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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.]]
==Shingled geometry of deltas==
==Shingled geometry of deltas==
Shingled geometries are common in deltas ([[:file:M91FG188.JPG|Figure 7]]). Shingled motifs may be seen on regional seismic lines and large-scale correlation diagrams.<ref name=Bhattacharyaandwalker_1992>Bhattacharya, J. P., and R. G. Walker, 1992, Deltas, in R. G. Walker and N. P. James, eds., Facies models response to sea level change: Geological Association of Canada, 2d edition, p. 157–177.</ref> There is evidence from some fields that the shingled patterns can result in compartmentalization and bypassed oil volumes; for example, the Teal field, offshore Louisiana,<ref name=Sibleyandmastoris_1994>Sibley, D. M., and S. S. Mastoris, 1994, Rethinking conventional field development: 3-D shows direct detection of bypassed oil reserves: Presented at the International Petroleum Conference and Exhibition of Mexico, Society of Petroleum Engineers, October 10–13, 1994, Veracruz, Mexico, [https://www.onepetro.org/conference-paper/SPE-28719-MS SPE Paper 28719], 11 p.</ref><ref name=Hartetal_1997>Hart, B. S., D. M. Sibley, and P. B. Flemings, 1997, [http://archives.datapages.com/data/bulletns/1997/03mar/0380/0380.htm Seismic stratigraphy, facies architecture and reservoir character of a Pleistocene shelf-margin delta complex, Eugene Island Block 330 field, offshore Louisiana]: AAPG Bulletin, v. 81, no. 3, p. 380–397.</ref> the Sirikit field, onshore Thailand,<ref name=Ainsworthetal_1999>Ainsworth, R. B., M. Sanlung, and S. T. C. Duivenvoorden, 1999, [http://archives.datapages.com/data/bulletns/1999/10oct/1535/1535.htm Correlation techniques, perforation strategies, and recovery factors: An integrated 3-D reservoir modeling study, Sirikit field, Thailand]: AAPG Bulletin, v. 83, no. 10, p. 1535–1551.</ref> and the Ivishak Formation of the Prudhoe Bay field, Alaska.<ref name=Tyeetal_1999 />
==Coarse-grained 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.
===Braid deltas===
A braid delta is a coarse-grained delta fed by a braided river.<ref name=Mcphersonetal_1988>McPherson, J. G., G. Shanmugan, and R. J. Moiola, 1988, Fan deltas and braid deltas: Conceptual problems, in W. Nemec and R. J. Steel, eds., Fan deltas: Sedimentology and tectonic settings: Glasgow, Blackie and Son, p. 14–22.</ref> Braid-delta sediments from the Tertiary of the Apsheron Peninsula, Azerbaijan, have been described by Reynolds et al.<ref name=Reynoldsetal_1998>Reynolds, A. D., et al., 1998, [http://archives.datapages.com/data/bulletns/1998/01jan/0025/0025.htm Implications of outcrop geology for reservoirs in the Neogene productive series: Apsheron Peninsula, Azerbaijan]: AAPG Bulletin, v. 82, no. 1, p. 25–29.</ref> as the onshore outcrop analog for the reservoir interval for offshore oil fields in the South Caspian Basin. This technical paper is an excellent example of reservoir characterization in practice. The sedimentology has been analyzed with a focus on making predictions of the flow-geology behavior for the offshore fields.
[[file:M91FG189.JPG|thumb|300px|{{figure number|8}}Delta-front facies association from the Tertiary of the Apsheron Peninsula, Azerbaijan. A stacked succession of mouth-bar and channel sandstones is vertically sealed by delta-front siltstones. Internally, the sandstones display both coarsening-upward and fining-upward trends with low internal heterogeneity (from Reynolds et al.<ref name=Reynoldsetal_1998 />).]]
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.
* 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.
* 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 deltaic sandstone reservoirs==
Deltas comprise stratigraphically complex jigsaw-puzzle and labyrinth reservoirs with significant potential for bypassed oil. Tyler et al.<ref name=Tyleretal_1987>Tyler, N., J. Crispin Gholston, and W. A. Ambrose, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0010/1150/1171.htm Oil recovery in a low-permeability, wave-dominated, Cretaceous, deltaic reservoir, Big Wells (San Miguel field, South Texas)]: AAPG Bulletin, v. 71, no. 10, p. 1171–1195.</ref> commented on the recovery efficiencies of the various types of delta systems in Texas. Fluvial-dominated reservoirs show low to average recoveries because of the predominance of fluvial channels and an overall labyrinthine geometry. Wave-dominated deltas show more lateral continuity and have significantly higher recoveries. They commonly have large aquifers, and as a result these reservoirs will be supported by strong water drives.
Tyler and Ambrose<ref name=Tylerandambrose_1986>Tyler, N., and W. A. Ambrose, 1986, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0070/0007/0800/0809.htm Facies architecture and production characteristics of strand-plain reservoirs in North Markham-North Bay City field, Frio Formation, Texas]: AAPG Bulletin, v. 70, no. 7, p. 809–829.</ref> described the sweep patterns in a wave-dominated delta system from the Cayce reservoir of the North Markham-North Bay City field in Texas. Two distinct patterns occur. Water flows preferentially along fluvial channels. The beach ridge, delta front, and shoreface sandstones are more laterally continuous and show broader zones of edge-water influx.