Geological characteristics of Kibiro, Uganda & Ngozi- Songwe, - - PowerPoint PPT Presentation

Geological characteristics of Kibiro, Uganda & Ngozi- Songwe, Tanzania geothermal prospects

Western Branch of EARS Workshop, Kigali, Rwanda March 9-11, 2016

Kenneth B. (Keg) Alexander, Geologist Middle Earth Geoscience, Auckland, New Zealand keg@middleearthgeo.co.nz

Presentation agenda

• Project background and organization
• Objectives
• Areas of interest and regional setting (Kibiro and Ngozi)
• For each project:
• Summary of previous geological surveys
• Current approach and focus
• Preliminary understanding of geological conceptual model

2

Project background and

• rganization
• In 2015, requests for technical assistance submitted by:
• Uganda Directorate of Geological Survey and Mines (DGSM)
• Tanzania Geothermal Development Company (TGDC)
• Funding by African Rift Geothermal Development (ARGeo)
• Implementation by UN Environment Programme (UNEP)
• Technical advice and equipment provided by Geothermal

Development Company of Kenya (GDC)

• 3 advisors: Bill Cumming, Luigi Marini, Keg Alexander

3

Project objectives

• Develop an integrated conceptual model for each site

using geology, geochemistry, and geophysics

• Review of existing surveys
• Collection of new data
• What are the potential heat sources and fluid pathways?
• Recommend locations and targets for exploration wells
• Tentative project completion date: May 2016

4

Areas of interest

• Kibiro
• SE shore of Lake

Albert

• Ngozi
• Part of the

Rungwe Volcanic Province at intersection of 3 rifts

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Kibiro Ngozi

Source: Chorowisz, 2005

Geological survey focus areas

• Thermal manifestations
• Evidence of a geothermal system
• Structural geologic setting
• Evidence of potential fluid pathways
• Evaluation of current stress regime
• Petrography and XRD
• Evidence of hydrothermal alteration
• Recent or relict?
• Hydrogeology
• Understanding groundwater flow, and recharge and discharge areas

6

Kibiro – overview

• Kibiro hot springs located in sediments at foot of eastern

escarpment of Lake Albert graben (northernmost rift basin)

• Extension related normal faulting has resulted in

significant topographic features – footwall rises more than 350 m above rift basin

• NE-SW escarpment divides the

study area into two entirely different geological environments

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Kibiro –

• verview

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Source: Ring, 2008

Kibiro

• To the SE, the geology is

dominated by Pre-Cambrian crystalline basement: granitic gneisses, schists, quartzites

• To the NW, the rift valley consists
• f sequences of sediments, up to

5.5 km thick

• No volcanic rocks near Kibiro;

closest location are Rwenzori Mts

Kibiro –

• 9

Kibiro – previous geological studies

• 1993: DGSM and UNDP. Focus on geology and
• geochemistry. Heat source from rift basin sediments?
• 1999-2002: DGSM and IAEA. Focus on isotope hydrology.

Recharge from higher ground. Heat source from basement?

• 2004-2005: ICEIDA. Focus on geology, TEM, gravity and
• magnetics. Low resistivity and gravity anomalies point to

possible heat source in basement to SE?

• 2012-2013: DGSM. Focus on Toro-Bunyoro Fault, thermal

manifestations, and mafic dykes. Upflow through main fault scarp?

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Kibiro – heat source

• ptions

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NW SE DRC Uganda

5 km 1500 m Base Kisegi Base Kaiso Pre-Rift Basement Based on Karp et al (2012)

Kibiro hot springs

Rift basin sediments Pre-Cambrian Basement Pre-Cambrian Basement Pre-Cambrian Basement

? ? ?

Kibiro – TG wells

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2006: DGSM and ISOR drilled 6 TG wells

Kibiro – results of TG wells

• Max T = 35.1 °C
• Thermal gradients low; continental average: 25 – 30 °C/km
• No hydrothermal alteration observed in any cuttings
• Low resistivity anomalies not caused by heat anomaly

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Well No. Location (WGS84 Zone 36 N) Elevation (masl) Depth (m) Maximum temperature (C) Thermal gradient (C/km) Easting Northing KIBH-1 305960 179150 1039 300 29.4 16.0 KIBH-2 306620 180140 1023 300 29.2 16.0 KIBH-3 303820 179200 959 300 29.7 16.0 KIBH-4 306335 184074 914 262 35.0 27.3 to 31.3 KIBH-5 305420 183420 931 300 35.1 22.0 KIBH-6 304800 179110 1000 290 29.4 16.0

