Role of gamma-ray logs in deciphering geochemical and geological aspects

What is it about?

Spectral gamma-ray tools present the concentrations of the three radioactive elements responsible for the natural gamma radiation, which are thorium (Th), uranium (U), and potassium (K), separately. This separation can be interpreted geochemically and geologically to identify some formation characteristics, such as lithologies, clay types, fractures, unconformities, and hydrocarbon potentialities. In this study, the geochemical and geological contributions of the natural spectral gamma-ray spectrometry (NGS) log were interpreted to characterize the Rudeis Formation in well Amal-9, Amal field, the southern province of the Gulf of Suez, Egypt. The interpretation was supported by other logging tools, cuttings description, and Rock-Eval pyrolysis. The study revealed that reducing conditions were predominant during the deposition of most of the stratigraphic sequence of the Rudeis Formation. The sediments of the Rudeis Formation were identified into four lithofacies; each one has its own diagnostic spectral gamma radiation. The lithofacies are composed of sandstone (lowest radiation), limestone, shale, and marl (highest radiation). The U content can be considered as the main contributor to the radioactivity of the Rudeis Formation. Furthermore, there is a relationship between the U and the TOC contents, implying that U can be used as a proxy for organic richness shale and as an indicator for good source rock intervals in this formation. As indicated from several crossplots, the clay minerals are mainly represented by montmorillonite with a low percentage of kaolinite and illite. Several unconformities were detected, based on the dipmeter data and observing the abrupt change in both the Th/K ratio and U curves.

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