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  | isbn    = 0891813721

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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.

==Deltaic sediments often form complex reservoirs==

==Deltaic sediments often form complex reservoirs==

Delta systems make heterogeneous reservoirs, typically with a jigsaw-puzzle to labyrinthine sediment geometry (Table 1). There may also be considerable structural complexity. Many rivers, particularly the larger ones, dump very large quantities of sediment into deltas on top of an unstable substrate containing mobile salt and/or shale. Salt deformation features and growth faulting are common in deltaic sediments, and these can result in numerous segmented reservoirs, such as in the Tertiary Niger Delta in west Africa.<ref name=Evamyetal_1978>Evamy, B. D., J. Haremboure, P. Kamerling, W. A. Knaap, F. A. Molloy, and P. H. Rowlands, 1978, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0062/0001/0000/0001.htm Hydrocarbon habitat of Tertiary Niger Delta]: AAPG Bulletin, v. 62, no. 1, p. 1–39.</ref><ref name=Tuttleetal_1999>Tuttle, M. L. W., R. R. Charpentier, and M. E. Brownfield, 1999, [http://greenwood.cr.usgs.gov/energy/WorldEnergy/OF99-50H/ChapterA.html The Niger Delta petroleum system: Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa: U.S. Geological Survey Open File Report 99-501], 210 p.</ref>

Delta systems make heterogeneous reservoirs, typically with a jigsaw-puzzle to labyrinthine sediment geometry (Table 1). There may also be considerable structural complexity. Many rivers, particularly the larger ones, dump very large quantities of sediment into deltas on top of an unstable substrate containing mobile salt and/or shale. Salt [[deformation]] features and growth faulting are common in deltaic sediments, and these can result in numerous segmented reservoirs, such as in the Tertiary Niger Delta in west Africa.<ref name=Evamyetal_1978>Evamy, B. D., J. Haremboure, P. Kamerling, W. A. Knaap, F. A. Molloy, and P. H. Rowlands, 1978, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0062/0001/0000/0001.htm Hydrocarbon habitat of Tertiary Niger Delta]: AAPG Bulletin, v. 62, no. 1, p. 1–39.</ref><ref name=Tuttleetal_1999>Tuttle, M. L. W., R. R. Charpentier, and M. E. Brownfield, 1999, [http://greenwood.cr.usgs.gov/energy/WorldEnergy/OF99-50H/ChapterA.html The Niger Delta petroleum system: Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa: U.S. Geological Survey Open File Report 99-501], 210 p.</ref>

{| class = "wikitable"

{| class = "wikitable"

| 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 || -

| Tidal-dominated delta || - || Low recoveries caused by complex geometry and numerous mud and silt baffles

| Tidal-dominated delta || - || Low recoveries caused by complex geometry and numerous mud and silt baffles

|-

|-

| Distributary channels form narrow sand bodies || - || May be missed by wells in fields with a large well spacing; difficult to locate injection wells

| [[Distributary channel]]s form narrow sand bodies || - || May be missed by wells in fields with a large well spacing; difficult to locate injection wells

|-

|-

| Distributary channel sands commonly the highest permeability facies association in deltas || Can be the most productive intervals in a delta || -

| Distributary channel sands commonly the highest permeability facies association in deltas || Can be the most productive intervals in a delta || -

| 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 || -

|-

|-

| Mouth bars separated vertically by shales || Individual mouth bars can be targeted by horizontal wells; shale prevents water influx from swept units above and below || Poor to no vertical connectivity between mouth bars

| Mouth bars separated vertically by shales || Individual mouth bars can be targeted by [[horizontal well]]s; shale prevents water influx from swept units above and below || Poor to no vertical connectivity between mouth bars

|-

|-

| Coarser grained distributary channels cutting finer-grained delta-front sandstones || - || Can result in preferential water ingress into the delta-front area, bypassing oil in the delta-front sediments

| Coarser grained distributary channels cutting finer-grained delta-front sandstones || - || Can result in preferential water ingress into the delta-front area, bypassing oil in the delta-front sediments

==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 coals 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==

==Distributary channels==

==Distributary channels==

Distributary channels are so called because of the way in which they branch off from the main feeder river and distribute water and sediment across the delta. Where the distributary channels split off from the main feeder river, the volume of water carried by individual channels will be a fraction of that in the main river. By comparison to fluvial channels, distributary channels tend to be narrower and shallower. Gibling<ref name=Gibling_2006>Gibling, M. R. 2006, Width and thickness of fluvial channel bodies and valley fills in the geological record: A literature compilation and classification: Journal of Sedimentary Research, v. 76, p. 731–770.</ref> noted that distributary channels show a common width range of 10–300 m (33–984 ft) (see Table 2). Distributary channels tend to be straight where they incise a mud substrate and more sinuous within a sand substrate.<ref name=Sneideretal_1978>Sneider, R. M., C. N. Tinker, and L. D. Meckel, 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, v. 30, no. 11, p. 1538–1546.</ref>

