The overall geological environment is a sand-shale sequence. The shalier intervals are indicated by their higher GR, low Res, and the N–D separation (high D due to low porosity and high N due to significant lattice-bound H which attenuates neutrons and some H in the clay-bound water fraction). The sand intervals have lower GR and convergence of N–D (decrease in D relative to shale due to higher porosity and drop in N due to absence of lattice-bound H versus shale).

Resistivity measurements in the shalier intervals exhibit curve separation with dominant cause due to differential resolution between specific curves, with high-frequency phase at highest resolution and low-frequency attenuation at lowest resolution. 

Sand intervals also exhibit curve separation, some of which is still due to differential resolution. The two low-frequency curves both exhibit conductivity and with the porosity indications from N–D suggest water-bearing sand. However, the two high-frequency curves exhibit higher resistivity, and markedly so for the high-frequency attenuation curve, compared to the two low-frequency curves. Is it hydrocarbons? Well, that might be a dangerous assumption if we didn’t have the two low-frequency curves, which contradict a hydrocarbon assessment. (Note that some clients may not purchase the 4–curve resistivity suite, opting to save money and only purchasing the higher-resolution high-frequency curves. That preference can be dangerous, however, due to in general the greater susceptibility of processing or other effects influencing high-frequency measurement. For example, invasion might be an initial consideration. However, an invasion profile with resistive mud in a water-bearing sand would exhibit high-frequency phase measurement as the highest resistivity due to its shallowest depth of investigation, rather than high-frequency attenuation as the highest value in the sands.

Recall from the Lundi Log 001 interpretation that curve generation for propagation resistivities assumes cylindrical “behavior”—i.e., assumption that the formation is a perfect homogeneous cylinder, the borehole as a perfect cylinder, and the tool as perfectly centered in that borehole. In Lundi Log 001, the perfect cylinder model was violated due to the moderate relative dip, meaning formational boundaries are not perpendicular to the tool and borehole but rather inclined, thus generating the polarization effect in the more resistive strata (in that case the shales).

Here note that the tool diameter is 8.25 inches–optimally designed for an 8.5 inch hole, with stabilizers effectively keeping the tool centered in an non-overgauged hole. However, a 12.25 inch bit diameter was used and the hole is slightly overgauged (averaging maybe about 0.5 inch), so the borehole exhibits signficantly greater diameter than the tool and stabilizers, making centering of the tool an unlikely situation, especially for an inclined hole. Rather the tool is likely eccentered thus violating the perfect cylinder model. 

Eccentricity effects occur when to tool is not centered within the borehole and a high resistivity contrast exists between the borehole fluid and the formation. For example, low-resistivity formations with oil based/synthetic muds, high-resistivity zones with highly-conductive muds, and conductive media with significant conductivity difference. This example is low-resistivity formation (water-bearing sands) and high-resistivity synthetic mud.

All propagation resistivities can be affected by eccentricity but higher-frequency 2 MHz LWD measurements are much more affected than the lower-frequency 400 kHz LWD measurements. When using a resistive mud in a conductive formation, the high-frequency attentuation measurement is influenced most strongly affected, followed by high-frequency phase. Both of these curves show elevated resistivity compared to the two low-frequency curves. (Curiously, local effects on the short-spaced high-frequency phase curve cause a drop in resistivity relative to the two low-frequency curves—this can be a useful check to confirm eccentricity.)

Note that eccentricity is not a major factor in wireline logging as the tool design is based on an eccentered tool (push the tool against the borehole wall) and curve generation generally accounts for that inherent eccentricity.