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Lithofacies maps show the areal variation in the depositional patterns that make up each genetic sequence within the reservoir interval. The method for constructing lithofacies maps involves extrapolating the lithofacies from the wells into the gaps between the wells. This is not easy, as there will be mostly vertical or near vertical well data in the field. The vertical facies profile may be determined reasonably confidently; however, the lateral facies progression will have to be inferred by analog and other means.

Lithofacies maps show the areal variation in the depositional patterns that make up each genetic sequence within the reservoir interval. The method for constructing lithofacies maps involves extrapolating the lithofacies from the wells into the gaps between the wells. This is not easy, as there will be mostly vertical or near vertical well data in the field. The vertical facies profile may be determined reasonably confidently; however, the [[lateral]] facies progression will have to be inferred by analog and other means.

==Use of analogs for lithofacies mapping==

==Use of analogs for lithofacies mapping==

However, there are limitations to the use of modern analogs. Certain macroforms lack preservation potential and may not be common in the subsurface because of erosion. Conditions today may not be anything like the prevailing conditions when a given interval of reservoir sediments formed. The present day has a specific climate, tectonic variability, relative position of sea level, and rate of sea level change.<ref name=Grammeretal_2004>Grammer, G. M., P. M. Harris, and G. P. Eberli, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER1/CHAPTER1.HTM Integration of outcrop and modern analogs in reservoir modeling: Overview with examples from the Bahamas], ''in'' G. M. Grammer, P. M. Harris, and G. P. Eberlie, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 1-22.</ref> For example, the continents are mountainous and widely dispersed, with a tendency for shorelines to cut predominantly north–south across the world's climate zones. The Earth currently has ice caps but has lacked them for large parts of its geological history.

However, there are limitations to the use of modern analogs. Certain macroforms lack preservation potential and may not be common in the subsurface because of erosion. Conditions today may not be anything like the prevailing conditions when a given interval of reservoir sediments formed. The present day has a specific climate, tectonic variability, relative position of sea level, and rate of sea level change.<ref name=Grammeretal_2004>Grammer, G. M., P. M. Harris, and G. P. Eberli, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER1/CHAPTER1.HTM Integration of outcrop and modern analogs in reservoir modeling: Overview with examples from the Bahamas], ''in'' G. M. Grammer, P. M. Harris, and G. P. Eberlie, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 1-22.</ref> For example, the continents are mountainous and widely dispersed, with a tendency for shorelines to cut predominantly north–south across the world's climate zones. The Earth currently has ice caps but has lacked them for large parts of its geological history.

[[file:M91Ch11FG69.JPG|thumb|300px|{{figure number|1}}An alluvial fan in China's Xinjuang Province. The diameter of the fan is about 170 km (106 mi). Courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. Shown is the approximate size of a billion-barrel oil field.]]

[[file:M91Ch11FG69.JPG|thumb|300px|{{figure number|1}}An [[alluvial]] fan in China's Xinjuang Province. The diameter of the fan is about 170 km (106 mi). Courtesy of the [http://www.earthasart.gsfc.nasa.gov NASA Web site]. Shown is the approximate size of a billion-barrel oil field.]]

Modern and ancient analogs can be investigated by referring to technical papers and outcrop studies, or the examination of aerial photographs.<ref name=Tye_2004>Tye, R. S., 2004, [http://archives.datapages.com/data/bulletns/2004/08aug/1123/1123.HTM Geomorphology: An approach to determining subsurface reservoir dimensions]: AAPG Bulletin, v. 88, no. 8, p. 1123-1147.</ref> The latter is a particularly vivid source of information ([[:file:M91Ch11FG69.JPG|Figure 1]]). Geologists working on carbonate reservoirs, for example, should have a look at the AAPG publication on modern day carbonate environments.<ref name=Harrisandkowalik_1994>Harris, P. M., and W. S. Kowalik, eds., 1994, Satellite images of carbonate depositional environments: [http://store.aapg.org/detail.aspx?id=130 AAPG Methods in Exploration Series 11], 147 p.</ref> The book includes a series of aerial photographs along with a transparent overlay giving the outlines of typical carbonate fields. This gives a very good impression of the size of depositional environments relative to the field scale. The photographs illustrate the fact that many depositional environments cover a much larger area than oil fields. This is an important observation. Because depositional environments extend over a greater area than the field, the geologist should investigate the sedimentology at the larger (basin) scale to get a realistic idea of the basinal controls influencing the sediments at the field scale.

