MSc defence by Ngereja Myabi Mgejwa
Ngereja Myabi Mgejwa, MSc Fellow in Geology at the University of Iceland will give a lecture on his MSc project on Thurday 26th May, 2016 at 14:30 at University of Iceland, room 132.
The title of the project is:
Structural Mapping of Olkaria Domes Geothermal Field using Geochemical Soil Gas Surveys, Remote Sensing and GIS
Samuel's supervisors are:
Enikö Bali from University of Iceland, Björn S. Harðarson from ISOR and Anette K. Mortensen from Orkuveita Reykjavíkur.
The external examiner will be Dr. Hjalti Franzson at ISOR.
Everyone's welcome to attend.
Abstract
The Menengai geothermal field is one of the high temperature geothermal systems in Kenya. It is seated within the Great East Africa Rift System in the Central Kenyan Rift Valley and covered by quarternary volcanics. The study wells, MW-03, MW-09 and MW-20, were drilled inside the Menengai Caldera, which is characterized by ring faults, the Molo TVA which trends NNW-SSE and the Solai TVA which trends NNE-SSW direction. The volcano formed about 200,000 years ago and the prominent 12 x 8 km caldera about 8000 years ago. Binocular and petrographic microscopes, XRD-analysis, ICP analysis and temperture logs were applied for the research. In the study wells, fine-coarse grained trachytes, pyroclastics, tuffs, basalt and intrusives (syenite) were observed. Based on Al2O3 concentration and total K2O + Na2O, the analysed rocks are metaluminous compared to the neighbouring wells MW-04, MW-06 and MW-07. The geochemical evolution of Menengai rocks seems to be mainly controlled by fractional crystallization however, more than one process are involved. The realionship between trace elements with depth in wells MW-03, MW-09 and MW-20 depict four volcanic episodes which may be related to Menengai caldera formation.
The study defined seven hydrothermal alteration zones; Unaltered, Zeolite-Smectite (40-180°C), Quartz (above 180°), Illite (220°C), Chlorite (above 230°C), Epidote (240°C) and Wollastonite-Actinolite zone (above 280°C). Chlorite was noted in MW-03 and MW-20, illite zone was defined in MW-09 and MW-20. In MW-03, illite and actinolite occur at the bottom of the well and Wollastonite-Actinolite zone is thinning towards MW-03. Based on the alteration minerals and formation temperatures, the wells show indications of heating, where MW-03 appears to be heating more from approximately 1500m, and MW-09 from approximately 800-1400m. Seven aquifers/feed zones were identified in MW-03, six in MW-09 and five in MW-20.These feed zones are linked to lithological boundaries, fractures and faults and permeable formations.
Calcite looks to be the dominant alteration mineral in the wells and tends to deposit later than higher temperature alteration minerals at some depths. Calcite forms by replacing primary minerals such as feldspar, pyroxene and volcanic glass; boiling of the reservoir fluid that causes to the loss of CO2 leads to calcite precipitates in veins or open spaces and calcite precipitates in veins or fractures when hotter fluid mixes with circulating ground water. Epidote appears to be a rare alteration mineral in the Menengai geothermal field, which could be due to high concentrations of CO2, low permeability and low contents of iron in the rocks which are important for epidote formation.