Abnormally high natural radioactivity zones in the main borehole of the Continental Scientific Drilling Project of Cretaceous Songliao Basin: Geophysical log responses and genesis analysis

SK-2 East Borehole (SK-2e) is the main borehole of Continental Scientific Drilling Project of Cretaceous Songliao Basin, which is designed to acquire the geophysical log data in the whole borehole, to provide accurate, comprehensive geophysical information for resource exploration and research on paleoclimate and paleoenvironment. Among various well logging methods, natural gamma-ray logging and natural gamma-ray spectral logging can measure natural gamma-ray intensity and uranium (U), thorium (Th), potassium (K) contents, and are effectively applied to radioactive deposit exploration, especially to uranium exploration and research. Conventional logging (including resistivity, acoustic, density and neutron logging) and special logging (including nuclear magnetic resonance, resistivity imaging and elemental capture spectroscopy logging) can obtain lithologic and physical parameters of strata, offering strong support for uranium exploration. In this study, the natural gamma-ray log data was used to identify abnormally high natural radioactivity zones. Based on conventional log data, special log data and previous geological research achievements, the characteristics and genesis of radioactive abnormality were analyzed. The results show that two abnormally high natural radioactivity zones are located in deep-seated Yingcheng Formation. And their depths are 3096. 8 similar to 3102. 8m (Layer I) and 3168. 3 similar to 3170. 9 m (Layer II) respectively. According to the comprehensive analysis of various log data and core data, Layer I is conglomerate, with the maximum gamma of 360 API and high uranium content (20. 5 similar to 29. 3 ppm). Therefore, Layer I has uranium mineralization potential. Analysis shows that tectonic conditions and epigenetic alteration are probably the key factors affecting uranium enrichment which causes abnormally high gamma-ray intensity. It was speculated that fault movements, volcanic activity, and the basin's uplift and erosion provide uranium-bearing groundwater, oil and gas with the migration channel to I layer. The epigenetic reduction of oil and gas eventually leads to uranium enrichment of Layer I. Layer II consists of agglomerate lava and tuff, with the maximum gamma of 250 API, high thorium content (22.4 similar to 37. 3 ppm) and high uranium content (5. 9 similar to 11. 0 ppm). The rhyolitic components of high thorium content and uranium adsorption by clay minerals may cause the abnormally high gamma-ray intensity in Layer II. The abnormally high radioactivity zones of Yingcheng Formation in SK-2e are characterized by deep burial depths and high uranium contents, indicating uranium prospecting potential in deep zones of Songliao Basin.