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Ciftci et al.<ref name=Ciftcietal_2004>Ciftci, B. N., A. A. Aviantara, N. F. Hurley, and D. R. Kerr, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER12/CHAPTER12.HTM Outcrop-based three-dimensional modeling of the Tensleep Sandstone at Alkali Creek, Bighorn Basin, Wyoming], in G. M. Grammer, P. M. Harris, and G. P. Eberli, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 235–259.</ref> attributed the poor recovery in the Tensleep Sandstone to low permeability baffles and barriers along bounding surfaces within the eolian dune sets. Bounding surfaces are subhorizontal to inclined discontinuities that divide eolian cross-beds into subsets, sets, and cosets ([[:file:M91FG171.JPG|Figure 2a]]). These form as a result of dune migration at the smaller scale and from regional discontinuities at the larger scale. Bounding surfaces have a tendency to act as baffles or barriers as a result of facies and grain size contrasts across them.<ref name=Shebi_1995>Shebi, M. A., 1995, The impact of reservoir heterogeneity on fluid flow in the Tensleep Sandstone of the Bighorn Basin: Resources of southwestern Wyoming, field conference guidebook: Wyoming Geological Association, p. 343–359.</ref> Perhaps the considerable difference in recoveries between the eolian reservoirs of Northwestern Europe and the United States is a function of how bounding surfaces influence fluid flow in each area. These features may provide less of an impedance to the flow of highly mobile gas in the Northwestern Europe gas fields than they do for the viscous oil found in the Tensleep Sandstone of Wyoming and Montana.

Ciftci et al.<ref name=Ciftcietal_2004>Ciftci, B. N., A. A. Aviantara, N. F. Hurley, and D. R. Kerr, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER12/CHAPTER12.HTM Outcrop-based three-dimensional modeling of the Tensleep Sandstone at Alkali Creek, Bighorn Basin, Wyoming], in G. M. Grammer, P. M. Harris, and G. P. Eberli, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 235–259.</ref> attributed the poor recovery in the Tensleep Sandstone to low permeability baffles and barriers along bounding surfaces within the eolian dune sets. Bounding surfaces are subhorizontal to inclined discontinuities that divide eolian cross-beds into subsets, sets, and cosets ([[:file:M91FG171.JPG|Figure 2a]]). These form as a result of dune migration at the smaller scale and from regional discontinuities at the larger scale. Bounding surfaces have a tendency to act as baffles or barriers as a result of facies and grain size contrasts across them.<ref name=Shebi_1995>Shebi, M. A., 1995, The impact of reservoir heterogeneity on fluid flow in the Tensleep Sandstone of the Bighorn Basin: Resources of southwestern Wyoming, field conference guidebook: Wyoming Geological Association, p. 343–359.</ref> Perhaps the considerable difference in recoveries between the eolian reservoirs of Northwestern Europe and the United States is a function of how bounding surfaces influence fluid flow in each area. These features may provide less of an impedance to the flow of highly mobile gas in the Northwestern Europe gas fields than they do for the viscous oil found in the Tensleep Sandstone of Wyoming and Montana.

One other factor may contribute to poorer oil than gas recoveries in eolian sediments. In oil fields, a significant volume of capillary-trapped oil can result from the waterflooding of dune-bedded sandstones. Huang et al.<ref name=Huangetal_1995 /> showed that between 30 and 55% of the oil was trapped in a coreflood experiment on cross-laminated eolian sandstone under conditions of low-rate flooding.

One other factor may contribute to poorer oil than gas recoveries in eolian sediments. In oil fields, a significant volume of capillary-trapped oil can result from the waterflooding of dune-bedded sandstones. Huang et al.<ref name=Huangetal_1995>Huang, V., P. S. Ringrose, and K. S. Sorbie, 1995, Capillary trapping mechanisms in water-wet laminated rocks: SPE Reservoir Engineering, v. 10, SPE Paper 28942, p. 287–292.</ref> showed that between 30 and 55% of the oil was trapped in a coreflood experiment on cross-laminated eolian sandstone under conditions of low-rate flooding.

