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.

| Shingled geometry || - || Results in bypassed oil in individual shingles

| Shingled geometry || - || Results in bypassed oil in individual shingles

|-

|-

| Increasing marine reworking of delta front || Creates increasing lateral connectivity in the delta-front sediments || -

| Increasing marine reworking of delta front || Creates increasing [[lateral]] connectivity in the delta-front sediments || -

|-

|-

| Wave-dominated delta || More continuous, may have an aquifer || -

| Wave-dominated delta || More continuous, may have an aquifer || -

| Stacked distributary channels || Larger sand bodies with good vertical connectivity and sweep || -

| Stacked distributary channels || Larger sand bodies with good vertical connectivity and sweep || -

|-

|-

| Mouth bars contain extensive mudstone laminae || - || Mouth bars may have poor vertical connectivity and sweep

| Mouth bars contain extensive [[mudstone]] laminae || - || Mouth bars may have poor vertical connectivity and sweep

|-

|-

| Stacked mouth bars || Larger sand bodies with good vertical connectivity and sweep || -

| Stacked mouth bars || Larger sand bodies with good vertical connectivity and sweep || -

==Deltas are often gas reservoirs==

==Deltas are often gas reservoirs==

Many deltaic reservoirs, particularly long-lived Tertiary to present-day delta areas, contain more gas than oil. This is because they can be particularly rich in [[coal]]s and woody kerogen, which form gas-prone humic source material. Gas fields are found in the Mackenzie, Nile, and Irrawady deltas, for instance. Deltas can contain oil or mixed oil and gas where sandstones interfinger with a marine source rock.<ref name=Gallowayandhobday_1996>Galloway, W. E., and D. K. Hobday, 1996, Terrigenous clastic depositional systems: Applications to petroleum, coal, and uranium exploration: New York, Springer-Verlag, 489 p.</ref>

Many deltaic reservoirs, particularly long-lived Tertiary to present-day delta areas, contain more gas than oil. This is because they can be particularly rich in [[coal]]s and woody [[kerogen]], which form gas-prone humic source material. Gas fields are found in the Mackenzie, Nile, and Irrawady deltas, for instance. Deltas can contain oil or mixed oil and gas where sandstones interfinger with a marine source rock.<ref name=Gallowayandhobday_1996>Galloway, W. E., and D. K. Hobday, 1996, Terrigenous clastic depositional systems: Applications to petroleum, coal, and uranium exploration: New York, Springer-Verlag, 489 p.</ref>

 

 

==Types of delta==

==Types of delta==

Deltas have been categorized into three classes in terms of sedimentary process: wave dominated, tidal dominated, and fluvial dominated ([[:file:M91FG185.JPG|Figure 2]]).<ref name=Galloway_1975>Galloway, W. E., 1975, Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in M. L. Broussard, ed., Deltas, models for exploration: Houston Geological Society, p. 87–98.</ref> Coarse-grained deltas include fan deltas and braid deltas. Each specific environment has its own geometries and typical reservoir characteristics. The geometrical patterns shown by the various types of delta can often be recognized on isochore, net-sand, and log-facies maps.<ref name=Colemanandwright_1975>Coleman, J. M., and L. D. Wright, 1975, Modern river deltas: Variability of processes and sand bodies, in M. L. Broussard, ed., Deltas, models for exploration: Houston Geological Society, p. 99–149.</ref> For example, a wave-dominated delta will show a T motif on these maps as a result of fluvial lineaments converging at a high angle to a shoreline trend (see [[:file:M91Ch11FG71.JPG|Figure 3]]). The lobate shape of the delta front may also be recognized.

Deltas have been categorized into three classes in terms of sedimentary process: wave dominated, tidal dominated, and fluvial dominated ([[:file:M91FG185.JPG|Figure 2]]).<ref name=Galloway_1975>Galloway, W. E., 1975, Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in M. L. Broussard, ed., Deltas, models for exploration: Houston Geological Society, p. 87–98.</ref> Coarse-grained deltas include fan deltas and braid deltas. Each specific environment has its own geometries and typical reservoir characteristics. The geometrical patterns shown by the various types of delta can often be recognized on isochore, net-sand, and log-facies maps.<ref name=Colemanandwright_1975>Coleman, J. M., and L. D. Wright, 1975, Modern river deltas: Variability of processes and sand bodies, in M. L. Broussard, ed., Deltas, models for exploration: Houston Geological Society, p. 99–149.</ref> For example, a wave-dominated delta will show a T motif on these maps as a result of fluvial lineaments converging at a high angle to a shoreline trend (see [[:file:M91Ch11FG71.JPG|Figure 3]]). The lobate shape of the delta front may also be recognized.

==Depositional environments==

==Depositional environments==

Sand is deposited within linear distributary channels as side bars. In the modern-day Mahakam Delta, Borneo, side bars alternate on both sides of the distributary channels. These form elliptical sand pods, 5–8 km (3–5 mi) or more long and up to 1 km (0.6 mi) wide.<ref name=Allenandchambers_1998>Allen, G. P., and J. L. C. Chambers, 1998, Sedimentation in the modern and Miocene Mahakam delta: Indonesian Petroleum Association, 236 p.</ref> Channel fills typically show an upward-fining sediment profile and an upward-decreasing permeability profile. From the base upward, a distributary channel comprises the active channel fill, showing decimeter-scale trough cross-bedded sets; a partial abandonment fill with mainly centimeter-scale cross-beds; and sometimes an abandonment channel fill of thinly interbedded fine sand, silt, and shale.

Sand is deposited within linear distributary channels as side bars. In the modern-day Mahakam Delta, Borneo, side bars alternate on both sides of the distributary channels. These form elliptical sand pods, 5–8 km (3–5 mi) or more long and up to 1 km (0.6 mi) wide.<ref name=Allenandchambers_1998>Allen, G. P., and J. L. C. Chambers, 1998, Sedimentation in the modern and Miocene Mahakam delta: Indonesian Petroleum Association, 236 p.</ref> Channel fills typically show an upward-fining sediment profile and an upward-decreasing permeability profile. From the base upward, a distributary channel comprises the active channel fill, showing decimeter-scale trough cross-bedded sets; a partial abandonment fill with mainly centimeter-scale cross-beds; and sometimes an abandonment channel fill of thinly interbedded fine sand, silt, and shale.

==Mouth bars==

==Mouth bars==

| colspan="11" | *''From Reynolds.<ref name=Reynolds_1999 /> N = number.''

| colspan="11" | *''From Reynolds.<ref name=Reynolds_1999 /> N = number.''

|}

|}

[[file:M91FG187.JPG|thumb|300px|{{figure number|6}}Idealized log and permeability profiles for deltaic sand bodies (from Sneider et al.<ref name=Sneideretal_1978 />). Reprinted with permission from, and &copy; by, the Society of Petroleum Engineers.]]

[[file:M91FG187.JPG|thumb|300px|{{figure number|6}}Idealized log and permeability profiles for deltaic sand bodies (from Sneider et al.<ref name=Sneideretal_1978 />). Reprinted with permission from, and &copy; by, the Society of Petroleum Engineers.]]

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.]]

==Sweep patterns in distributary channels==

==Sweep patterns in distributary channels==

[[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.

==Sweep patters in delta-front sediments==

==Sweep patters in delta-front sediments==