The main reservoir can be affected by high radioactivity minerals in the T oil field of Middle East; as a result, shale content cannot be calculated exactly by natural gamma log data. In addition, it is difficult to confirm clay types of each main oil-bearing reservoir because of insufficient core analysis data. To tackle these problems, the authors used the close relationships between clay minerals and uranium, thorium, potassium from natural gamma ray spectrometry log to reasonably calculate argillaceous matter content by thorium and potassium content. According to the interpretation charts of Schlumberger, the clay types were defined by using Th-K crossplots, and depositional environment was studied by making Th/U histogram, with satisfactory results achieved. The results obtained by the authors provide dependable reservoir evaluation data for late development of oil field.