Carbonate Cements In Reservoir Sandstones Of Paleogene And Genesis Of Lishui Sag

By use of thin-section analysis, cathodeluminecenme analysis, and isotope analysis and so on, the carbonate cements and its formation mechanism in the early Tertiary sandstone of LiShui sag are discussed in this paper.Based on all the experiments, we research the classification, distribution patter, petrology and geochemistry of carbonate cements of the sandstone reservoir in Paleocene and Eocene (Ojiang Formation).Carbonates are the most abundant cements in the Palaeogene sandstones of Lishui Sag, East China Sea basin, which mainly consist of ankerite and calcite and have distinctive stratigraphic distribution and geometry. The ankerite cement just occurs beneath the unconformity of Ojian movement and nearby coal beds; and the calcite occurs at relatively deep bed far from the unconformity. The stable-isotope data for ankerite are δ ¹³C = -2.1 to -2.5‰ VPDB, ⁸⁷Sr/⁸⁶Sr=0.708416 and 0.708520, δ ¹⁸O = -3.7 to -4.1‰ VPDB (precipitation temperature is 37 to 39℃); and those for calcite are δ ¹³C = -2.4 to -17.3‰ VPDB (most of the samples are less than -10‰ VPDB), 8 ¹⁸O = -6.8 to -15.3‰ VPDB (precipitation temperature for most of samples is more than 90℃).On the basis of geologic setting, province, and buried history, we have the following suggestions: a) the precipitation of ankerite was from meteoric fluids and some kind of acids from coal bed during early diagenesis, input into sediments during times of relative sea-level fall; b) the source of ankerite cement was dissolution of coeval carbonate matierals; c) the precipitation of calite cement was from buried fluids during later diagenesis; d) the source of calite cement was transformation of clay minerals and dissolution of feldspar and other aluminosilicates; and e) the earlier cementation by ankerite have enhanced the mechanical strength of rocks and resistance ability to compaction, which is positive significance for the Preservation of porosity in deeply buried condition.