Hydrothermal hydrocarbon gases: 2, application in the East African Rift System

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 MORE 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 C-1/C-2 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. (C) 1998 NERC. Published by Elsevier Science Ltd. All rights reserved.