Changes

===Cementation===

===Cementation===

Cementation, the filling of original pore space by cements, may occur early or late in the diagenetic history of a rock.<ref name=pt06r117>Scholle, P. A., Schluger, P. R., eds., 1979, Aspects of Diagenesis: SEPM Special Publication, n. 26, 443 p.</ref><ref name=pt06r86>McDonald, D. A., Surdam, R. C., eds., 1984, Clastic Diagenesis: AAPG Memoir 37, 434 p.</ref> Table 2 lists some common cement types. Precipitation of authigenic minerals usually reduces reservoir quality; however, early formation of some authigenic minerals can preserve the original porosity by protecting the rock from later degradation by compaction or cementation.<ref name=pt06r158>Wilson, M. D., Pittman, E. D., 1977, Authigenic clays in sandstones—recognition and influence on reservoir properties and paleoenvironmental analysis: Journal of Sedimentary Petrology, v. 47, p. 3–31.</ref>

Cementation, the filling of original pore space by cements, may occur early or late in the diagenetic history of a rock.<ref name=pt06r117>Scholle, P. A., Schluger, P. R., eds., 1979, Aspects of Diagenesis: SEPM Special Publication 26, 443 p.</ref><ref name=pt06r86>McDonald, D. A., Surdam, R. C., eds., 1984, [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/m37.htm Clastic Diagenesis]: AAPG Memoir 37, 434 p.</ref> Table 2 lists some common cement types. Precipitation of authigenic minerals usually reduces reservoir quality; however, early formation of some authigenic minerals can preserve the original porosity by protecting the rock from later degradation by compaction or cementation.<ref name=pt06r158>Wilson, M. D., Pittman, E. D., 1977, Authigenic clays in sandstones—recognition and influence on reservoir properties and paleoenvironmental analysis: Journal of Sedimentary Petrology, v. 47, p. 3–31.</ref>

{| class = "wikitable"

{| class = "wikitable"

===Dissolution===

===Dissolution===

Dissolution of less chemically stable minerals in sandstones and carbonates can sometimes significantly increase both the rock porosity and the permeability.<ref name=pt06r116>Schmidt, V., McDonald, D. A., 1980, Secondary Reservoir Porosity in the Course of Sandstone Diagenesis: AAPG Continuing Education Course Note Series No. 12, 125 p.</ref> Dissolution tends to be especially important in carbonates that are buried to shallow depths and sandstones that are deeply buried.

Dissolution of less chemically stable minerals in sandstones and carbonates can sometimes significantly increase both the rock porosity and the permeability.<ref name=pt06r116>Schmidt, V., McDonald, D. A., 1980, [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/cn12.htm Secondary Reservoir Porosity in the Course of Sandstone Diagenesis]: AAPG Continuing Education Course Note Series No. 12, 125 p.</ref> Dissolution tends to be especially important in carbonates that are buried to shallow depths and sandstones that are deeply buried.

===Recrystallization===

===Recrystallization===

===[[Wettability]]===

===[[Wettability]]===

Wettability in an oil reservoir controls reservoir quality by affecting the amount of water production (see [[Wettability]]). When the reservoir rock is oil-wet, water is located in the central portion of the pores and will flow through the pore system with the oil. Conversely, in a water-wet reservoir, the water is restricted to the perimeter of the pores and will not (''low'' through the pore system until much of the oil has been removed. In addition, the irreducible water saturations of oil-wet reservoirs tend to be much lower than those of water-wet reservoirs.

Wettability in an oil reservoir controls reservoir quality by affecting the amount of water production. When the reservoir rock is oil-wet, water is located in the central portion of the pores and will flow through the pore system with the oil. Conversely, in a water-wet reservoir, the water is restricted to the perimeter of the pores and will not flow through the pore system until much of the oil has been removed. In addition, the irreducible water saturations of oil-wet reservoirs tend to be much lower than those of water-wet reservoirs.

===[[Capillary pressure]]===

===[[Capillary pressure]]===

Modern three-dimensional seismic data<ref name=pt06r17>Brown, A. R., 1986 Interpretation of three-dimensional seismic data: [http://store.aapg.org/detail.aspx?id=1025 AAPG Memoir 42], 194 p.</ref> can sometimes assist in predicting reservoir quality away from well control. Careful processing of seismic data allows a conversion of the seismic reflection amplitudes to estimates of acoustic impedance. Because lithology, porosity, and fluid saturations affect the acoustic impedance of a rock, a relationship can then be established between the seismic estimates of impedance and the rock properties determined from the logs or in the laboratory. (For information on comparing seismic data to rock properties, see [[Seismic inversion]].)

Modern three-dimensional seismic data<ref name=pt06r17>Brown, A. R., 1986 Interpretation of three-dimensional seismic data: [http://store.aapg.org/detail.aspx?id=1025 AAPG Memoir 42], 194 p.</ref> can sometimes assist in predicting reservoir quality away from well control. Careful processing of seismic data allows a conversion of the seismic reflection amplitudes to estimates of acoustic impedance. Because lithology, porosity, and fluid saturations affect the acoustic impedance of a rock, a relationship can then be established between the seismic estimates of impedance and the rock properties determined from the logs or in the laboratory. (For information on comparing seismic data to rock properties, see [[Seismic inversion]].)

Wireline logs can be classified into three different groups based on the information they provide: (1) lithology indicators—gamma ray, sonic, density, and neutron logs, (2) porosity logs—sonic, density, and neutron logs, and (3) fluid saturation logs—resistivity logs.<ref name=pt06r6>Asquith, G., Gibson, C. 1982, Basic well log analysis for geologists: AAPG Methods in Exploration Series, 216 p.</ref> (For more on the information that wireline logs can provide, see [[Standard interpretation]].)

Wireline logs can be classified into three different groups based on the information they provide: (1) lithology indicators—gamma ray, sonic, density, and neutron logs, (2) porosity logs—sonic, density, and neutron logs, and (3) fluid saturation logs—resistivity logs.<ref name=pt06r6>Asquith, G., Gibson, C. 1982, [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me3.htm Basic well log analysis for geologists]: AAPG Methods in Exploration Series 3, 216 p.</ref> (For more on the information that wireline logs can provide, see [[Standard interpretation]].)

In addition to lithology, porosity, and fluid saturations, permeability sometimes can be inferred from log responses or a combination of log responses. The spontaneous potential log is most often used as a qualitative indicator of the permeability of a formation. (For more on wireline log response to reservoir properties, see [[Quick-look lithology from logs]].)

In addition to lithology, porosity, and fluid saturations, permeability sometimes can be inferred from log responses or a combination of log responses. The spontaneous potential log is most often used as a qualitative indicator of the permeability of a formation. (For more on wireline log response to reservoir properties, see [[Quick-look lithology from logs]].)