Changes

  | isbn    = 0891813721

  | isbn    = 0891813721

}}

}}

[[Fluvial]] reservoirs are difficult for the production geologist to understand, characterize, and model. One major problem involves trying to classify fluvial reservoirs in the [[subsurface]]. The system used in this article broadly categorizes fluvial systems into meandering and [[braided fluvial reservoirs]]. Although this is a classification used by many production geologists, not all experts are happy with this approach; some believe the classification to be too prescriptive. They consider that only limited inferences can be made from core and log data as to the overall geometry of a fluvial reservoir in the subsurface (e.g., Bridge<ref name=Bridge_2003>Bridge, J. S., 2003, Rivers and flood plains: Forms, processes and sedimentary record: Oxford, Blackwell, 491 p.</ref>). Because of this, some geologists prefer to use a simple nongeneric description by classifying subsurface fluvial geometries as either sheets or ribbons.<ref name=Friendetal_1979>Friend, P. F., M. J. Slater, and R. C. Williams, 1979, [http://jgs.geoscienceworld.org/content/136/1/39.abstract Vertical and lateral building of river sandstone bodies, Ebro Basin, Spain]: Journal of the Geological Society of London, v. 136, p. 39–46.</ref>

[[Fluvial]] reservoirs are difficult for the production geologist to understand, characterize, and model. One major problem involves trying to classify fluvial reservoirs in the [[subsurface]]. The system used in this article broadly categorizes fluvial systems into meandering and [[braided fluvial reservoirs]]. Although this is a classification used by many production geologists, not all experts are happy with this approach; some believe the classification to be too prescriptive. They consider that only limited inferences can be made from core and log data as to the overall geometry of a fluvial reservoir in the subsurface (e.g., Bridge<ref name=Bridge_2003>Bridge, J. S., 2003, Rivers and flood plains: Forms, processes and sedimentary record: Oxford, Blackwell, 491 p.</ref>). Because of this, some geologists prefer to use a simple nongeneric description by classifying subsurface fluvial geometries as either sheets or ribbons.<ref name=Friendetal_1979>Friend, P. F., M. J. Slater, and R. C. Williams, 1979, [http://jgs.geoscienceworld.org/content/136/1/39.abstract Vertical and [[lateral]] building of river sandstone bodies, Ebro Basin, Spain]: Journal of the Geological Society of London, v. 136, p. 39–46.</ref>

[[file:M91Ch11FG70.JPG|thumb|300px|{{figure number|1}}A point bar cut into the underlying Ivan limestone as picked out by varying seismic amplitudes on a horizon display, late Pennsylvanian to Early Permian, Baylor County, Texas (from Burnett<ref name=Burnett_1996>Burnett, M., 1996, [http://archives.datapages.com/data/specpubs/study42/ch05/0062.htm 3-D seismic expression of a shallow fluvial system in west central Texas], in P. Weimer and T. L. Davis, eds.: AAPG Studies in Geology 42 and SEG (Society of Exploration Geophysicists) Geophysical Developments Series 5, p. 45–56.</ref>). Reprinted with permission from the AAPG.]]

[[file:M91Ch11FG70.JPG|thumb|300px|{{figure number|1}}A point bar cut into the underlying Ivan [[limestone]] as picked out by varying seismic amplitudes on a horizon display, late Pennsylvanian to Early Permian, Baylor County, Texas (from Burnett<ref name=Burnett_1996>Burnett, M., 1996, [http://archives.datapages.com/data/specpubs/study42/ch05/0062.htm 3-D seismic expression of a shallow fluvial system in west central Texas], in P. Weimer and T. L. Davis, eds.: AAPG Studies in Geology 42 and SEG (Society of Exploration Geophysicists) Geophysical Developments Series 5, p. 45–56.</ref>). Reprinted with permission from the AAPG.]]

