Calcite is one the most widely spreading cements in deep sandstone reservoirs in Daqing Placanticline.It has also been regarded as the main factor for facilitating the formation of tight reservoirs in oil-bearing Fuyu layer.Thin section

scanning electron microscope

X-ray diffraction

fluid inclusion and carbon/oxygen isotope analyses were carried out on calcite from cored wells penetrating the layer to identify its isotope composition and genesis.The results show that the reservoirs are mainly feldspar lithic and lithic feldspar sandstone with clay minerals represented by I/S+I+K assemblage.The cement has its δ

13

C(PDB)value ranging between -20.3‰ and -3.2‰ and the δ

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O(PDB)value varying from -25.3‰ to -20.3‰ with an average value of -22.67‰.Calculation shows that the paleosalinity Z is between 73.92 and 109.89

indicating a fresh water environment with high salinity for the formation of carbonate.The temperature for the calcite to be formed is between 80 ℃ and 100 ℃ based on converted temperature of isotopes and homogenization temperature of fluid inclusions.Isotope diagrams reveal that the formation of the calcite cement is probability connected with organic acid decarboxylation

inferring an “organic carbon” origin for the carbon in the cement.