Page 149 - Petroleum Geology

126 I I t
$ I

Sl.tlC

Fig. 6-10. Spontaneous potential diagram.

the lattice (not interstitial) by residual charges due to the substitution of one
positive atom in a clay mineral by another of lower valency (e.g., Mg2' re-
placing A13'). 0- tends to occur on the outer margins of the layers, with the
result that mudstones are more permeable to Na' ions than to C1- ions. Na'
tends to pass through the clay from the more saline solution (assumed for
discussion to be the formation water) to the less saline solution in the bore-
hole, and this is the direction of the electrical current (+). This is known as
the membrane potential.
Within the permeable bed, however, invasion of mud filtrate gives rise
to contrasting salinities in the pore spaces, across the interface between the
mud filtrate and the formation water. C1- are more mobile than Na' ions, so
there is a net flow of C1- from the more saline formation water to the less
saline mud filtrate, with a resulting current flow in the opposite direction.
This is known as the liquid junction potential.
These two components give rise to an electric current that circulates
around the interface between the mudstone and the permeable unit near the
borehole, and the potential of this current is measured in the borehole. The
intensity of the electrochemical potential is greatest at the level of the inter-
face in the borehole. If the static SP could be measured, that is, the SP that
is not reduced by the resistance encountered by the current flowing through
the rocks and their fluids, it would be rectilinear and mark the changes
of lithology at the borehole precisely. Nevertheless, the slope of the SP as
recorded is proportional to the electromotive force in the borehole, and the
lithological boundary is indicated by the infZexion point of the curve. This
may well not coincide with the mean value of the deflexion. The SP log is
therefore of fundamental geological value in locating the boundaries of rock
units penetrated by the borehole.
When the salinity of the mud and its filtrate is greater than that of the
formation water, the direction of the currents is reversed, and the deflexion
opposite the permeable beds will be positive with respect to that opposite
a mudstone or shale. Fresh-water aquifers, commonly in the higher part of
the borehole, are so revealed (see Fig. 6-1). Holes drilled with sea water or a