Page 49 - The Geological Interpretation of Well Logs

- SELF-POTENTIAL OR SP LOGS -

calcium and magnesium as well as sodium ions are pre-
__ (Rae
sent. The effects of calcium and magnesium are especially
important at high resistivities in ‘fresh’ waters. Corrections
S(SP) = SP value: this should be the SSP (static SP) must be made to the formulae shown above.
(Re = equivalent mud filtrate resistivity (for the SSP
Shale volume from SP (Pseudo-static SP)
equation) closely related to R.
It is considered that the volume of shale V., in a water-
Je
(A, = equivalent formation water resistivity (for the
wet, shaly sandstone can be simply calculated using the
SSP equation) closely related to R..
SP as follows:
X = temperature-dependent coefficient, as an average,
71 at 25°C (65 + 0.24 x T°C) (cf. Desbrandes, 1982).
The preceding method allows an approximation of the HV y(%) =[1-E5E |x 100
SSP
resistivity of formation water (see Figure 5.11). However,
it is based on the ionic activity.of NaC1 solutions, although
PSP = pseudo-static spontaneous potential = the SP read
it is generally observed that salinities of both mud filtrates
in the water-bearing shaly sand zone.
and of formation waters are due to ionic mixtures and that
SSP = static spontaneous potential = maximum SP value
0 in a clean sand zone.
|
This simply assumes that the SP deffection between the
shale base line (100% shale) and the static SP in a clean
sand (0% shale) is proportional] to the shale volume
(Figure 5.12). This relationship is certainly true qualita-

tively, but quantitatively there is no theoretical basis. The
SP-derived V,, is probably over-estimated.
ohm-meter 5.5 Qualitative uses

irue NaC} Permeability recognition
If there is even a slight deflection on the SP, the bed
“| cesist ivity
RESISTIVITY ‘mee opposite the deflection is permeable. All deftections (with

some rare exceptions) on the SP indicate, the priori, a
XX
jequvalent
permeable bed. The amount of deflection, however, does
\ resistivity
|
|
not indicate the amount of permeability: a very slightly
oor
G-0 o4 tO 10
permeable bed will give the same value as a permeable
IONIC ACTIVITY [g-ions!] [Na* total]
bed (other values being equal).
Figure 5.11 Graph of the relationship between water
Naturally, the reverse is not true; not all permeable
resistivity and SP deflection (ionic activity) for salt solutions.
This is the basis for using the SP to calculate formation water beds give an SP deflection, although these cases are rare
resistivity, R|. (From Gondouin et al., 1957). (Figure 5.10).
O% —
S.P LOG
3 Ze LITHOLOGY
wo 2=

SKALeE
3
LIGNITE
ao
18 so% | | —
BLACK SHALE
SAND WITH
DISSEMINATED PYRITE
SHALE

%
WO RHYOLITE
ssp 50% SHALE
OIABASE
SAND LINE BASE LINE
9% VSh % 100%
Figure 5.12 Graph of the clay volume-SP relationship. The Figure 5.13 Identification of some minerals and lithologies
shale baseline represents 100% shale and the SSP 0% shale. using the SP curve. The log is idealized. (Redrawn from
The relative deflection then depends on the clay volume as Pirson, 1963).
shown by the graph.
39