The link between clay composition and the location of a geothermal system reservoir: a case of Rantau Dedap geothermal system

The geothermal industry has extensively used clay minerals as indicators for changes in temperature and circulation zones in geothermal systems. The latter includes marking the extent of the reservoir section and permeability within the system. Methylene blue (MeB) stain test value and geophysical resistivity data are often the primary tools to locate the reservoir section, especially in the early geothermal exploration stage. The reservoir section is marked by delineating the encompassing clay cap based on the distribution of smectite clay, indicated by a higher MeB value and the existence of a conductive layer. In this paper, we compare the MeB and magnetotelluric (MT) resistivity values with the clay minerals content of rocks, among others, smectite. Whereas the identification of clay mineral composition has typically relied on X-ray diffraction (XRD), we used three analytical methods (i.e. non-imaging infrared spectroscopy (IRS), infrared imaging spectroscopy (IRIS), and XRD). Our research used extracted clay fraction of ground cuttings and the original cuttings taken from the Rantau Dedap geothermal system in Sumatra (Indonesia). The IRS and XRD analyses were done on the same clay fraction, first on air-dried fraction and then on the ethylene glycol treated fraction, while IRIS was done on the original cuttings. The clay minerals identification from the three analytical methods showed similar results. Then, the comparison with MeB test results showed that smectite has the most effect on this test results, followed by mixtures of smectite-chlorite and smectite-illite. The MT resistivity value is a function of clay mineral types and lithology. Different types of clay are associated with varying resistivity ranges; from lowest to highest are smectite, mixtures between smectite and chlorite/illite, kandites, illite and chlorite. Comparing clay composition and MeB and MT resistivity data has given a better understanding of the reservoir location in Rantau Dedap.