Page 30 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

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Figure 5. Horizontal section through a permeable hydrocarbon-bearing formation and the concomitant resistivity profiles which
occur when there is invasion by either freshwater- or saltwater-based drilling muds (see Fig. 4 for resistivity profiles ina
water- bearing formation).
Freshwater Muds—Because the resistivity of both the mud filtrate (R,,;) and residual hydrocarbons (RH) is much greater
than formation water (R,,). the resistivity of the flushed zone (R,,) is comparatively high (remember that the
flushed zone has mud filtrate and some residual hydrocarbons).
Beyond its flushed part (R,,), the invaded zone (Rj) has a mixture of mud filtrate (Ry). formation water (R,,), and
some residual hydrocarbons (RH). Such a mixture causes high resistivities. In some cases, resistivity of the
invaded zone (R,} almost equals that of the flushed zone (R,,).
The presence of hydrocarbons in the uninvaded zone causes higher resistivity than if the zone had only formation
water (R,,), because hydrocarbons are more resistant than formation water. So, R, > R,. The resistivity of the
uninvaded zone (R,) is normally somewhat less than the resistivity of the flushed and invaded zones (R,, and Rj).
However, sometimes when an annulus profile is present, the invaded zone’s resistivity (Rj) may be slightly lower
than the uninvaded zone’s resistivity (R,).
To summarize: therefore, R,, > R; 2 R, in hydrocarbon-bearing zones.
Saltwater Muds—Because the resistivity of the mud filtrate (R,,-) is approximately equal to the resistivity of formation
water (Ry = Ry). and the amount of residual hydrocarbons (RH) is low, the resistivity of the flushed zone (R,,) is
low.
Away from the borehole as more hydrocarbons mix with mud filtrate in the invaded zone, the resistivity of the
invaded zone (R;) begins to increase.
Resistivity of the uninvaded zone (R,) is much greater than if the formation was at 100% water saturation (R,,)
because hydrocarbons are more resistant than saltwater. Resistivity of the uninvaded zone is greater than the
resistivity of the invaded (Rj) zone. So, R, > Rj > Ryy.
Both the above examples assume that the water saturation of the uninvaded zone is much less than 60%.