Despite the above difficulties, the production geologist will nevertheless try and find some basis for providing a predictive model for the subsurface geology of a fluvial reservoir. Seismic data can help to determine the planform geometry where it is of sufficient resolution ([[:file:M91Ch11FG70.JPG|Figure 1]]). Fluvial geometries can sometimes be well differentiated on horizon slice amplitude displays (e.g., Brown et al.,<ref name=Brownetal_1981>Brown, A. R., C. G. Dahm, and R. J. Graebner, 1981, A stratigraphic case history using three-dimensional seismic data in the Gulf of Thailand: Geophysical Prospecting, v. 29, no. 3, p. 327–349.</ref> Rijks and Jauffred,<ref name=Rijksandjauffred_1991>Rijks, E. J. K., and J. C. E. M. Jauffred, 1991, Attribute extraction: An important application in any detailed 3D interpretation study: Leading Edge, v. 10, no. 9, p. 11–19.</ref> Noah et al.,<ref name=Noahetal_1992>Noah, J. T., G. S. Hofland, and K. Lemke, 1992, Seismic interpretation of meander channel point-bar deposits using realistic seismic modeling techniques: The Leading Edge, v. 11, p. 13–18.</ref> Carter<ref name=Carter_2003>Carter, D. C., 2003, [http://archives.datapages.com/data/bulletns/2003/06jun/0909/0909.HTM 3-D seismic geomorphology:  Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea]: AAPG Bulletin, v. 87, no. 6, p. 909–934.</ref>).

Despite the above difficulties, the production geologist will nevertheless try and find some basis for providing a predictive model for the subsurface geology of a fluvial reservoir. Seismic data can help to determine the planform geometry where it is of sufficient resolution ([[:file:M91Ch11FG70.JPG|Figure 1]]). Fluvial geometries can sometimes be well differentiated on horizon slice amplitude displays (e.g., Brown et al.,<ref name=Brownetal_1981>Brown, A. R., C. G. Dahm, and R. J. Graebner, 1981, A stratigraphic case history using three-dimensional seismic data in the Gulf of Thailand: Geophysical Prospecting, v. 29, no. 3, p. 327–349.</ref> Rijks and Jauffred,<ref name=Rijksandjauffred_1991>Rijks, E. J. K., and J. C. E. M. Jauffred, 1991, Attribute extraction: An important application in any detailed 3D interpretation study: Leading Edge, v. 10, no. 9, p. 11–19.</ref> Noah et al.,<ref name=Noahetal_1992>Noah, J. T., G. S. Hofland, and K. Lemke, 1992, Seismic interpretation of meander channel point-bar deposits using realistic seismic modeling techniques: The Leading Edge, v. 11, p. 13–18.</ref> Carter<ref name=Carter_2003>Carter, D. C., 2003, [http://archives.datapages.com/data/bulletns/2003/06jun/0909/0909.HTM 3-D seismic geomorphology:  Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea]: AAPG Bulletin, v. 87, no. 6, p. 909–934.</ref>).

==Geometry of meander belts==

==Geometry of meander belts==

Meandering rivers deposit sand and mud within well-defined meander belts. The appearance of a meander belt in plan and cross section is of a complex labyrinth of interlocking sand bodies on the scale of hundreds of meters, embedded within varying volumes of mud ([[:file:M91FG173.JPG|Figure 2]]). The mud can make up 50% or more of the volume. Channel features, where they survive, tend to be plugged with clay ([[:file:M91FG173.JPG|Figure 2]], [[:file:M91FG174.JPG|Figure 3]]).

Meandering rivers deposit sand and mud within well-defined meander belts. The appearance of a meander belt in plan and [[cross section]] is of a complex labyrinth of interlocking sand bodies on the scale of hundreds of meters, embedded within varying volumes of mud ([[:file:M91FG173.JPG|Figure 2]]). The mud can make up 50% or more of the volume. Channel features, where they survive, tend to be plugged with clay ([[:file:M91FG173.JPG|Figure 2]], [[:file:M91FG174.JPG|Figure 3]]).

