Keywords: 
• cell parameters /
• carbon oxygen isotope

Abstract: Based on observation of core of the Middle and Lower Ordovician carbonate in Wells S85 and S79, a wide rangegeochemical methods has been used to characterize the diagenetic settings of paleocaves and fractures filling & vugs calcites and to decipher their origin, which includes Xray diffraction of crystal cell, carbon and oxygen isotope composition, fluid inclusion, hydrogen isotope of fluid inclusion and chemical elements analysis. Two diagenetic environments are defined and believed to be prevailing in the formation of paleocaves and fractures filling & vugs calcites. the first is diagenetic environment with prevailing hypergen fresh water, mostly related to phreatic zones in subaerial process and of characteristic of crystal cells c=17.057~17.062 and Fe2O3＝0.06%~0.07%，Sr=53.3×106~96.2×106,Ba=19×106~28×106，Mg/Ca(×103)＝1.94~5.14，Sr/Ba＝2.12~5.24 , the lowest values of δ18OPDB (15‰~17‰) , the considerable lower values of δ13C PDB (4‰~2‰) for calcites, without or with few hydrocarbon inclusions and δD PDB＝94.99~109.54‰ of its fluid inclusion; the second is believed to occur in freshwaterseawater mixing and formed during some degree of burial environment, and of features of crystal cell parameters (c)=17.064~17.065(exceptional for 17.212),the considerable high values Fe2O3＝0.06%~0.18%，Sr=111×106~208×106、Ba=215×106~479×106，the considerable lower ratios Mg/Ca(×103)＝1.53~1.76，Sr/Ba＝0.49~0.58 for calcites, the lower values of δ18OPDB (9.6‰~13.7‰)for calcites, the considerable higher δD PDB＝77.5‰~88.2‰ of fluid inclusion. In addition, the study also demonstrated that differential hydrogeochemical systems had been developed in the formation of the gigantic speleothem calcite crystal at the 5 960～5 966 m depth of burial with more than 6 m in height palaeocave in Well S85 in Tahe oil field.