The distribution and isotopic composition of sulfur in organic-rich sedimentary rocks

Sulfur concentration in bituminous chalks of the Maastrichtian-Campanian Ghareb Formation is high (1.8-2.6%), its concentration and isotope composition were determined for the main S-bearing phases: pyrite, extractable bitumen and kerogen. Pyritic sulfur accounts for only 20-54% of total S content. Sulfur concentration in the bitumen and kerogen fractions are 8.9-16.4% and 6.5-12.1%, respectively. Two distinct types of isotope composition reflect different depositional environments and diagenetic processes; type-1 pyrite (-19.4 to -23.8‰) is much lighter than organically bonded S in the bitumen and kerogen (-4.2 and -8 to -10.7‰, respectively). Type 2, all sulfur compounds are heavier than type 1 and the difference between species smaller; pyritic sulfur (+6.1‰), bitumen and kerogen (+1.7 to +8.7 and +1.2 to +8.3‰, respectively). Sulfur-isotopic composition of the pyrite is as a rule the lightest species and indicates that bacterial activity is involved in pyrite formation. The heavier sulfur in organic compounds suggests a secondary enrichment, probably via a radical mechanism. The marked differences between the two types of isotope composition may be due to differences in the sulfate reduced, or to escape of light, reduced sulfur. Sulfur-enrichment experiments in kerogen showed that high degree of enrichment is associated with loss of oxygen; low-S kerogen is enriched more than high-S kerogen. Decrease in ESR N_g-value is associated with secondary sulfur enrichment.