ABSTRACT

Intergranular porosity and associated permeability are directly related to particle grade sizes. Clastic sediments are largely defined by particle size and are logged by grade size symbols. Carbonates should be similarly treated if porosity/permeability relationships are to be properly understood.

Grade size terminology requires clarification, particularly for carbonates. This can be done by accepting the Wentworth grade limits for all rocks, including carbonates, and by using objective grade names, free from petrogenetic implication. Except for one term (megagrade), conventional terminology like "fine grained, cryptocrystalline," etc., can be converted to objective grade names and aligned with the Wentworth scale. Well known genetic names like oolite, etc., are also necessary.

Recrystallisation can make accurate grade percentage estimates impossible, especially from bit samples, but complete obliteration of textures originally coarser than fine grade is likely to be rare. Post-tectonic dolomitisation is common in the Canadian Rockies. Dolomitisation in general requires much study. Crystalline grade size is easily determined in dolomites, however. Two Mississippian examples and one Devonian example of secondary dolomites in Canada suggest that dolomite crystal grade is directly related to original limestone grade, and that sections containing the two rock types can be successfully correlated on this basis.

A symbolic system, consisting of variations of standard limestone and dolomite symbols, permits representation of carbonate grade sizes comparable to that of clastics. This greatly improves interpretation of porosity/permeability relationships in individual wells and assists inter-well correlation, so facilitating genetic interpretation. Even low accuracy grade determinations can give fair results.