[[Distributary channel]]s are so called because of the way in which they branch off from the main feeder river and distribute water and sediment across the delta. Where the distributary channels split off from the main feeder river, the volume of water carried by individual channels will be a fraction of that in the main river. By comparison to fluvial channels, distributary channels tend to be narrower and shallower. Gibling<ref name=Gibling_2006>Gibling, M. R. 2006, Width and thickness of fluvial channel bodies and valley fills in the geological record: A literature compilation and classification: Journal of Sedimentary Research, v. 76, p. 731–770.</ref> noted that distributary channels show a common width range of 10–300 m (33–984 ft) (see Table 2). Distributary channels tend to be straight where they incise a mud substrate and more sinuous within a sand substrate.<ref name=Sneideretal_1978>Sneider, R. M., C. N. Tinker, and L. D. Meckel, 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, v. 30, no. 11, p. 1538–1546.</ref>

{| class = "wikitable"

{| class = "wikitable"

| Valley fills on bedrock unconformities || 12-1400 m (39-4593 ft); most < 500 m (1640 ft) || 75 m-52 km (246 ft-32 mi); most < 10 km (6 mi) || 2-870; highly variable; mainly 2-100

| Valley fills on bedrock unconformities || 12-1400 m (39-4593 ft); most < 500 m (1640 ft) || 75 m-52 km (246 ft-32 mi); most < 10 km (6 mi) || 2-870; highly variable; mainly 2-100

|-

|-

| Valley fills within alluvial and marine strata || 2-210 m (6-689 ft); most < 60 m (197 ft) || 0.1-105 km (0.06-65 mi); common range 0.2-25 km (0.1-15 mi) || 4.6-3640; highly variable; common range 10-1000; many from 100 to 1000

| Valley fills within [[alluvial]] and marine strata || 2-210 m (6-689 ft); most < 60 m (197 ft) || 0.1-105 km (0.06-65 mi); common range 0.2-25 km (0.1-15 mi) || 4.6-3640; highly variable; common range 10-1000; many from 100 to 1000

|-

|-

| colspan="4" | ¹''From Gibling,<ref name=Gibling_2006 /> Journal of Sedimentary Research. Reprinted with permission from, and &copy; by, the SEPM (Society for Sedimentary Geologists).''

| colspan="4" | ¹''From Gibling,<ref name=Gibling_2006 /> Journal of Sedimentary Research. Reprinted with permission from, and &copy; by, the SEPM (Society for Sedimentary Geologists).''

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==

Mouth bars form where a distributary channel enters a standing body of water and sediment drops out. A shoaling to emergent sand body grows at the channel mouth. The resulting obstruction can cause the channel to bifurcate at the upstream head of the mouth bar. Mouth bars show an arcuate fan shape in plan view and a wedge-shaped profile in cross section. Reynolds<ref name=Reynolds_1999 /> gave average dimensions for mouth bars of about 3 km (1.8 mi) wide and about 6.5 km (4 mi) long (see [[:file:M91FG123.png|Figure 4]]; Table 3). Relatively straight distributary channels building out into deep water will form more linear deposits known as bar fingers ([[:file:M91FG186.JPG|Figure 5]]).<ref name=Fisk_1961 />

Mouth bars form where a distributary channel enters a standing body of water and sediment drops out. A shoaling to emergent sand body grows at the channel mouth. The resulting obstruction can cause the channel to bifurcate at the upstream head of the mouth bar. Mouth bars show an arcuate fan shape in plan view and a wedge-shaped profile in [[cross section]]. Reynolds<ref name=Reynolds_1999 /> gave average dimensions for mouth bars of about 3 km (1.8 mi) wide and about 6.5 km (4 mi) long (see [[:file:M91FG123.png|Figure 4]]; Table 3). Relatively straight distributary channels building out into deep water will form more linear deposits known as bar fingers ([[:file:M91FG186.JPG|Figure 5]]).<ref name=Fisk_1961 />

{| class = "wikitable"

{| class = "wikitable"

| 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.

==Transgressive sandstones==

==Transgressive sandstones==

Delta lobes will persist as areas of active sedimentation for a period of time, eventually becoming abandoned once the locus of sediment input switches elsewhere. The lobe eventually founders as a result of subsidence and a marine transgression follows. The transgressive sediments are thin, forming a distinctive facies association consisting of a series of coarse-grained, shelly beach ridges and barrier-bar inlet complexes.<ref name=Gallowayandhobday_1996 /> These units are laterally extensive and can be important marker beds in delta systems where the reservoir comprises stacked delta lobes. The generally heterogeneous nature of delta sediments can make correlation difficult otherwise. Other important marker horizons are thin marine shales, impure limestones, and coal beds.

