Methods are presented for incorporating capillary pressure, pore throat aperture radii, height above the free-water table, and Win land r₃₅ values on Pickett plots. The techniques involve the use of log-log plots of effective porosity vs. resistivity combined with em pirical equations for calculating capillary pressure written as a func tion of permeability, porosity, and water saturation.

I show that a crossplot of porosity vs. true resistivity (in some cases apparent resistivity or true resistivity affected by a shale group) should result in a straight line for intervals with constant capillary pressure and constant pore throat aperture radii. Key ad vantages of the proposed methods are (1) the capillary pressure at any point on the Pickett plot is consistent with porosity, perme ability, and water saturation at that particular point; (2) the value of R_w does not have to be known in advance, provided that the reservoir contains some water-bearing intervals; and (3) core data are not essential, although it is strongly recommended to have cores to properly calibrate the equations presented in this article. If cap illary pressures from cores are available, it is possible to estimate the value of R_w even if there are not water-bearing intervals in the reservoir.

Pore throat aperture radii (r₃₅) values computed using the em pirically derived Winland equation compare reasonably well with pore throat aperture radii (r_p35) calculated from techniques pre sented in this article. This is significant because the data sets used to establish these empirical equations come from different areas, different reservoirs, and different lithologies and were evaluated in dependently at different times. A mathematical relationship is de veloped between Winland r₃₅ values and the pore throat aperture r_p35 presented in this article.

The methods are illustrated using two case histories. The first one is a Gulf Coast high-porosity sand-shale sequence. The second is a limestone oil reservoir from the Lansing Kansas City formation.

The integration of permeability, capillary pressures, pore-size classes, and geometry of the pores on a log-log graph of porosity vs. resistivity makes the Pickett plot one of the most formidable formation evaluation tools yet devised.