It is discovered by the experimental study on Shan-2 Paleozoic reservoir of Shan-141 well area that the resistivity factor F in Archie's Formula is not only related with the porosity and tortuousity of the pore but also related with the throat of the pore as well. When the throat rate =1

the minimum of n is 1

when the throat becomes larger

n becomes larger accordingly. It is concluded that the smaller the ratio of the pore throat

the lower the Value F is. The size of Value n of the reservoir resistivity index is chiefly effected by the ratio of the pore throat.The theory is that when the pore is full of oil

the layer of the conductor decreases and the length of it increases.