Mold pore of anhydrite nodule is the major pore type in the 5

th

member of Ordovician Majiagou Formation

Ordos 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 δ

18

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 δ

18

O and δ

13

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 δ

18

O and δ

13

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 constrast

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.