==Vertical permeability barriers in eolian sandstones==

==Vertical permeability barriers in eolian sandstones==

Baffles of limited areal extent in interdune areas are described by Shebi<ref name=Shebi_1995 /> from the Tensleep Sandstone of the Bighorn Basin in northwestern Wyoming and southwestern Montana. These are thin, discrete intervals of dolomite and anhydrite, about 0.15–0.7 m (0.5–2 ft) thick and with lateral dimensions on the scale of a few meters to tens of meters. The dolomite and anhydrite intervals are interpreted as sabkha deposits, which formed in wet interdune areas and playa lakes.

Baffles of limited areal extent in interdune areas are described by Shebi<ref name=Shebi_1995 /> from the Tensleep Sandstone of the Bighorn Basin in northwestern Wyoming and southwestern Montana. These are thin, discrete intervals of dolomite and anhydrite, about 0.15–0.7 m (0.5–2 ft) thick and with lateral dimensions on the scale of a few meters to tens of meters. The dolomite and anhydrite intervals are interpreted as sabkha deposits, which formed in wet interdune areas and playa lakes.

Studies in the western United States have shown that some sabkha units can be traced for several kilometers within the Mesozoic eolian sediments.<ref name=Crabaughandkocurek_1993 /> Cyclic climatic conditions resulted in alternating dune sandstone and widespread sheet-like fluvial deposits in the Jurassic Kayenta-Navajo Formations of northeastern Arizona.<ref name=Herries_1993 />

Studies in the western United States have shown that some sabkha units can be traced for several kilometers within the Mesozoic eolian sediments.<ref name=Crabaughandkocurek_1993>Crabaugh, M., and G. Kocurek, 1993, [http://sp.lyellcollection.org/content/72/1/103.abstract Entrada Sandstone: An example of a wet eolian system], in K. Pye, ed., Dynamics and environmental context of eolian sedimentary systems: Geological Society (London) Special Publication 72, p. 103–126.</ref> Cyclic climatic conditions resulted in alternating dune sandstone and widespread sheet-like fluvial deposits in the Jurassic Kayenta-Navajo Formations of northeastern Arizona.<ref name=Herries_1993>Herries, R. D., 1993, [http://sp.lyellcollection.org/content/73/1/199.abstract Contrasting styles of fluvial-eolian interaction at a downwind erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, U.S.A.], in C. P. North and D. J. Prosser, eds., Characterization of fluvial and eolian reservoirs: Geological Society Special Publication 73, p. 199–218.</ref>

The degree of layering within an eolian reservoir can therefore range from moderate to intense.<ref name=Krystinik_1990 /> Probably a major control on this is as to whether dry desert or wet desert conditions prevail, the latter associated with extensive fluvial, sabkha, and lacustrine interbeds.

The degree of layering within an eolian reservoir can therefore range from moderate to intense.<ref name=Krystinik_1990>Krystinik, L. F., 1990, Development geology in eolian reservoirs, in S. G. Fryberger, L. F. Krystinik, and C. J. Schenk, eds., Modern and ancient eolian deposits: Petroleum Exploration and Production, Rocky Mountain section, SEPM, Denver, Colorado, p. 13-1–13-12.</ref> Probably a major control on this is as to whether dry desert or wet desert conditions prevail, the latter associated with extensive fluvial, sabkha, and lacustrine interbeds.

