geological characteristics of kibiro, uganda & ngozi- songwe,...
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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
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
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Project background and organization
- 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
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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
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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
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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 – overview
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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 of sequences of sediments, up to 5.5 km thick - No volcanic rocks near Kibiro; closest location are Rwenzori Mts
Kibiro – -
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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 options
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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
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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
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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
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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)
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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
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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?
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