A stratigraphic partition of delta 13 Corg values is shown in the Maykop Series. For Upper Priabonian-Lower Rupelian and Lower Miocene deposits, a negative correlation is observed between HI values and delta 13 C values of kerogens. This relation is due to a more marked reduction of 13 C in marine organic matter than in continental organic matter at that time. The marine organic matter remained poorer in 13 C than the continental organic matter during Lower Miocene. Consequently the inversion of isotopic composition between the marine and terrestrial kerogens, poorly dated, would have occurred in the region investigated at the end of the Lower Miocene. An important 13 C enrichment in marine kerogens, whose delta 13 C values approach those of the continental organic matter, occurred during Upper Rupelian. The less negative values of delta 13 C in kerogens having high HI values, are due to an important increase of the primary productivity during that period, leading to the coeval enrichment in marine organic matter observed in the Maykop Series.