It is a common law that mudstones have higher GR values and conventional sandstones have relatively lower GR values in clastic rock formations. Therefore, the GR log is widely used to calculate apparent shale content novel for clastic rock. Based on the principles of rock naming, sandstone and mudstone can be distinguished by the apparent shale content cut-off value of 50%. However, according to the principles of GR log, the GR value only reflects the rock’s radioactive character. Thus, some sandstone reservoirs with relatively high radioactive features are often found to be symbiotic of conventional sandstone reservoirs. In order to distinguish conventional sandstone reservoir from high GR sandstone reservoir, the sandstone reservoirs with apparent shale content greater than the cut-off value of 50% are defined as high GR sandstone reservoirs.

According to the GR log characters of conventional sandstone and mudstone in the study area, when the GR value is 110API, the apparent shale content is approximated to be 50%. Thus, in the study area the specific GR value criterion for high GR reservoir is 110API.3. Genesis of High GR Sandstone ReservoirsThe feldspar, clay minerals, and tuff in sandstones usually contain or absorb radioactive elements, so the accumulation of feldspar, clay minerals, and tuff will lead to high GR character for sandstones [4�C11]. In order to determine whether feldspar, clay minerals, and tuff are the direct causes for high GR sandstone reservoirs, core samples are selected from conventional sandstone reservoirs and high GR sandstone reservoirs simultaneously.

Laboratory experiments, such as rock thin section analysis, X-ray diffraction analysis, and whole rock analysis, are processed on these core samples. Comparisons of experimental results between conventional sandstone reservoirs and high GSK-3 GR sandstone reservoirs show that these two kinds of sandstones are feldspathic lithic sandstones (Figure 1). The tuff contents (Table 1), clay minerals contents and types, and feldspar contents and types (Figure 2 and Table 1) are almost the same in high GR sandstone reservoirs and conventional sandstone reservoirs, which indicates that the tuff, feldspar, and clay minerals are not the main and direct causes for high GR sandstone reservoirs.Figure 1The lithology chart: I quartz sandstone, II feldspar quartz sandstone, III lithic quartz sandstone, IV feldspar sandstone, V lithic feldspar sandstone, VI feldspar lithic sandstone, and VII lithic sandstone.Figure 2The feldspar and clay mineral types for high GR sandstone reservoir and conventional sandstone reservoir.Table 1The tuff content, feldspar content, clay mineral content, U, Th, and 40K in conventional sandstone reservoir, and high GR sandstone reservoir.