Changes

==Influence of textural parameters on porosity==

==Influence of textural parameters on porosity==

Primary porosity in clastic and some carbonate rocks (such as oolites) is a function of grain size, packing, shape, 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, 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., 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 (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., 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.

 

[[file:porosity_fig4.png|thumb|{{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 <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 <ref name=pt05r69 />.)]]

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., 10., 1086/jg., 1935., 43., issue-8</ref> and Beard and Weyl<ref name=pt05r23>Beard, D. C., Weyl, P. K., 1973, 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, 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, [[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 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., 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., 10., 1086/jg., 1935., 43., issue-8</ref> and Beard and Weyl<ref name=pt05r23>Beard, D. C., Weyl, P. K., 1973, 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, 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, [[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 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., 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.