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''Porosity'' determines reservoir storage capacity. It is defined as the ratio of void space, commonly called pore volume, to bulk volume and is reported either as a fraction or a percentage. Almost all hydrocarbon reservoirs are composed of sedimentary rocks in which porosity values generally vary from 10 to 40% in sandstones and from 5 to 25% in carbonates.<ref name=pt05r36>Coneybeare, C. E. B., 1967, Influence of compaction on stratigraphic analysis: Canadian Petroleum Geology Bulletin, v. 15, p. 331–345.</ref><ref name=pt05r92>Keelan, D. K., 1982, Core analysis for aid in reservoir description: Journal of Petroleum Technology, v. 34, p. 2483–2491., 10., 2118/10011-PA</ref>

''Porosity'' determines reservoir storage capacity. It is defined as the ratio of void space, commonly called pore volume, to bulk volume and is reported either as a fraction or a percentage. Almost all hydrocarbon reservoirs are composed of sedimentary rocks in which porosity values generally vary from 10 to 40% in sandstones and from 5 to 25% in carbonates.<ref name=pt05r36>Coneybeare, C. E. B., 1967, Influence of compaction on stratigraphic analysis: Canadian Petroleum Geology Bulletin, v. 15, p. 331–345.</ref><ref name=pt05r92>Keelan, D. K., 1982, Core analysis for aid in reservoir description: Journal of Petroleum Technology, v. 34, p. 2483–2491, DOI: [https://www.onepetro.org/journal-paper/SPE-10011-PA 10.2118/10011-PA].</ref>

==Definition of porosity terms==

==Definition of porosity terms==

[[file:porosity_fig1.png|left|thumb|{{figure number|1}}Schematic of a pore system relating mineralogy, water content, and porosity assessment. (Notes: *lf sample is completely disaggregated during measurement. “Varies as a function of height above the free water level.) (After <ref name=pt05r33>Chatzis, I., Morrow, N. R., Lim, H. T., 1983, Magnitude and detailed structure of residual oil saturation: Society Petroleum Engineers Journal, v. 23, p. 311–326., 10., 2118/10681-PA</ref>; modified from Hill et al., 1969.)]]

[[file:porosity_fig1.png|left|thumb|{{figure number|1}}Schematic of a pore system relating mineralogy, water content, and porosity assessment. (Notes: *lf sample is completely disaggregated during measurement. “Varies as a function of height above the free water level.) (After Chatzis et al.<ref name=pt05r33>Chatzis, I., Morrow, N. R., Lim, H. T., 1983, Magnitude and detailed structure of residual oil saturation: Society Petroleum Engineers Journal, v. 23, p. 311–326., 10., 2118/10681-PA</ref>; modified from Hill et al., 1969.{{citation needed}})]]

Discrepancies often exist between laboratory determined porosity values and porosities derived from downhole logs. Some of these discrepancies result from differences inherent in comparing direct measurements of physical properties made on small samples with indirect assessments of averaged properties. Many of these discrepancies, however, can be explained by noting differences in the definition and assessment of porosity ([[:file:porosity_fig1.png|Figure 1]]).

Discrepancies often exist between laboratory determined porosity values and porosities derived from downhole logs. Some of these discrepancies result from differences inherent in comparing direct measurements of physical properties made on small samples with indirect assessments of averaged properties. Many of these discrepancies, however, can be explained by noting differences in the definition and assessment of porosity ([[:file:porosity_fig1.png|Figure 1]]).

===Total porosity===

===Total porosity===

''Total porosity'' includes all void space regardless of whether the pores are interconnected or isolated. There is no practical ''way'' in the laboratory to measure isolated pore volume routinely on rocks. However, it can be determined by disaggregating the samples. If the disaggregated rocks contain smectite, the technique used to dry the samples can affect porosity values and the oven-dried total porosity will be larger than the humidity-dried total porosity (see Effective porosity below). Total porosity from a density log would equate with the disaggregated oven-dried total porosity from cores. The neutron log, however, would enlarge the definition to include structural hydroxyl chemistry.

''Total porosity'' includes all void space regardless of whether the pores are interconnected or isolated. There is no practical ''way'' in the laboratory to measure isolated pore volume routinely on rocks. However, it can be determined by disaggregating the samples. If the disaggregated rocks contain smectite, the technique used to dry the samples can affect porosity values and the oven-dried total porosity will be larger than the humidity-dried total porosity (see Effective porosity below). Total porosity from a density log would equate with the disaggregated oven-dried total porosity from cores. The [[Basic open hole tools#Compensated neutron|neutron log]], however, would enlarge the definition to include structural hydroxyl chemistry.

===Effective porosity===

===Effective porosity===

==Pore types==

==Pore types==

[[file:porosity_fig2.png|left|thumb|{{figure number|2}}Idealized sandstone porosity system showing four basic pore types: intergranular, microporosity, dissolution, and fracture. (After <ref name=pt05r127 />.)]]

[[file:porosity_fig2.png|left|thumb|{{figure number|2}}Idealized sandstone porosity system showing four basic pore types: intergranular, microporosity, dissolution, and fracture. (After Pittman.<ref name=pt05r127 />)]]

Basic clastic and carbonate pore types can be identified by integrating data from [[core description]]s, thin section petrography, scanning electron microscopy, and [[capillary pressure]] tests. These analyses indicate that significant differences exist between clastic and carbonate pore types.

Basic clastic and carbonate pore types can be identified by integrating data from [[core description]]s, thin section petrography, scanning electron microscopy, and [[capillary pressure]] tests. These analyses indicate that significant differences exist between clastic and carbonate pore types.

[[file:porosity_fig3.png|thumb|{{figure number|3}}Idealized carbonate porosity system showing three basic porosity groups: fabric selective, not fabric selective, and fabric selective or not. (After <ref name=pt05r34 />.)]]

[[file:porosity_fig3.png|thumb|{{figure number|3}}Idealized carbonate porosity system showing three basic porosity groups: fabric selective, not fabric selective, and fabric selective or not. (After Choquette and Pray.<ref name=pt05r34 />)]]

===Sandstone pore systems===

===Sandstone pore systems===

==Influence of textural parameters on porosity==

==Influence of textural parameters on porosity==

[[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 />.)]]

[[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 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, 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.