GEOTHERMAL ENERGY POTENTIAL FOR LONGONOT PROSPECT, KENYA By Mariita N. O. Kenya Electricity Generating Company
PRESENTATION OUTLINE INTRODUCTION REGIONAL SETTING GEOLOGY GEOTHERMAL MANIFESTATIONS HYDROGEOLOGY GEOCHEMISTRY GEOPHYSICS CONCEPTUALIZED GEOTHERMAL MODEL CONCLUSIONS
REGIONAL SETTING
REGIONAL SETTING. Longonot Quaternary volcano in the Kenyan rift. 60 km NW of Nairobi. Olkaria west, L. Naivasha north, Suswa south and Kijabe hill east.
OBJECTIVES FOR THE INVESTIGATIONS AT LONGONOT PROSPECT To determine the geothermal potential at the volcano by studying the structural patterns, lithologic outcrops, volcanology, hydrogeology Geophysics and Geochemical signatures and geothermal manifestations. To determine whether a resource exists and if so propose sites for exploratory drilling. To develop a geothermal conceptual model of the area.
METHODOLOGY The methods used during the investigations included: Photogeology Ground geology -Field mapping Landsat imagery Geophysics, Geochemistry
Geological Setting The volcano is located close to the eastern escarpment where curved faults that mimic the volcano structure occur. Caldera collapse was succeeded by development of central volcano with crater on the caldera floor and a major NW-SE tectono-volcanic axis.
GEOLOGY Calderas ~11 km diameter The shield building magmatism (trachytic) started 0.2 Ma BP. Caldera collapse at 100,000 yr Post caldera activity produced central volcano at the edge of the caldera Youngest activity, dated about 200 yrs BP
LONGONOT GEOLOGICAL HISTORY LONGONOT PRODUCTS AGE (yrs BP) EVENT Shield volcanics >0.4 m Pre-caldera Lower trachyte ignimbrite 9150±110 Syn-caldera Trachyte pumice lapilli and ash Upper trachyte ignimbrite Base surge trachyte pumice and ash --- Post-caldera Trachyte pumice Trachyte ash Mixed basalt/trachyte lava --- Trachyte lava and interflow pumice Trachyte ash and pumice 3280 ±120 Mixed basalt trachyte lava Trachyte lava 180 ±50
VOLCANOLOGICAL INFERENCE The volcanological inference from Longonot data indicate tapping from a single large long lived magma chamber as the ICE ratios from all phases of volcanisms are very similar.
STRUCTURES Structures found in Longonot are: Tectono Volcanic alignments (TVA s) trending NNW-SSE and NW-SE Caldera wall Lineament Faults
LONGONOT STRUCTURAL MAP 9906 208 210 212 214 216 218 220 222 224 9906 9904 9904 LONGONOT 9902 9900 ` LONGONOT CRATER ERERI 9902 9900 Tectono Volcanic alignments 9898 9898 9896 KANARU 9896 9894 9894 9892 9892 9890 LEGEND Caldera wall Crater Fault AKIRA RANCH MAAI MAHIU RANCH 9890 9888 9886 Volcanic centers Main road Earth road Railway line Fumaroles LONGONOT EARTH SATTELITE EWASO KEDONG VALLEY 0 1 2KM 9888 9886 208 210 212 214 216 218 220 222 224
GEOTHERMAL MANIFESTATIONS The area around Longonot marked by active and extinct geothermal manifestations. They occur in form of: Fumaroles Altered/warm grounds Sulphur and/or SiO 2 deposition.
HYDROGEOLOGY The hydrogeology of Longonot area is strongly controlled by; 36 o 00'E 36 o 30'E > Eastern rift flank faults > Fractures, > Rift floor faults, > Rock types and > Tectono volcanic axes. Lake Nakuru Lake Elmenteita EBURRU GEOTHERMAL FIELD NDABIBI PLAINS Lake Naivasha NAIVASHA TOWN 0 o 30'S Hell's Gate Piezometric contour (masl) Lateral flow Surface water divide OLKARIA GEOTHERMAL FIELD Ol'Njorowa Gorge LONGONOT 1 o 00'S
GEOCHEMISTRY Radon counts are generally moderate to low within the caldera but high southwards and at the crater floor. CO 2 /radon222 ratios are high within the caldera Gas geothermometry:199-357 o C for TDP and T H2S, respectively. T CO2 =295 o C.
