Abstract: The ultra?low permeability reservoir of Chang81 in Zhenbei oilfield of Ordos basin is characterized by distinct formation resistivity which in west part of the reservoir is obviously lower than that in east part, even part of it closes to that of aqueous layers, which allows the western reservoir’s pay zone to be difficultly identified and developed. This paper analyzed the genesis of the resistivity differences in this reservoir. The results show that the causes of lower resistivity in the western reservoir compared with the eastern reservoir include three aspects as below:1) The salinity of formation water in the western reservoir is higher than that in the eastern reservoir;2)the western reservoir contains higher content of illite and chlorite with stronger capacity of cation exchange and additional conductivity; and3) it contains higher clay minerals and more complex pore structures as well as weaker hydrocarbon accumulation dynamics, which result in higher irreducible water saturation in it. This could be of significance for exploration and development of ultra?low permeability reserfvoirs