Hydrothermal Clay Minerals in the Ulubelu Geothermal Field, Lampung, Indonesia

Abstract

Hydrothermal reservoir minerals, such as clays, wairakite and epidote, occur in samples from wells UBL-01, UBL-02 and UBL-03 drilled in the Ulubelu geothermal field. Well UBL-01 encountered andesite lavas at depths from 680 to 1200 m and tuff at 520 m. The hydrothermal clays consist of chlorite, illite, kaolinite, and interlayered clay and These are mostly present in andesite now intensely altered. Moderately porous rocks (5 - 15%) in well UBL-02 consist of lithic tuffs, from 80 to 400 m, above andesite lavas at 420 to 609 m and lithic tuff at 520 m depth. The rocks have been strongly altered with alteration intensities in the range 70 to 90%. The hydrothermal minerals here are dominated by interlayered clays consisting of plus discrete chlorite, illite, kaolinite and possibly talc. Well UBL-03, at depths from 320 to 607 m, mostly encountered tuffbreccias with an intensity of alteration of about 75% and porosities around 15%. The deeper parts consist of dacite lava that is intensely altered, about with a porosity of about 20%. The hydrothermal minerals in rocks of this well are mostly the same as those in the two other wells but also present are wairakite and epidote. The hydrothermal minerals delineate fluid flow that is strongly controlled by formation permeability. The interlayered clay minerals record this process. Specific temperature ranges and fluid compositions are interpreted from the presence of the hydrothermal minerals.