Identification of an oil-bearing layer by formation water resistivity: A case study of the Jurassic reservoir, Southwest Ordos Basin

In the southwestern Ordos Basin, the formation water salin-ity and resistivity of Jurassic sandstone reservoirs are charac-terized by rapid changes and great variations, which lead to difficulties in the identification of reservoir fluid types and the development and exploration of the hydrocarbons in this area. To explore appropriate methods for reservoir fluid iden-tification provided with complex formation water salinity and resistivity, we have conducted three research steps. First, using formation water testing and well-logging data, the formation water salinity, spontaneous potential, and reservoir resistivity are used to calculate the formation water resistivity. Second, the calculation results were used to separate the reservoir fluid types. Third, the calculated formation water resistivities are classified and the relationship between the formation water re-sistivity and the oil-producing interval is established. The re-sults indicate that, for oil-producing intervals, the formation water resistivity calculated using the reservoir resistivity is the highest, whereas that using the formation water salinity is the lowest. Based on this understanding, two identification methods of reservoir fluid types are developed, 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 differen-tiation of formation water resistivity. Compared with some ear-lier quantitative methods, the ternary graph method can achieve an efficiency of 87.7% in identifying the type of res-ervoir fluid. Finally, the reliability of both methods is verified by case studies.