Residual oil zones several hundred feet thick are found beneath many San Andres and Grayburg oil reservoirs in the Permian Basin, West Texas, suggesting that these reservoirs once contained a much larger oil volume. Oil has migrated out of these reservoirs, causing the oil-water contact and the zero capillary pressure level (zcpl) to rise, placing the reservoir in imbibition rather than drainage mode. Therefore, saturation profiles should be characterized by the imbibition capillary pressure curve. Imbibition curves have lower water saturations in the productive interval and significantly smaller transition zones than predicted by drainage curves. The reduction in the transition zone is a function of the rise of the zcpl, which is equal to the thickness of the residual oil zone. A more accurate estimate of original oil in place can be obtained if imbibition curves are used to model fluid saturations.

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