Page 68 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -—

Table 6.9 Construction of the average Archie Formula.

Symbol Character Derived from

6 Porosity Sonic log, neutron log, density log,
cross—plots, etc.

R 0.62 F calculated using empirical formulae
° ath (Humble Formula) (e.g. Humble Formula) and porosity as above

RK, Formation-water SP or laboratory measurements
resistivities of water samples

R, Rock resistivity saturated R,=FXR,
100% with formation water (can only be calculated,
cannot be measured with logs)

RX, True formation resistivity Induction logs
Laterologs (deep resistivities)

Ss Water saturation of pores S, hydrocarbons — R,
§,, 100% water R

Ry Ry Ry
vor ? salt fresh ydracardons (gas)
sal light 6 =9

no

& 1$%

q
'
1
'
1
1
'
'
medium resistivily Rat constant
deep rasislivily
Figure 6.23 Schematic illustration of the behaviour of resistivity logs over the same reservoir bed but with different fluids and, in
the last case, no porosity.

and values for the unknowns may be obtained as shown in interpretation are: gross indications — deep logs; texture -
Table 6.9. intermediate logs; texture and structure — micro-logs.

Textures
6.8 Qualitative uses
The resistivity of a rock is intimately related to texture.
General indications for resistivity log interpretation The simplest expression of this is the variation of resistiv-
To interpret the geological significance of resistivity logs ity with porosity changes. When the porosity decreases,
it is essential to realize that the same porous bed can have the resistivity increases other things being equal (Figure
a multitude of resistivity responses, depending on fluid 6.24). This is in fact the basis of the porosity-resistivity
content (Figure 6.23). In petrophysical terms, F will cross-plots (Hingle and Pickett), in which a departure
remain constant while X, varies (see Figure 6.6). No from a constant porosity to cesistivity relationship indi-
porous bed can be said to have a typical resistivity - this cates a change in water saturation and the presence of
is a genera] principle for qualitative geological work. hydrocarbons (see Asquith, 1982).
General notions of depth of investigation and bed However, as discussed previously (Section 6.2), the
resolution as previously described (see ‘Log character- influence a rock (as opposed to fluids) has on resistivity
istics’) must also be considered. The indications for can be expressed by F, the formation resistivity factor,

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