| Summary: | Gemurah Besar is one of the blocks located in Lumut-Balai Geothermal
Prospect Area, South Sumatra Province. The objective of this research is to
characterize of subsurface geology and hydrothermal alteration in research area based
on data from wells of LMB 1-1, LMB 1-3, LMB 3-1 and LMB 3-3 are located within
the Gemurah Besar Block.
The research method is focused on the petrographic study and x-ray
diffraction (XRD) analysis of cores and cutting samples from research wells. 88
samples of core and cutting rocks have been selected for petrographic study and 41
samples of rocks of cutting sample have been selected for x-ray diffraction analysis.
Secondary data will be used is the well temperature data at this time and gross
permeability test.
The results showed that subsurface lithology composing research area is
lapilli-stone, lapilli tuff and andesite. It has weakly � very strongly altered (20 � 95%)
and shows the increased alteration intensity as the depth increases. Based on the
distribution of hydrothermal alteration mineral formed in certain depth, zonation for
hydrothermal alteration may then be made in each well: Smectite-Cristobalite
Alteration Zone, Smectite-Quartz Alteration Zone, Illite/Smectite-Chlorite-Quartz
Alteration Zone, and Illite±Illite/Smectite-Chlorite-Epidote Alteration Zone.
Subsurface temperature shows the presence of decreased temperature
(cooling) from paleotemperature to current temperature. Subsurface permeability
shows the presence of relatively good permeability zone. Subsurface fluid is
dominated by chloride of neutral pH. It is observed from the presence of alteration
minerals that can be used as an indicator of temperature, permeability and fluid type.
Geothermal systems in the research area is composed of cap rock on smectitecristobalite
alteration zones, smectite-quartz alteration zones and illite/smectitechlorite-
quartz alteration zones
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