Maturity studies in the Carboniferous Dobroudja coal basin (northeastern Bulgaria): coalification, clay diagenesis and thermal modelling

In northeastern Bulgaria Upper Paleozoic coals are only found in the Dobroudja Basin. This coal basin belongs to a Variscan foredeep which is at present covered by a thick (1300–1500 m) Mesozoic-Cenozoic complex (Moesian platform). The coal seams are associated with alternating sandstones, siltstones and shales deposited under fluvio-deltaic environmental conditions. Previous coalification studies on the basin have recorded high volatile to low volatile bituminous ranks. The construction of coalification profiles (increase of vitrinite reflectance with depth) in ten wells from the Dobroudja coal basin has enabled us to interpret the paleotemperature regime. The simulation of the burial and thermal history of the Upper Carboniferous formations in the Dobroudja coal basin yielded paleogeothermal gradients between 45 and 71°C/km. The results show an increase of the paleogeothermal regime from the northwest towards the southeast. The existence of a high paleoheatflow is in accordance with data from other Carboniferous basins in Europe. Due to high vertical paleogeothermal gradients and deep burial, the maturation of the formations was completed during the Late Carboniferous. The relatively large maturation range of the Upper Carboniferous sediments enables comparison among different thermal indicators (vitrinite reflectance, spectral fluorescence maximum, pyrolysis indices, illite crystallinity and bulk clay mineralogy) and their behavior in the course of diagenesis