Bandung, West Java Province, with a total resource potential of approximately 400 Mwe (Megawatt electric). With such potential, seven power plants have been developed in Unit-1, capable of producing 60 MWe of electricity, and are currently being developed in stages for Unit-2. In the development of the Unit-2 power plant, there was a problem where collapse occurred around the drill hole area, especially since the color of the material that came out during drilling was red. We call this material the red material layer (a layer composed of red material). This problem also occurs in Unit-1. This research aims to characterize the type of red material layer to see why this problem occurs in this layer. We studied the petrological and mineralogical characteristics of the red material layer by cutting rock samples from six drill holes in Unit-1. The analytical methods used were megascopic observation, petrography, and XRD (X-ray diffraction) to determine the mineral content texture and identify the material’s origin, including clay minerals, alteration, and oxidation mineral aggregates. The petrological and mineralogical analysis found that the red-colored material is dominated by hematite, with some samples containing magnetite. We found four characteristic types of red layer material and four types of textures that appeared in iron oxide minerals: rim, vein, matrix replacements, and fragment replacements. We conclude from the visible vein texture that oxidation occurs due to oxygen penetration through gaps of a fault/fracture/joint. This oxidizes the area around the vein and then undergoes hydrothermal change. Meanwhile, those that do not have a vein texture are considered material resulting from an oxidation process on the surface, which is then deposited and undergoes hydrothermal change to contain oxide minerals such as hematite and magnetite. Apart from that, XRD observations also found high amounts of clay minerals such as chlorite, smectite, kaolinite, illite-smectite mixed layers, and chlorite-smectite mixed layers. The presence of these clay minerals is thought to cause a collapse in the area around the drill hole because its swelling and exfoliation properties are reactive to fluids, especially drilling fluids.