22
Sub-surface geology and hydrothermal alteration of wells LA-9D and LA-10D in the Aluto Langano geothermal field, Ethiopia. Selamawit Worku [email protected]
Sub-surface geology and hydrothermal alteration of wells LA-9D and LA-10D in
the Aluto Langano geothermal field, Ethiopia.
Selamawit Worku [email protected]
Introduction
Location of Aluto Langano
Surface geology
Objectives
Methodology
Results and discussion
Conclusion and recommendations
Outline
Ethiopian rift system:
Afar depression (triple
junction)
MERS
Geothermal resources are present
in MERS
More than 16 prospect areas have
potential for electricity production
ALGF is located within the AVC
The first geothermal field to be
developed
Introduction
Introduction cont...
Geology of AVC
AVC located In
CMERS
Covers 100 km2
The youngest flow
was dated 2 ka.
ELC, 2015
Introduction cont...
The Kefar River
basin and the
Munesa
highlands are
the major
recharge zones
for the
geothermal
reservoir.
Total mean
annual rainfall
is 725mm.
Introduction cont...
Objectives
Identify lithological units of the study wells LA-9D and
LA-10D.
Identify the hydrothermal alteration minerals and the
history of alteration, and to understand the thermal
evolution of the wells.
Identify and categorize the feed zones in the study
wells. To understand the permeability of the well.
Determine the distribution of temperature in the study
wells together with the neighbouring wells.
Methodology
Binocular Microscopy analysis
Identify rock lithology, alteration minerals, porosity etc.
Petrographic analysis
Confirm rock type(s), texture, porosity, vein fillings and
alteration minerals
XRD – Analysis
To Identifying clays minerals
ICP-OES analysis
Specifically to identify rock units.
Results and Discussion
• The study wells LA-9D and LA-10D are the first
directional wells in Ethiopia with a depth of
1920m and 1940m.
• The cutting samples were taken every five
meter during drilling.
Alteration zones LA-9D
Unaltered zone:
(0-120m)
Smectite zone:
(121-500m)
Illite/chlorite zone:
(501-1020m )
Illite/chlorite/epidote
zone: (1021-1920m)
Results and Discussion cont...
Alteration zones LA-10D
Unaltered zone:
(0-129m)
Smectite zone:
(130-500m)
Illite/chlorite zone:
(501-859m)
Illite/chlorite/epidote
zone: (860-1940m)
Results and Discussion cont...
Alteration zones
distribution in wells LA-3,
LA-4, LA-6, LA-7, LA-8, LA-
9D and LA-10D of Aluto
Langano geothermal field.
Results and Discussion cont...
Feed zones LA-9D
Identified on the basis of:
Temperature recovery profiles
Intensity of alteration
Abundance of calcite and pyrite
Lithological contacts and
Circulation losses.
Related to fault activity
Seven minor (760m, 830m, 940m,
1000m, 1140m, 1640m, 1800m) and
One Major feed zones (1350m)
Results and Discussion cont...
Feed zones LA-10D
Identified on the basis of:
Temperature recovery profiles
Intensity of alteration
Abundance of calcite and pyrite
Lithological contacts and
Circulation losses
Related to fault activity
Six minor (920m, 1160m, 1280m,
1320m, 1620m, 1760m ) and
One Major feed zones (700m)
Results and Discussion cont...
Main lithological units
in ALGF
From older to younger
formations:
Ignimbrite (2.3Ma)
Basalt (1.5 Ma)
Sediments (0.1Ma)
Thinner towards east
Fault activity
Silicic tuff and breccia
Permeability related with
lithological contacts and
faults.
Results and Discussion cont...
Hydrothermal
alteration zone vs
temperature
Unaltered zone : <50°C
Smectite zone :
75-150°C
Illite/chlorite zone:
180-250°C
Illite/chlorite/epidote:
>250°C
At LA-4, LA-5 and LA-7
<250°C
Results and Discussion cont...
Temperature distribution
at 800m.a.s.l
Up-flow zone within
NNE-SSW faulting
Temperature reversal
in LA-4 & LA-5
NNE-SSW & caldera
faults
Temperature reversal
in LA-7
Cold inflow
N-S and NNW-SSE
faults
Results and Discussion cont...
Conceptual model
of ALGF
Reservoir temp:
> 250°C
Max. T° recorded
335°C
The main upflow
zone controlled by
NNE-SSW faults
High temperature
geothermal fluid
travel along NNE-
SSW faults
Results and Discussion cont...
Conclusions • The major rock units identified in the study wells LA-9D and LA-
10D are pyroclastics, silicic tuff and breccia, sediment, rhyolite,
trachyte, basalt and ignimbrite.
• Scarcity of intermediate rock units are noted. Intermediate
compositions should be noted with caution as they may
possibly be caused by mixing of drill cuttings in the wells.
• Low to high temperature hydrothermal alteration minerals are
observed in the study wells.
• Four alteration zones are identified by the abundance and first
appearance of alteration minerals.
• Alteration minerals, depositional sequences and formation
temperature indicated that the wells (LA-9D and LA-10D) are
heating up.
Recommendations
• A more complete conceptual model can be achieved through
further incorporation of geochemical data and data from other
geophysical surveys e.g. seismic monitoring and gravity surveys.
• Detailed fluid inclusion studies need to be carried out,
specifically in wells LA-9D and LA-10D to supplement information
about alteration and measured formation temperatures, to
understand the evolution of the geothermal system.
• Detailed surface and borehole petrochemical studies in Aluto
Langano geothermal field are highly recommended to understand
the geochemical evolution of the rocks.
