Molecular parameters of maturation in the Toarcian shales, Paris Basin, France—II Evolution of metalloporphyrins

The distributions of nickel (II) and vanadyl alkyl porphyrins in a suite of Toarcian shales from the Paris Basin have been examined as the demetallated compounds by high performance liquid chromatography and mass spectrometry. The suite covers a wide range of burial history. Among the major changes which occur in the vanadyl species with increasing maximum depth of burial are: (1) conversion of DPEP components to etio components, (2) a decrease in the chain lengths of the β alkyl substituents, and (3) a decrease in the ratio of Ni to VO porphyrins. In addition, evidence is presented for the generation of higher molecular weight species (C 35 C 39 ) below 2000 m maximum burial depth. These appear to arise by thermal cracking of the kerogen. Within the shallow samples the Ni species undergo the DPEP to etio conversion and decrease of alkyl chain length before the VO species. These changes indicate, for the shallow samples, an increase in the extent of maturation from north to south across the basin.