A new parameter called "a" is calculated from capillary-pressure data. "a" = ( m - 1)/2, where m is the slope of a line segment on a log-log plot of the difference between maximum cumulative intrusion volume and cumulative intrusion volume versus capillary pressure. "a" has been previously interpreted as a measure of pore-throat shape: "a" has the value I for sheetlike pore throats, 2 for cylindrical pore throats, and 3 for equant pore throats. An alternative, and more likely, interpretation is that "a" measures pore-throat surface roughness. This inference is based on the observations that (1) "a" is calculated by a method analogous to that of fractal dimension, and fractal dimension is commonly interpreted as a roughness measure, (2) most "a" values in Smackover reservoir rocks from Alabama are low (< 1) and most pore throats in these strata are smooth, and (3) pore throats in samples of Smackover reservoir rocks with "a" values near 2 are not cylindrical. "a" is independent of gas permeability. If "a" is correctly interpreted as a measure of pore-throat roughness, then it probably affects liquid permeability and could influence producibility of oil and gas-condensate.

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