Delta lobes will persist as areas of active sedimentation for a period of time, eventually becoming abandoned once the locus of sediment input switches elsewhere. The lobe eventually founders as a result of subsidence and a marine transgression follows. The transgressive sediments are thin, forming a distinctive facies association consisting of a series of coarse-grained, shelly beach ridges and barrier-bar inlet complexes.<ref name=Gallowayandhobday_1996 /> These units are laterally extensive and can be important marker beds in delta systems where the reservoir comprises stacked delta lobes. The generally heterogeneous nature of delta sediments can make correlation difficult otherwise. Other important marker horizons are thin marine shales, impure limestones, and [[coal beds]].

==Tidal processes==

==Tidal processes==

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.]]

==Coarse-grained deltas==

==Coarse-grained deltas==

===Fan deltas===

===Fan deltas===

Fan deltas are coarse-grained deltas that form where alluvial fans deliver sediments into a lake or the sea. Geometries vary from wedging to tabular to sheet-like. Dreyer<ref name=Dreyer_1993>Dreyer, T., 1993, Geometry and facies of large-scale flow units in fluvial-dominated fan-delta-front sequences, in M. Ashton, ed., Advances in reservoir geology: Geological Society Special Publication 69, p. 135–174.</ref> described exposures of fan-delta sediments in the Miocene Ridge Route Formation of California as an analog for the Tilje Formation of the mid-Norwegian shelf. The main permeability barriers occur as transgressive prodelta mudstones separating individual regressive fan-delta sedimentary episodes. Minor heterogeneity within individual fan-delta mouth bars is provided by intramouth bar shales and carbonate-cemented sandstones.

Fan deltas are coarse-grained deltas that form where [[alluvial]] fans deliver sediments into a lake or the sea. Geometries vary from wedging to tabular to sheet-like. Dreyer<ref name=Dreyer_1993>Dreyer, T., 1993, Geometry and facies of large-scale flow units in fluvial-dominated fan-delta-front sequences, in M. Ashton, ed., Advances in reservoir geology: Geological Society Special Publication 69, p. 135–174.</ref> described exposures of fan-delta sediments in the Miocene Ridge Route Formation of California as an analog for the Tilje Formation of the mid-Norwegian shelf. The main permeability barriers occur as transgressive prodelta mudstones separating individual regressive fan-delta sedimentary episodes. Minor heterogeneity within individual fan-delta mouth bars is provided by intramouth bar shales and carbonate-cemented sandstones.

===Braid deltas===

===Braid deltas===

Four facies associations were defined:

Four facies associations were defined:

* Alluvial braided river, comprising stacked units of fine to coarse-grained, poorly sorted, cross-bedded sandstone. There are no shales present to act as permeability barriers to vertical flow. Excellent sweep could result from a wide well spacing in these rocks. However, there may be preferential flow through the coarser base of the sandstones, and the flood front could be unstable at the laminar scale as a result of grain size variation in the cross-beds.

* [[Alluvial]] braided river, comprising stacked units of fine to coarse-grained, poorly sorted, cross-bedded sandstone. There are no shales present to act as permeability barriers to vertical flow. Excellent sweep could result from a wide well spacing in these rocks. However, there may be preferential flow through the coarser base of the sandstones, and the flood front could be unstable at the laminar scale as a result of grain size variation in the cross-beds.

* Delta plain characterized by multistory sandstone channels and laterally extensive delta-plain siltstones. The bases of the channels commonly show silty conglomerate lags that are likely flow barriers or baffles. The persistent, thick siltstone intervals impart a strong degree of flow layering and create several stacked hydraulic units within this facies association.

* Delta plain characterized by multistory sandstone channels and laterally extensive delta-plain siltstones. The bases of the channels commonly show silty [[conglomerate]] lags that are likely flow barriers or baffles. The persistent, thick siltstone intervals impart a strong degree of flow layering and create several stacked hydraulic units within this facies association.

* Proximal delta front ([[:file:M91FG189.JPG|Figure 8]]). This comprises a series of thick mouth-bar sandstones and channel systems separated by extensive delta-front siltstones and mudstones. The latter can potentially form permeability barriers and create stacked hydraulic units.

* Proximal delta front ([[:file:M91FG189.JPG|Figure 8]]). This comprises a series of thick mouth-bar sandstones and channel systems separated by extensive delta-front siltstones and mudstones. The latter can potentially form permeability barriers and create stacked hydraulic units.

* Distal delta front. This facies association comprises interbedded thin fine-grained sandstone, siltstone, and mudstone beds. The thin sandstones are extensive, highly layered, and have low permeability.

* Distal delta front. This facies association comprises interbedded thin fine-grained sandstone, siltstone, and mudstone beds. The thin sandstones are extensive, highly layered, and have low permeability.

==Sweep patterns in distributary channels==

==Sweep patterns in distributary channels==

Distributary channels 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 />).]]

[[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==