Spontaneous Potential Revisited

L. Dunham
Consulting Petrophysicist ([email protected])

This paper will prove SP theory is incorrect. Current SP theory dictates the curve responds to temperature, mud filtrate and formations water resistivity. To examine this theory, visualize in one well, a clean moderately large grained water sand, a clean silt sized water sand, a shaly water sand and a clean water filled carbonate formation with large variations in primary porosity. If these formations contain identical mud filtrate and formations water resistivity, current theory dictates the SP of all will have identical shape. It is well known, however, that the shape and magnitude of the SP will be different in all of these formations, which proves SP theory is incorrect. Amazingly, current theory even fails to explain why the SP becomes smaller when shale is added to sand.

This paper will illustrate that SP theory is incorrect because two basic laws of physics have been ignored. Current theory assumes the shape and dimensions of water in the pore space are constant and, additionally, does not consider the resistive path through the pore space. Both of these assumptions are false.

This paper will prove the SP responds to the resistance of a formation filled with mud filtrate and formation water. The new equation is

E = - K log matrix R(mf)/matrix R(w)

Where matrix R(mf) and matrix R(w) are the resistance of the matrix [clean or shaly formation] filled with mud filtrate and formation water. Matrix R(mf) and matrix R(w) are measured in ohms.

AAPG Search and Discovery Article #90903©2001 AAPG Mid-Continent Meeting, Amarillo, Texas