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A systematic procedure for the petrophysical identification and interpretation of low-resistivity and low-resistivity-contrast pay zones in intergranular reservoirs is founded upon an analysis of case histories for different reservoir types in diverse areas of the world. The approach acknowledges that a reservoir rock is a coupled physico-chemical system. The proposed 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 world-wide illustrate how failure to recognize low-resistivity pay can result in much loss of potential value. A principal thrust is to facilitate the re-evaluation of other wells within the same reservoir system without the need for excessive acquisition of additional data. However, the proposed interpretation framework does allow the incorporation of new logging technology as this becomes established. The end-product is a flexible petrophysical interpretation scheme for these unconventional reservoirs that benefits from cost-effectiveness, portability, a higher degree of exactness and consequently a much reduced uncertainty.

Petrophysical measurements on effectively clean, desaturated core plugs from different fields have shown that the assumption of a constant saturation exponent for a given sample frequently is violated, although the conventional bilogarithmic distribution of resistivity-index/water-saturation data is often approximately linear. These variations in saturation exponent are attributed to the effects of pore geometry, in particular the nonuniform distribution of electrolyte within a heterogeneous pore system as desaturation progresses. The data indicate that, if unrecognized, a variable saturation exponent can induce errors of more than 10 saturation units (s.u.) in the petrophysical evaluation of water saturation. Procedures are outlined for the identification of those physicochemical reservoir conditions that can give rise to a variable saturation exponent and for accommodating these variations in reservoir evaluation.

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