Since the early days of the technique of well logging and the original water saturation equation developed by Archie, low-resistivity pay was, practically speaking, a contradiction in terms. Through the years, however, this critical topic and its practical implication for the exploration and development of oil & gas reservoirs has become recognized as a worldwide phenomenon of particular and challenging interest.

For this specific study, we will focus in the continental deposits, mainly composed of eolian facies, of the pre-Cretaceous Nia formation that we can further sub-divide into two main prospective dry gas units. The idiosyncrasy of these two siliciclastic bodies is that they are exhibiting a lack of resistivity contrast between sands and adjacent shales and also with the water filled bottom section of the reservoir, resulting in complex formation evaluation process and problematical reserve calculation because the standard and practical "a, m & n" values are not anymore appropriate.

Despite a uniform mineralogy and clay content, the overall small grain size and relatively elevated amount of feldspars contribute actively to the presence of irreducible water which is not directly related to clay content but to micro-porosity, that creates a "low contrast resistivity" pay (LCP) effect. Furthermore, when micro-pores are present, they exert a gradually increasing influence as Sw decreases. Their effect is to decrease the saturation exponent "n" and this reduction is related to the degree of surface conduction constraints on current flow.

Therefore, we will present in this work a petrophysical approach that was adopted and validated in order to overcome these difficulties by combining a systematic and exhaustive laboratory core calibration along with proficient logging techniques (i.e. mineralogical & NMR). In these relatively freshwater shaly sands logged conductivity is imperfectly modeled and unsuitable to derive an exact water saturation necessary for a precise OGIP calculation. Uncertainties will arise from a combination of various phenomenon including lack of resistivity contrast between pay and non-pay, dissimilarity in shape and size of the sand grains, variable irreducible water content and lack of knowledge of the effective concentration of clay exchange cations. Nevertheless, we will illustrate and evidence that it is still possible to resolve this LCP challenge with the integration of discrete core data, reliable continuous specialized electrical logs and non-linear saturation equation.

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