HYDROTHERMAL HYDROCARBON GASES - 2, APPLICATION IN THE EAST-AFRICAN RIFT SYSTEM

Hydrocarbon gases are ubiquitous in the hydrothermal systems of the Ea st African Rift System (EARS), though often found at very low concentr ations in the 'volcanic' eastern branch as compared to the 'sedimentar y' 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 ri ft units. Consideration of C and He isotopic evidence points to a pred ominantly crustal thermogenic origin for the hydrocarbons, there being no evidence of mantle inputs in either the MORE or 'hotspot' sectors of the EARS. Temperature information from geothermal wells has been ut ilised to investigate the relationship between reservoir temperatures and ratios of CH4 to C2H6. The general C-1/C-2 geothermometric relatio nship proposed in Part 1 of this study holds reasonably well, and is s hown to give results equal to or better than the 'inorganic' gas geoth ermometers presently in use, both in the wellfields and undeveloped hi gh-enthalpy geothermal areas. Results from low-enthalpy hot spring sys tems are less well correlated with apparent deep temperatures, but con sistent with data from similar systems elsewhere in the world. (C) 199 8 NERC. Published by Elsevier Science Ltd. All rights reserved.