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[[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>).

[[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]]).

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.