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       Radioactive Sands --Granite Wash - Bald High        Uranium-Ricj Sands       Granite Reservoirs

RADIOACTIVE SANDS
Radioactive sands are often mistaken for shale  Radioactivity can come from uranium or feldspar. Weathering of granite can produce porosity and normal migration paths may bring oil to the porous reservoir. There are very large granite reservoirs offshore Viet Nam and smaller ones in many other countries, including the USA.  These reservoirs are often called "bald highs".
If weathered granite is moved by normal erosional processes, the resulting reservoir is often called a "granite wash".

Most real reservoirs exhibit more than one problem to be solved by the analyst. Here we have a sand that is radioactive because it is a feldspar sand. Feldspar contains potassium which is radioactive, so the zone looks like a shale on the gamma ray log. The density neutron porosity curves on sandstone scale however show zero separation, so this interval cannot be a shale. If you think like a detective, the answers usually come to light; gather the evidence, assess the evidence, discard the impossible, select the most probable from what remains. In general, the simplest solution is often the best choice.

Composite raw data plot for radioactive feldspar sand.

This example has no gamma ray spectral log. It would have been a help, but is not essential since we know the area and the density neutron curves answer all the questions about porosity and lithology.

There is a second issue though, and that is the low resistivity over most of the sand interval. suggesting a water zone. The highest resistivities are in tight anhydrite and dolomite. The best resistivity in the sand is just 2 ohm-m and the water zone is 0.4 ohm-m, a contrast of 5:1. An old rule of thumb suggests that a ratio of 3:1 or better means we should complete the well, as long as the porosity is about the same in both the water and oil legs.

A test on the top of the sand in the well on the left, below, produced clean oil, but water cut increased after about six months production. A second well at the right was drilled and tested water with some oil. The resistivity log signature is only very slightly different than the first well. Visually there is not much difference between them.

Detailed petrophysical analysis does show subtle differences. The well on the left shows a 2 meter pay zone on either a long transition zone or a depleted oil zone of about 7 meters. The second well shows only a half meter of pay on top of the same transition zone. The test and production results are confirmed by the fluid distribution in the two wells. And there is not much that can be done to improve the oil production.

 
Actual saturation (blue curve in Track 3) compared to irreducible water saturation (black curve) in two wells. Where the two curves are close together, little water will be produced at initial completion. Where they are separated, water will flow with the oil. Production histories on these two wells bear out this interpretation: the well on the left produced clean oil for six months, the other tested water with oil immediately.

A good wellsite geologist will correlate his description to the shape of the drilling time log. Later, the sample depths may be adjusted to the open hole logs, especially gamma ray, resistivity, and density logs. In this pair of wells, the first hint of the feldspar sands is in the wellsite sample descriptions as shown below.

 
Log analysis lithology plot (left) in a complex sequence, and sample description plot (right) over the same interval. Although the lithology description is not usually quantitative, it is an essential ingredient in choosing the correct mineral mixture for the log analysis lithology calculation. A little care is needed to read these logs. In this case, the word "SAND" describes the rock texture, not its mineralogy. This is a radioactive sand so it must contain feldspar (decomposed granite) and possibly some quartz, as well as the dolomite and anhydrite layers above the sand. Shale, of course must be handled by an appropriate method. In this case, shale cannot be found using the GR inside the radioactive sand interval.
 

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