The shape and amplitude of S.P. deflections may be influenced by several factors. These are:

1.Influence of salinity of fluids in contact

The S.P. essentially depends on the difference in salinity of the fluids in contact, the mud filtrate and the formation water. Various situations may arise:

  • The formation water is more saline than the mud filtrate Rmf > Rw. Such S.P. is considered normal.This is the case we have considered so far. The positive here is opposite the clays and the deflection toward the negative signifies the presence of a porous and permeable formation. The deflection increases with an increase in contrast of salinities

 

  • The resistivity of the filtrate is close to that of the formation water Rmf = Rw. The S.P. is termed FLAT. The log shows no clear difference between the clays and the porous and permeable beds.

 

  • The formation water is less saline than the mud filtrate Rmf < Rw. In this case, we obtain an INVERTED S.P..The curve reveals a positive opposite the permeable layers and a negative opposite the clays. This phenomenon is often observed in groundwater boreholes.

 

2.Influence of resistivity

  • If the permeable bed has a high resistivity, S.P. currents do not develop readily. The S.P. deflection and bed boundaries are less clearly indicated. Charts can be used to correct this effect

 

3.Influence of clay

  • The presence of clay in the reservoir formation reduces the amplitude of S.P. deflection. The attenuation is a linear fonction of the percentage of clay dispersed in the rock.This property also enables us to calculate the percentage of clay in permeable formation.

4.Influence of thickness of permeable bed Generally

The boudaries between the clay and the permeable formation correspond to the points of inflection of the S.P curve. An increase of borehole diameter reduces the S.P. deflection.

  • If the layer is thick, the deflection is maximum and appears as a plateau
  • If the layer is thin, the curve is recorded as a narrow peak and the static potential is not reached. In this case to find the maximum theoretical value of S.P. deflection, correction charts must be used. To use these charts we must know the thickness of the permeable layer, which can be obtained from other log.
  • As the invasion increases, the S.P. deflection is reduced.

 

5.Effect of compact formations

  • Clay beds interbedded between the compact layers, for example, limestone, are observed by a change in the slope of the S.P. curve.