On the question of geological prerequisites of Dnieper-Donets depression hydrodynamic traps formation

Formulation of the problem. Hydrocarbon pools in hydrodynamic traps are known within many oil and gas provinces. The Dnieper-Donets basin is one of the regions where non-vault traps of almost all types and associated with them hydrocarbon pools are widespread, while HT in DDD are not yet typical, although the geological structure of the region indicates the possibility of their formation. Thus, substantiation of the role of geological preconditions in the processes of hydrodynamic type traps formation is the main task of the work. Analysis of recent research and publications. In the monograph of O. Plotnikov (2001) among the promising regions for the search for HT defined DDD. The reason for this was the data on the presence in some productive horizons, in particular in the Sorochinsky and Rudenkivsky fields, of formation waters that lie above gas-bearing deposits. It is believed by some researchers, that the leading role in the formation of gas accumulations at these fields belongs to formation waters. According to the results of geological and geophysical study of the southern slope of the Kalmius-Torets depression and the Krasnoarmijska monocline in the south-eastern part of the DDD (A. Terdovidov (2000), S. Pavlov (2004), favorable geological prerequisites for the formation of HT in Paleozoic sediments established. Selection of previously unsolved parts of the overall problem. The general problem of searching for HT in the DDD as a whole task has not been solved so far. Formulation of the purpose of the article. Estimation of possibilities of a new direction of search and reconnaissance works in DDD, namely on non-traditional non-vault traps of hydrodynamic type substantiation. Results. The Visean dominant productive complex, regionally extended on the monocline slopes of the northern and southern sides of the depression and lithologically represented mainly by sandy horizons, was selected for priority research. The significant content of alluvial sandstones in Visean sediments section allowed the authors to consider them as special reservoirs of fluids, and paleoalluvial systems to study in the rank of independent search objects. Their territorial location is logically related to the existing deflections of the surface of the crystalline basement, most of which are associated with modern river systems, which indicates the current functioning of their relationship. The authors made an attempt to substantiate within the northern side of the DDD the following forecast water exchange systems: 1) expultional overflowing; 2) docking regime (both in the terminology of O. Plotnikov) in Paleozoic sediments; 3) infiltration water exchange in Mesozoic sediments, as necessary conditions for catching migrating upward hydrocarbons. The decrease in the infiltration waters velocity to insignificant values is interpreted by the authors as a typical phenomenon for "closed" monoclines, within which there was an equilibrium between the pressure of infiltrative waters and the resistance of expultional fluids. This equilibrium is seen as a term of creating conditions for hydrodynamic trapping. The conclusions emphasize the need for comprehensive research to confirm the predictive models of water exchange and obtain data for mapping piezometric minimums, where there is an presumable accumulation of hydrocarbons.