Carbon isotope composition of lower palaeozoic kerogen: Effects of maturation

The carbon isotope composition of type I kerogen from a marine, organic-rich black shale deposit of Late Cambrian age (the Alum Shale Formation of southern Scandinavia) varies from -28 to -30% PDB. The thermal maturity is rated by HI-OI index, T_max, PI and vitrinite reflectance values and published conodont alteration data. The results range from immature conditions with regard to oil generation in central Sweden to overmature and submetamorphic conditions in southern Sweden, Denmark and Norway. In the immature areas, the shale is found to be extremely hydrogen rich and oil prone (hydrocarbon potential up to 85 kg per ton rock). The observed δ¹³-variations are not correlated with maturity. It is therefore concluded that thermal maturation processes have no influence on the isotopic composition of a purely marine kerogen. The observed range in Lower Palaeozoic δ¹³ (kerogen)-values (-27 to -31%) is in accordance with published results from other sedimentary basins.