Influencing factors and identification method of low-resistivity pay zone in Jurassic, Huanxian area, Ordos Basin

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.