GEOTHERMAL ENERGY POTENTIAL FOR LONGONOT PROSPECT, KENYA. By Mariita N. O. Kenya Electricity Generating Company

Transcription

1 GEOTHERMAL ENERGY POTENTIAL FOR LONGONOT PROSPECT, KENYA By Mariita N. O. Kenya Electricity Generating Company

2 PRESENTATION OUTLINE INTRODUCTION REGIONAL SETTING GEOLOGY GEOTHERMAL MANIFESTATIONS HYDROGEOLOGY GEOCHEMISTRY GEOPHYSICS CONCEPTUALIZED GEOTHERMAL MODEL CONCLUSIONS

3 REGIONAL SETTING

4 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.

5 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.

6 METHODOLOGY The methods used during the investigations included: Photogeology Ground geology -Field mapping Landsat imagery Geophysics, Geochemistry

7 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.

8 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

9 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

10 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.

11 STRUCTURES Structures found in Longonot are: Tectono Volcanic alignments (TVA s) trending NNW-SSE and NW-SE Caldera wall Lineament Faults

12 LONGONOT STRUCTURAL MAP LONGONOT ` LONGONOT CRATER ERERI Tectono Volcanic alignments KANARU LEGEND Caldera wall Crater Fault AKIRA RANCH MAAI MAHIU RANCH Volcanic centers Main road Earth road Railway line Fumaroles LONGONOT EARTH SATTELITE EWASO KEDONG VALLEY 0 1 2KM

13 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.

14 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

15 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: o C for TDP and T H2S, respectively. T CO2 =295 o C.

16 GEOPHYSICS -MT (2 km bsl) MEQ Caldera Crater MT 2km bsl shows a body of <20 ohm m in the southern slopes of the central volcano km 2 within the caldera floor MW TVA TVA1 Scale 1: m ProposedMEQ Longonot JGI Occam Resistivity at 3000mbsl ohm.m

17 GEOPHYSICS -MT (2 km bsl) Potential area: >25 km 2 Northings (km) Ohm-m Eastings (km)

18 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.) lmt50 lmt06 lmt09 Lmt12 LMT14 lmt Distance (m) NE1 N Horizontal Scale 1 : lmt25 lmt29 Vertical Scale 1 : Vertical Exaggeration = 1 lmt37 lmt39 lmt45 lmt49 LMT51 lmt53 lmt55 lmt57 Longonot LMT124 MT Sounding Longonot JGI

19 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

20 GEOTHERMAL MODEL

21 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.

22 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.

23 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.

24 Proposed Exploration Well Sites 9905 Ohm-m Northings (km) EP-2 EP-1 EP T V A Eastings (km)

25 What then??? And hopefully be happy, thereafter Thank you for your kind attention