Argon

Abstract

N NATURAL CASSedimentary accumulations of gas contain a limited number of compounds: saturated light hydrocarbons from methane to pentane, C02 , H2 S, N2 (Durand, 1993) and traces of noble gases, mercury and hydrogen (Sokolov, 1974). All of these gases, except mercury and the noble gases, may come from organic matter, either from bacterial activity, or by thermal degradation. Inert gases may also have mineral origins: N 2 from ammonium of shales or from the deeper crust, H2 S from thermo-reduction of sulfates (with hydrocarbon as a reducing agent), and C02 from thermal alteration of carbonates or mantle contamination. Nevertheless, they are clearly identified in some fields as generated from organic matter. H2S:Since organic matter contains sulfur in various amounts, H2 S is generated during the whole sequence of gas generation, by bacteria during the diagenesis stage, and by thermal stress during catagenesis. The first generated H2 S results from the breaking of sulfides, later complemented by the breaking of thiophenic compounds (aromatic sulfur incorporated). Finally, organic pyrite may contribute to H2 S generation at high maturation levels.C02 : Oxygen is present in all types of kerogens, but mainly in type III/humic organic matter. The amount of organic C02 may reach 20% of the total gas generated, and is characterized by light carbon isotopic signature, as compared to other C0 2 generation processes. Nevertheless, bacterial or chemical oxidation of organic matter also produces isotopically light C02, distinct from mineral origins.