LAUSR

In the southwestern Ordos Basin, the formation water salinity and resistivity of Jurassic sandstone reservoirs are characterized by rapid changes and great variations, which lead to difficulties in the identification of reservoir fluid types and in the development and exploration of hydrocarbons in this area. To explore appropriate methods for reservoir fluid identification provided with complex formation water salinity and resistivity, we conducted three research steps. Firstly, using formation water testing and well logging data, the formation water salinity, spontaneous potential, and reservoir resistivity were used to calculate the formation water resistivity. Secondly, the calculation results were used to separate reservoir fluid types. Thirdly, the calculated formation water resistivities were classified and the relationship of the formation water resistivity and oil-producing interval was established. The results showed that, for oil-producing intervals, the formation water resistivity calculated using the reservoir resistivity is the highest, while that using the formation water salinity is the lowest. Based on this understanding, two identification methods of reservoir fluid types were proposed, including the qualitative curve overlap method and the quantitative ternary graph method. The curve overlap method can be applied to visually distinguish reservoir fluid types through the differentiation of formation water resistivity. Compared with some earlier quantitative methods, the ternary graph method can achieve an efficiency of 87.7% in identifying the type of reservoir fluid. Finally, the reliability of both methods was verified by case studies.