Hydrothermal hydrocarbon gases: 2, Application in the East African Rift System |
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| Institution: | 1. British Geological Survey, Wallingford, Oxfordshire OX10 8BB, UK;1. Research Institute for Geo-Hydrological Protection, National Research Council of Italy (CNR), Via Amendola 122/I, 70126 Bari, Italy;2. Geology and Petroleum Geology, University of Aberdeen, Aberdeen, UK;3. Department of Earth and Geoenvironmental Sciences, University of Bari, Via Orabona 4, 70125 Bari, Italy;4. Institute of Geosciences and Earth Resources, National Research Council of Italy (CNR), Via G. Moruzzi 1, 56124 Pisa, Italy;5. Institute of Geosciences and Earth Resources, National Research Council of Italy (CNR), Via La Pira 4, 50121 Florence, Italy;1. Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia;2. National Centre for Petroleum Research and Development, A.T.B.U, Bauchi State, Nigeria;3. Geology Department, Faculty of Applied Science, Taiz University, 6803, Taiz Yemen;4. Department of Geology, Ekiti State University, P.M.B. 5363, Ado-Ekiti, Nigeria;1. Applied Chem Data, 22410 Bridgehaven Drive, Katy, TX 77494, USA;2. SGS, Oil, Gas and Chemicals, 87101 New Trails Drive, Suite 175, The Woodlands, TX 77381, USA;1. State Key Laboratory of Organic Geochemistry (SKLOG), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China;2. University of Chinese Academy of Sciences, Beijing 10039, PR China;1. Department of Physics, College of Natural and Computational Sciences, University of Gondar, P.O. Box 196, Gondar, Ethiopia;2. Department of Earth Sciences, College of Natural Science, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia |
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| Abstract: | Hydrocarbon gases are ubiquitous in the hydrothermal systems of the East African Rift System (EARS), though often found at very low concentrations in the ‘volcanic’ eastern branch as compared to the ‘sedimentary’ western branch. Study of the chemical and isotopic compositions of these hydrocarbons from sites in Kenya, Ethiopia, Djibouti and Uganda reveals considerable homogeneity over hundreds of km of the various rift units. Consideration of C and He isotopic evidence points to a predominantly crustal thermogenic origin for the hydrocarbons, there being no evidence of mantle inputs in either the MORB or ‘hotspot’ sectors of the EARS. Temperature information from geothermal wells has been utilised to investigate the relationship between reservoir temperatures and ratios of CH4 to C2H6. The general C1/C2 geothermometric relationship proposed in Part 1 of this study holds reasonably well, and is shown to give results equal to or better than the ‘inorganic’ gas geothermometers presently in use, both in the wellfields and undeveloped high-enthalpy geothermal areas. Results from low-enthalpy hot spring systems are less well correlated with apparent deep temperatures, but consistent with data from similar systems elsewhere in the world. |
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