Changes

==Eolian lithofacies associations==

==Eolian lithofacies associations==

Eolian lithofacies associations include dune, interdune, fluvial, and sabkha environments ([[:file:M91FG170.JPG|Figure 1]]). Dunes form where large volumes of dry sand are blown across the landscape. Lying between the dunes are the interdune areas, which are flat-lying belts or depressions. These areas may be subjected to either erosion or deposition. In wetter conditions, [[alluvial]] fans may extend outwards from upland areas, and fluvial sediments can be deposited by ephemeral streams. Large damp to wet areas between the dunes may dry out to form flat-lying evaporitic crusts called [[sabkha]]s. Playa lakes are desert lakes that episodically dry out.

Eolian [[lithofacies]] associations include dune, interdune, fluvial, and sabkha environments ([[:file:M91FG170.JPG|Figure 1]]). Dunes form where large volumes of dry sand are blown across the landscape. Lying between the dunes are the interdune areas, which are flat-lying belts or depressions. These areas may be subjected to either erosion or deposition. In wetter conditions, [[alluvial]] fans may extend outwards from upland areas, and fluvial sediments can be deposited by ephemeral streams. Large damp to wet areas between the dunes may dry out to form flat-lying evaporitic crusts called [[sabkha]]s. Playa lakes are desert lakes that episodically dry out.

Several different dune types are found in deserts. They can form as crescentic dunes (including barchan dunes), as long linear dunes, or as star dunes with crest lines radiating from one or two central peaks. Different dune types may be superimposed to form complex dunes, whereas the same type of dune is superimposed to form compound dunes. The term draa has been used to refer to large compound or complex dunes.<ref name=Kocurek_1996>Kocurek, G., 1996, Desert eolian systems, in H. G. Reading, ed., Sedimentary environments: Processes, facies and stratigraphy, 3d ed.: Oxford, Blackwell Science, p. 125–153.</ref>

Several different dune types are found in deserts. They can form as crescentic dunes (including barchan dunes), as long linear dunes, or as star dunes with crest lines radiating from one or two central peaks. Different dune types may be superimposed to form complex dunes, whereas the same type of dune is superimposed to form compound dunes. The term draa has been used to refer to large compound or complex dunes.<ref name=Kocurek_1996>Kocurek, G., 1996, Desert eolian systems, in H. G. Reading, ed., Sedimentary environments: Processes, facies and stratigraphy, 3d ed.: Oxford, Blackwell Science, p. 125–153.</ref>

In deserts, dune sediments aggrade laterally and vertically as large-scale sand blankets. These may be very thick (more than 300 m [1000 ft]), and cover hundreds of square kilometers.<ref name=Richardsonetal_1988a>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1988, Eolian dunes: Journal of Petroleum Technology, v. 40, no. 1, p. 11–12.</ref> Internally, dunes comprise thick, cross-bedded intervals of well-rounded, well-sorted sandstones. They are normally the most productive lithofacies in eolian reservoir systems. Flatter-lying eolian sand sheets may be found along the margins of dune systems.<ref name=Kocurekandnielson_1986>Kocurek, G., and J. Nielson, 1986, [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.1986.tb00983.x/abstract Conditions favourable for the formation of warm-climate eolian sand sheets]: Sedimentology, v. 33, no. 6, p. 795–816.</ref>

In deserts, dune sediments aggrade laterally and vertically as large-scale sand blankets. These may be very thick (more than 300 m [1000 ft]), and cover hundreds of square kilometers.<ref name=Richardsonetal_1988a>Richardson, J. G., J. B. Sangree, and R. M. Sneider, 1988, Eolian dunes: Journal of Petroleum Technology, v. 40, no. 1, p. 11–12.</ref> Internally, dunes comprise thick, cross-bedded intervals of well-rounded, well-sorted sandstones. They are normally the most productive lithofacies in eolian reservoir systems. Flatter-lying eolian sand sheets may be found along the margins of dune systems.<ref name=Kocurekandnielson_1986>Kocurek, G., and J. Nielson, 1986, [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.1986.tb00983.x/abstract Conditions favourable for the formation of warm-climate eolian sand sheets]: Sedimentology, v. 33, no. 6, p. 795–816.</ref>

Interdune and fluvial sediments generally show poorer reservoir characteristics by comparison to dune lithofacies. They are poorly sorted and are more likely to contain evaporite cements. Intercalated fine-grained sand and silt laminations together with diagenetic cementation tend to produce reservoir intervals with very poor vertical permeability. Nagtegaal<ref name=Nagtegaal_1979>Nagtegaal, P. J. C., 1979, [http://onlinelibrary.wiley.com/doi/10.1111/j.1747-5457.1979.tb00699.x/abstract Relationship of facies and reservoir quality in Rotliegends desert sandstones, southern North Sea region]: Journal of Petroleum Geology, v. 2, p. 145–158.</ref> used multivariate analysis to determine the factors controlling the porosity of eolian sediments from the Permian of the southern North Sea. He found that the main control on porosity is grain sorting, which varies from well sorted in dune sandstones to less well sorted in the other associated sediments. The relationship between original sedimentary texture and porosity has survived even after extensive diagenesis. The very poor permeability characteristics of interdune sediments are commonly reported. Lindquist<ref name=Lindquist_1983>Lindquist, S. J., 1983, [https://www.onepetro.org/journal-paper/SPE-10993-PA Nugget Formation reservoir characteristics affecting production in the overthrust belt of southwestern Wyoming]: Journal of Petroleum Technology, v. 35, no. 7, p. 1355–1365.</ref> found a contrast in permeability of four to five orders of magnitude between interdune and dune deposits in the Nugget Sandstone of southwestern Wyoming.

Interdune and fluvial sediments generally show poorer reservoir characteristics by comparison to dune lithofacies. They are poorly sorted and are more likely to contain evaporite cements. Intercalated fine-grained sand and silt laminations together with diagenetic cementation tend to produce reservoir intervals with very poor vertical permeability. Nagtegaal<ref name=Nagtegaal_1979>Nagtegaal, P. J. C., 1979, [http://onlinelibrary.wiley.com/doi/10.1111/j.1747-5457.1979.tb00699.x/abstract Relationship of facies and reservoir quality in Rotliegends desert sandstones, southern North Sea region]: Journal of Petroleum Geology, v. 2, p. 145–158.</ref> used multivariate analysis to determine the factors controlling the porosity of eolian sediments from the Permian of the southern North Sea. He found that the main control on porosity is grain sorting, which varies from well sorted in dune sandstones to less well sorted in the other associated sediments. The relationship between original sedimentary texture and porosity has survived even after extensive [[diagenesis]]. The very poor permeability characteristics of interdune sediments are commonly reported. Lindquist<ref name=Lindquist_1983>Lindquist, S. J., 1983, [https://www.onepetro.org/journal-paper/SPE-10993-PA Nugget Formation reservoir characteristics affecting production in the overthrust belt of southwestern Wyoming]: Journal of Petroleum Technology, v. 35, no. 7, p. 1355–1365.</ref> found a contrast in permeability of four to five orders of magnitude between interdune and dune deposits in the Nugget Sandstone of southwestern Wyoming.

==Geometry==

==Geometry==

[[File:M91FG171.JPG|thumb|300px|{{figure number|2}}The influence on fluid flow by heterogeneity in eolian sediments.]]

[[File:M91FG171.JPG|thumb|300px|{{figure number|2}}The influence on fluid flow by heterogeneity in eolian sediments.]]

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>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.

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.

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.

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.

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>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>

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>

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>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.

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==