Gibling<ref name=Gibling_2006>Gibling, M. R. 2006, [http://jsedres.geoscienceworld.org/content/76/5/731?related-urls=yes&legid=jsedres;76/5/731 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> provided data on width and thickness relationships for fluvial systems in various settings from [[Quaternary]] and older outcrops (Table 1). He found that meandering rivers do not generally create thick sedimentary packages. The maximum thickness for meandering river deposits in his database is only 38 m (124 ft), with 4–20 m (13–65 ft) as a common thickness range. Gibling makes the comment that despite their familiarity in the modern landscape, meandering river deposits probably constitute only a minor portion of the fluvial rock record by comparison to braided systems. This may be because the organized flow patterns associated with meandering rivers rarely persist for long periods.

Gibling<ref name=Gibling_2006>Gibling, M. R. 2006, [http://jsedres.geoscienceworld.org/content/76/5/731?related-urls=yes&legid=jsedres;76/5/731 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> provided data on width and thickness relationships for fluvial systems in various settings from [[Quaternary]] and older outcrops (Table 1). He found that meandering rivers do not generally create thick sedimentary packages. The maximum thickness for meandering river deposits in his database is only 38 m (124 ft), with 4–20 m (13–65 ft) as a common thickness range. Gibling makes the comment that despite their familiarity in the modern landscape, meandering river deposits probably constitute only a minor portion of the fluvial rock record by comparison to braided systems. This may be because the organized flow patterns associated with meandering rivers rarely persist for long periods.

| 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).''

==Meandering fluvial macroforms==

==Meandering fluvial macroforms==

[[Macroform]]s found in meander belts include point bars, crevasse splays, and mud-rich channel plugs within a background of floodplain muds. Coals are found in fluvial systems with high water tables. Levees sometimes border rivers but they do not appear to be a major feature preserved in the subsurface.<ref name=Gibling_2006 />

[[Macroform]]s found in meander belts include point bars, crevasse splays, and mud-rich channel plugs within a background of floodplain muds. [[Coal]]s are found in fluvial systems with high water tables. Levees sometimes border rivers but they do not appear to be a major feature preserved in the subsurface.<ref name=Gibling_2006 />

[[file:M91FG175.JPG|thumb|300px|{{figure number|4}}Point bars within background floodplain shales and crevasse splays, Ebro basin, Spain.]]

[[file:M91FG175.JPG|thumb|300px|{{figure number|4}}Point bars within background floodplain shales and crevasse splays, Ebro basin, Spain.]]

[[file:M91FG176.JPG|thumb|300px|{{figure number|5}}Flow geology influences in meandering river sediments.]]

[[file:M91FG176.JPG|thumb|300px|{{figure number|5}}Flow geology influences in meandering river sediments.]]

Jordan and Pryor<ref name=Jordanandpryor_1992>Jordan, D. W., and W. A. Pryor, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0010/0000/1601.htm Hierarchical levels of heterogeneity in a Mississippi river meander belt and application to reservoir systems]: AAPG Bulletin, v. 76, no. 10, p. 1601–1624.</ref> made detailed measurements on sediment body dimensions along a 10-mi (16-km) stretch of the Mississippi River meander belt system in southeastern Missouri. The point bars here are 15–45 m (49–147 ft) thick, a few kilometers long (typically 3 km; 2 mi), and between 600 and 1800 m (1968 and 5905 ft) wide. The Mississippi is a continental-size river stretching the length of the United States. As a general rule, big rivers like the Mississippi will tend to deposit large point bar sand bodies; lesser rivers will tend to produce smaller point bars ([[:file:M91FG176.JPG|Figure 5a]]). For example, the typical width of individual point bars in the 35-1 sand of the Widuri field in the Java Sea is 1200-1500 m.<ref name=Carter_2003>Carter, D. C., 2003, [http://archives.datapages.com/data/bulletns/2003/06jun/0909/0909.HTM 3-D seismic geomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea]: AAPG Bulletin, v. 87, no. 6, p. 909–934.</ref> A well located in one of the point bars has produced 3.2MM bbls of oil. By contrast the fluvial sands in the Jonah Gas field of Wyoming are estimated to have a P50 width ranging from only 60 to 210 m.<ref name=Shanley_2004>Shanley, K. W., 2004, [http://archives.datapages.com/data/specpubs/study52/CHAPTER10/CHAPTER10.HTM Fluvial reservoir description for a giant, low-permeability gas field: Jonah field, Green River Basin, Wyoming, U.S.A.], in J. W. Robinson and K. W. Shanley, eds., Jonah field: Case study of a giant tight-gas fluvial reservoir: AAPG Studies in Geology 52, p. 159–182.</ref>

