Characterisation of the Lower Palaeozoic sediments in NE Bulgaria for hydrocarbon potential assessment by integrating geochemical and geophysical research data

Abstract

The Lower Palaeozoic marine sediments in NE Bulgaria could be considered not only as a potential hydrocarbon source rock, but also as a potential unconventional oil and gas reservoir. Several wells have penetrated this succession composed of mudstones, shales, siltstones, minor limestones, and marls. The knowledge on the major parameters of these rocks is rather obscure and is based on scarce rock sample data, because only a limited interval was drilled by coring, while most of the section was covered only by logging. Therefore, the current study combines geological, geophysical, and geochemical methods to characterise and assess the hydrocarbon potential of this succession. In the studied samples, one of the main parameters – total organic carbon, varies between 0.5% and 2.61%, which defines the rocks as having fair to good potential. A relatively positive correlation is observed between total organic carbon content and geological depth. The Lower Devonian–Silurian sediments exhibit generally higher total organic carbon content than the other part of the section. The applied method of using well-loggings allows the identification of vertical and lateral distribution of organic matter. A good correlation was observed between the total organic carbon data yielded by the geochemical method and the different well-log calculations. The obtained data on the type of organic matter and the degree of maturity show mainly gas potential of the shale succession. Different approaches, using Rock-Eval analysis and classical methodology of vitrinite reflectance (Ro), were applied for thermal maturity evaluation. Such combination and integration of different methods provide a more detailed and wide-ranging assessment of the Lower Palaeozoic sediments, and the data received could change the prospect and exploration strategy of a potential unconventional hydrocarbon resource.