Both Pate (1989) and Ehrenberg (1989) are correct; my method of estimating the amount of original porosity destroyed by compaction processes and cementation ignores the fact that bulk sediment volume is dynamically reduced as compaction proceeds. However, I do not believe that their solutions are superior to my method for the following reasons.

At the most fundamental level, I believe that diagrams such as mine should be tools that allow the objective presentation of data. My original diagram (Houseknecht, 1987, his Figure 4) allows petrographers to plot two variables (intergranular volume and cement, both expressed as percentages of the present whole rock volume) that can be objectively estimated by point counting or image analysis of thin sections. Petrographers must be able to communicate in terms of objectively defined and collected variables; intergranular volume (IGV) and cement are two such variables. Moreover, the relative locations at which samples plot on my diagram have distinct physical significance, as I demonstrated with thin section and scanning electron micrographs in Figures 2 and 3 of my 1987 paper. With on y limited experience, a petrographer can form a mental image of a sedimentary rock--its packing and cement characteristics--simply by looking at data plotted on the IGV-cement diagram.

Estimating the percent of original porosity destroyed by compaction and cementation is a procedure aided by my diagram, but it is an interpretative derivation that necessarily introduces subjectivity. For this reason, I chose to create separate axes (Houseknecht, 1987, his Figure 4) that are not as prominently emphasized as the IGV and cement axes. The subjectivity involves two assumptions: (1) the original IGV of a sand is 40%, and (2) the bulk volume of a sand does not decrease as compaction proceeds. Neither assumption is strictly valid; however, better assumptions are difficult to make because both involve unresolved research problems. I shall examine each in turn.

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