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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>).]] + + + + +
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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==