Abstract

Dr N. L. Watts writes: Dr Selley's paper raises three main questions which must be considered before application of his model is attempted. These are: 1. How reliable is the relationship between porosity and permeability? As is well known, and was discussed by several earlier speakers, reduction of porosity during diagenesis results in variable losses of permeability depending on (a) the original sediment type, and (b) the mechanism of pore occlusion; e.g. porosity reduction due to illite authigenesis may give a greater loss of permeability than by the development of quartz (and feldspar) overgrowths. It should be noted, however, that the development of quartz overgrowths at pore throats may create appreciable permeability losses, whereas only illite neoformation has a significant effect on poroperm reduction. During the questioning Dr Selley produced a graph showing the relationship between porosity and permeability and concluded that, because of the high correlation between the variables, the inclusion of a permeability scale on his depth/porosity plots was justified. The points on his graph, however, were average values and it is arguable whether such a good correlation would have been obtained had all the original values been plotted. In any case, a permeability/depth scale, if used, can only be applied confidently to a single lithological type. It should not be used for a range of rocks from different environments because original lithology (mineralogy, grain size, sorting, etc.) exerts a major control on the way that permeability is reduced during diagenesis.

What effect does early diagenesis have on

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