MIXING TRENDS AND SOLUTE GEOTHERMOMETRY OF BOREHOLE WATERS FROM THE PAKA GEOTHERMAL PROSPECT, KENYA. Kizito Maloba Opondo Kenya Electricity Generating Company
Geothermal Prospects and fields in Kenya - Paka is situated 25 km north of Lake Baringo and ~20 km east of Chemlingot Village at 00 o 25 N and 36 o 12 E - Surface studies carried out in 2006-2007 2
Geological setting of Paka geothermal prospect Paka is composed of trachytic, basaltic lavas and pyroclastic deposits A shield volcano with craters at the top and the north east. It is dominated by a zone of intense normal faulting on the eastern and northeastern flanks and to the south 3
Boreholes Location in Paka Geothermal Prospect Paka Topo.srf 4
CHEMICAL COMPOSITION OF BORE HOLE WATER ( in ppm) SAMPLE SITES Nginyang SDA TEMP ph Na K Ca Mg SiO2 CO2 SO4 Cl 39 7.3 33.2 11.1 1.6 2.1 81.8 146.7 18.3 66 Orus 38 6.7 141 9.1 30.8 7.8 29.6 242 ND 71 Dabalikow 35 7.1 93.1 13.4 5.31 0.65 49.2 251.6 18.3 94 Kokwototo 31 7.6 71.4 12.5 8.16 10.6 44.2 409 4.0 28 Chemlingot 35 8.5 13.1 4.36 2.61 1.08 44.8 304 23.8 97 Boreholes fall in Eastern and Western parts of the Paka Geothermal prospects The bore hole sampled at Nginyang SDA has relatively high silica content at the measured temperature of the water 5
CL-SO4-HCO3 Ternary plot CL Steam heated waters SO4 HCO3 0 25 50 75 100 6 75 50 25 0 100 75 Mature waters 50 Volcanic-waters Q 25 Orus Chemlingot DO's Office Nginyang' SDA Kokwototo Dabalikow Peripheral-waters 0 100
Mixing trends: Chloride- Boron plot Boron ppm 0.32 0.28 0.24 0.2 0.16 0.12 0.08 Kokwototo Dabalikow Nginyang SDA Orus Chemolingot D.O's Office 55 60 65 70 75 80 85 Chloride ppm Chloride and boron concentrations are low in cold water, but higher in geothermal waters. Mixing involves simple lowering of ratios without affecting the Cl/B ratio High Chloride concentration in Chemlingot water, low in Kokwototo Boiling accompanied by mixing 7
Mixing trends- Chloride- sulphate Sulphate ppm 25 20 15 10 5 0 Kokwototo Nginyang SDA Chemolingot D.O's Office Dabalikow 20 40 60 80 100 Chloride ppm High chloride and high sulphate contents High sulphate could result from oxidation of hydrogen sulphide by atmospheric oxygen This occurs during mixing of shallower ground waters e.g Kokwototo and hotter waters e.g Chemlingot The borehole waters are mixed waters 8
Mixing trends; Magnesium - sulphate Sulphate, ppm Chemolingot D.O's Office 24 20 16 12 8 Dabalikow Nginyang SDA High magnesium contents is a good indicator of ground or surface waters Kokwototo water highest in Magnesium contents Waters from Kokwototo could be the diluting waters 4 Kokwototo 0 2 4 6 8 10 12 Magnesium, ppm 9
Mixing Trends: Chloride-Enthalpy 1800 Nginyang SDA Chemolingot D.O's Office Takes into account of both boiling and mixing Enthalpy kj/kg 1600 1400 1200 Kokwototo Dabalikow Relates analysed chloride and water enthalpy Enthalpy was derived from NaK temperatures 1000 55 60 65 70 75 80 85 Chloride (ppm) Hotter water represented by samples at Chemlingot mix with colder waters in kokwototo 10
Solute geothermometry temperatures Borehole site TNaK (G,19 88) TNaK (F, 1979) TNa-K- Ca (F&T, 1973 Tqtz F&P 1982a Nginyang 348 351 266 126 SDA Orus 148 200 164 79 Dabalikow 250 263 209 101 Kokwototo 270 281 221 97 Chemlingo t 348 351 265 97 NaK yields very high temperatures except samples from Orus Na-K-Ca temperatures lower than Na-K temperatures Quartz temperatures much lower. Probably due to lack of attainment of equilibrium between water and rock minerals 11
Relationship between TNaK & Tqtz temperatures Chemical thermometry for warm springs and boiling springs differs TNa-K temperatures much higher than Tqtz temperatures. Major cation concentration in warm mixed waters are interpretated to be due to leaching. Assuming fluid mineral equilibrium may not be correct and this gives unrealistic geothermometry temperatures. 12
CONCLUSIONS The borehole waters in Paka are mixed bicarbonatechloride type which could have derived from sediment deposits and evaporate processes. The solute geothermometry temperatures of NaK, Na-Ca- K and quartz show high discrepancy. Assumptions of equilibrium between mineral and water may not be correct. Mixing trends suggest the borehole waters mix from waters in Chemlingot to waters in Kokwototo. 13
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