Vertical permeability barriers can also be formed by diagenetic cements. Chandler et al.<ref name=Chandleretal_1989 /> noted that meteoric water can seep along bounding surfaces with the preferential formation of carbonate and silicate cements. Where the lower part of a dune is below a water table, early cementation may form a permeability barrier.<ref name=Northandprosser_1993 />

Vertical permeability barriers can also be formed by diagenetic cements. Chandler et al.<ref name=Chandleretal_1989>Chandler, M. A., G. Kocurek, D. J. Goggin, and L. W. Lake, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0005/0650/0658.htm Effects of stratigraphic heterogeneity on permeability in eolian sandstone sequence, Page Sandstone, northern Arizona]: AAPG Bulletin, v. 73, no. 5, p. 658–668.</ref> noted that meteoric water can seep along bounding surfaces with the preferential formation of carbonate and silicate cements. Where the lower part of a dune is below a water table, early cementation may form a permeability barrier.<ref name=Northandprosser_1993>North, C. P., and D. J. Prosser, 1993, [http://sp.lyellcollection.org/content/73/1/1.extract Characterization of fluvial and eolian reservoirs: Problems and approaches], in C. P. North and D. J. Prosser, eds., Characterization of fluvial and eolian reservoirs: Geological Society Special Publication 73, p. 1–6.</ref>

==Lateral permeability anisotropy within dune sandstones==

==Lateral permeability anisotropy within dune sandstones==

Horizontal and vertical permeability can be highly variable at the laminar scale in dune sandstones (e.g., Prosser and Maskall<ref name=Prosserandmaskall_1993 />). This results from the configuration of the three basic strata types in dune sandstones: wind-ripple, grain-flow, and grain-fall deposits.<ref name=Hunter_1977 />

Horizontal and vertical permeability can be highly variable at the laminar scale in dune sandstones (e.g., Prosser and Maskall<ref name=Prosserandmaskall_1993>Prosser, D. J., and R. Maskall, 1993, [http://sp.lyellcollection.org/content/73/1/377.abstract Permeability variation within eolian sandstone: A case study using core cut sub-parallel to slipface bedding, the Auk field, central North Sea, UK: Characterization of fluvial and eolian reservoirs]: Geological Society Special Publication 73, p. 377–397.</ref>). This results from the configuration of the three basic strata types in dune sandstones: wind-ripple, grain-flow, and grain-fall deposits.<ref name=Hunter_1977>Hunter, R. E., 1977, [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.1977.tb00128.x/abstract Basic types of stratification in small eolian dunes]: Sedimentology, v. 24, no. 3, p. 361–387.</ref>

 

[[File:M91FG172.JPG|thumb|300px|{{figure number|3}}Dune-interdune relationships in the Entrada Sandstone, northern Utah and Colorado. Interdune sediments act as discontinuous baffles in eolian sediments (from Kocurek<ref name=Kocurek_1981>Kocurek, G., 1981, [http://www.sciencedirect.com/science/article/pii/0031018281900547 Erg reconstruction: The Entrada Sandstone (Jurassic) of northern Utah and Colorado]: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 36, no. 1–2, p. 125–153.</ref>). Reprinted with permission from, and © by, Elsevier. Satellite photograph of Namib Desert, Namibia, Courtesy of [http://www.earthasart.gsfc.nasa.gov NASA].]]

Sand grains migrate over dunes, forming rippled surfaces. The grains pack together relatively closely in wind-rippled strata, and the porosity is lower in these units. Inverse grading is common with low-permeability pin-stripe laminae reducing the vertical permeability. Chandler et al.<ref name=Chandleretal_1989 /> found in the Page Sandstone of Arizona that the grain size ratio from the coarse-grained to the fine-grained parts of each wind-rippled strata averages 3:1 and can be as much as 7:1. The permeability ratio between the coarse and fine laminae gives an average value of 11:1 with a maximum value of 75:1.

Sand grains migrate over dunes, forming rippled surfaces. The grains pack together relatively closely in wind-rippled strata, and the porosity is lower in these units. Inverse grading is common with low-permeability pin-stripe laminae reducing the vertical permeability. Chandler et al.<ref name=Chandleretal_1989 /> found in the Page Sandstone of Arizona that the grain size ratio from the coarse-grained to the fine-grained parts of each wind-rippled strata averages 3:1 and can be as much as 7:1. The permeability ratio between the coarse and fine laminae gives an average value of 11:1 with a maximum value of 75:1.

