Types and origin of pore-fillings from the 5 th member of the Ordovician Majiagou Formation in Ordos Basin

Mold pore of anhydrite nodule is the major pore type in the 5th member of Ordovician Majiagou Formation ,Or-dos Basin .The fillings of the pore consist mainly of calcite ,very fine crystalline dolomite ,and coarse crystalline dolomite , as well as a small amount of quartz ,anhydrite,kaolinite,etc.The calcite,fully filling the pores,is mostly coarse crystalline and shows yellow color under cathodoluminescence .Its moderate Mn content ,a relatively low Fe content and the obviously negative δ18 O values indicate that the calcite deposited in atmospheric environment .The very fine crystalline dolomite , usually partly filling the pores ,is similar to matix dolomite in mineralogy and origin .It shows two color types ( one is light orange and the other is dark red similar to matix dolomite ) under cathodoluminescence .The very finecrystalline dolomite can be divided into two parts under electron probe backscatter:the core and the cortex .The Fe and Mn content of the core are close to that of the matrix dolomite ,while the cotex has the highest Fe and Mn content in carbonate minerals . Furthermore,its δ18 O and δ13 C values are more negative than that of the matrix dolomite .These features illustrate the change of diagenetic fluids .The coarse crystalline dolomite , blue after dyeing , has higher Mn and Fe content , relatively negative δ18 O andδ13 C values,suggesting a possble late hydrothermal fluid origin .Different pore-fillings in the reservoir are the results of different fluids .The calcite is possibly the result of meteoric water filling in hypergenic stage and has certain relatonship with karst paleogeomorphology .It commonly occurs in low-lying areas with surficient fluids .In con-strast,the very fine crystalline dolomite is related to sedimentary environments and alterated by hydrothermal fluids .The fromation of coarse crystalline dolomite as a result of hydrothermal fluids associated with fractures .