Jordan and Pryor<ref name=Jordanandpryor_1992>Jordan, D. W., and W. A. Pryor, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0010/0000/1601.htm Hierarchical levels of heterogeneity in a Mississippi river meander belt and application to reservoir systems]: AAPG Bulletin, v. 76, no. 10, p. 1601–1624.</ref> made detailed measurements on sediment body dimensions along a 10-mi (16-km) stretch of the Mississippi River meander belt system in southeastern Missouri. The point bars here are 15–45 m (49–147 ft) thick, a few kilometers long (typically 3 km; 2 mi), and between 600 and 1800 m (1968 and 5905 ft) wide. The Mississippi is a continental-size river stretching the length of the United States. As a general rule, big rivers like the Mississippi will tend to deposit large point bar sand bodies; lesser rivers will tend to produce smaller point bars ([[:file:M91FG176.JPG|Figure 5a]]). For example, the typical width of individual point bars in the 35-1 sand of the Widuri field in the Java Sea is 1200-1500 m.<ref name=Carter_2003>Carter, D. C., 2003, [http://archives.datapages.com/data/bulletns/2003/06jun/0909/0909.HTM 3-D seismic geomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea]: AAPG Bulletin, v. 87, no. 6, p. 909–934.</ref> A well located in one of the point bars has produced 3.2MM bbls of oil. By contrast the fluvial sands in the Jonah Gas field of Wyoming are estimated to have a P50 width ranging from only 60 to 210 m.<ref name=Shanley_2004>Shanley, K. W., 2004, [http://archives.datapages.com/data/specpubs/study52/CHAPTER10/CHAPTER10.HTM Fluvial reservoir description for a giant, low-permeability gas field: Jonah field, Green River Basin, Wyoming, U.S.A.], in J. W. Robinson and K. W. Shanley, eds., Jonah field: Case study of a giant tight-gas fluvial reservoir: [http://store.aapg.org/detail.aspx?id=866 AAPG Studies in Geology 52], p. 159–182.</ref>

Several technical papers give cross plots of fluvial sand-body widths versus the maximum bankful depth of rivers (e.g., Bridge and Mackey<ref name=Bridgeandmackey_1993>Bridge, J. S., and S. D. Mackey, 1993, A theoretical study of fluvial sandstone body dimensions, in S. S. Flint and I. D. Bryant, eds., Geological modeling of hydrocarbon reservoirs: International Association of Sedimentologists, Special Publication 15, p. 213–236.</ref>) These plots have been used to model thickness-to-width ratios for 3-D geological models.

Several technical papers give cross plots of fluvial sand-body widths versus the maximum bankful depth of rivers (e.g., Bridge and Mackey<ref name=Bridgeandmackey_1993>Bridge, J. S., and S. D. Mackey, 1993, A theoretical study of fluvial sandstone body dimensions, in S. S. Flint and I. D. Bryant, eds., Geological modeling of hydrocarbon reservoirs: International Association of Sedimentologists, Special Publication 15, p. 213–236.</ref>) These plots have been used to model thickness-to-width ratios for 3-D geological models.

Miall<ref name=Miall_2006>Miall, A. D., 2006, [http://archives.datapages.com/data/bulletns/2006/07jul/BLTN05065/BLTN05065.HTM Reconstructing the architecture and sequence stratigraphy of the preserved fluvial record as a tool for reservoir development]: A reality check: AAPG Bulletin, v. 90, no. 7, p. 989–1002.</ref> criticized the use of empirical relationships for fluvial geometries in too prescriptive a manner. He suggests that they should only be used as approximate guidelines for developing alternative scenarios of fluvial reservoirs for modeling purposes. Shanley,<ref name=Shanley_2004 /> characterizing the Jonah field in Wyoming, preferred to estimate a range of possible dimensions for fluvial bodies instead of using a single unique value for the width-to-thickness ratio.

