| 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., and H. J. Fraser, 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 effects of grain shape on primary porosity were investigated by Fraser<ref name=pt05r59>Fraser, H. J., 1935, Experimental study of porosity and permeability of clastic sediments: Journal of Geology, v. 43, p. 910–1010, DOI: [http://www.jstor.org/discover/10.2307/30058422?uid=3739848&uid=2&uid=4&uid=3739256&sid=21103791335533 10.1086/jg.1935.43.issue-8].</ref> and Beard and Weyl.<ref name=pt05r23>Beard, D. C., Weyl, P. K., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0002/0300/0349.htm Influence of texture on porosity and permeability of unconsolidated sand]: AAPG Bulletin, v. 57, p. 349–369.</ref> In general, porosity decreases as sphericity increases due to tighter packing arrangements associated with spherical grains. Numerous studies<ref name=pt05r59 /><ref name=pt05r136>Rogers, J. J., Head, W., 1961, [http://jsedres.geoscienceworld.org/content/31/3/467.abstract Relationship between porosity, median size and sorting coefficients of synthetic sands]: Journal of Sedimentary Petrology, v. 31, p. 467–470.</ref><ref name=pt05r23 /><ref name=pt05r131>Pryor, W. A., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0001/0150/0162.htm Permeability-porosity patterns and variations in some Holocene sand bodies]: AAPG Bulletin, v. 57, n. 1, p. 162–189.</ref> indicate that porosity generally increases with [[Core_description#Maturity|sorting]]. Gaither<ref name=pt05r62>Gaither, A., 1953, A study of porosity and grain relationships in experimental sands: Journal of Sedimentary Petrology, v. 23, p. 180–195, DOI: [http://jsedres.geoscienceworld.org/content/23/3/180.abstract 10.1306/D4269602-2B26-11D7-8648000102C1865D].</ref> showed that when two grain sizes are mixed, porosity is reduced until both grain sizes are present in approximately equal amounts. | + | The effects of grain shape on primary porosity were investigated by Fraser<ref name=pt05r59>Fraser, H. J., 1935, Experimental study of porosity and permeability of clastic sediments: Journal of Geology, v. 43, p. 910–1010, DOI: [http://www.jstor.org/discover/10.2307/30058422?uid=3739848&uid=2&uid=4&uid=3739256&sid=21103791335533 10.1086/jg.1935.43.issue-8].</ref> and Beard and Weyl.<ref name=pt05r23>Beard, D. C., and P. K. Weyl, 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0002/0300/0349.htm Influence of texture on porosity and permeability of unconsolidated sand]: AAPG Bulletin, v. 57, p. 349–369.</ref> In general, porosity decreases as sphericity increases due to tighter packing arrangements associated with spherical grains. Numerous studies<ref name=pt05r59 /><ref name=pt05r136>Rogers, J. J., and W. Head, 1961, [http://jsedres.geoscienceworld.org/content/31/3/467.abstract Relationship between porosity, median size and sorting coefficients of synthetic sands]: Journal of Sedimentary Petrology, v. 31, p. 467–470.</ref><ref name=pt05r23 /><ref name=pt05r131>Pryor, W. A., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0001/0150/0162.htm Permeability-porosity patterns and variations in some Holocene sand bodies]: AAPG Bulletin, v. 57, n. 1, p. 162–189.</ref> indicate that porosity generally increases with [[Core_description#Maturity|sorting]]. Gaither<ref name=pt05r62>Gaither, A., 1953, A study of porosity and grain relationships in experimental sands: Journal of Sedimentary Petrology, v. 23, p. 180–195, DOI: [http://jsedres.geoscienceworld.org/content/23/3/180.abstract 10.1306/D4269602-2B26-11D7-8648000102C1865D].</ref> showed that when two grain sizes are mixed, porosity is reduced until both grain sizes are present in approximately equal amounts. |