In some intervals of the Devonian in the Polignac Basin, in Algeria, there is a fairly large variation in the salinity of the interstitial water at, or near, the oil-water contact; the water is fresh below the contact, and salty above, with probably an intermediate zone where the salinity increases progressively with the distance above the oil-water contact. As the producing interval is made up of several distinct reservoirs separated by thin shale beds, several SP base line shifts can take place over a vertical distance of no more than two or three hundred feet. It seems that in most cases the theory can satisfactorily account for the behavior of the SP curve, from which it is possible to derive reliable information about the salinities; however the determination of Rw is unusually difficult and the SP curve does not give all the answers.

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