Miall<ref name=Miall_2006>Miall, A. D., 2006, [http://archives.datapages.com/data/bulletns/2006/07jul/BLTN05065/BLTN05065.HTM Reconstructing the architecture and sequence stratigraphy of the preserved fluvial record as a tool for reservoir development: A reality check]: AAPG Bulletin, v. 90, no. 7, p. 989–1002.</ref> criticized the use of empirical relationships for fluvial geometries in too prescriptive a manner. He suggests that they should only be used as approximate guidelines for developing alternative scenarios of fluvial reservoirs for modeling purposes. Shanley,<ref name=Shanley_2004 /> characterizing the Jonah field in Wyoming, preferred to estimate a range of possible dimensions for fluvial bodies instead of using a single unique value for the width-to-thickness ratio.

Werren et al.<ref name=Werrenetal_1990>Werren, E. G., R. D. Shew, E. R. Adams, and R. J. Stancliffe, 1990, Meander-belt reservoir geology, mid-dip Tuscaloosa, Little Creek field, Mississippi, in J. H. Barwis, J. G. McPherson, and R. J. Studlick, eds., Sandstone petroleum reservoirs: Berlin, Springer, p. 85–107.</ref> described a vertical profile for a point bar deposit in the Cretaceous reservoir of the Little Creek field in Mississippi. An erosional base is overlain by channel lags with intraformational shale rip-up clasts. Above this are large-scale cross-bedded sandstones, which pass upward to beds showing horizontal and small-scale ripple cross [[laminae]], clay drapes, micaceous and carbonaceous streaks, local mud balls, and [[intraclast]]s. The overall pattern is fining upward.

Werren et al.<ref name=Werrenetal_1990>Werren, E. G., R. D. Shew, E. R. Adams, and R. J. Stancliffe, 1990, Meander-belt reservoir geology, mid-dip Tuscaloosa, Little Creek field, Mississippi, in J. H. Barwis, J. G. McPherson, and R. J. Studlick, eds., Sandstone petroleum reservoirs: Berlin, Springer, p. 85–107.</ref> described a vertical profile for a point bar deposit in the Cretaceous reservoir of the Little Creek field in Mississippi. An erosional base is overlain by channel lags with intraformational shale rip-up clasts. Above this are large-scale cross-bedded sandstones, which pass upward to beds showing horizontal and small-scale ripple cross [[laminae]], clay drapes, micaceous and carbonaceous streaks, local mud balls, and [[intraclast]]s. The overall pattern is fining upward.

[[file:M91FG177.JPG|thumb|300px|{{figure number|6}}Upward-decreasing permeability profile in a point bar sandstone in the Peoria field, Colorado (from Chapin and Mayer<ref name=Chapinandmayer_1991>Chapin, M. A., and D. F. Mayer, 1991, [http://archives.datapages.com/data/sepm_sp/csp3/Constructing_a_Three-DimensionalOp.htm Constructing a three-dimensional rock property model of fluvial sandstones in the Peoria field, Colorado], in A. D. Miall and N. Tyler, eds., The three dimensional facies architecture of terrigenous clastic sediments and its implication for hydrocarbon discovery and recovery: SEPM Concepts in Sedimentology and Paleontology 3, p. 160–171.</ref>). Reprinted with permission from, and &copy; by, the SEPM (Society for Sedimentary Geology).]]

[[file:M91FG177.JPG|thumb|300px|{{figure number|6}}Upward-decreasing permeability profile in a point bar sandstone in the Peoria field, Colorado (from Chapin and Mayer<ref name=Chapinandmayer_1991>Chapin, M. A., and D. F. Mayer, 1991, [http://archives.datapages.com/data/sepm_sp/csp3/Constructing_a_Three-DimensionalOp.htm Constructing a three-dimensional rock property model of fluvial sandstones in the Peoria field, Colorado], in A. D. Miall and N. Tyler, eds., The three dimensional facies architecture of terrigenous clastic sediments and its implication for hydrocarbon discovery and recovery: SEPM Concepts in Sedimentology and Paleontology 3, p. 160–171.</ref>). Reprinted with permission from, and &copy; by, the SEPM (Society for Sedimentary Geology).]]

