Since the first measurement of self-potential in the wellbore in the early 1930's, a large effort has been directed toward determining the origin of this potential. Very little effort, however, has been directed toward studying the effect of hydrocarbon saturations on the SP. Numerous field examples of logs with reduced SP due to high oil or gas saturations are available, yet essentially no laboratory measurements of the phenomenon have been reported outside the work by Kerver and Prokop. A laboratory investigation was conducted to study the effect of hydrocarbon saturation on the SP across samples from eleven different sandstone formations. The SP was measured across samples from each of these eleven formations for a wide range of hydrocarbon saturations. Without exception, the presence of hydrocarbon shifted the SP in the direction of the shale potential. The shift was most pronounced at the highest hydrocarbon saturation. Also it was observed that hydrocarbons had a greater effect on the potential across shaly sands than across clean sands. Capillary pressure versus saturation relations were determined for the eleven sandstone formations and were compared with the shift in SP due to the hydrocarbon saturation. A plot of capillary pressure and a plot of SP versus hydrocarbon saturation were similar.

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