Modern and ancient analogs can be investigated by referring to technical papers and outcrop studies, or the examination of aerial photographs.<ref name=Tye_2004>Tye, R. S., 2004, [http://archives.datapages.com/data/bulletns/2004/08aug/1123/1123.HTM Geomorphology: An approach to determining subsurface reservoir dimensions]: AAPG Bulletin, v. 88, no. 8, p. 1123-1147.</ref> The latter is a particularly vivid source of information ([[:file:M91Ch11FG69.JPG|Figure 1]]). Geologists working on carbonate reservoirs, for example, should have a look at the AAPG publication on modern day carbonate environments.<ref name=Harrisandkowalik_1994>Harris, P. M., and W. S. Kowalik, eds., 1994, Satellite images of carbonate depositional environments: [http://store.aapg.org/detail.aspx?id=130 AAPG Methods in Exploration Series 11], 147 p.</ref> The book includes a series of aerial photographs along with a transparent overlay giving the outlines of typical carbonate fields. This gives a very good impression of the size of depositional environments relative to the field scale. The photographs illustrate the fact that many depositional environments cover a much larger area than oil fields. This is an important observation. Because depositional environments extend over a greater area than the field, the geologist should investigate the sedimentology at the larger (basin) scale to get a realistic idea of the basinal controls influencing the sediments at the field scale.

| Sabkha lake || Tunisia || 33&deg;41'28.31"N || 8&deg;28'21.40"E

| Sabkha lake || Tunisia || 33&deg;41'28.31"N || 8&deg;28'21.40"E

|-

|-

| Alluvial fans || United States || 36&deg;10'42.30"N || 116&deg;54'12.46"E

| [[Alluvial]] fans || United States || 36&deg;10'42.30"N || 116&deg;54'12.46"E

|-

|-

| Braided river || Madagascar || 21&deg;45'32.20"S || 43&deg;53'57.55"E

| Braided river || Madagascar || 21&deg;45'32.20"S || 43&deg;53'57.55"E

Outcrops allow the geometrical patterns shown by lateral facies variation to be inspected. They may be less reliable as a direct analog for reservoirs by comparison to modern-day environments because their origin requires interpretation, and this introduces an element of subjectivity. Nevertheless, they will provide sedimentological information directly comparable to cores from the subsurface. Perhaps the ideal analog database involves a combination of images from modern-day environments for geometrical data and outcrops for a sedimentological comparison.

Outcrops allow the geometrical patterns shown by lateral facies variation to be inspected. They may be less reliable as a direct analog for reservoirs by comparison to modern-day environments because their origin requires interpretation, and this introduces an element of subjectivity. Nevertheless, they will provide sedimentological information directly comparable to cores from the subsurface. Perhaps the ideal analog database involves a combination of images from modern-day environments for geometrical data and outcrops for a sedimentological comparison.

[[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., N. Tyler, R. Tyler, S. K. Raeuchle, M. H. Holtz, J. Yeh, M. Uxcategul, T. Jimenez, A. Salazar, C. E. Cova, R. Barbato, and A. Rusic, 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>).]]

[[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 channel]]s 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., N. Tyler, R. Tyler, S. K. Raeuchle, M. H. Holtz, J. Yeh, M. Uxcategul, T. Jimenez, A. Salazar, C. E. Cova, R. Barbato, and A. Rusic, 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>).]]

