Characteristics and genesis of high-gamma sandstones in the 6th to 9th oil groups of the Triassic Yanchang Formation, Wuqi area, Ordos Basin

Sandstones with high natural gamma-ray (GR) values (also referred to as high-gamma sandstones) occur in the 6th to 9th oil groups of the Yanchang Formation (collectively referred to as the Chang 6‒9 oil groups) in the Wuqi area, Ordos Basin. By conventional log interpretations, these sandstones are often mistakenly identified as siltstones or even mudstones, leading to an underestimation of the effective reservoir thickness. In this study, we conduct a fine-scale lithological division by combining core observations with grain size data. Accordingly, the logging responses, petrological characteristics, physical properties, and oil-bearing properties of high-gamma sandstones in the study area are summarized, followed by an analysis and exploration of their origin. The results indicate that the high-gamma sandstones exhibit significant conventional logging responses, including high GR values, elevated sonic interval transit time, and pronounced negative spontaneous potential (SP) anomalies. In the spectral GR logs, these sandstones show the characteristics of high uranium (U) and thorium (Th) concentrations, along with low potassium (K) concentration. Compared to common sandstones, the high-gamma sandstones generally contain higher contents of feldspar, mica, and clay minerals. Despite comparable porosity, the high-gamma sandstones show slightly lower permeability and oil saturation than common sandstones. Furthermore, their oil-bearing properties tend to decrease with an increase in the concentrations of radioactive elements such as U, Th, and K. The primary factors controlling the formation of the high-gamma sandstones include volcanic activity, sedimentary environment, and clay mineral type. The debris produced by volcanic eruptions supplies abundant radioactive materials for the study area. The northeastern and southwestern parts of the study area show differences in the migration, accumulation, and preservation conditions of radioactive elements, especially U and Th, which exhibit distinct degrees of enrichment under different sedimentary environments. Clay minerals exhibit varying adsorption capacities for radioactive elements. Specifically, illite and mixed-layered illite-montmorillonite demonstrate higher adsorption capacities, whereas kaolinite and chlorite exhibit limited adsorption capacities. Fine-scale investigations of the lithological and developmental characteristics of high-gamma sandstones in the Wuqi area hold great practical significance for increasing hydrocarbon reserves and enhancing productivity in the Ordos Basin, while also providing a key geological basis and theoretical guidance for the exploration and exploitation of similar reservoirs in other basins of China.