Editing Well logging (section)

====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.