Fining-upward profiles are typical for point bars; the permeability declines upward with decreasing grain size ([[:file:M91FG177.JPG|Figure 6]]). The decrease in permeability commonly occurs in a step-like fashion rather than showing a gradual upward decrease.

Fining-upward profiles are typical for point bars; the permeability declines upward with decreasing [[grain size]] ([[:file:M91FG177.JPG|Figure 6]]). The decrease in permeability commonly occurs in a step-like fashion rather than showing a gradual upward decrease.

Upward-decreasing permeability profiles are unfavorable to efficient sweep. Water will flood through the high-permeability basal part of the point bar leaving the uppermost section unswept ([[:file:M91FG176.JPG|Figure 5b]]).

Upward-decreasing permeability profiles are unfavorable to efficient sweep. Water will flood through the high-permeability basal part of the point bar leaving the uppermost section unswept ([[:file:M91FG176.JPG|Figure 5b]]).

[[file:M91FG179.JPG|thumb|300px|{{figure number|8}}Net sand isochore map of the Q reservoir in the Little Creek field in Mississippi. The reservoir comprises three connected point bar sandstones in a background of floodplain mudstones and siltstones. Just to the north is the Sweetwater field, which produces from a depositionally isolated point bar in the same meander belt system (from Werren et al., 1990). Reprinted with permission from, and &copy; by, Springer Ltd.]]

[[file:M91FG179.JPG|thumb|300px|{{figure number|8}}Net sand isochore map of the Q reservoir in the Little Creek field in Mississippi. The reservoir comprises three connected point bar sandstones in a background of floodplain mudstones and siltstones. Just to the north is the Sweetwater field, which produces from a depositionally isolated point bar in the same meander belt system (from Werren et al., 1990). Reprinted with permission from, and &copy; by, Springer Ltd.]]

Technical papers indicate that connectivity in meander belt sediments can be highly variable and prone to chance factors. An example of this is the Little Creek field in Mississippi.<ref name=Werrenetal_1990 /> The lower reservoir unit comprises three connected point bar sandstones ([[:file:M91FG179.JPG|Figure 8]]). The Sweetwater field immediately to the north is believed to form part of the same fluvial system and produces from a fourth point bar sand body along the same trend. Nevertheless, the Sweetwater field is isolated from the Little Creek field on the evidence of a 24-m (79-ft) higher oil-water contact. The two fields are thought to be separated by a shale plug or an area with relatively high capillary displacement pressure. A similar observation was made by Carter<ref name=Carter_2003 /> for a meander belt reservoir in the Widuri field in the Java Sea. Following the depletion of a well on the updip side of a 100-m (328-ft)-wide abandoned channel, a second well was drilled on the opposite site of the clay plug. A full oil column was found in the new well, unaffected by production from the previous well. In the Saddle Lake area of Alberta, Canada, oil and gas pools are restricted to point bars completely surrounded by clay plugs.<ref name=Edieandandrichuk_2003>Edie, R. W., and J. M. Andrichuk, 2003, [http://bcpg.geoscienceworld.org/content/51/3/253.short Meander belt entrapment of hydrocarbons at Saddle Lake, Alberta and an untested in situ combustion scheme for recovery of heavy oil]: Bulletin of Canadian Petroleum Geology, v. 51, no. 3, p. 253–274.</ref>

