Abstract: Ordovician reservoir is the most important exploration and production block of Tahe Oilfield of Tarim Basin，which
has undergone multiple-period tectonic movements and paleo-karstification effects and become karst-vuggy carbonate reservoir.
The main lithologies of Yijianfang and Yingshan Formation group are calcite and dolomite. The reservoir is typical super-low
porosity and low-permeability. Investigations of outcrops，cores and scanning electron microscope（SEM）images show that in
this kind of reservoir，matrix is almost non-porous and non-permeable. The main reserving spaces，however，are fractures and
vugs，which resulted from tectonic deformation movement and，pores，caves，which occurred through karstification effects.
Among these，caves are most important spaces for hydrocarbon. Fracture is another key element for this reservoir，which has
double effects，one is that the fracture provides reserving space by itself，and the other，it is tunnel for hydrocarbon migration.
Because of the pore morphology，different size and uneven distribution of reserving spaces，this kind of reservoir has very
distinguished heterogeneous properties. We studied the characteristics of the fractures，and analysis the reason of fractures
occurring，the distribution law of fractures. By using geological simulation software，we adopted sequence Gaussian randomizing
methods，to model the fracture parameters，such as porosity，permeability and open width. The three-dimensional images
of fracture parameters model shows seriously heterogeneous of this reservoir. The simulation result shows that the fractures
modify the permeability of the Ordovician reservoir evidently.

Key words: Tahe Oilfield, Ordovician Formation, Carbonate reservoir, fracture, randomizing modeling