Page 46 - Basic Well Log Analysis for Geologist

THE SPONTANEOUS POTENTIAL LOG

Figure 10. Examples of SP deflection from the shale baseline.

10A—SP deflection with different resistivities of mud filtrate (R,,-) and formation water (Ry). Where resistivity of
the mud filtrate (Ry) is equal to the resistivity of the formation water (R,,) there is no deflection, positive or
negative, from the shale baseline.
Where R,,- is greater than R,,, the SP line kicks to the left of the shale baseline (negative deflection). Where Rupr
greatly exceeds Ry. the deflection is proportionately greater.
Where Ry is less than Ry, the kick is to the right of the shale baseline. This is called positive deflection.
Remember, the spontaneous potential log (SP) is used only with conductive (saltwater-based) drilling muds.
10B—SP deflection with resistivity of the mud filtrate (R,,) much greater than formation water (R,,). SSP (static
spontaneous potential) at the top of the diagram, is the maximum deflection possible in a thick, shale-free, and
water-bearing (“‘wet”’) sandstone for a given ratio of R,,/R,,. All other deflections are less, and are relative in
magnitude.
SP (spontaneous potential) is the SP response due to the presence of thin beds and/or the presence of gas. PSP
(pseudo-static spontaneous potential) is the SP response if shale is present.
Note at bottom of diagram: A formula for the theoretical calculated valuc of SSP is given. SSP = —K x log
(Raf R,), where K = (.133 X T,) + 60.

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