Maturation patterns in Palaeozoic rocks of north-eastern Bulgaria based on conodont colour alteration index (CAI) data

Abstract. CAI data have been used to estimate the thermal history of Palaeozoic sedimentary rocks in five boreholes in north-eastern Bulgaria. Gen-erally, the highest CAI values (4.5) have been found in Lower Devonian sediments, and the lowest values in the Lower Carboniferous (1.5), which sug-gests pre-tectonic coalification. The Palaeozoic rocks attained the observed thermal maturity during Late Carboniferous times. The post-Carboniferous heating should have had no influence on coalification of Palaeozoic sediments. It seems likely that the palaeogeothermal gradient was higher than the present one of ~ 30 °C/km. The economic potential for hydrocarbons in the Devonian sediments of north-eastern Bulgaria is questionable, as most CAI values suggest burial temperatures of about 110–200 °C and 190–300 °C (CAI 3 and 4, respectively) resulting in the supramature state of the organic mat-ter. The occurrence of hydrocarbons might be possible in some of the Carboniferous sediments, based on their estimated palaeotemperatures between 50 °C and 90 °C.