Role of rock carbonation in formation of hydrocarbon deposits in Western Siberia

The paper discusses relation of carbonate content in Jurassic and Neocomian reservoirs of certain West Siberian regions (northern part of Middle Ob, Urengoy) to the formation of oil pools by means of water-oil emulsion vertical migration in carbon dioxide medium from the deep-seated “intermediate” reservoirs. Because of pressure decrease and СО2 release, the higher carbonate content (dolomite, calcite, siderite) is formed in oil-bearing beds. They are capable of luminescing in yellow colour, which is typical of oil. In pay zones having good reservoir properties and substantial volume, this phenomenon is more notable (e.g., БС10, БС11, БС12, Ю1 beds); while in the lenticular beds having the worse reservoir properties (Achimov sequence, Ю2 bed) it is less notable. Owing to the fact that pressure of the deep-seated mixture in reservoir zones having higher reservoir quality and net volume decreased most significantly, these areas are enriched with carbonates. The direct relationship of carbonate content and oil occurrence allows concluding that vertical migration of gas-water-oil emulsion influenced the formation of oil pools in Western Siberia, and carbon dioxide that provides the mobility of oil, played an important role. The direct relationship of scattered carbonate content in reservoir rocks and porosity was occasionally observes, which is also caused by deposition of carbonate cement in the most permeable reservoir parts. This type of relationships is observed for sandstone having higher porosity. At the same time, the opposite, more usual trend of porosity decrease with rock carbonate content increase is also documented. This results in sealing low-porosity rock’s pores with carbonates; in certain conditions this causes formation of carbonate barriers that contribute to formation of combination-type oil pools. Hence, why the wide studies of terrigenous-carbonate rocks of reservoir formations is one of the topical tasks of the applied petroleum geology