I read that theSpectral Gamma Ray tool is preferred for logging micaceous sandstones.They appear shaly on a standard gamma ray log but can be distinguished from the results of a spectral gamma ray log because the radiation is all from radioactive potassium.Clay minerals such as Illite are also rich in radioactive potassium.
The reason formentioning micaceous sandstone is that with reference to the excellent paper by Oesterlen and Millsteed (1994), the Alternations Member (Upper Angwa) is agraded sandstone, and the main rock type is fine-grained micaceous sandstone,and is well sorted.
The sandstone grades upwards to siltstone/mudstone composed mainly of kaolinite and Illite.Thin beds of micritic limestone can also occur frequently.Large swamps were also a feature of the Alternations Member flood plains, their presence manifested by the coal and carbonaceous mudstone.
Once identified from the spectral gamma ray log, mica will cause an increase in resistivity of sandstones.Illite has the opposite effect of decreasing resistivity, making Sw appear too large. The petrophysicist perform log corrections to remove the effect of these minerals.
