Changes

[[file:porosity_fig4.png|thumb|300px|{{figure number|4}}Schematic diagram of packing arrangements for spheres. Porosity values are calculated for cubic (47.6%), orthorhombic (39.5%), rhombohedral (26%), and tetragonal (30.2%) packing. (After Berg;<ref name=pt05r25>Berg, R. R., 1970, Method for determining permeability from reservoir rock properties: Transactions Gulf Coast Association of Geological Societies, v. 20, p. 303–317.</ref>; modified from Graton and Fraser.<ref name=pt05r69 />)]]

[[file:porosity_fig4.png|thumb|300px|{{figure number|4}}Schematic diagram of packing arrangements for spheres. Porosity values are calculated for cubic (47.6%), orthorhombic (39.5%), rhombohedral (26%), and tetragonal (30.2%) packing. (After Berg;<ref name=pt05r25>Berg, R. R., 1970, Method for determining permeability from reservoir rock properties: Transactions Gulf Coast Association of Geological Societies, v. 20, p. 303–317.</ref>; modified from Graton and Fraser.<ref name=pt05r69 />)]]

Primary porosity in clastic and some carbonate rocks (such as oolites) is a function of grain size, packing, shape, [[Core_description#Maturity|sorting]], and amount of intergranular matrix and cement.<ref name=pt05r124>Pettijohn, F. J., 1975, Sedimentary rocks, 3rd ed.: New York, Harper and Row, p. 628.</ref> In theory, porosity is independent of grain size. Changes in grain size, however, affect grain shape and sorting. Because these variables directly affect porosity, changes in grain size indirectly affect porosity.

Primary porosity in clastic and some carbonate rocks (such as oolites) is a function of [[grain size]], packing, shape, [[Core_description#Maturity|sorting]], and amount of intergranular matrix and cement.<ref name=pt05r124>Pettijohn, F. J., 1975, Sedimentary rocks, 3rd ed.: New York, Harper and Row, p. 628.</ref> In theory, porosity is independent of grain size. Changes in grain size, however, affect grain shape and sorting. Because these variables directly affect porosity, changes in grain size indirectly affect porosity.

The theoretical effects of grain size and packing on porosity were investigated by Graton and Fraser<ref name=pt05r69>Graton, L. C., Fraser, H. J., 1935, Systematic packing of spheres with particular reference to porosity and [[permeability]]: Journal of Geology, v. 43, p. 785–909, DOI: [http://www.jstor.org/discover/10.2307/30058420 10.1086/jg.1935.43.issue-8].</ref> who computed the porosity of various packing arrangements of uniform spheres. The theoretical maximum porosity for a cubic packed rock, regardless of the value assigned to grain radius, is 47.6%. Porosity values for other packing arrangements ([[:file:porosity_fig4.png|Figure 4]]) can be calculated.

The theoretical effects of grain size and packing on porosity were investigated by Graton and Fraser<ref name=pt05r69>Graton, L. C., Fraser, H. J., 1935, Systematic packing of spheres with particular reference to porosity and [[permeability]]: Journal of Geology, v. 43, p. 785–909, DOI: [http://www.jstor.org/discover/10.2307/30058420 10.1086/jg.1935.43.issue-8].</ref> who computed the porosity of various packing arrangements of uniform spheres. The theoretical maximum porosity for a cubic packed rock, regardless of the value assigned to grain radius, is 47.6%. Porosity values for other packing arrangements ([[:file:porosity_fig4.png|Figure 4]]) can be calculated.