HYDROTHERMAL HYDROCARBON GASES - 1, GENESIS AND GEOTHERMOMETRY

Various sources for hydrothermal CH4 have been proposed over the years . While C isotope studies have narrowed the possibilities, enough high er hydrocarbon gas data now exist both to supplement the isotopic data and to permit additional deductions regarding origins. Comparison of typical C-1-C-6 data for gases of various origins (from sedimentary an d crystalline rocks, and hydrothermal systems) reveals certain charact eristics. Apart from isotopic differences, hydrothermal hydrocarbons d iffer from sedimentary hydrocarbons mainly in possessing tendencies to wards a relative excess of CH4, higher normal/iso ratios for butane an d pentane, and relatively high amounts of C-6 gases. Despite these dif ferences, consideration of the evidence indicates that hydrothermal hy drocarbon gases in most cases originate like sedimentary basin gases b y thermal degradation of organic matter in the relatively shallow subs urface. The principal characteristic of these hydrothermal gases, ''ex cess'' CH4, appears to have a geothermometric function. The following empirical relationship has been derived: t degrees C = 57.8 log(CH4/C2 H6) + 96.8, which fits moderately well a range of geothermal fields wo rldwide. This gas geothermometer may be particularly applicable during geothermal exploration in areas where there is little direct knowledg e of subsurface Conditions. (C) 1998 NERC. Published by Elsevier Scien ce Ltd. All rights reserved.