2,803
edits
Changes
Jump to navigation
Jump to search
Line 42:
Line 42: + + + + + + + + + + + + + + + + + + + + + + − + − + − +
no edit summary
==Vertical permeability barriers in eolian sandstones==
==Vertical permeability barriers in eolian sandstones==
Fluvial and sabkha sediments deposited in interdune areas can be permeability barriers and baffles within eolian sediments ([[:file:M91FG171.JPG|Figure 2b]]). These may either be confined to interdune areas and of limited extent or they can be laterally extensive on a basin scale.
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 />
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.
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 />
==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 />
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.
When the wind-blown sediments reach the brinkline of the dune, the wind speed drops and the grains fall onto the leeward dune slip face, coming to rest as grain-fall deposits. These form parallel-laminated, slightly tapering strata. Grain size can vary between the individual strata.
On steep surfaces, avalanches may occur, forming grain-flow deposits. These develop as cone- or tongue-shaped geometries at the base of the slipface. Grain-flow cross strata are thicker than other eolian strata, up to a maximum of 2–5 cm (0.7–1.9 in.) thick. They are internally structureless or show subtle grading. The grain packing of grain-flow deposits is relatively loose as a result of the very rapid deposition of the grains. Thus, grain-flow deposits tend to be more porous and permeable compared to the other strata types.
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.
[[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 />). Reprinted with permission from, and © by, Elsevier. Satellite photograph of Namib Desert, Namibia, Courtesy of [http://www.earthasart.gsfc.nasa.gov NASA].]]
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.
==See also==
==See also==
* [[
* [[Meandering fluvial reservoirs]]
* [[
* [[Braided fluvial reservoirs]]
* [[
* [[Deltaic reservoirs]]
==References==
==References==