Spontaneous Potential Log

By Richard Yeomans

The Spontaneous Potential (SP) Log is is a fairly standard tool when performing a LWD procedure. It measures the spontanious potential, or the electrical energy difference accross the formation. This comes about due to the salt content difference between the wellbore drilling mud and the conate formation water. The apparent electrical charge potential between these two fluids allows us to get an idea of what rock formation we are dealing with subsurface. It should be noted here that the SP log is a more qualitative method for determining the formations subsurface and need to be compared with a variety of other logging tools in order to get an accurate description down hole.

SP log analysis is not particularly difficult process and can be done without much prior knowledge or experience.

  • Deflections to the right (highly positive) for the spontaneous potential log we should expect shale formations.
  • SP deflections to the left (negative) would imply cleaner sands for example sandstones.



sp_res_log.jpg
E. Ross (2000)


If we look at the SP log above (LHS blue track) we can observe two primary voltage levels. One approximately at 80 mV and the other at 20 mV. The 80 mV value is what the base line case for a shale formation will be, so any significant deflections away from this we can assume would be cleaner sands. Try to find them yourself. These clean sand area's are what we should be looking for in petroleum exploration because this is where production would be best.