Changes

  | isbn    = 0891813721

  | isbn    = 0891813721

}}

}}

[[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.

Weber<ref name=Weber_1982>Weber, K. J., 1982, Influence of common sedimentary structures on fluid flow in reservoir models: Journal of Petroleum Technology, v. 34, no. 3, [https://www.onepetro.org/journal-paper/SPE-9247-PA SPE Paper 9247], p. 655–672.</ref> stated that shales are not so laterally extensive in deltas by comparison to shoreface systems as fluvial and tidal channels commonly erode them. In distributary channels, the shale breaks can be short, commonly less than 10 m (33 ft) laterally.

Weber<ref name=Weber_1982>Weber, K. J., 1982, Influence of common sedimentary structures on fluid flow in reservoir models: Journal of Petroleum Technology, v. 34, no. 3, [https://www.onepetro.org/journal-paper/SPE-9247-PA SPE Paper 9247], p. 655–672.</ref> stated that shales are not so laterally extensive in deltas by comparison to shoreface systems as fluvial and tidal channels commonly erode them. In distributary channels, the shale breaks can be short, commonly less than 10 m (33 ft) laterally.

Mudstones may be more extensive along the delta front. Tye et al.<ref name=Tyeetal_1999 /> found that mudstones deposited following delta lobe abandonment formed locally significant flow barriers between delta lobes within the Ivishak Formation, the basal reservoir interval of the Prudhoe Bay field in Alaska.

Mudstones may be more extensive along the delta front. Tye et al.<ref name=Tyeetal_1999 /> found that mudstones deposited following delta lobe abandonment formed locally significant flow barriers between delta lobes within the Ivishak Formation, the basal reservoir interval of the [[Prudhoe Bay field]] in Alaska.

[[file:M91FG188.JPG|thumb|300px|{{figure number|7}}Seaward-dipping shingles in the Ivishak Formation, Prudhoe Bay field, Alaska (from Tye et al.<ref name=Tyeetal_1999 />). GR = gamma ray.]]

[[file:M91FG188.JPG|thumb|300px|{{figure number|7}}Seaward-dipping shingles in the Ivishak Formation, Prudhoe Bay field, Alaska (from Tye et al.<ref name=Tyeetal_1999 />). GR = gamma ray.]]

[[Distributary channel]]s tend to be narrow with a common width range of 10–300 m (33–984 ft).<ref name=Gibling_2006 /> It is possible that hydrocarbons in isolated distributary channels may be missed in fields with larger well spacings. Richardson et al.<ref name=Richardsonetal_1989 /> stated that it could be impractical to try and locate both injection and production wells to sweep individual distributary channels. The sweep efficiency in the distributary channels will be low without direct injection support, particularly if the sand bodies are isolated. There is a better chance of improving recovery by waterflooding the delta-front sandstones.

[[Distributary channel]]s tend to be narrow with a common width range of 10–300 m (33–984 ft).<ref name=Gibling_2006 /> It is possible that hydrocarbons in isolated distributary channels may be missed in fields with larger well spacings. Richardson et al.<ref name=Richardsonetal_1989 /> stated that it could be impractical to try and locate both injection and production wells to sweep individual distributary channels. The sweep efficiency in the distributary channels will be low without direct injection support, particularly if the sand bodies are isolated. There is a better chance of improving recovery by waterflooding the delta-front sandstones.

[[file:M91FG190.JPG|thumb|300px|{{figure number|9}}Sweep patterns in delta sediments. (a) Preferential water ingress along channel sediments can result in bypassed oil in the surrounding sediments. From the South Pass Block 27 field, offshore Louisiana (from Hartman and Paynter<ref name=Hartmanandpaynter_1979>Hartman, J. A., and D. D. Paynter, 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, [https://www.onepetro.org/journal-paper/SPE-7532-PA SPE Paper 7532], v. 31, no. 10, p. 1313–1322.</ref>). Reprinted with permission from, and &copy; by, the Society of Petroleum Engineers. (b) Horizontal well drilled to target oil within a mouth bar in the Ivishak Formation, Prudhoe Bay field. Bay shales above the mouth bar act to prevent gas ingress from a gas cap immediately above (from Tye et al.<ref name=Tyeetal_1999 />).]]

[[file:M91FG190.JPG|thumb|300px|{{figure number|9}}Sweep patterns in delta sediments. (a) Preferential water ingress along channel sediments can result in bypassed oil in the surrounding sediments. From the South Pass Block 27 field, offshore Louisiana (from Hartman and Paynter<ref name=Hartmanandpaynter_1979>Hartman, J. A., and D. D. Paynter, 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, [https://www.onepetro.org/journal-paper/SPE-7532-PA SPE Paper 7532], v. 31, no. 10, p. 1313–1322.</ref>). Reprinted with permission from, and &copy; by, the Society of Petroleum Engineers. (b) Horizontal well drilled to target oil within a mouth bar in the Ivishak Formation, [[Prudhoe Bay field]]. Bay shales above the mouth bar act to prevent gas ingress from a gas cap immediately above (from Tye et al.<ref name=Tyeetal_1999 />).]]

==Sweep patters in delta-front sediments==

==Sweep patters in delta-front sediments==