Kibiro – thermal manifestations

• Kibiro hot springs
• Main area is Mukabiga located near intersecting faults at base of

escarpment

• Max T = 86.5 °C, total flow = 4 L/s, gas bubbling and strong H2S smell, oily odor
• Second group of springs downstream (Mwibanda)
• Range of T’s: 33 to 72 °C, total flow = 2.5 L/s
• Muntere springs occur in pits excavated for salt production
• Max T = 39.5 °C
• No other active hot springs known to exist in Kibiro study

area nor along southeastern margin of Lake Albert graben

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Kibiro – other manifestations

• Sulphur deposits indicate that some H2S is being released
• No active fumaroles
• Deposition of secondary minerals of uncertain origin in Kachuru and Butiaba
• Surface alteration
• Limited areas of argillic alteration (major clay type is smectite)
• Smectite can form due to weathering or low T hydrothermal alteration
• New samples currently being analyzed
• Oil seeps
• Petroleum-bearing source rocks present
• Migration pathways to surface

15

Kibiro – petroleum in Lake Albert basin

• Potential source rock kitchen (area of petroleum

generation) on DRC side of Lake Albert

• Nearest oil well is 15 km to NE (Taitai-1)
• - Max T of 61 °C at depth of 971 m

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Source: Tullow, 2007

Kibiro

Kibiro – structural geologic setting

• Lake Albert has undergone substantial tectonic

movements and thick sediments have been deposited

• Full, asymmetrical graben; shallow lake, max depth = 58 m

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Source: Karp et al, 2012 Note: Seismic Line 57 tied to Well Waki-B1 in footwall of Butiaba Fault for stratigraphy

Kibiro – structural geologic setting (cont.)

• Recent tectonic history
• Rifting originated in late Oligocene or Early Miocene (~23 Ma)
• Compression during mid-Miocene (~15 Ma)
• Second phase of rifting during Pliocene (~2.6 to 5.3 Ma)
• Compression during the Pleistocene (~0.01 to 2.6 Ma)
• Present day: normal faulting regime (extensional)
• Lake Albert rift developed as a pull-apart in a sinistral-

transtensional environment

• Complicates interpretation due to different stress regimes

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Tectonic history source: Delvaux & Barth, 2010

Kibiro – current stress regime

• Normal faulting with WNW–ESE extension (based on focal

mechanisms)

• SHmax direction is NNE-SSW

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Kibiro

Kibiro – influence of faults

• N. Toro Bunyoro Fault
• Butiaba Fault
• Accommodation and

transfer zones = increased fracture density

• Deformation of upper

crust between faults influenced drainage and sedimentation

• Minor faults intersect

Toro Bunyoro Fault at Kibiro

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Source: Karp et al, 2012

Kibiro

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Kibiro – minor fault intersections Kachuru fault – NNE-SSW Kitawe fault – WNW-ESE

Kibiro – hydrogeology

• Regional uplift of escarpment has reversed drainage

systems and helped form Lakes Victoria and Kyoga

• Lake elevation has decreased ~100 m over past 2,000 yrs
• Recharge from area above Rift escarpment, through

fractured metamorphics with discharge into lake

• Well KIBH-4 (~1.1 km SSE of hot springs) encountered high

permeability in fractured rock at depth of 114 m (800 masl)

• Kibiro springs (639 masl) mix of geothermal water and

shallow brackish water. Likely separate hydraulic connection from groundwater in metamorphics.

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Kibiro – status of current geological survey

• DGSM conducting field surveys with focus on Kachuru and

Kitawe faults and surface alteration – results pending

• GDC of Kenya analyzing 11 rock samples for thin section

petrography and XRD - results pending

• Compilation and interpretation of groundwater data -

results pending

• Meeting with DGSM and GDC in Entebbe to evaluate

recent field results (14-18 March 2016)

• Integration of data to prepare geologic conceptual model

23

Kibiro – preliminary geological conceptual model

• Lake Albert rift basin
• Rifting led to normal graben faulting and thinner crust
• Similar to Basin & Range setting
• Active and deep (>30 km) seismicity beneath Lake Albert
• Earthquakes >M4.5

since 1900:

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Kibiro – preliminary geological conceptual model (cont.)

