KINETIC MODEL OF HYDROGEN SULFIDE GENERATION DURING DIAGENESIS AND CATAGENESIS OF ORGANIC MATTER

Abstract

The paper addresses the problem of hydrogen sulfide generation from organically bound sulfur in fossil organic matter during diagenesis and catagenesis. The relevance of the study is determined by the fact that sulfur incorporated into humic acids, kerogen and other geopolymeric components of sedimentary organic matter substantially affects the pathways and temperature limits of its transformation, as well as the generation of gaseous products, the composition of bitumoids, oils and asphaltene fractions. A review of published data shows that in sulfur-enriched systems thermally unstable sulfide, disulfide and polysulfide fragments are important precursors of H₂S, whereas catagenetic evolution is accompanied by the relative enrichment of organic matter in more thermally stable thiophenic structures. These regularities are characteristic not only of sulfur-rich kerogen and solid bitumens, but also of coal-bearing organic matter in which the proportion of thiophenic and non-thiophenic sulfur changes systematically with increasing maturity. Taking these features into account, a simplified kinetic model of hydrogen sulfide generation during the hydrolysis of labile sulfide groups in a geopolymeric organic matrix under water excess is proposed. This assumption corresponds to a pseudo-first-order approximation and makes it possible to consider the contribution of one specific mechanism of sulfur transformation separately from radical, thermolytic and microbial pathways. The model describes a two-stage conversion of organically bound sulfur through the intermediate formation of thiol groups followed by H₂S generation. A system of differential equations is derived for the proposed reaction scheme, and its analytical solution is obtained. The solution makes it possible to trace the temporal evolution of sulfide sulfur, thiol sulfur and gaseous hydrogen sulfide, as well as to evaluate the limiting redistribution of sulfur among the components of the system. It is shown that, over sufficiently long time intervals in water-saturated rocks, the amount of sulfide sulfur decreases monotonically, whereas the amount of hydrogen sulfide increases toward a limiting value controlled by the initial sulfur distribution. The model also predicts that the concentration of intermediate thiol groups may pass through a maximum, and the time required to reach this maximum depends on the ratio of the rate constants of the two stages, the effective water content and the initial abundance of reactive sulfur-bearing groups. Thus, the model can be used for assessing the dynamics of hydrolytic destruction of fossil organic matter during diagenesis and early catagenesis, including peat-forming, coal-bearing and other carbonaceous sedimentary systems. The proposed scheme is regarded as a first approximation. It does not take into account microbial sulfate reduction, radical and thermolytic pathways of H₂S generation, reverse reactions, mineral trapping of hydrogen sulfide into FeS/FeS₂, secondary incorporation of sulfur into the organic matrix, or mass-transfer limitations. Nevertheless, it provides a useful theoretical framework for describing one of the important mechanisms of sulfur evolution in fossil organic matter and may be applied in further studies aimed at estimating Arrhenius parameters and validating the model by hydropyrolysis experiments and natural maturity series.