Editing Well logging (section)

===Lithology logs===

====Gamma ray====
{{See also|Gamma ray logging}}
A log of the natural radioactivity of the formation along the borehole, measured in [[American Petroleum Institute#Standards and certification|API units]], particularly useful for distinguishing between sands and shales in a siliclastic environment.<ref name="darling">
{{cite book
  | last = Darling
  | first = Toby
  | title = Well Logging and Formation Evaluation
  | publisher = Elsevier
  | year = 2005
  | location = Oxford, UK
  | url = https://books.google.com/books?id=vKdIGNAE8JYC
  | isbn = 0-7506-7883-6
  | page = 5 p }}</ref>  This is because sandstones are usually nonradioactive quartz, whereas shales are naturally radioactive due to potassium isotopes in clays, and adsorbed uranium and thorium.

In some rocks, and in particular in carbonate rocks, the contribution from uranium can be large and erratic, and can cause the carbonate to be mistaken for a shale. In this case, the carbonate gamma ray is a better indicator of shale content. The carbonate gamma ray log is a gamma ray log from which the uranium contribution has been subtracted.

====Self/spontaneous potential====
{{See also|Spontaneous potential logging}}
The Spontaneous Potential (SP) log measures the natural or [[Spontaneous potential|spontaneous potential difference]] between the borehole and the surface, without any applied current.  It was one of the first wireline logs to be developed, found when a single potential [[electrode]] was lowered into a well and a potential was measured relative to a fixed reference electrode at the surface.<ref name="Etnyre">
{{cite book
  | last = Etnyre
  | first = L.M.
  | title = Finding Oil and Gas from Well Logs
  | publisher = Kluwer Academic Publishers 
  | year = 1989
  | isbn = 978-0442223090
  | page = 220 p }}</ref>

The most useful component of this potential difference is the [[electrochemical potential]] because it can cause a significant deflection in the SP response opposite permeable beds. The magnitude of this deflection depends mainly on the [[salinity]] contrast between the drilling mud and the formation water, and the clay content of the permeable bed. Therefore, the SP log is commonly used to detect permeable beds and to estimate clay content and formation water salinity.
The SP log can be used to distinguish between impermeable shale and permeable shale and  porous sands.