EVOLUTION OF THE TYPE OF CARBONATE CEMENTS IN LOWER TERTIARY SANDSTONE OF GAO YOU DEPRESSION IN NORTHERN JIANGSU AND ITS GEOLOGICAL SIGNIFICANCE

The thin section study and electronic scanning microscopy were applied to carbonate cements in sandstones of oil layers in lower Tertiary of Gao You depression in northern Jiangsu.Based on the characteristics of evolution,five types of carbonate cements are determined.1.Micritic carbonate 2.recrystalliz-ed carbonate 3.dissolution carbonate relics 4.ferri-carbonate precipitates 5,Mosaic replaced carbonate.The relationship between the origins of various types of caronates and diagenetic stages and the affects on reservoirs were discussed.The author thought ferri-carbonate is likely to precipitate in the forms of Fe(OH)₃ and H₂S in the process of acidulation during field deve-loment,which would damage oil formation.