--> The Role of Hypogenic Karst in Formation of Carbonate Reservoirs and Development of Oil Deposits

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The Role of Hypogenic Karst in Formation of Carbonate Reservoirs and Development of Oil Deposits


It is known that the world's largest oil and gas fields in carbonate reservoirs are located in the Middle East, Europe, Africa, Asia, Australia, North and South America. Many the oil and gas deposits are found in karst reservoirs from Proterozoic to Miocene. Study of karst process in the reservoirs has been founded that both pervasive faults and hydrocarbons’ vertical migration with thermochemical sulfate reduction (TSR) play an important role in formation and evolution of the hypogenic (deep) karst. The available geological data of the fields show that the volume of large cavities and karst caves in such carbonate reservoirs can reach 38%, and the daily oil production of wells more than several 10,000 bpd. Vertical migration of light hydrocarbons from oil-gas reservoirs to the surface is essential to all founded oil and gas deposits. Possible mechanism of the vertical migration involves the vertical ascent of ultra-small gas bubbles through a network of interconnected micro fractures, fractures and faults, that have been called the vertical fracture corridors. High pressure and displacement of rocks during the folds formation promote the development of “mechanic chemical reaction” between hydrocarbons and sulfates. First mechanic chemical reactions have been investigated by P.W. Bridgman (1949). Using modern equipment, we reproduced the TCP in the laboratory. It was found that under conditions of high pressures and shear deformations, the reduction of sulfate by hydrocarbons proceeds even under conditions of low temperatures. Thus, at high pressures and shear deformations, it is not the thermochemical reduction of sulfates (TSR), but the chemical reduction of sulfates (CSR). These reactions are proceeded under high pressure and shearing stress which can arise into vertical fracture zones during the tectonic activity of the folds. The reaction between hydrocarbons and sulfates proceeds in two stages according to Engler-Gofer scheme: P+mechanical activity CaSO4 + CH4 → CaS + CO2 + 2H2O CaS + CO2 + H2O → CaCO3 + H2S Ascending fluids carrying CO2 and H2S reach the surface in the vicinity of the karsting carbonate rocks. The CO2 and carbonic acid dissolution is balanced by subsequent calcite cementation. The H2S arrives to the oxidation zone, sulfuric acid is formed which subsequently causes major dissolution and karstification. Significant pyrite mineralization during cementation filling the fractures is a strong indication of H2S formation after generation fractures. The reduction of sulfates by hydrocarbons according to this CSR scheme is confirmed by isotope determination of the secondary calcite and sulfur in karstified carbonate oil and gas reservoirs. We can use this new understanding of fracture-karst zones formation and its distribution to give clear and valuable recommendations for improvement of exploration and development of carbonate reservoirs in different oil-gas bearing basins.