Changes

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 /> 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 /> 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 intraclasts. 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 intraclasts. 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 />). 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.

[[file:M91FG178.JPG|thumb|300px|{{figure number|7}}Mud-lined lateral accretion surfaces in the upper section of a point bar sandstone, Ebro basin, Spain. The point bar grew by accretion from left to right.]]

[[file:M91FG178.JPG|thumb|300px|{{figure number|7}}Mud-lined lateral accretion surfaces in the upper section of a point bar sandstone, Ebro basin, Spain. The point bar grew by accretion from left to right.]]

Sweep will be retarded where the upper sections of point bars form mud sheets along lateral accretion surfaces ([[:file:M91FG176.JPG|Figure 5c]], [[:file:M91FG178.JPG|Figure 7]]). These are inclined surfaces formed by the lateral growth of the point bar as the meander loop migrates. Mud-lined lateral accretion surfaces develop by mud deposition during ponding at low river stages.<ref name=Jordanandpryor_1992 /> The inclined mud drapes form a series of shingled barriers to both vertical and horizontal flow.<ref name=Maetal_1999 /><ref name=Pranteretal_2007 />

Sweep will be retarded where the upper sections of point bars form mud sheets along lateral accretion surfaces ([[:file:M91FG176.JPG|Figure 5c]], [[:file:M91FG178.JPG|Figure 7]]). These are inclined surfaces formed by the lateral growth of the point bar as the meander loop migrates. Mud-lined lateral accretion surfaces develop by mud deposition during ponding at low river stages.<ref name=Jordanandpryor_1992 /> The inclined mud drapes form a series of shingled barriers to both vertical and horizontal flow.<ref name=Maetal_1999>Ma, S., J. Zhang, N. Jin, and Z. Wang, 1999, The 3-D architecture of point bar and the forming and distribution of remaining oil: Presented at the Society of Petroleum Engineers Asia Pacific Improved Oil Recovery Conference, October 25–26, Kuala Lumpur, Malaysia, [https://www.onepetro.org/conference-paper/SPE-57308-MS SPE Paper 57308], 7 p.</ref><ref name=Pranteretal_2007>Pranter, M. J., A. I. Ellison, R. D. Cole, and P. E. Patterson, 2007, [http://archives.datapages.com/data/bulletns/2007/07jul/BLTN06102/BLTN06102.HTM Analysis and modeling of intermediate-scale reservoir heterogeneity based on a fluvial point-bar outcrop analog, Williams Fork Formation, Piceance Basin, Colorado]: AAPG Bulletin, v. 91, no. 7, p. 1025–1051.</ref>

Detailed data integration analysis has been made for oil recovery from meander belt sandstones in the Daqing field in China.<ref name=Xue_1986 /><ref name=Fuzhiguoetal_1998 /> After 30 yr of production from one of the reservoir units, the sweep efficiency is only 29.8%. It was found that although the basal intervals of the fluvial sandstones are well swept, there is much oil left behind in the upper part. Water cuts in the production wells can reach 90% with only the basal section of the fluvial sandstones contacted by the waterflood. A horizontal well was drilled as a pilot trial to determine whether this could recover the oil in the upper sections of the point bar sandstones. The well found an estimated net pay of 2–4 m (7–13 ft) but produced less than expected as a result of a combination of formation damage and poorer than predicted vertical permeability.<ref name=Fuzhiguoetal_1998 />

Detailed data integration analysis has been made for oil recovery from meander belt sandstones in the Daqing field in China.<ref name=Xue_1986>Xue, P., 1986, A point bar facies reservoir model—Semi-communicated sand body: Presented at the International Meeting on Petroleum Engineering, Society of Petroleum Engineers, March 17–20, Beijing, China, [https://www.onepetro.org/conference-paper/SPE-14837-MS SPE Paper 14837], 13 p.</ref><ref name=Fuzhiguoetal_1998 /> After 30 yr of production from one of the reservoir units, the sweep efficiency is only 29.8%. It was found that although the basal intervals of the fluvial sandstones are well swept, there is much oil left behind in the upper part. Water cuts in the production wells can reach 90% with only the basal section of the fluvial sandstones contacted by the waterflood. A horizontal well was drilled as a pilot trial to determine whether this could recover the oil in the upper sections of the point bar sandstones. The well found an estimated net pay of 2–4 m (7–13 ft) but produced less than expected as a result of a combination of formation damage and poorer than predicted vertical permeability.<ref name=Fuzhiguoetal_1998 />

Meander belt sediments may be better suited as gas reservoirs than oil reservoirs. The labyrinth of numerous dead ends in these systems will not tend to trap nearly so much gas as oil. The expansion of gas on the reduction of pressure with depletion will cause much of the gas to spill out of the dead ends in fluvial reservoirs. Gas can also flow through the low-permeability connections that exist in fluvial systems, which would otherwise not allow oil to pass.

Meander belt sediments may be better suited as gas reservoirs than oil reservoirs. The labyrinth of numerous dead ends in these systems will not tend to trap nearly so much gas as oil. The expansion of gas on the reduction of pressure with depletion will cause much of the gas to spill out of the dead ends in fluvial reservoirs. Gas can also flow through the low-permeability connections that exist in fluvial systems, which would otherwise not allow oil to pass.