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Line 57: − − [[file:M91FG185.JPG|thumb|300px|{{figure number|2}}Three categories of delta can be defined according to the dominant sedimentary process. These are wave-dominated, tide-dominated, and fluvial-dominated deltas. Courtesy of the [http://www.earthobservatory.com NASA Web site].]] − − [[file:M91Ch11FG71.JPG|thumb|300px|{{figure number|3}} A gross sandstone thickness map can give an idea of the depositional dip and strike of the sedimentary system. In the Budare field of Venezuela, north–south strike elements correspond to distributary channels in the bottom part of the map. An east–west arcuate depositional element in the north of the map corresponds to a wave-dominated delta front (from Hamilton et al.<ref name=Hamiltonetal_2002>Hamilton, D. S., et al., 2002, [http://archives.datapages.com/data/bulletns/2002/07jul/1237/1237.htm Reactivation of mature oil fields through advanced reservoir characterization: A case history of the Budare field, Venezuela]: AAPG Bulletin, v. 86, no. 7, p. 1237–1262.</ref>).]] + + + + + Line 93:
Line 94: − − [[file:M91FG123.png|thumb|300px|{{figure number|4}}A schematic delta showing a range of sand body types at their average dimensions, together with several oil and gas fields at the same scale. The delta front is divided into three segments that are storm-, fluvial-, and tidal-dominated, respectively. The delta and its divisions are not to scale (from Reynolds<ref name=Reynolds_1999>Reynolds, A. D., 1999, [http://archives.datapages.com/data/bulletns/1999/02feb/0211/0211.htm Dimensions of paralic sandstone bodies]: AAPG Bulletin, v. 83, no. 2, p. 211–229.</ref>).]] − − [[file:M91FG186.JPG|thumb|300px|{{figure number|5}}Fluvial-dominated delta environment, Mississippi Delta. Photograph courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. The inset box on the photograph measures 34 times 42 km (21 times 26 mi). The lower diagram is a box diagram showing the sedimentological relationships within the inset box (after Fisk<ref name=Fisk_1961>Fisk, H. N., 1961, [http://archives.datapages.com/data/specpubs/sandsto1/data/a055/a055/0001/0000/0029.htm Bar-finger sands of the Mississippi delta], in J. A. Peterson and J. C. Osmond, eds., Geometry of sandstone bodies: AAPG Symposium, SP22, p. 29–52.</ref>).]] Line 143:
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==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.
<gallery mode=packed heights=450px widths=450px>
file:M91FG185.JPG|{{figure number|2}}Three categories of delta can be defined according to the dominant sedimentary process. These are wave-dominated, tide-dominated, and fluvial-dominated deltas. Courtesy of the [http://www.earthobservatory.com NASA Web site].
file:M91Ch11FG71.JPG|{{figure number|3}} A gross sandstone thickness map can give an idea of the depositional dip and strike of the sedimentary system. In the Budare field of Venezuela, north–south strike elements correspond to distributary channels in the bottom part of the map. An east–west arcuate depositional element in the north of the map corresponds to a wave-dominated delta front (from Hamilton et al.<ref name=Hamiltonetal_2002>Hamilton, D. S., et al., 2002, [http://archives.datapages.com/data/bulletns/2002/07jul/1237/1237.htm Reactivation of mature oil fields through advanced reservoir characterization: A case history of the Budare field, Venezuela]: AAPG Bulletin, v. 86, no. 7, p. 1237–1262.</ref>).
</gallery>
==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.''
|}
|}
<gallery mode=packed heights=450px widths=400px>
file:M91FG123.png|{{figure number|4}}A schematic delta showing a range of sand body types at their average dimensions, together with several oil and gas fields at the same scale. The delta front is divided into three segments that are storm-, fluvial-, and tidal-dominated, respectively. The delta and its divisions are not to scale (from Reynolds<ref name=Reynolds_1999>Reynolds, A. D., 1999, [http://archives.datapages.com/data/bulletns/1999/02feb/0211/0211.htm Dimensions of paralic sandstone bodies]: AAPG Bulletin, v. 83, no. 2, p. 211–229.</ref>).
file:M91FG186.JPG|{{figure number|5}}Fluvial-dominated delta environment, Mississippi Delta. Photograph courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. The inset box on the photograph measures 34 times 42 km (21 times 26 mi). The lower diagram is a box diagram showing the sedimentological relationships within the inset box (after Fisk<ref name=Fisk_1961>Fisk, H. N., 1961, [http://archives.datapages.com/data/specpubs/sandsto1/data/a055/a055/0001/0000/0029.htm Bar-finger sands of the Mississippi delta], in J. A. Peterson and J. C. Osmond, eds., Geometry of sandstone bodies: AAPG Symposium, SP22, p. 29–52.</ref>).
</gallery>
[[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 © 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 © by, the Society of Petroleum Engineers.]]