Abstract: Carbonate rock strata in well Xichen-1, Xisha islands consist of 3 layers of dolostone and 4 layers of limestone. In dolostone layers, the micritic dolomite, containing 91.1%-100% of dolomite, can distribute in all kinds of contents such as framework grains and inter-grain fillings. Isotopes of the dolomite are as follows, δ¹³C varies from 0.80‰ to 3.16‰, averaging 2.24‰; δ¹⁸O varies from 0.56‰ to 5.23‰, averaging 2.56‰. These characteristics reflect that dolomitization occurred in synsedimentary heavy brine environment when paleosalinity is 36.88% and paleotemperature is 1.07℃. The limestone layers with dominant calcsparite contain dolomite in general less than 25%, in which dolomite are concentrated in pore or fissure fillings and absent in framework grains. The fluid inclusion homogenization temperature measured in domolite range 102.5-296℃ with an average of 156.3℃; the salinity is 0.55%-6.25%, averaging 3.75%; the density is 0.95-0.99 g/cm³, averaging 0.97 g/cm³; the pressure is 20.65-42.65 MPa, averaging 33.65 MPa. These characteristics reflect that dolomitization may be in connection with post diagenetic hydrothermal event.

Key words: carbonate rock, dolomitization, formation mechanism, well Xichen-1 in Xisha islands