Abstract: Extra low permeability tight oil & gas reservoirs widely exist in middle-deep formation of the western South China Sea. This type of reservoirs presents good gas measurement and oil bearing display, medium porosity and high resistance characteristics. But the pressure measuring points are mainly dry points and tight points. The pressure fluidity and core permeability are very low. So it is difficult to obtain undisturbed formation fluids through conventional sampling, which brings great difficulties to logging fluid identification and reservoir evaluation. Therefore, based on core analysis data, this artical focuses on analyzing of the cause of extra low permeability reservoirs combined with logging curves, pressure measurement and formation testing. Through the proven extra low permeability tight oil layers, a set of lower limit standards about reservoir parameters based on measures is established. Based on logging curve characteristics of mud invasion under different drilling fluid systems, the reservoir permeability is analyzed and the fluid properties are ascertained. Finally, combined with the data of phase permeability and semi-permeability diaphragm, the regional irrestrictable water and residual oil saturation model is established, and the fine evaluation of the reservoir is carried out to find the sweet spot of the reservoir, which lays the foundation for development and production.