Dolomite of the Dengying Formation in southern Sichuan Basin has been studied a lot as an important kind of reservoir rocks for oil and gas.However

origin of the dolomite is still in controversy.This paper studied the geochemical characteristics and fluid origin of the dolomite in the Dengying Formation in southern Sichuan Basin through analyses of C-O isotopes and trace elements.The results show that the main types of dolomites are micritic dolomite

fine-medium dolomite and breccia-dolomite.The δ

13

C and δ

18

O values are 0.52‰~2.06‰ (PDB) and -11.9‰~-2.14‰ (PDB)

respectively

which are lower than the original values of δ

13

C (4.43‰) and δ

18

O (-0.62‰) of carbonates of the same period.The dolomite has high U/Th ratio

low Sr and high Eu anomalies.It is believed that the dolomite formed in a burial diagenetic environment and was altered by hydrothermal fluid later.REE distribution patterns of the dolomite is similar to the dolomite originated from seawater.Salinity indexes(Z)of the dolomite is higher than those of normal seawater.Therefore

it can be inferred that the dolomitization fluids mainly originate from the condensed seawater trapped in the Formation.