GEOELF SULFUR ANALYZER - QUANTIFICATION OF THERMALLY EXTRACTABLE AND PYROLYSABLE ORGANIC AND MINERAL SULFUR IN-SOURCE ROCKS

Sedimentary rocks are routinely characterised by Rock-Eval pyrolysis. A sediment with a high S-2 yield and a high hydrogen index (HI) indica tes a source rock with a good hydrocarbon generation potential. Sulphu r, organically bound in kerogen or as an inorganic mineral (e.g. pyrit e), is often found in source rocks. The presence of abundant organic s ulphur 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 nor, only th e total amount of sulphur present in a source rock, but more specifica lly the amount of organically bound sulphur. The development of a pyro lysis unit, using a sulphur chemiluminescence detector (SCD), has made it possible to detect three distinct forms of sulphur during temperat ure-programmed pyrolysis. These three forms are: thermally extractable sulphur components released after heating to 300 degrees C (SS1), pyr olysable organic sulphur (SS2) and pyrolysable mineral sulphur (SS3) g enerated during pyrolysis from 300 to 600 degrees C. Determination of sulphur content by thermal desorption and pyrolysis of different sourc e rocks with type I, II, II-S and III kerogens shows a wide range of y ields of pyrolysable organic and mineral sulphur. Siliciclastic source rocks with type II kerogen, predominant mineral sulphur (iron sulphid e, SS3) and subordinate amounts of organically bound sulphur (SS2), gi ve sulphur index values (SI) less than 100 mg S-org g(-1) TOC. Nonsili ciclastic source rocks with type II-S kerogen and substantial enrichme nt in pyrolysable organic sulphur, display SI-values as high as 360 mg S-org g(-1) TOC and are depleted in mineral sulphur. Freshwater/lacus trine source rocks with type I kerogen are almost devoid of both pyrol ysable organic and mineral sulphur. The SS2-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 ker ogens can be distinguished. In conclusion, the prediction of source ro ck generation and expulsion behaviour would be enhanced by using a 3D definition of kerogen (HI vs OI vs SI) instead of the currently applie d 2D definition (i.e. HI vs OI or the pseudo van Krevelen diagram). Su ch complementary information could also be used to predict the composi tion and the nature of the fluids generated and expelled (sulphur-rich or sulphur-poor hydrocarbons). (C) 1998 Elsevier Science Ltd. All rig hts reserved.