The contrast in grain size and sorting between the individual sand streaks in dune beds results in large variations in permeability. Permeability differences can be exacerbated by early diagenesis. Fine-grained laminae can potentially draw in cementing solutes by capillary action.

The contrast in grain size and sorting between the individual sand streaks in dune beds results in large variations in permeability. Permeability differences can be exacerbated by early diagenesis. Fine-grained laminae can potentially draw in cementing solutes by capillary action.

All three strata types may be found on dune foresets. By contrast, wind-ripple lamination dominates the interdune sediments. These are typically poorly sorted and finely laminated. Interdune sediments probably create an interleaving network of permeability baffles, which serve to create tortuous flow pathways upward through stacked dune reservoirs ([[:file:M91FG171.JPG|Figure 2c]], [[:file:M91FG172.JPG|Figure 3]]). They can act to inhibit coning in thick dune sandstone reservoirs.<ref name=Weber_1987 />

All three strata types may be found on dune foresets. By contrast, wind-ripple lamination dominates the interdune sediments. These are typically poorly sorted and finely laminated. Interdune sediments probably create an interleaving network of permeability baffles, which serve to create tortuous flow pathways upward through stacked dune reservoirs ([[:file:M91FG171.JPG|Figure 2c]], [[:file:M91FG172.JPG|Figure 3]]). They can act to inhibit coning in thick dune sandstone reservoirs.<ref name=Weber_1987 />

Eolian dune sets show strong lateral permeability anisotropy within the reservoir. Reservoir fluids flowing across the wind-flow direction are impeded by pin-stripe lamination of fine-grained material along the dune cross sets. By contrast, the individual layers and laminae are much more continuous along the depositional strike trend of the dune system, perpendicular to the wind flow direction ([[:file:M91FG171.JPG|Figure 2d]]).<ref name=Weber_1987 /> Krystinik<ref name=Krystinik_1990 /> stated that, in most eolian reservoirs, the anisotropy permeability ratio is between 4:1 and 25:1, although the overall range may be approximately 1: 1 and up to 200:1 locally. He recommends that horizontal core plugs should be taken both along and perpendicular to the wind-flow direction in order to assess the lateral permeability anisotropy in dune sandstones. Follows<ref name=Follows_1997 /> described how a horizontal well was planned to be drilled along depositional dip in the Auk oil field in the UK North Sea. The intention was to connect up the highest number of grain-flow sets between bounding laminae so as to maximize production.

Eolian dune sets show strong lateral permeability anisotropy within the reservoir. Reservoir fluids flowing across the wind-flow direction are impeded by pin-stripe lamination of fine-grained material along the dune cross sets. By contrast, the individual layers and laminae are much more continuous along the depositional strike trend of the dune system, perpendicular to the wind flow direction ([[:file:M91FG171.JPG|Figure 2d]]).<ref name=Weber_1987 /> Krystinik<ref name=Krystinik_1990 /> stated that, in most eolian reservoirs, the anisotropy permeability ratio is between 4:1 and 25:1, although the overall range may be approximately 1: 1 and up to 200:1 locally. He recommends that horizontal core plugs should be taken both along and perpendicular to the wind-flow direction in order to assess the lateral permeability anisotropy in dune sandstones. Follows<ref name=Follows_1997>Follows, E., 1997, [http://pg.geoscienceworld.org/content/3/1/43.short Integration of inclined pilot hole core with horizontal image logs to appraise an eolian reservoir, Auk field, central North Sea]: Petroleum Geoscience, v. 3, p. 43–55.</ref> described how a horizontal well was planned to be drilled along depositional dip in the Auk oil field in the UK North Sea. The intention was to connect up the highest number of grain-flow sets between bounding laminae so as to maximize production.

==See also==

==See also==