Curve Description

The Volume of Shale curve for the North Slope wells is based on a clean sand (or rarely a clean carbonate) baseline and a shale baseline for the "ordinary" shales in the interval. The "hot" shales are not used in defining the shale baseline. Any minor excursion of the gamma ray curve below the VSH_GR value of zero or any excursion of the VSH_GR curve above 1.0 has not been not truncated so as to facilitate correlation of the Pebble Shale and any other highly radioactive beds.

Advantages

Raw gamma ray curves are difficult to work with because the readings in the same lithology may be quite different from run to run and from well to well due to borehole effects and tool calibration issues. Often the gamma ray from surface casing (down to about 2500 feet) is nearly useless for cross sections because there may only a difference of 20-25 API units between sandstones and shales.

The VSH_GR curve gives a consistent picture of sands and shales, whether it is displayed on cross sections or using a 3D seismic work station.

Using the standardized VSH_GR curve, it is possible to make a great variety of highly accurate sandstone/shale maps almost instantly with batch-mode log analysis software. These include the standard net sand, gross sand and sand percent maps using a whole spectrum of cut-off levels. A different sort of sand count can be done by summing the VSH_GR values lower than some cut-off value. The ratio of clean sands to shaly sands defines depositional energy, which is in turn related to deltaic depositional patterns, marine bar development, reservoir quality and other important factors. Water saturation calculations can be made faster and more consistently using this standardized volume of shale curve.

Curve Derivation

The VSH_GR curve takes into account stratigraphic changes, as well as changes in tool response for the different logging runs. These include the following.

Cretaceous sandstones are arkosic, with typical uncorrected gamma ray values of 20 to 30 API units. Ivishak conglomerates and sandstones contain few radioactive clasts, with typical uncorrected gamma ray values for zero shale of about 15 API units. Lisburne carbonates have zero shale values of about 10 API units.

In some areas, Cretaceous shales typically have values of 60 to 80 API units, whereas the Kavik shale is 100 to 120 API units. In other areas, the Cretaceous and Kavik shales have much the same API values. The Kingak shale has values intermediate between the Cretaceous Brookian shales and the Kavik Shale. Lisburne shales have values similar to that of the Kavik.

In each interval, the sand and shale values for that interval are used to define VSH_GR.

The gamma ray curve was not borehole corrected before VSH_SP was created. One reason for this is that in highly enlarged portions of the hole, the available correction methods over-correct, producing hot streaks that do not exist. In a few cases it has been possible to define a shale baseline that partially corrects for large wash-outs. In most cases the washout effects remain, so that a shale which should read 1.0 may read as low as .80. Since the sandstones and hard silts do not wash out, this does not create any errors in any sand mapping that uses the VSH_GR curve.

Cautionary Notes

While gamma ray interpretation is straight forward in most wells, in some wells it is not. Typically these more difficult wells have many run changes, poor hole conditions, etc. at the base of the hole where there are major changes in lithology over short intervals. As much as possible, ambiguities in gamma ray response were resolved by reference to other curves that were available over the interval. Because all of the VSH_GR interpretations were made by one experienced petrophysicst/stratigrapher, they are likely to be more consistent and accurate than similar interpetations made piecemeal by different individuals.