Maturation patterns in Palaeozoic rocks of north-eastern Bulgaria based on conodont colour alteration index (CAI) data
Bulletin of Geosciences, volume 80, issue 3;
pages: 223 - 237;
Received 17 December 2004;
Accepted in revised form 11 May 2005;
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CAI data have been used to estimate the thermal history of Palaeozoic sedimentary rocks in five boreholes in north-eastern Bulgaria. Generally, the highest CAI values (4.5) have been found in Lower Devonian sediments, and the lowest values in the Lower Carboniferous (1.5), which suggests 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 matter. The occurrence of hydrocarbons might be possible in some of the Carboniferous sediments, based on their estimated palaeotemperatures between 50 °C and 90 °C.