Packing is a parameter that described how grains are organized in a clastic sedimentary rock with respect to each other. In first order, clastic rocks can be:

matrix-supported, when grains are not in contact and are suspended in finer matrix
grain-supported: when grains are in contact with each other.

This difference is important, because it indicates the type of transport a sediment have experienced. Selective transport (e.g. rivers) removes matrix and sorts specific grain sizes, while non-selective transport (e.g. glaciers) produce deposits with much matrix around grains. Recognizing how grains are packed is useful to understand how much matrix is still present in the interstices between grains.

In a theoretical packing of grains with spherical shape and point contacts, the empty pores between grains contain between 47.6% and 25.9% of available space over the rock volume (porosity), depending on the type of packing. The available pore spaces, which can be filled by matrix, cement or remain empty, progressively reduce if the contacts between grains are not point contacts. Here are the possible situations:
– point contacts: grains touch only in a point
– long contacts: long sides of grains touch each other
– concavo-convex contacts: concave sides of grains penetrates into convex sides of other grains
– sutured contacts: wriggly boundaries with grains interpenetrating each other.

The type of contacts between grains determines the porosity, and hence the percentage of matrix, if pores are filled. Point contacts and long contacts can form at deposition, whereas concavo-convex and sutured contacts indicate progressive compaction (reduction of pore spaces) during diagenesis. A rock with prevailing long contacts, concavo-convex contacts, and sutured contacts has pore spaces, and eventually matrix, < 15%. This can be useful to recognize mature, low-matrix rocks like arenites e orthoconglomerates.