Sandstone oil reserves are composed of a variety of clay minerals, including kaolinite, illite, and chlorite. These clay minerals have a significant effect of reservoir quality. The upper sandstone member (USS) of Zubair Formation is the most plentiful reservoir of the field and it’s part of a large anticline that belongs to an enormous clastic sandstone formation, from the Lower Cretaceous period. A spectral gamma-ray (SGR) log was used to identify the type of clay minerals, depositional environment, and the relationship between total organic matter with uranium concentration. SGR log indicated that USS is composed mainly of chlorite, smectite, and illite clay minerals with the presence of kaolinite as a dominant clay mineral component. Th/U ratio varies between 2.55 and 8.52 and 1.11 to 11.68 in the north and south parts of the field, respectively. The USS had a fluvially dominated, sand-rich deltaic environment based on the Th/U ratio. Furthermore, Th/K cross-plot was found that the south part was more affected by illite compared with the north part, although the presence of kaolinite.