Closed-system dolomitization and the significance for petroleum and economic geology:An example from Feixianguan carbonates,Triassic NE Sichuan basin of China.

Dolomitization and origin of dolomite are still controversial in carbonate sedimentology.Dolomitization can enhance porosity and permeability of tight limestone and dolomites are the most important host rocks for base metal mineral deposits.Therefore, dolomitization and origin of dolomite are paid more attention by petroleum and economic geologists.Crystalline dolomite acts as the good reservoir in Feixianguan Formation of Triassic,NE Sichuan Basin of China,which formed by the dolomitization in a closed-system through the burial diagenesis.The high porosity in the rock is related to the dolomitization reaction in the closed-system:2CaCO+3+ Mg~(2+)= CaMg(CO_3)_2+Ca~(2+),which produced the pore fluid with very high Sr concentrations.The thermochemieal sulphate reduction occurred in deep buried diagenesis is responsible for the further enrichment of Sr.The origin of the large scale or super-large scale eelestite deposits in Triassic is also related to the closed-system dolomitization and TSR.Very low Mn concentrations(average 20 ppm,approximately)and theδ~(13)C values similar to coeval seawater in crystalline dolomite show that major source for dolomitization fluid must be marine fluid,and there is no meteoric water intervention during dolomitization.Relatively lowerδ~(18)O values indicate that dolomitization could occur during the time of later burial diagenesis.Most of dolomitization models are based on the subaerial diagenesis of carbonate,which should be dominated by an open environment.The dolomitization(or dolomite precipitation)in an open-system may take place by the reaction:CaCO_3+CO_3~(2-)+Mg~(2+)=CaMg(CO_3)_2,which can not enhance porosity and permeability of limestone,and is not benefit for the stratabound deposits.The dolomitization for micrite dolomite and grainy dolomite(with original texture)in Feixianguan Formation of Triassic,NE Sichuan Basin of China could be controlled by an open-system dolomitization with the influence of meteoric water.