ABSTRACT

This paper discusses interpretation methods presently applied in Jurassic Sandstones of the Northern North Sea. It should be directly applicable to other areas with similar complex lithology. The formations are briefly described: they often contain mica but also other minerals that may seriously affect log interpretation because they are either heavy or radioactive. When these additional minerals are ignored the results of log interpretation are much too low both in porosity and in hydrocarbon saturation. A recommended logging suite is given. In addition to the standard resistivity and porosity logs it should include the Natural Gamma Ray Spectroscopy and Litho-Density logs. The formations are first divided into zones of reasonably comparable lithology. An automatic method is suggested for this purpose. A Quick Look interpretation can then be obtained in each separate zone. It should provide an estimation of the main minerals and of their relative amounts. Practical overlay methods are proposed to obtain this information. A more complete interpretation is then provided by the Global* method with a set of mineralogical components different for each zone and defined by the previous study. The results of this first Global output should then be used to refine the logging parameters, possibly reduce the importance of a poor log and revise the list of minerals computed in each zone either by adding a new mineral recognized as important or dropping one that only occurred in small amounts. A new Global pass should provide the final interpretation. This method has been developed on theoretical basis but optimized by comparison with the results of laboratory measurement on cores. A few examples of application are discussed.

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