Abstract: The Chang 6₁ reservoir in the Jiyuan area, the Ordos Basin, is of typically low porosity and low permeability. Its complex pore structure, high irreducible saturation and formation water salinity make the oil reservoir with low contrast and bring great difficulties to logging identification. The genetic mechanism of the low contrast Chang 6₁ oil reservoir was analyzed in terms of rock lithology, reservoir properties and formation water properties, and two types of genetic mechanisms are:First, the high content and complex distribution of the clay in the interstitial material in the Chang 6₁ reservoir resulting in complex pore structure and high irreducible saturation which leads to additional conductivity; second, high water salinity in the Chang 6₁ reservoir. These two factors contributed to the low contrast on logging data, and resulting in the low contrast oil reservoir. Then based on the identified genetic mechanisms, the normal resistivity distribution of apparent formation water and the Fisher discrimination and analysis method were used to identify fluid properties and solve the problem in logging identification. And finally a water productivity prediction model is established based on the abrupt change in the linear neuron Sigmoid function to calculate water productivity and conduct quantitative classification and evaluation of the Chang 6₁ reservoir, and the detailed classification and evaluation standards are proposed for fluid properties. Application to 64 wells in the H57 well area in the study area indicates that the calculated and evaluated water productivity results from using the standards proposed in this paper are correlated to the initial water productivity by a factor of 0.917. This proves that the model precision and classified results meet the need of practical exploration and production.

Key words: Ordos Basin, Jiyuan area, low contrast oil reservoir, genetic mechanism, identification method, classification and evaluation