Lithology Interpretation

Spontaneous Potential (SP) or/and Gamma Ray (GR) curves are included in nearly all Log.

There is no direct relationship between the value of permeability and porosity and the magnitude of the SP deflection.

SP scale is in mV (20mV). Positive reading will deflect to the right and negative reading will deflect to the left. In the case of fresh water mud and saline water in the formation (limestone), the curve will deflect to the left. If the salinity of the mud is higher than the water in the sand formation (shale), then the curve will deflect to the left.

In quantitative formation evaluation, SP and GR curves are used to determine potentially porous and permeable reservoir rocks like sandstone, limestone, and dolomite from non-permeable clays and shales as well as defining bed boundary.

GR curve is the measure of natural radioactivity of the formation. Because radioactive components tend to concentrate in shales, the GR normally reflects the shales content of the formation. Low reading reflects limestone, sandstone, and dolomite. However, a clean shale-free sandstone can produce high gamma ray response if it contains radioactive contaminants such as micas, potassium feldspars, glauconite, or uranium-rich formation waters which it this case will need a gamma ray spectral log to avoid the sandstone being overlooked. This GR spectral log will identify the concentrations of potassium, thorium, and uranium. Ratios of Th/U is used for identifying environment of deposition. U/K indicate potential source rock, Th/K is useful in typing clay.





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