• Changes in regional stress regime over past 20 Ma
• Episodes of rifting and compression
• Development of pull apart basin with deep-seated, parallel boundary faults
• Kibiro hot springs
• Located at intersection of faults
• Only springs along SE margin Lake Albert rift basin
• Hydrologic system separate from groundwater flow in metamorphics
• Oil seeps show fluid migration through nearby sediments

and up N. Toro Bunyoro Fault

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Kibiro – preliminary geological conceptual model (cont.)

• Fluids gain heat through deep circulation beneath Lake

Albert

• Heated fluids rise to base of sediments in basin and then

up deep-seated rift border fault (possibly Butiaba Fault)

• Heated fluids flow updip through sediments towards

Bunyoro Fault

• Discharge at Kibiro springs at zone of weakness

(intersection of Bunyoro Fault and minor faults)

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Ngozi – overview

• Ngozi volcano is part of the Rungwe Volcanic Province in

southern Tanzania

• Located at triple rift junction of NW-SE trending Rukwa

and Malawi Rifts and the NE-SW trending Usangu Basin

• Primary geothermal features in the study area:
• Ngozi caldera lake – thermal water discharges up to 89 °C on lake bottom
• Songwe River thermal area – hot springs located >40 km WNW of Ngozi,

numerous small and large springs, max T = 75-80 °C, total flow estimated to be 75 L/s

27

Ngozi – overview (cont.)

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Songwe

Source: Fontijn et al, 2012

33.5°E

Ngozi – lithology

• Travertine – active deposition over past 360 ka
• Miocene – Recent (past 10 ± 2 Ma) Rungwe volcanics
• Miocene – Recent lacustrine and fluvial sediments
• Cretaceous (~66-145 Ma) Red Sandstone Group formed in

rift valleys

• Permian – Triassic (~200-300 Ma) Karoo conglomerates in

rift valleys

• Pre-Cambrian (>600 Ma) metamorphic basement (exposed

to NW and SE of RVP)

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Ngozi – previous geological studies

• Tanzanian government (MEM, GST, and TANESCO) in

cooperation with the German Institute for Geosciences and Natural Resources (BGR) completed:

• 2006-09: GEOTHERM Phase I
• Comprehensive pre-feasibility study
• Surveys identified presence of active faults, young volcanic heat source, and a reservoir in basal

volcanics, above the basement metamorphics

• 2009-13: GEOTHERM Phase II
• Focus on geophysics (MT and TEM)
• Bathymetric survey of Lake Ngozi showed high bottom temperatures
• The conceptual model indicated a >200 °C geothermal

reservoir beneath Ngozi with outflows mainly to Songwe

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Ngozi – thermal manifestations

• Thermal manifestations consist of hot and warm springs, and

cold gas emissions (e.g., intense bubbling at Shiwaga)

• Can be divided by elevation and locality
• Warm springs and cold gas (CO2) emissions at higher elevations on flanks of

Ngozi

• Significant hot spring discharges at lower elevations distal from RVP (Songwe to

NW and Kilambo to SE)

• No fumaroles are known to occur
• No thermal manifestations within Ngozi caldera
• Very little surface alteration
• Fluid flow controlled by fracture permeability along active faults

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Ngozi – thermal manifestations (Songwe)

• Songwe River thermal area
• Numerous springs discharge neutral, Na-HCO3 water between 50 and 80 °C
• Spring locations fault-controlled in general NNW-SSE alignment
• Spring elevations at Songwe are 30 to 50 m higher than Songwe River
• Total discharge estimated at 50 to 75 L/s
• Songwe travertine deposits
• Springs discharge through extensive travertine deposits, 5 to 70 m thick

covering an area of about 13 km2

• Travertine overlies Cretaceous Red Sandstone Group
• Travertine formation possibly due to hot water circulating through 100 Ma,

carbonatite-rich volcanic rocks

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Songwe – springs

• Songwe River

Thermal Area springs

• Songwe (60 – 75 °C)
• Madibira (~55 °C)
• Kaguri (~48 – 67 °C)
• Ilatile (65 – 80 °C)
• Over time springs

and travertine deposition moving toward NW

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Adapted from: Roberts et al, 2004

Songwe Madibira Kaguri Ilatile

RVP – volcanic activity

• Late Cretaceous (~70-100 Ma) with Alkali-carbonatitic

magmatism (emplacement of Panda Hill and Songwe scarp)

