Using the Modified Resistivity–Porosity Cross Plot Method to Identify Formation Fluid Types in Tight Sandstone with Variable Water Salinity

It is generally difficult to identify fluid types in low-porosity and low-permeability reservoirs, the Chang 8 Member Ordos Basin a typical example. In of Yanchang Formation Zhenyuan area Basin, affected by lithology physical properties, resistivity oil layer water are close, which brings great difficulties type identification. this paper, we first analyzed geological petrophysical characteristics study area, found that high clay content one reasons for low-resistivity pay layer. Then, formation salinity were studied. The was mainly CaCl2, had difference ranging from 7510 ppm 72,590 ppm, main cause According reservoir logging response characteristics, saturation boundary layer, oil–water determined be 30%, 65% 80%, respectively. We modified traditional resistivity–porosity cross plot method based on Archie’s equations, established three basic plates with variable salinity, above used application results indicate agrees well perforation test data, can effectively improve accuracy plate 88.1%. findings help better understanding identification evaluation.