==Three-dimensional seismic geomorphology==

==Three-dimensional seismic geomorphology==

Carbonate sediments produce distinctive seismic facies with reefs and marginal reef environments commonly well imaged.<ref name=Fontaineetal_1987>Fontaine, J. M., R. Cussey, J. Lacaze, R. Lanaud, and L. Yapaudjian, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0003/0250/0281.htm Seismic interpretation of carbonate depositional environments]: AAPG Bulletin, v. 71, no. 3, p. 281–297.</ref> <ref name=Masaferroetal_2003>Masaferro, J. L., R. Bourne, and J.-C. Jauffred, 2003, 3D visualization of carbonate reservoirs: The Leading Edge, v. 19, no. 1, p. 18–25.</ref><ref name= Eberlietal_2004>Eberli, G. P., J. L. Masaferro, and J. F. Sarg, 2004, [http://archives.datapages.com/data/specpubs/memoir81/INTRODUCTION/INTRODUCTION.HTM Introduction], ''in'' G. P. Eberli, J. L. Masaferro, and J. F. Sarg, eds., Seismic imaging of carbonate reservoirs and systems: [http://store.aapg.org/detail.aspx?id=659 AAPG Memoir 81], p. 1–9.</ref> Deep-water marine deposits typically show sharp lithological contrasts between sand bodies and the encasing deep marine mudstones. These enable the sandstones to be picked on horizon slice amplitude and semblance displays.<ref name=Varnai_1998>Varnai, P., 1998, [http://archives.datapages.com/data/bulletns/1998/05may_b/0986/0986.htm Three-dimensional seismic stratigraphic expression of Pliocene–Pleistocene turbidite systems, northern Green Canyon (offshore Louisiana), northern Gulf of Mexico]: AAPG Bulletin, v. 82, no. 5B, p. 986–1012.</ref> <ref name=Salleretal_2004>Saller, A. H., J. T. Noah, A. P. Ruzuar, and R. Schneider, 2004, [http://archives.datapages.com/data/bulletns/2004/01jan/0021/0021.HTM Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems]: AAPG Bulletin, v. 88, no. 1, p. 21–46.</ref> Semblance displays (also known as coherence cubetrade displays) are computed from seismic data by comparing the similarity of each seismic trace with its neighbors within a specific window of interest. Significant changes in the response corresponding to sand pinch-outs or faults are highlighted as edges.<ref name=Bahorichandfarmer_1985>Bahorich, M., and S. Farmer, 1995, 3-D seismic discontinuity for faults and stratigraphic features: The coherency cube: The Leading Edge, v. 14, p. 1053–1058.</ref> <ref name=Marfurtetal_1998>Marfurt, K. J., R. L. Kirlin, S. L. Farmer, and M. S. Bahorich, 1998, 3-D seismic attributes using a semblance-based coherency algorithm: Geophysics, v. 63, no. 4, p. 1150–1165.</ref>

Carbonate sediments produce distinctive seismic facies with reefs and marginal reef environments commonly well imaged.<ref name=Fontaineetal_1987>Fontaine, J. M., R. Cussey, J. Lacaze, R. Lanaud, and L. Yapaudjian, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0003/0250/0281.htm Seismic interpretation of carbonate depositional environments]: AAPG Bulletin, v. 71, no. 3, p. 281–297.</ref> <ref name=Masaferroetal_2003>Masaferro, J. L., R. Bourne, and J.-C. Jauffred, 2003, 3D visualization of carbonate reservoirs: The Leading Edge, v. 19, no. 1, p. 18–25.</ref><ref name= Eberlietal_2004>Eberli, G. P., J. L. Masaferro, and J. F. Sarg, 2004, [http://archives.datapages.com/data/specpubs/memoir81/INTRODUCTION/INTRODUCTION.HTM Introduction], ''in'' G. P. Eberli, J. L. Masaferro, and J. F. Sarg, eds., Seismic imaging of carbonate reservoirs and systems: [http://store.aapg.org/detail.aspx?id=659 AAPG Memoir 81], p. 1–9.</ref> Deep-water marine deposits typically show sharp lithological contrasts between sand bodies and the encasing deep marine mudstones. These enable the sandstones to be picked on horizon slice amplitude and semblance displays.<ref name=Varnai_1998>Varnai, P., 1998, [http://archives.datapages.com/data/bulletns/1998/05may_b/0986/0986.htm Three-dimensional seismic stratigraphic expression of Pliocene–Pleistocene turbidite systems, northern Green Canyon (offshore Louisiana), northern Gulf of Mexico]: AAPG Bulletin, v. 82, no. 5B, p. 986–1012.</ref> <ref name=Salleretal_2004>Saller, A. H., J. T. Noah, A. P. Ruzuar, and R. Schneider, 2004, [http://archives.datapages.com/data/bulletns/2004/01jan/0021/0021.HTM Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems]: AAPG Bulletin, v. 88, no. 1, p. 21–46.</ref> Semblance displays (also known as coherence cubetrade displays) are computed from seismic data by comparing the similarity of each seismic trace with its neighbors within a specific window of interest. Significant changes in the response corresponding to sand pinch-outs or faults are highlighted as edges.<ref name=Bahorichandfarmer_1985>Bahorich, M., and S. Farmer, 1995, 3-D seismic discontinuity for faults and stratigraphic features: The coherency cube: The Leading Edge, v. 14, p. 1053–1058.</ref> <ref name=Marfurtetal_1998>Marfurt, K. J., R. L. Kirlin, S. L. Farmer, and M. S. Bahorich, 1998, 3-D seismic attributes using a semblance-based coherency algorithm: Geophysics, v. 63, no. 4, p. 1150–1165.</ref>

