Secondary alterations of deep buried Lower Carboniferous terrigenous rocks in some areas of the Dnieper-Donets basin

Abstract

Problematics. Due to the ambiguous understanding of the nature and void space distribution within reservoir rocks, prediction of prospective deep buried and low permeable reservoirs becomes more uncertain. There are many factors influencing on the pore space, such as primary rock composition, hydrogeological regime, tectonic processes, geological time, epigenetic processes etc. Moreover, in explanation of different factors the influence of secondary reservoirs formation, systematization problems become more essential. Either, it is important to identify ‘working’ conditions of influencing factors to make the prediction of hydrocarbon deposits at great depth more certain. Finding universal criteria of prospective secondary reservoir rocks identification will allow us to enhance the discovery rate of deep buried hydrocarbon deposits. Analysis of previous studies. Many researches were devoted to epigenetic processes in the second part of XXth century. There were described major secondary alterations which may occur at great depth. Recent publications showed the expansion of previous knowledge about epigenetic processes and their influence on the secondary deep buried reservoir formation. Some scientists state that major influencing factors on reservoir quality are tectonic and primary mineral composition, others bring epigenetic imposed processes to the top of influencing factors. Purpose. The purpose of the research is to study secondary alterations and find possible zonation based on the alteration intensity of Carboniferous (Viséan and Serpukhovian) rocks within Central part of the Dnieper-Donets basin. Methods. During this research we studied 43 thin sections of Carboniferous sandstones from cores of Central part of the Dnieper-Donets basin, using polarizing microscope. Alteration stage analyses were used for determination of mineral sequence transformations. Results. Carboniferous (Viséan and Serpukhovian) sandstones discovered by deep wells within Solokhivsko-Dykanskyi and Berezivsko-Kotelevskyi archs in Central part of the Dnieper-Donets depression have been studied. Microscopic studies have shown widespread development of imposed epigenetic alterations in terrigenous rocks. The involvement of laboratory evaluation of porosity and permeability, as well as the values of formation temperatures, allowed us to determine the main trends of secondary processes effect on reservoir properties of rocks. It is shown that the development of secondary microfracturing, kaolinitization and sideritization increases the value of porosity and permeability. Dissolution, regeneration and redeposition of quartz are accompanied by calcitization and lead to the significant decrease of the reservoir properties of terrigenous rocks and formation of secondary epigenetic seals. Sideritization facilitates the development of secondary reservoirs at great depth, meanwhile calcitization leads to decrease of the pore space. Bitumoids and pyrite presence may be the evidence of hydrocarbon migration process. Combination of the research results with thermobaric data retrieved directly from borehole measurements as well as theoretically calculated on Bilske gas condensate field shows that formation of epigenetic seal in the temperature interval of 110-120 °C is caused by quartz dissolution and intensive calcitization of terrigenous rocks. Consequently, secondary reservoir rocks below the epigenetic seal have local spreading caused by microfracturing, sideritization and calcitization. Specific secondary sandy reservoirs of Berezivske gas condensate field are located within 110 and 120 °C isotherm. They occurred in acid environment leading to dissolution of calcite cement and intense sandstone kaolinitization. Such favorable conditions for secondary porosity formation may occur as a result of possible tectonic movements inversion which triggered the development of regressive epigenesis.