Depth Shifting

In general curves from the main wireline resistivity tool and the SP were considered to be on depth with each other. The deep resistivity curve was used as the depth master. If there was no wireline data, the deep resistivity curve from the measured-while-drilling tool was the depth master.

The gamma ray and porosity curves were depth shifted with respect to each other and to the master deep resistivity curve. If measured-while-drilling resistivity and porosity curves were present in addition to the wireline curves, these were depth shifted with respect to the wireline curves.

The caliper curve was not depth shifted because it can be expected to match the other curves only in an imprecise way. You will observe that in some cases, it is clearly evident that the caliper curve is off depth with the shifted curves.

SBL Curve

This is the straightened SP curve with the shale baseline set at zero. If the SP is "reversed" in that part of the hole, the sandstones have positive SBL values. If the polarity is "normal" the sandstones have negative values.

VSH_SP Curve

In Cook Inlet, all sandstones include at least some fairly radioactive volcanic clasts and in some sandstones the gamma ray values for sandstones may be similar to adjacent shales. As a result, using the gamma ray curve to separate sandstones from shales, or distinguishing clean sandstones from shaly sandstones in Cook Inlet is problematic. In the older wells drilled with fresh water muds, the SP provides a much more reliable sandstone/shale indicator. These older wells include some 80% to 90% of the wells in Cook Inlet, as well as nearly all key wildcat wells. The SP is absent or meaningless in more recent wells drilled with oil-base muds and absent in wells logged while drilling.

In general Cook Inlet is an environment in which the salinity of the formation water varies a good deal. In a stack of sands with similar formation waters, some isolated sands may have been charged with a much more saline water. Or vice versa, some isolated sands may have retained less saline water as compared with most surrounding sands. As a practical matter, it may be difficult to separate the effect of shale in a sandstone from highly localized changes in formation water. As can be observed, the clean sand baseline varies a good deal, often not very systematically.

In defining the clean sand baseline, I have done my best to take into account all the other available curves. I have paid special attention to the separation between deep and shallow resistivity, to the gamma ray curve and to the caliper curve. In some cases, the relative change in resistivity between sandstones and shales was helpful and in other cases it was not.

In some wells, the SBL curve changes polarity in different intervals due to changes in formation water properties. Zones where the clean sandstone line approaches zero have poor definition for the VSH_SP curves. In come cases I have nulled out this zone of low reliability. In other cases, this zone of low reliability simply appears as a mostly "shale" interval, regardless of there real sand/shale pattern.

If the SP curve is totally meaningless and swings wildly, I may not have created an SBL curve. If the SP was semi-meaningless, usually I created an SBL curve purely for display purposes. If I felt that it was not possible to make a meaningful Volume of Shale interpretation from the SP curve, there will be no VSH_SP curve, although there may be an SBL curve.

All volume of shale interpretations were done by myself. I believe that the VSH_SP curve represents a reasonable and consistent interpretation of the volume of shale in a challenging lithologic/formation water environment. However they are only an interpretation.

In making cross sections, the VSH_SP curve is more useful because water salinity effects are mostly eliminated and meaningless SP or SBL curves simply won't show up since the VSH_SP is not defined for these intervals. The Volume of Shale curve solves the problem of the low-relief SP curve above the Hemlock.