Geometrical patterns that allow depositional sedimentary environments to be recognized can sometimes be picked out by seismic facies analysis.<ref name=Posamentier_2004 /> Seismic facies analysis involves the analysis of seismic character to help predict the depositional environment. One method uses a computer-based neural network analysis of waveform character within a window of seismic data. A map is made showing the areal distribution of the waveform character classes, and this can be correlated with lithofacies variation.

Geometrical patterns that allow depositional sedimentary environments to be recognized can sometimes be picked out by seismic [[facies analysis]].<ref name=Posamentier_2004 /> Seismic facies analysis involves the analysis of seismic character to help predict the depositional environment. One method uses a computer-based neural network analysis of waveform character within a window of seismic data. A map is made showing the areal distribution of the waveform character classes, and this can be correlated with lithofacies variation.

Semblance and spectral decomposition methods were used to pick out individual macroforms in Pleistocene deltaic sediments in the Gulf of Mexico.<ref name=Lopezetal_1997>Lopez, J. A., G. Partyka, N. L. Haskell, and S. E. Nissen, 1997, Identification of deltaic facies with 3-D seismic coherency and the spectral decomposition cube: A study from South Marsh Island Area, Gulf of Mexico: Gulf Coast Association of Geological Societies Transactions, v. 47, p. 305–309.</ref> Spectral decomposition is a way of breaking down a seismic trace into its discrete component frequencies.<ref name=Partykaetal_1999>Partyka, G., J. Gridley, and J. Lopez, 1999, Interpretational applications of spectral decomposition in reservoir characterization: The Leading Edge, v. 18, no. 3, p. 353–360.</ref> Certain stratigraphic features can be picked out because they are more sensitively tuned to specific frequencies although they may not be obvious in the seismic trace as a whole.

Semblance and spectral decomposition methods were used to pick out individual macroforms in Pleistocene deltaic sediments in the [[Gulf of Mexico]].<ref name=Lopezetal_1997>Lopez, J. A., G. Partyka, N. L. Haskell, and S. E. Nissen, 1997, Identification of deltaic facies with 3-D seismic coherency and the spectral decomposition cube: A study from South Marsh Island Area, Gulf of Mexico: Gulf Coast Association of Geological Societies Transactions, v. 47, p. 305–309.</ref> Spectral decomposition is a way of breaking down a seismic trace into its discrete component frequencies.<ref name=Partykaetal_1999>Partyka, G., J. Gridley, and J. Lopez, 1999, Interpretational applications of spectral decomposition in reservoir characterization: The Leading Edge, v. 18, no. 3, p. 353–360.</ref> Certain stratigraphic features can be picked out because they are more sensitively tuned to specific frequencies although they may not be obvious in the seismic trace as a whole.

==Determining the basin topography==

==Determining the basin topography==

[[File:M91Ch11FG73.JPG|thumb|300px|{{figure number|5}}Lithofacies map for the upper Piper Sand interval of the Scott field, UK North Sea (from Guscott et al.<ref name=Guscottetal_2003>Guscott, S., K. Russell, A. Thickpenny, and R. Poddubiuk, 2003, The Scott field, Blocks 15/21a, 15/22, UK North Sea, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 467–481.</ref>). Reprinted with permission from the Geological Society.]]

[[File:M91Ch11FG73.JPG|thumb|300px|{{figure number|5}}Lithofacies map for the upper Piper Sand interval of the Scott field, UK North Sea (from Guscott et al.<ref name=Guscottetal_2003>Guscott, S., K. Russell, A. Thickpenny, and R. Poddubiuk, 2003, The Scott field, Blocks 15/21a, 15/22, UK North Sea, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 467–481.</ref>). Reprinted with permission from the Geological Society.]]