• RVP alkaline volcanism began during Miocene (10 ± 2 Ma)
• RVP volcanoes have been active during Holocene:
• Rungwe – 1 Plinian eruption ~4 ka and >7 explosive eruptions in last ~4 ka
• Ngozi – 2 Plinian eruptions in last ~10 ka (Kitulo Pumice and <1 ka Ngozi

Tuff)

• Kiejo – Effusive eruptions ca. 200 years ago
• Ngozi lavas are mainly phonolitic-trachytic with some

alkali-basaltic composition at lower elevations

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Source: Delalande-Le Moullic et al (2015)

Ngozi – intrusive carbonatite outcrops

• Panda Hill (113 ± 6 Ma)
• Songwe scarp (100 ± 10 Ma)
• Dyke structure 6 – 30 m thick along Mbeya Front fault
• Outcrops for ~20 km along base of 1000-m high scarp between

metamorphics of Mbeya Range to NE and Songwe river valley to SW

• Emplaced in a zone of weakness (likely prior rift faulting) during period of

normal faulting regime

• Implies deep fault connection with near mantle source
• No thermal features along Songwe scarp (Mahombe springs at N end of

Mbeya Front fault is N of carbonatite outcrop)

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Source: Brown, 1964

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Songwe

Ngozi – structural geologic setting

• Development of Western

resulted in major transfer zones between normal rift faults

• Field investigations show that

faulting in the Rukwa Rift during the Late Cenozoic are normal

• Regime changes rapidly into

strike-slip faulting in the Rungwe volcanic area at the junction between the Rukwa and Malawi Rifts

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Source: Corti, 2012

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Source: Delvaux et al, 2010

Ngozi – current stress regime

• Since the Middle Pleistocene, the stress regime is dextral

strike-slip faulting along (partly pre-existing) NW-SE to N-S faults

• Compressional stress regime with SHmax in ENE-WSW

direction

• Focal mechanisms consistent with compressional regime

and associated strike-slip faulting within RVP

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Source: Fontijn et al, 2012

Regional stress regime

• Albertine Rift

(#11)

• Mbeya

(#16)

40

Source: Delvaux & Barth, 2010

Ngozi – Mbeya Front- Mbaka Faults

• Mbeya Front fault to NW of RVP, Mbaka Fault to SE
• Current tectonic stress regime implies that faults are under

E-W to ENE-WSW horizontal compression

• Dominant structural features, but are they connected?
• Near perfect alignment
• Numerous springs and maars along Mbaka Fault; none along Mbeya Front
• Carbonatite not found along Mbaka Fault
• Mbeya Front fault – deep-seated, substantial throw, >1000-m scarp
• Regardless of connection, Mbeya Front fault is likely a

barrier to horizontal groundwater flow

41

Ngozi – hydrogeology

• Ngozi Caldera lake has no surface inlets or outlets
• Water enters via precipitation or groundwater flow and

discharges by groundwater outflow and evaporation

• Surface drainage indicative of groundwater flow direction
• West side of Ngozi Caldera lake drains toward the south
• Drainage to NW likely captured by Mbaliza River watershed
• There is a hydrologic divide between Mbaliza and Songwe Rivers
• Therefore, outflow from Ngozi is unlikely to reach Songwe

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Source: Kraml et al, 2014

Ngozi – status of current geological survey

• TGDC currently evaluating data collected during recent field

surveys – results pending

• Lithic sample assessment (26 lithic samples)
• Interpretation of groundwater data
• Evaluation of depth to basement at Ngozi
• Updated geological and structural map
• GDC of Kenya analyzing 50 rock samples for thin section

petrography and XRD – final results pending

• Meeting with TGDC and GDC in Mbeya to evaluate recent field

results (20-25 March 2016)

• Integration of data to prepare geologic conceptual model

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Ngozi – preliminary geological conceptual model

• Ngozi
• Young volcanic heat source; thermal discharge at lake bottom
• Rungwe volcanics directly overlie metamorphic basement rock
• Geothermal reservoir likely within Older Volcanics on north flank of Ngozi
• Outflow likely to NW toward Mbeya
• Structural and hydrologic barriers to flow from Ngozi to Songwe
• Songwe
• Separate, fault-controlled geothermal system
• Fluids gain heat through deep circulation (likely through SW margin fault of

Rukwa Rift)

• Recharge likely through fractured metamorphics of Chumwa Range to SW

with possible input from SSE (Panda Hill)

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Thanks to Geology Teams at DGSM, TGDC and GDC! Questions?

Keg@middleearthgeo.co.nz

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