Significance of δ³⁴S and evaluation of its imprint on sedimentary sulfur rich organic matter II: Thermal changes of kerogens type II-S catagenetic stage controlled mechanisms. A study and conceptual overview

Kerogens of type II-S are rich in sulfur, containing up to 10-12% organically bound sulfur. Most of this sulfur is thermally unstable due to the presence of catanated poly-S-S linkages. The δ³⁴S values for these kerogens carry the imprint of the pore water polysulfides introduced into the organic matter at the diagenetic stage as described in the previous review (Part I). The catagenetic stage, covered in Part II, is mostly driven by the increase of temperature, leading to rearrangement of both carbon and sulfur bonds that are reformed thermally to stabilized alicyclic and aromatic sulfur-containing structures. The controlling factors for δ¹³C changes during these modifications are the release of CO₂ and C₁-C₅ hydrocarbons, mostly CH₄. The sulfur stabilization releases H₂S and S° during the forming of the C-S-C moieties and their aromatization. The present report and review examines the influence of the above geochemical changes and the mechanisms controlling them on the stable isotope distribution of the thermally derived products. The understanding of these changes can lead to a better correlation between potential source rocks (PSR) and petroleum generated from them. The released carboncontaining molecules, i.e. CO₂ and CH₄ are chemically stable and not reactive (non-reversible reactions); in contrast at elevated temperature the sulfur released (H₂S, S°) can re-react with the organic matter if not removed. It is therefore very important to examine the sulfur functionality changes during catagenesis that are thermally controlled through the mechanisms leading to sulfur isotope ratios variation. In addition, whether the system is open or closed influences the free radical restructuring of the organic matter and hence will influence the isotopic distribution of sulfur. The thermal cleavage and restructuring of kerogen to produce oil has an impact on the δ¹³C of the asphalts and petroleum of 2%. Moreover, the various fractions such as gas (CH₄), saturates, aromatics, resins and asphaltenes show different δ¹³C ranges. The most depleted in ¹²C is methane. Despite the recognized impact of maturity on carbon isotopes ratios, it has been previously suggested that the decrease in concentration of sulfur from kerogens of type II-S during maturation to generate oil does not cause δ³⁴S changes. While many hydrous pyrolysis and "dry" pyrolysis thermal simulation experiments were carried out and the thermal behavior of kerogens (type II-S) was studied, very few of these experiments were monitored for δ³⁴S changes. However, in the last 15 years some studies showed that the loss of sulfur and associated thermal stabilization is reflected in ³²S enrichment in H₂S and concurrently, ³⁴S enrichment of the petroleum produced. Based on these experiments we will offer mechanisms for the observed trend. Some new laboratory experiments performed by us in both closed and open systems are reported. Only very rough examination of the various organic sulfur-containing fractions was carried out in these studies. Some natural (geological) sites such as the Monterey Formation (Miocene, California, USA) and Senonian Formation (Dead Sea Area, Israel) are presented for comparison. In the general scheme, the δ³⁴S signature recorded in the kerogen changes during catagenesis to form petroleum depleted in sulfur and isotopically heavier. This enrichment in ³⁴S could amount to + 4 to + 8% relative to the kerogen of the PSR in thermally controlled experiments. In a field-based source rock to oil generated comparison, the isotope discrimination could be even higher, leading also to secondary metal sulfides (including relatively heavy pyrite). The chemically controlled thermal sulfate reduction ( ≧ 200°C) is discussed only briefly.