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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.

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<p>The Ordos Basin is located in the central and western part of China, which is rich in oil resources in Mesozoic strata. Huanxian area is located in the west of the Ordos Basin, covering an area of about 3000 km<sup>2</sup>. With the wide distribution of Jurassic low resistivity reservoir, it is difficult to identify reservoir fluid by logging, which restricts the efficient promotion of oil resources exploration and development in this area to a certain extent.</p><p>Based on the basic geological law, this study makes full use of the data of oil test conclusion, production performance and formation water analysis to deeply analyze the genesis of low resistivity reservoir in this area. The average formation water salinity of Jurassic in Huanxian area is 63.5g/l. Through the correlation analysis of mathematical methods such as fitting and regression, the formation water salinity and reservoir apparent resistivity show a good negative correlation in the semi logarithmic coordinate, and the correlation coefficient is 0.78. Therefore, it is considered that the main controlling factor for the widespread development of low resistivity reservoir in this area is the high formation water salinity. Irreducible water saturation, clay mineral content and nose bulge structure amplitude are the secondary controlling factors for the development of low resistivity reservoir in this area, and their correlation coefficients with apparent resistivity are 0.23, 0.25 and 0.31, respectively.</p><p>On the basis of clarifying the genesis of Jurassic low resistivity reservoir in Huanxian area, the comprehensive identification of reservoir fluid type by logging is carried out. For the whole area, there are obvious differences in geological characteristics, so conventional methods such as cross plot method of acoustic time difference and apparent resistivity can not effectively identify reservoir fluid. According to the main controlling factors of reservoir apparent resistivity, the salinity of formation water is combined with apparent resistivity and resistivity index of reservoir respectively to establish the cross plot. Using these two kinds of cross plot, the accuracy of reservoir fluid type identification is 62.9% and 88.6% respectively. This method can meet the accuracy requirements of reservoir fluid identification, realize the rapid identification of reservoir fluid types in the whole area, and provide technical support for efficient exploration and development of Jurassic low resistivity reservoir in this area.</p>

Huanxian area is in the west of the Ordos Basin and contains a widely distributed Jurassic low-resistivity pay zone. Due to the lack of a systematic summary of the influencing factors of the Jurassic low-resistivity pay zone in this location, oilfield engineers have not established a logging rapid identification method, increasing the complexity of oilfield exploration and development. Therefore, we reveal the main influencing factors and propose a corresponding logging identification method. Through a groundwater chemical composition test, phase permeability experiment, cast thin-section analysis, seismic data, and logging data, we have obtained the groundwater salinity, irreducible water saturation, clay mineral content, tectonic amplitude, and resistivity and apparent resistivity index of sandstone. The results of the correlation analysis indicated that the groundwater salinity is the main control factor for the development of the Jurassic low-resistivity pay zone in the study area, whereas other factors (such as irreducible water saturation) are secondary influencing factors. We have created a crossplot for groundwater salinity versus sandstone resistivity to identify the Jurassic low-resistivity pay zone based on clarifying the main control factor. The identification accuracy of this crossplot has reached approximately 65% through oil testing conclusions in the study area. To further improve the application, we have replaced sandstone resistivity with an apparent resistivity index in the crossplot. As a result, we have improved the identification accuracy of the Jurassic low-resistivity pay zone to more than 80%, which can meet the production demand. The results of this study can provide technical support for the exploration and development of the Jurassic low-resistivity pay zone, which is significant for optimizing various oil and gas resource production structures and coproduction methods in this location.