2,803
edits
Changes
Jump to navigation
Jump to search
Line 29:
Line 29: + + + + + + + + + + + +
no edit summary
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 /> 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 /> 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 /> 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 /> 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==
Eolian sediments form layer-cake to jigsaw-puzzle geometries. Dune sand bodies may intercalate with or pinch out into poorer quality sabkha and fluvial facies associations. Eolian environments tend to occur on a large scale and dune sandstones can be greater in length than typical well spacings. Weber<ref name=Weber_1987 /> described outcrops of eolian sandstone in the Permian De Chelly Sandstone in northern Arizona. The average thickness of cross-bed sets is about 6 m (20 ft). The width-to-thickness ratio is estimated as between 50:1 and 100:1. The length-to-thickness ratio is estimated as 200 to 1.
==Heterogeneity of eolian sandstones==
It is difficult to make generalizations about the effect of heterogeneity on hydrocarbon recovery from eolian sandstones as recovery factors seem to vary enormously worldwide. It is possible that eolian sandstones are much better suited as gas than oil reservoirs. Some of the Western European Permian desert sandstone gas reservoirs show very high recoveries; a 91% recovery factor is quoted for the Leman gas field for instance.<ref name=Hillier_2003 /> By comparison, the viscous oil reservoirs of the Pennsylvanian to Permian Tensleep Sandstone of Wyoming and Montana show low recoveries by primary production. In the Little Buffalo Basin field of Wyoming, well spacing has been reduced successively from 40- to 20- to 10- to 5-ac spacing, in some cases without interwell interference.<ref name=Mccaleb_1979 /><ref name=Ahlbrandtandfryberger_1982 />
[[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 /> 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 /> 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.
==Vertical permeability barriers in eolian sandstones==