Geoelf sulphur analyser: quantification of thermally extractable and pyrolysable organic and mineral sulphur in source rocks

Abstract

Sedimentary rocks are routinely characterised by Rock-Eval pyrolysis. A sediment with a high S 2 yield and a high hydrogen index (HI) indicates a source rock with a good hydrocarbon generation potential. Sulphur, organically bound in kerogen or as an inorganic mineral (e.g. pyrite), is often found in source rocks. The presence of abundant organic sulphur in a source rock suggests that petroleum generation will begin earlier than in classical type II source rocks. This will then lead to sulphur-rich oils, and it will be important to determine not only the total amount of sulphur present in a source rock, but more specifically the amount of organically bound sulphur. The development of a pyrolysis unit, using a sulphur chemiluminescence detector (SCD), has made it possible to detect three distinct forms of sulphur during temperature-programmed pyrolysis. These three forms are: thermally extractable sulphur components released after heating to 300°C (SS 1), pyrolysable organic sulphur (SS 2) and pyrolysable mineral sulphur (SS 3) generated during pyrolysis from 300 to 600°C. Determination of sulphur content by thermal desorption and pyrolysis of different source rocks with type I, II, II-S and III kerogens shows a wide range of yields of pyrolysable organic and mineral sulphur. Siliciclastic source rocks with type II kerogen, predominant mineral sulphur (iron sulphide, SS 3) and subordinate amounts of organically bound sulphur (SS 2), give sulphur index values (SI) less than 100 mg S org g −1 TOC. Nonsiliciclastic source rocks with type II-S kerogen and substantial enrichment in pyrolysable organic sulphur, display SI-values as high as 360 mg S org g −1 TOC and are depleted in mineral sulphur. Freshwater/lacustrine source rocks with type I kerogen are almost devoid of both pyrolysable organic and mineral sulphur. The SS 2-yield and derived sulphur index (SI) provide extremely useful and complementary data to the well-known hydrogen (HI) and oxygen (OI) indices. The plot of (HI) vs (SI) clearly shows that various classes of source rocks and associated kerogens can be distinguished. In conclusion, the prediction of source rock generation and expulsion behaviour would be enhanced by using a 3D definition of kerogen (HI vs OI vs SI) instead of the currently applied 2D definition (i.e. HI vs OI or the pseudo van Krevelen diagram). Such complementary information could also be used to predict the composition and the nature of the fluids generated and expelled (sulphur-rich or sulphur-poor hydrocarbons).