In unfractured reservoirs, low-resistivity pay zones can usually be identified with one or more of the following: laminated reservoir/non-reservoir sequences, formations with multi-modal pore-size characteristics, sediments with anomalously high surface area, and reservoir characteristics that extend beyond the range of applicability of interpretative algorithms, e.g. reservoirs containing very fresh formation waters. In all these cases, dry oil has been produced in the presence of high interpreted water saturations, in many different parts of the world. It is therefore important to have a generalised facility for recognising low-resistivity pay as early as possible in the life of a prospect.

A systematic procedure is described for the identification of low-resistivity and low-resistivity-contrast pay zones in primary reservoirs. The approach acknowledges that a reservoir rock is a coupled physicochemical system, the method is generic and robust, it is conceptually simple, and it is structured in a manner that is easy to understand. The scheme is modular and it is arranged hierarchically to reflect maturing data scenarios. Therefore it can be progressively refined during the appraisal and development stages. The essence of the method is the definition and calibration of reliable interpretative procedures through quality-assured reference data from key wells by admitting only validated reservoir characteristics. Examples illustrate how failure to do this can result in much loss of value.

The principal thrust is to facilitate the re-evaluation of other wells within the same reservoir system without the need for additional logging programmes. The proposed interpretation scheme also constitutes a basis for the incorporation of new logging technology when this becomes established. The end-product is a flexible petrophysical interpretation scheme for these complex reservoirs that benefits from cost-effectiveness, portability, a higher degree of exactness and consequently a reduced uncertainty.

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