Formula 2 of Ehrenberg (1989) is indeed the correct formula for calculating the amount of porosity lost by compaction. Fortunately, as noted by Ehrenberg, our use of the erroneous formula 1 does not alter our main conclusions (Wilson and McBride, 1988).

Knowledge of the initial porosity of a sandstone sample is essential to the kinds of calculations that we (Wilson and McBride, 1988) and Houseknecht (1987) have made. Ehrenberg (1989) suggests that the data of Beard and Weyl (1973) provided a more accurate means of estimating the initial porosity of a sample than by assuming an initial porosity as we did. Beard and Weyl (1973) provided a set of visual textural comparators that can be used to estimate the Trask sorting coefficient of a sample from a thin section. The sorting value then can be translated to porosity using Ehrenberg's sorting-porosity plot (Ehrenberg, 1989, his Figure 3).

However, the comparators of Beard and Weyl (1973) are of questionable value in accurately estimating the sorting of sands that have undergone moderate to strong compaction. Figure 1 shows the thin section view of uncompacted, randomly packed, spherical, sand-size grains of uniform diameter (i.e., perfect sorting). During compaction, as framework grains move into tighter packing and porosity is lost, the average apparent diameter of the population of framework grains visible in a thin section will change. As a result, the sorting of a sand will appear to change as compaction proceeds. The error introduced by using the comparators of Beard and Weyl (1973) to estimate sorting and then porosity for strongly compacted sandstones is not easily computed and possibly is nonlinear. The changes in apparent sorting will depend on the relative degrees of packing by grain rearrangement, ductile deformation, and intergranular pressure solution. We do not believe the comparator technique is demonstrably better than estimating the initial porosity of a sandstone using as references Pryor's (1973) data for modern sands plus data from thin sections of 125 field-impregnated modern sands in our departmental collection.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.