Diagenetic facies of dolomite and geochemical characteristics across the Cambrian-Ordovician transitions in the Central Uplift Zone, Tarim Basin

Abstract

The identification and division of diagenetic facies is useful for the classification, evaluation and prediction of carbonate reservoirs.To clarify the type and characteristics of the diagenetic facies of dolomite near the Cambrian-Ordovician transitions in the Central Uplift Zone of Tarim Basin, in this work, the lithographic characteristics of dolomite were studied in detail, and the category of diagenetic facies was divided by core description, conventional and casting thin section observation, X diffraction analysis and grain size analysis.Based on this, the geochemical characteristics of different diagenetic facies were discussed, and their formation models were established by testing stable isotopes, which included oxygen and carbon isotope as well as major and trace elements(Fe, Mn, Sr, Li and U).The results show that the diagenetic facies of Cambrian-Ordovician dolomite in the Central Uplift Zone, Tarim Basin, can be divided into five types, including penecontemporaneous dolomatization diagenetic facies(PD-F), shallow latent reflux dolomatization diagenetic facies(SLRD-F), shallow active reflux dolomatization diagenetic facies(SARD-F), weathered crust karstification diagenetic facies(WK-F) and tectonic fracture diagenetic facies(TF-F).Among them, the dolomitic fluid of PD-F mode dolostones mainly come from saltwalized seawater and is affected by freshwater, which shows an obviously negative bias of oxygenisotopes and high Fe, Mn, Sr and Li contents. The dolostone formed in the SARD-F mode shows a consistent value with Ordovician sea water and relatively low Fe, Mn, and Sr contents because it mainly formed in normal sea water environment and individually experienced burial dissolution and recrystallization and was affected by freshwater.Because the diagenetic environment of dolostone formed in SLRD-F mode is relatively closed, this dolostone is characterized by consistent values of oxygenisotopes, relatively low Fe and Mn contents but high Sr contents.Dolostone samples formed in WK-F mode have high Fe and U but low Sr contents because they experienced dissolution of freshwater and burial dolomitization.The dolostone formed in the TF-F model is characterized by low Fe, Mn, U and Li because of the influence of structural hydrothermal fluids.

Keywords

• diagenetic facies of dolomite
• geochemistry
• cambrian-ordovician
• tarim basin