Abstract: Effect of Shape Sorting on Prediction of Porosity in Carbonate Sands

R. J. Vinopal, A. H. Coogan

The porosity of a carbonate sand of varied-shaped particles has been unpredictable because of the unknown effect of shape distribution on packing. Porosity was measured for 250 single, dual, and multicomponent packs of varied-shaped, mud-free, particulate, natural, and artificial carbonate sands. Single-shape component packs have average porosities as follows: disks, 35%; spheres, 43%; and rods, 46%, where the particles correspond to Zingg shapes and are moderately well-sorted for size. Porosities of single-shape packs of various radical-shaped particles (e.g., bivalves) range from 65 to 83%.

Dual-shape component packs show porosities proportional to the percent of the dominant particle, making the porosity of a two-component mixture essentially predictable. Sorting of three of more shapes tends to cancel the effects of bridging of radical-shaped particles which in single-component packs results in a wide porosity range. Carbonate sedimentologic factors normally produce a bimodal shape distribution with average porosities approaching 41 and 72% respectively for packs of simple and radical-shaped particles.

Lacking evidence that sedimentologic forces which molded the composition and fabric of a carbonate sand preserved a high porosity deposit of substantially radical-shaped particles one can presume that an undefined carbonate sand has 41% porosity, subject to correction after analysis of shape-mixture limiting factors. This 41% value is within the 39 to 57% porosity range determined for modern clastic sands. It is close to the 39.5% value for orthorhombically packed ordered spheres.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC