Shaly Sand Evaluation Using Gamma Ray Spectrometry, Applied To The North Sea Jurassic Available to Purchase

In formations where radioactive minerals other than clay are present, their effects on log responses result in a reduction of the accuracy of determination of the shale fraction. The Gamma Ray log, which is one of the primary indicators for shaliness determination, is the most affected; other logs used in shaliness indicators are also influenced, particularly when heavy minerals are present, such as those encountered in the micaceous sandstones of the North Sea Jurassic. For comparison purposes, possible ways to correct for heavy radioactive minerals using a conventional logging suite are described. Computer processed examples illustrate the results obtained. A different approach is through an analysis of the natural gamma ray spectrum of the formation, as determined with the Gamma Ray Spectrometry tool. Natural gamma rays originate from the radioactive isotope of Potassium and the radioactive elements of the Uranium and Thorium series. Each of these three groups of elements (Potassium, Uranium, Thorium) contributes its distinctive spectrum to that of the formation in proportion to its abundance. Thus, by analysis of the formation spectrum, the presence of each can be detected and its amount estimated. This makes possible quantitative corrections to the shaliness indicators. A computer program which performs the necessary computations is described, and several log examples using this technique are presented. The results from this approach have been good, and the Gamma Ray Spectrometry tool is indicated to be a valuable addition to the logging program, particularly for the case of micaceous sands. Promising results have also been obtained in other cases of radioactive sands, such as those encountered in Nigeria. With new computer techniques the Gamma Ray Spectrometry log leads to faster and more accurate determination of clay content and hence of?