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(A) Figure showing gamma-ray and SP (self-potential or spontaneous potential) logs through an interbedded interval of sandstone (SS), shale (SH), limestone (LS), and dolomite (DOL). The effects of lithology are seen on the two logs in the picture. Gamma-ray counts (in API units of measure) are high for shales and low for sandstones and dolomite. The same is true for the SP log curve. Thus, both logs are lithology indicators. The gamma-ray log has a better resolving power than the SP log for detecting beds of a certain thickness, so the gamma-ray log provides a better characterization of lithology and bed thickness than does the SP log. (B) The density log measures the density of the rock and its contained fluids. Thus, the density log is sometimes referred to as a porosity log. Different fluids, particularly gas, can have a pronounced effect on the density measurement, as is shown on the diagram. Limestones and dolomites tend to have a higher density than do sandstones of the same porosity.
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