The electrical behaviour of fully electrolyte-saturated reservoir rocks has been represented through verified type curves as a continuum across a range of electrolyte resistivity. The type curves can be used with core and log data to infer whether a reservoir rock is behaving as effectively clean or shaly. In the case of effectively shaly sands, a single-point value of excess conductivity can be evaluated from log data alone.
More generally, the type curves allow different reservoirs to be compared in terms of their electrical behaviours. Where these are similar, it is reasonable to assume that the petrophysical interpretation procedures will be similar. Thus, the type curves allow a given reservoir-rock to be evaluated within the "big picture" of electrical behaviour. This, in turn, allows experience gained elsewhere to be brought to bear on the current appraisal, especially in terms of the suitability of available interpretation models.
Two of the primary needs of the oil industry are to accelerate the time taken for early reservoir appraisal, add to quantify and reduce uncertainty, especially in frontier areas. In order to do this, we require to become much more effective at applying lessons learnt elsewhere to a reservoir that is currently under appraisal. The fulfillment of this need is impeded by the empirical heritage of reservoir geoscience, with its implied formation and geographical dependence, by differences of approach within the industry as a whole, and by staff mobility within and between companies, which restricts the accumulation of reservoir- or region-specific knowledge. Considerable advances could be made if data subsets pertinent to quantitative reservoir evaluation could be compared easily within a common, non-subjective reference system that is not limited by rock type or geography and is easily understood by all.
The aim of this paper is to advance our appreciation of the commonality of the electrical characteristics of reservoir rocks. These data have an important role in the evaluation of hydrocarbons in place, and it is now becoming clearer that they form part of a continuum of electrical behaviour that accommodates many rock types. If we can describe this continuum using type curves, any appraisal reservoir can be identified within the "big picture" so established. Thus, reservoir behaviours might be compared directly and messages from an earlier appraisal exercise might be transposed to a current one.
This paper outlines the foundations of this continuum approach with a view to the evaluation of hydrocarbons in place. One of the key questions is whether the reservoir should be treated as effectively clean, in which case the Archie equations can be applied directly, or effectively shaly, in which case more sophisticated procedures are necessary (Fig. 1). We will seek to answer this question within the framework of the big picture that the continuum approach provides. These considerations will be confined to fully water-saturated reservoir rocks for which we have a sound understanding.