||KOESTONO, H., PRASETYO, I.M., NUSANTARA, V. D.M., THAMRIN, M.H., KAMAH, M.Y
||Hydrothermal alteration mineralogy of Well Pad C, Hululais Geothermal Field, Bengkulu, Indonesia Koestono, H., Prasetyo, I.M., Nusantara, V. D.M., Thamrin, M.H., Kamah, M.Y. Pertamina Geothermal Energy Email: firstname.lastname@example.org Hululais geothermal prospect is located at Bengkulu Province, Indonesia. The preliminary geoscientific survey (geology, geochemistry and geophysics) were completed in 1994. PGE carried out advance MT resistivity survey to get better understanding of the Hululais geothermal system and provide assistance for well targeting. Geologically, the main prospect is consist of quarternary rock of breccia, andesite and tuff. The Hululais prospect is situated within the Semangko Fault System (SFS) that trends NW-SE and divided onto several fault segments. One of the exploration well namely Pad C was spuded in 2011. Generally, the wells encountered interstratified of tuff, breccia and andesite. The rocks may part of the Hululais and Bt. Resam products. The alteration minerals are smectite, chlorite, illite, serisite, zeolite, quartz, calcite, anhydrite, epidote,actinolite and biotite. There are 7 (seven) alteration zonations identified, which are: smectite, chlorite-smectite, illite-smectite, chlorite-epidote, illite-epidote, epidote-actinolite and actinolite-biotite. The permeability eveidences were marked by the deposition of alteration minerals in veins, such as quartz, pyrite, chlorite, wairakite, calcite and epidote. Those minerals coincide with the presence of circulation losses during drilling. Partial circulation losses started at depth 1861 mMD/1462 mVD, followed by the total circulation losses from 2776 mKU/1783 mKT until Total Depth (3001 mMD/ 1784 mVD) .