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[[file:M91FG184.JPG|thumb|300px|{{figure number|1}}Photograph of the Lena delta, Russia. Courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. The delta is about 200 km (124 mi) across in this view. The photograph has been rotated such that north faces down the page. The lower diagram is a lithofacies map of the basal Ivishak Formation, Prudhoe Bay field, Alaska (from Tye et al.<ref name=Tyeetal_1999>Tye, R. S., J. P. Bhattacharya, J. A. Lorsong, S. T. Sindelar, D. G. Knock, D. D. Puls, and R. A. Levinson, 1999, [http://archives.datapages.com/data/bulletns/1999/10oct/1588/1588.htm Geology and stratigraphy of fluvio-deltaic deposits in the Ivishak Formation: Applications for development of Prudhoe Bay field, Alaska]: AAPG Bulletin, v. 83, no. 10, p. 1588–1623.</ref>).]]

[[file:M91FG184.JPG|thumb|300px|{{figure number|1}}Photograph of the Lena delta, Russia. Courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. The delta is about 200 km (124 mi) across in this view. The photograph has been rotated such that north faces down the page. The lower diagram is a [[lithofacies map]] of the basal Ivishak Formation, Prudhoe Bay field, Alaska (from Tye et al.<ref name=Tyeetal_1999>Tye, R. S., J. P. Bhattacharya, J. A. Lorsong, S. T. Sindelar, D. G. Knock, D. D. Puls, and R. A. Levinson, 1999, [http://archives.datapages.com/data/bulletns/1999/10oct/1588/1588.htm Geology and stratigraphy of fluvio-deltaic deposits in the Ivishak Formation: Applications for development of Prudhoe Bay field, Alaska]: AAPG Bulletin, v. 83, no. 10, p. 1588–1623.</ref>).]]

Deltas are major sites of sand and mud deposition. They contain significant volumes of hydrocarbons worldwide ([[:file:M91FG184.JPG|Figure 1]]). Major petroleum provinces include the Niger Delta in west Africa, the Mahakam Delta in Borneo, the Caspian Sea, and the Maracaibo Basin in Venezuela.

Deltas are major sites of sand and mud deposition. They contain significant volumes of hydrocarbons worldwide ([[:file:M91FG184.JPG|Figure 1]]). Major petroleum provinces include the Niger Delta in west Africa, the Mahakam Delta in Borneo, the Caspian Sea, and the Maracaibo Basin in Venezuela.

An upward-increasing [[grain size]] profile is characteristic for mouth bars. The lower parts are finer grained, more poorly sorted, and with common shale intercalations. Upward, the texture is coarser although there may be many laminations of clays and organic material. Permeability typically increases upward ([[:file:M91FG187.JPG|Figure 6]]).

An upward-increasing [[grain size]] profile is characteristic for mouth bars. The lower parts are finer grained, more poorly sorted, and with common shale intercalations. Upward, the texture is coarser although there may be many laminations of clays and organic material. Permeability typically increases upward ([[:file:M91FG187.JPG|Figure 6]]).

Mouth bars usually show lower overall permeabilities than distributary channel fills.<ref name=Richardsonetal_1989>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1989, Sand-rich deltas: Journal of Petroleum Technology, v. 41, no. 2, p. 157–158.</ref> For example, Tye et al.<ref name=Tyeetal_1999 /> gave average rock property values for the various lithofacies associations within the Ivishak Formation of the Prudhoe Bay field in Alaska. The mouth bars have a mean permeability of 151 md compared to 315 md for the distributary channel fills.

Mouth bars usually show lower overall permeabilities than distributary channel fills.<ref name=Richardsonetal_1989>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1989, Sand-rich deltas: Journal of Petroleum Technology, v. 41, no. 2, p. 157–158.</ref> For example, Tye et al.<ref name=Tyeetal_1999 /> gave average rock property values for the various [[lithofacies]] associations within the Ivishak Formation of the Prudhoe Bay field in Alaska. The mouth bars have a mean permeability of 151 md compared to 315 md for the distributary channel fills.

The coarsest and best sorted sediments in the mouth bars form near the stream mouth and along the bar margins adjacent to the distributary channels. Tye and Hickey<ref name=Tyeandhickey_2001>Tye, R. S., and J. J. Hickey, 2001, [http://archives.datapages.com/data/bulletns/2001/03mar/0459/0459.htm Permeability characterization of distributary mouth bar sandstones in Prudhoe Bay field, Alaska: How horizontal cores reduce risk in developing deltaic reservoirs]: AAPG Bulletin, v. 85, no. 3, p. 459–475.</ref> found an order of magnitude higher permeability in this part of the point bars in Prudhoe Bay field, Alaska. Outward and down slope, the sediment becomes finer grained. Downstream, along the outer edge of the mouth bar, fine sand and silts interfinger with prodelta muds.

The coarsest and best sorted sediments in the mouth bars form near the stream mouth and along the bar margins adjacent to the distributary channels. Tye and Hickey<ref name=Tyeandhickey_2001>Tye, R. S., and J. J. Hickey, 2001, [http://archives.datapages.com/data/bulletns/2001/03mar/0459/0459.htm Permeability characterization of distributary mouth bar sandstones in Prudhoe Bay field, Alaska: How horizontal cores reduce risk in developing deltaic reservoirs]: AAPG Bulletin, v. 85, no. 3, p. 459–475.</ref> found an order of magnitude higher permeability in this part of the point bars in Prudhoe Bay field, Alaska. Outward and down slope, the sediment becomes finer grained. Downstream, along the outer edge of the mouth bar, fine sand and silts interfinger with prodelta muds.