No abstract

Modern gas migration territorially coincides with discontinuous disturbances in the sedimentary cover and deposits of combustible fossils in previous epochs of sediment accumulation. The geodynamics of the region determined the stages of its formation, accompanied by platform transformations and tectonic-magmatic processes, as well as the formation of sedimentary complexes with shale, coal seams and oil and gas reservoirs. Vertical, relatively disturbed, and lateral, intraplate migration of hydrocarbons were the main factors in the formation of deposits in the region. The presence of discontinuities and stratigraphic unconformities in the deposits of the Pre-Dobrudzh trough and near them is regarded as a key factor in the low conservation of most of the paleo-reservoirs of these deposits. All samples contain an insignificant amount of hydrogen and C6 hydrocarbons, which indicates modern gas migrations to the Earth's surface, mainly in the zones of faults.

During the Upper Cretaceous, the land concerned in this study was part of an extensive area of monotonous tectofacies evolution involving a spatially uniform, rhythmic (mostly flysch), carbonate-terrigenous and volcanic-terrigenous (there was active volcanism in adjacent areas) sedimentation, the general monotony being broken by events of olistostrome deposition and consedimentation block movements restricted both in space and in time. From the north to the south, the post-Illirian structural pattern has been found to involve parts of the Illirian Luda-Kamčija synclinorium and the contemporaneous Kačulka anticline and Vrailec anticlinorium surrounding it from the south. By either thrust faulting or recumbent folding, the latter's western periclinorium was overlaid by the northeastern margin of the eastern end of the Laramide Srednogorie anticlinorium, the margin manifesting intense polyphase deformation and block fragmenting. The existence of the so-called Kozuina syncline, named after a mountain peak west of the town of Sliven, has not been confirmed in this study. Essential structural differences have been found between the Srednogorie anticlinorium and the adjacent major Illirian folds. In the area studied, there is the largest areal concentration of faults of Tvârdica orientation and Upper Cretaceous tectonic activity indicating that the Tvârdica strike-slip fault set axis active during the Upper Cretaceous geochron to the south of the Balkan Range should be sought there.