20 15 25 30 50 70 GEOPHYSICS -MT (2 km bsl) 9904000 30 40 50 200 70 150 9902000 100 70 50 40 10 5 7 3 15 10 25 20 15 30 10 15 3 20 20 20 25 25 30 25 30 30 5 10 7 25 30 15 20 30 40 30 100 70 9900000 100 9898000 50 40 70 9896000 50 150 200 100 70 50 40 40 70 9894000 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 40 150 200 50 100 9892000 70 50 40 50 9890000 25 20 10 40 7 9888000 9886000 MEQ2004 9904000 2000 1500 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 9902000 Caldera Crater MT 2km bsl shows a body of <20 ohm m in the southern slopes of the central volcano 16-25 km 2 within the caldera floor 240-375 MW 9900000 9898000 9896000 9894000 9892000 TVA TVA1 Scale 1:125000 0 2500 5000 7500 m ProposedMEQ Longonot JGI Occam Resistivity at 3000mbsl 9890000 9888000 9886000 ohm.m 1000 800 600 400 200 150 100 70 50 40 30 25 20 15 10 7 5 3
GEOPHYSICS -MT (2 km bsl) Potential area: >25 km 2 Northings (km) 9905 9900 9895 Ohm-m 61 42 32 25 20 15 9890 0 205 210 215 220 Eastings (km)
GEOPHYSICS -MT 2D model 2-D interpretation of the MT data indicates that the heat source is shallow Ohm.m Elevation (m a.s.l.) 3500 3000 2500 2000 1500 1000 500 0-500 -1000-1500 -2000-2500 -3000-3500 -4000-4500 -5000 lmt50 lmt06 lmt09 Lmt12 LMT14 lmt18 0 2000 4000 6000 8000 10000 12000 14000 16000 Distance (m) NE1 N Horizontal Scale 1 : 100000 lmt25 lmt29 Vertical Scale 1 : 100000 Vertical Exaggeration = 1 lmt37 lmt39 lmt45 lmt49 LMT51 lmt53 lmt55 lmt57 Longonot LMT124 MT Sounding Longonot JGI 3500 3000 2500 2000 1500 1000 500 0-500 -1000-1500 -2000-2500 -3000-3500 -4000-4500 -5000 3000 2000 1500 1000 700 500 300 200 150 100 70 50 40 30 25 20 15 10 7 5 3 2 1
3-D MODEL OF LONGONOT W e s t e r n c a l d e r a r i m S u m m i t c r a t e r E a s t e r n c a l d e r a b o u n d a r y??? K E Y T r a c h y t e p u m i c e a s h f a l l a n d s u r g e d e p o s i t s P r e - c a l d e r a t a r c h y t e a n d s h i e l d v o l c a n i c s L o w e r t r a c h y t e K i n a n g o p t u f f P r e K i n a n g o p / M a u t u f f v o l c a n i c s S y e n i t i c i n t r u s i v e I n f e r r e d c o l l a p s e d b r a c c i a C o l d r e c h a r g e H o t u p f l o w F a u l t F i g u r e 1 4. C o n c e p t u a l i z e d g e o l o g i c a l m o d e l o f L o n g o n o t
GEOTHERMAL MODEL
CONCLUSIONS The rock types found in Longonot include trachyte and mixed basalt/trachyte lavas and their pyroclastic equivalents. From the studies done on Longonot volcanics, trace and major elements indicate that the pre- and postcaldera lavas are comagmatic, thus suggesting presence of a large, highly evolved and long lived magmatic system under the caldera.
CONCLUSIONS CONT.. A heat source exists under the caldera and is probably shallow at the summit crater as evident from the strong geothermal manifestations and the most recent eruptions. Occurrence of active geothermal manifestations and hydrothermally altered lithics containing epidote indicate that a geothermal reservoir with temperatures in excess of 250 o C probably exist in Longonot.
CONCLUSIONS CONT.. The structural pattern in Longonot is to the NNW- SSE. Only the northwestern, western, southwestern and the southern caldera rim walls can be observed. In all the other areas it is obscure. The recharge zones include the rift flanks and the northern parts of the area. The outflow zone is probably to the south.
Proposed Exploration Well Sites 9905 Ohm-m 9902 85 60 Northings (km) 9899 9896 EP-2 EP-1 EP-3 40 35 30 T V A 25 20 9893 15 10 9890 0 210 213 216 219 Eastings (km)
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