Authigenic Carbonate Mineral In The Early Tertiary Sandstone Of South-central XiHu Depression

By means of thin-section analysis, cathodeluminecenme analysis, element analysis, isotope analysis and SEM examination, authigenic carbonate mineral in the early Tertiary sandstone of south-central XiHu depression was studied here. Based on those experiments, the petrology character, mineral character and geochemistry character of authigenic carbonate mineral is analysed, the effect on the reservoir quality for carbonate cements is evaluated, and its formation mechanism is discussed.Carbonate is the most abundant authigenic mineral in the early Tertiary sandstone of south-central XiHu depression, kaolinite secondly. The authigenic carbonate mineral mainly consists of calcite, ankerite and siderite. The higher amount of calcite cement, the lowerδ¹³C value, and the closer relationship with organic acid. Meantime, with lowerδ¹⁸O value, the formation temperature of mosaic calcite is higher, and the formation time is later. Theδ¹⁸O mean value for ankerite is 6.4‰higher than mosaic calcite, means that the the formation temperature of ankerite is lower, and its formation time is earlier. Theδ¹³C value for ankerite cement (mean value is -2.8%o) is higher than mosaic calcite cement visibly, and nearby theδ¹³C value of marine carbonate(0‰), that is to say, the material resource for ankerite cement is from marine carbonate and meteoric CO₂ partly.Compaction processes and cementation, as the two most important diagenesis effect factors on reduction the porosity, are judged here, and showed that compaction processes is more important for the reservoir sandstone on reduction porosity than cementation. With a graph of intergranular volume vs. cement assessed, the rocks, which are cemented by calcite or ankerite, is caused by cementation predominantly on its porosity reduction. Calcite cement occludes intergranular volume; the ankerite formed early stage with more secondly porosity can improve the mechanical strength and anti-compact power for rocks, and stick up to the futher compaction later.Little mosaic calcite with more intergranular volume is formed in the early diagenesis stage when the rocks did not compact effectively, and its formation mechanism is connected with the hydration of aluminosilicate; most mosaic calcite cement with high minus cement porosity is formed in the burial diagenesis stage when organic material developed extensively carboxylic acid and feldspar dissoluted widely. The ankerite cement deposited on early diagenesis stage with meteoric fluids dissolution, and partly came from marine carbonate material perhaps. Siderite deposited on iyngenesis stage.