Technical papers indicate that connectivity in meander belt sediments can be highly variable and prone to chance factors. An example of this is the Little Creek field in Mississippi.<ref name=Werrenetal_1990 /> The lower reservoir unit comprises three connected point bar sandstones ([[:file:M91FG179.JPG|Figure 8]]). The Sweetwater field immediately to the north is believed to form part of the same fluvial system and produces from a fourth point bar sand body along the same trend. Nevertheless, the Sweetwater field is isolated from the Little Creek field on the evidence of a 24-m (79-ft) higher oil-water contact. The two fields are thought to be separated by a shale plug or an area with relatively high capillary [[displacement pressure]]. A similar observation was made by Carter<ref name=Carter_2003 /> for a meander belt reservoir in the Widuri field in the Java Sea. Following the depletion of a well on the updip side of a 100-m (328-ft)-wide abandoned channel, a second well was drilled on the opposite site of the clay plug. A full oil column was found in the new well, unaffected by production from the previous well. In the Saddle Lake area of Alberta, Canada, oil and gas pools are restricted to point bars completely surrounded by clay plugs.<ref name=Edieandandrichuk_2003>Edie, R. W., and J. M. Andrichuk, 2003, [http://bcpg.geoscienceworld.org/content/51/3/253.short Meander belt entrapment of hydrocarbons at Saddle Lake, Alberta and an untested in situ combustion scheme for recovery of heavy oil]: Bulletin of Canadian Petroleum Geology, v. 51, no. 3, p. 253–274.</ref>

It seems from these case examples that clay plugs can be an important element limiting horizontal connectivity in meander belt sediments ([[:file:M91FG176.JPG|Figure 5d]]). Some point bars show flow connectivity with each other, others do not. Connectivity may be effective where the clay plug does not totally separate two point bars areally. Richardson et al.<ref name=Richardsonetal_1987>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1987, Meandering stream reservoirs: Journal of Petroleum Technology, v. 39, no. 12, p. 1501–1502.</ref> noted that there is commonly some sand or gravel underneath clay plugs that can allow communication. Similar observations have been made by Donselaar and Overeem.<ref name=Donselaarandovereem_2008>Donselaar, M. E., and I. Overeem, 2008, [http://archives.datapages.com/data/bulletns/2008/09sep/BLTN07079/BLTN07079.HTM Connectivity of fluvial point bar deposits: An example from the Miocene Huesca fluvial fan, Ebro basin, Spain]: AAPG Bulletin, v. 92, no. 9, p. 1109–1129.</ref> Clay plugs occur at the same level of the point bar that it partially encloses. If vertical connectivity is effective between incised point bars, the clay plug obstruction can be bypassed above and below. If no effective vertical communication occurs, then clay plugs are more likely to act as lateral flow barriers ([[:file:M91FG176.JPG|Figure 5e]]).

It seems from these case examples that clay plugs can be an important element limiting horizontal connectivity in meander belt sediments ([[:file:M91FG176.JPG|Figure 5d]]). Some point bars show flow connectivity with each other, others do not. Connectivity may be effective where the clay plug does not totally separate two point bars areally. Richardson et al.<ref name=Richardsonetal_1987>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1987, Meandering stream reservoirs: Journal of Petroleum Technology, v. 39, no. 12, p. 1501–1502.</ref> noted that there is commonly some sand or gravel underneath clay plugs that can allow communication. Similar observations have been made by Donselaar and Overeem.<ref name=Donselaarandovereem_2008>Donselaar, M. E., and I. Overeem, 2008, [http://archives.datapages.com/data/bulletns/2008/09sep/BLTN07079/BLTN07079.HTM Connectivity of fluvial point bar deposits: An example from the Miocene Huesca fluvial fan, Ebro basin, Spain]: AAPG Bulletin, v. 92, no. 9, p. 1109–1129.</ref> Clay plugs occur at the same level of the point bar that it partially encloses. If vertical connectivity is effective between incised point bars, the clay plug obstruction can be bypassed above and below. If no effective vertical communication occurs, then clay plugs are more likely to act as lateral flow barriers ([[:file:M91FG176.JPG|Figure 5e]]).

