Anhydrite columnar crystal and nodules of millimeter to centimeter sizes are widely developed in the crystal powder dolomite of Ma 5

1

-5

4

(the 1

st

to 4

th

submembers of the 5

th

member of Majiagou Formation) in the gas fields of the central Ordos Basin

and are critical to reservoir development.The occurrence

genesis and evolution of anhydrite were finely analyzed for the first time through detailed subsurface geological survey

observation of typical core section

system sampling and testing

indoor comparative analysis

and so on.The anhydrite columnar crystals and small nodules occur in crystal powder dolomite.The columnar crystals are scattered in the dolomite

while the small nodules have three typical distribution forms

namely medium nuclear diameter in middle-high nucleation number

small nuclear diameter in middle-high nucleation number and large nuclear diameter in low nucleation number.All of the anhydrite minerals mentioned above are the products of metasomasis of early dolomite during penecontemporaneous or shallow burial periods in an restricted (inside) continental shelf environment.of the amount of anhydrite minerals depends on the seawater salinity at the beginning of anhydrite precipitation

while their sizes are controlled by the duration of seawater salinity that is suitable for the growth of anhydrite minerals.Moreover

different evaporite mineral associations may experience different process of modification.Residual moldic pores and associated cracks are highly developed in the coarse crystal powder dolomite of medium nuclear diameter in middle-high nucleation number

which finally evolved into the high quality fractured-vuggy reservoir in the area.