THE SIGNIFICANCE OF d34S DETERMINATION AND EVALUATION IN ORGANIC RICH SEDIMENTS, II. INSIGHT TO THE MECHANISMS CONTROLLING CATAGENESIS OF TYPE II-S KEROGENS TO THE FORMATION OF ASPHALT AND PETROLEUM. STUDY & OVERVIEW

At the end of the diagenetic stage the kerogens of type II-S are rich in sulfur. Most of this sulfur is thermally unstable (poly-S-S). he catagentic stage is mostly controlled by increase of temperature, both carbon and sulfur bonds reform to the thermally stabilized structures. The controlling factors for d¹³C changes during these modifications are the release of CO₂ and C₁-C₅ hydrocarbons. Whereas the sulfur stabilization is producing H₂S and S⁰ with the aromatization of the C-S-C moieties formed, the released carbon containing molecules are "out of equilibrium": non-reversible reactions. In contrast, at elevated temperature the sulfur released could react with the organic phase if not removed. These modifications and restructuring were recorded to have a signature on the d¹³C of the asphalts and petroleum. Moreover, the various fractions, e.g., Saturates, Aromatics, Resins etc., show different d¹³C ranges (kerogen ® CH₄ d¹³C light). It has been suggested that the depletion in sulfur from kerogen to oils does not cause any d³⁴S changes. While many simulation thermal experiments were carried out (hydrous pyrolysis and "dry" pyrolysis) and the thermal behavior of kerogens (type II-S) was studied, very few of these experiments were monitored for d³⁴S changes. However, in the last 15 years some studies, though scant, showed that the loss of sulfur and thermal stabilization is marked in ³²S enrichment in H₂S and subsequently ³⁴S enrichment of the petroleum produced. We wish to present these works and try to offer some mechanistic explanations to the trend. Although we performed some laboratory experiments in closed and open systems, only very rough examination of the various fractions was carried out. Some natural (geological) samples will be presented for comparison to support the general concept. In the general scheme the sulfur isotope (d³⁴S) signature recorded in the kerogen is changing during catagenesis to form petroleum less rich in sulfur and heavier istopically. This enrichment in ³⁴S could cause +4 to +8‰ in thermally controlled experiments. In natural source rock to oil comparison, the isotope discrimination could be even higher, leading also to secondary metal sulfides (including heavy pyrite). The chemically controlled thermal sulfate reduction (TSR) (³200°C) will be discussed in a separate talk.