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 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 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 Kibiro SE shore of Lake Albert - Ngozi Part of the Rungwe Volcanic Province at intersection of 3 rifts Ngozi Source: Chorowisz, 2005 5
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 7
Kibiro – overview - To the SE, the geology is dominated by Pre-Cambrian crystalline basement: granitic gneisses, schists, quartzites Kibiro - 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 Source: Ring, 2008 8
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? 10
Kibiro – heat source options NW SE Uganda DRC Kibiro hot springs 1500 m Rift basin sediments Base Kaiso ? ? Pre-Cambrian Pre-Cambrian Base Kisegi Basement Basement Pre-Rift ? Basement Pre-Cambrian 5 km Basement Based on Karp et al (2012) 11
2006: DGSM and ISOR Kibiro – TG wells drilled 6 TG wells 12
Kibiro – results of TG wells Location (WGS84 Maximum Zone 36 N) Elevation Depth Thermal gradient Well No. temperature ( C/km) (masl) (m) ( C) 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 - 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 13
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 H 2 S 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 14
Kibiro – other manifestations - Sulphur deposits indicate that some H 2 S 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 Kibiro Source: Tullow, 2007 16
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 Note: Seismic Line 57 tied to Well Waki-B1 in footwall of Butiaba Fault for stratigraphy Source: Karp et al, 2012 17
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 Tectonic history source: Delvaux & Barth, 2010 18
Kibiro – current stress regime - Normal faulting with WNW – ESE extension (based on focal mechanisms) - SHmax direction is NNE-SSW Kibiro 19
Kibiro – influence of faults - N. Toro Bunyoro Fault - Butiaba Fault - Accommodation and transfer zones = increased fracture density - Deformation of upper crust between faults Kibiro influenced drainage and sedimentation - Minor faults intersect Toro Bunyoro Fault at Kibiro Source: Karp et al, 2012 20
Kibiro – minor fault intersections Kachuru fault – NNE-SSW Kitawe fault – WNW-ESE 21
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. 22
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: 24
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 25
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) 26
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