Nevertheless, it probably does not take much for the connectivity between the various sand bodies in a meander belt to be disrupted. The connections between the various macroforms are likely to be through apertures of limited cross-sectional area such as erosional windows, crossovers, and crevasse splay-point bar intersections. Carbonate cementation of the basal lag by circulating groundwater can create permeability barriers at the base of individual point bars. Precipitation of carbonate cement may be accentuated where calcrete fragments form part of the basal lag.<ref name=Mckieandaudretsch_2005>Mckie, T., and P. Audretsch, 2005, Depositional and structural controls on Triassic reservoir performance in the Heron cluster, ETAP, central North Sea, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), p. 285–297.</ref>

Nevertheless, it probably does not take much for the connectivity between the various sand bodies in a meander belt to be disrupted. The connections between the various macroforms are likely to be through apertures of limited cross-sectional area such as erosional windows, crossovers, and crevasse splay-point bar intersections. Carbonate cementation of the basal lag by circulating groundwater can create permeability barriers at the base of individual point bars. Precipitation of carbonate cement may be accentuated where calcrete fragments form part of the basal lag.<ref name=Mckieandaudretsch_2005>Mckie, T., and P. Audretsch, 2005, Depositional and structural controls on Triassic reservoir performance in the Heron cluster, ETAP, central North Sea, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), p. 285–297.</ref>

Abundant mud chips caked along the base of point bars have the potential to attenuate communication. For example, Chapin and Mayer<ref name=Chapinandmayer_1991 /> found that the vertical connectivity between stacked point bars was severely impeded by mudstone-rich lags at the base of individual point bars in the reservoir of the Peoria field in Colorado. Doyle and Sweet<ref name=Doyleandsweet_1995> Doyle, J. D., and M. L. Sweet, 1995, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0079/0001/0050/0070.htm Three-dimensional distribution of lithofacies, bounding surfaces, porosity, and permeability in a fluvial sandstone—Gypsy Sandstone of northern Oklahoma]: AAPG Bulletin, v. 79, no. 1, p. 70–95.</ref> found that mudclast lags at the base of point bar sandstones have a patchy distribution in the Gypsy Sandstone of Northern Oklahoma. They consider them more likely to form baffles to flow instead of continuous barriers. Shanley<ref name=Shanley_2004 /> noted that where basal lags contain abundant mudclasts, they can be mistaken for shales on the gamma-ray log. Caution should be taken where a shale-like wireline log response is seen within thick multistory fluvial sandstones.

Abundant mud chips caked along the base of point bars have the potential to attenuate communication. For example, Chapin and Mayer<ref name=Chapinandmayer_1991 /> found that the vertical connectivity between stacked point bars was severely impeded by [[mudstone]]-rich lags at the base of individual point bars in the reservoir of the Peoria field in Colorado. Doyle and Sweet<ref name=Doyleandsweet_1995> Doyle, J. D., and M. L. Sweet, 1995, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0079/0001/0050/0070.htm Three-dimensional distribution of lithofacies, bounding surfaces, porosity, and permeability in a fluvial sandstone—Gypsy Sandstone of northern Oklahoma]: AAPG Bulletin, v. 79, no. 1, p. 70–95.</ref> found that mudclast lags at the base of point bar sandstones have a patchy distribution in the Gypsy Sandstone of Northern Oklahoma. They consider them more likely to form baffles to flow instead of continuous barriers. Shanley<ref name=Shanley_2004 /> noted that where basal lags contain abundant mudclasts, they can be mistaken for shales on the gamma-ray log. Caution should be taken where a shale-like wireline log response is seen within thick multistory fluvial sandstones.

[[Coal]]s commonly act as significant flow barriers where they occur in more humid fluvial systems. The precursor [[peat]] deposits to coal occur as thick mats of flexible and intertwined plant material and these can withstand strong erosive forces to stay substantially intact.

[[Coal]]s commonly act as significant flow barriers where they occur in more humid fluvial systems. The precursor [[peat]] deposits to coal occur as thick mats of flexible and intertwined plant material and these can withstand strong erosive forces to stay substantially intact.

* [[Siliciclastic shorelines and barrier island reservoirs]]

* [[Siliciclastic shorelines and barrier island reservoirs]]

* [[Deep-water marine reservoirs]]

* [[Deep-water marine reservoirs]]

* [[Carbonate reservoirs]]

* [[Carbonate reservoir]]

==References==

==References==