Sandstone percentage maps are contour maps that show the percentage thickness of sandstone within a gross rock interval. These can give a good indication of the sediment dispersal patterns and lateral pinch-out edges. Gross sandstone thickness maps, which are maps of the total thickness of sandstone within an interval, help determine the locations of sediment depocenters. These maps can also be used to infer depositional strike and depositional dip. Depositional strike is the dominant direction along which sedimentary bodies tend to be elongated. Depositional dip is the direction perpendicular to the depositional strike.

Sandstone percentage maps are [[contour]] maps that show the percentage thickness of sandstone within a gross rock interval. These can give a good indication of the sediment dispersal patterns and lateral pinch-out edges. Gross sandstone thickness maps, which are maps of the total thickness of sandstone within an interval, help determine the locations of sediment depocenters. These maps can also be used to infer depositional strike and depositional dip. Depositional strike is the dominant direction along which sedimentary bodies tend to be elongated. Depositional dip is the direction perpendicular to the depositional strike.

Hamilton et al.<ref name=Hamiltonetal_2002 /> described the reservoir characterization of the Tertiary deltaic sediments in the Merecure unit A reservoir interval of the Budare field in Venezuela. The gross sandstone thickness map shows a dominant east–west grain defined by elongate, thick sandstone bodies. Narrower north–south lineaments are perpendicular to this main trend. The depositional environment is interpreted as a wave-dominated delta system. North–south-oriented features interpreted as distributary channels intersect at an oblique angle with arcuate to linear oriented trends interpreted as a marine reworked delta front ([[:file:M91Ch11FG71.JPG|Figure 3]]).

Hamilton et al.<ref name=Hamiltonetal_2002 /> described the reservoir characterization of the Tertiary deltaic sediments in the Merecure unit A reservoir interval of the Budare field in Venezuela. The gross sandstone thickness map shows a dominant east–west grain defined by elongate, thick sandstone bodies. Narrower north–south lineaments are perpendicular to this main trend. The depositional environment is interpreted as a wave-dominated delta system. North–south-oriented features interpreted as [[distributary channel]]s intersect at an oblique angle with arcuate to linear oriented trends interpreted as a marine reworked delta front ([[:file:M91Ch11FG71.JPG|Figure 3]]).

Log facies maps give a sense of the internal sedimentary character of a reservoir interval.<ref name=Shelton_1972>Shelton, J. W., 1972, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0056/0008/1500/1541.htm Correlation sections and log maps in determination of sandstone trends]: AAPG Bulletin, v. 56, no. 8, p. 1541–1544.</ref> For each well, a paper copy of the gamma-ray log is trimmed to the top and base of the reservoir unit of interest and pasted on a map. Computer applications are also available to help make these displays. Log facies maps give a visual impression of how the log facies varies over the field in terms of distribution, trends, and internal bedding characteristics. The various log patterns can be mapped across the field and then tied in to a lithofacies scheme ([[:file:M91Ch11FG72.JPG|Figure 4]]). In the Budare field example from Venezuela,<ref name=Hamiltonetal_2002 /> discrete zones of log character were mapped out. Blocky log profiles are related to distributary channel and aggradational mouth bar complexes, whereas multiple, thin serrated and subtly upward-coarsening log facies are interpreted as strand plain complexes flanking the delta front.

Log facies maps give a sense of the internal sedimentary character of a reservoir interval.<ref name=Shelton_1972>Shelton, J. W., 1972, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0056/0008/1500/1541.htm Correlation sections and log maps in determination of sandstone trends]: AAPG Bulletin, v. 56, no. 8, p. 1541–1544.</ref> For each well, a paper copy of the gamma-ray log is trimmed to the top and base of the reservoir unit of interest and pasted on a map. Computer applications are also available to help make these displays. Log facies maps give a visual impression of how the log facies varies over the field in terms of distribution, trends, and internal bedding characteristics. The various log patterns can be mapped across the field and then tied in to a lithofacies scheme ([[:file:M91Ch11FG72.JPG|Figure 4]]). In the Budare field example from Venezuela,<ref name=Hamiltonetal_2002 /> discrete zones of log character were mapped out. Blocky log profiles are related to distributary channel and aggradational mouth bar complexes, whereas multiple, thin serrated and subtly upward-coarsening log facies are interpreted as strand plain complexes flanking the delta front.