Eolian Environments | Eolian Environment in Petroleum Geology?

23 minutes read

Eolian-Environments, Sedimentology, geology

Reservoir Characteristics Eolian Environments

Eolianite sequences are commonly thought to offer some of the best reservoir characteristics. Though often true, this should not be a reflexive assumption. Among several misconceptions about this facies is the notion that they are, by definition, very well-sorted, lithologically homogeneous sandstones with excellent and vertically continuous porosities and permeabilities.

In fact, as discussed by Ahlbrandt and Fryberger (1982), dune sequences are known to be heterogeneous, complex reservoirs with substantial vertical and lateral variation in fluid conductivity. One reason for this is the differential texture that exists along individual crossbeds, which essentially influences differential fluid flow. Specifically, grain orientation normally takes place along each cross-bed, with micas and more lenticular sand grains aligned in the toeset plane. In addition, toeset beds often show alterations of such finer-grained material with much coarser sand that has avalanched down the slipface of the dune. These aspects introduce significant permeability heterogeneity. Moreover, cementation is also apparently influenced by these anisotropies and, therefore, differential porosity/permeability preservation has also resulted in some deposits. As a result, fluids may be best able to migrate along, rather than across, crossbed layers, i.e., horizontal permeabilities commonly exceed vertical permeabilities. Figure 1 (Porosity/permeability plot showing variety in general reservoir characteristics for the productive eolian Leman sands of the North Sea region) is a simple indication of the wide and continuous range in reservoir quality shown by eolianites.

Porosity permeability plot, Desert environment, Eolian Environments, dunes, sand, sandstone, erosion, abrasion, deflation, desertification, aeolian processes, windward, leeward, cross-bedding, slip face, ripple marks, yardangs, desert pavement, sand sheets, saltation, suspension, wind velocity, dune fields, ergs, transverse dunes, longitudinal dunes, barchan dunes, star dunes, blowouts, loess, dust storms, sand storms, haboobs, desert varnish, rock pedestals, hoodoos, arches, mesas, buttes, canyons, playas, pan dunes, sand shadows, wind tails, zibars, salt flats, salt pans, quicksand, wind ripples, deflation hollows, sand seas, desert ecology, aridity, erosion control, sand mining, wind turbines, dust devils, sand spouts, desert flora, desert fauna, geologic time, eolian sediments, fossil dunes, desert pavement, ergodic hypothesis, paleoclimate, desert hydrology, aeolian transport, ventifacts, desert geology, desertification control, wind erosion, eolian landforms, sand seas, dune migration, geomorphology, desertification, atmospheric sciences, aeolian dust, eolian climate archives, eolian soil, eolian deposits, aeolian landforms, eolian geomorphology, Eolian environment, Petroleum geology, Sediment transport, Aeolian deposition, Wind-blown sediments, Dune morphology, Cross-bedding, Sand accumulation, Dust deposition, Erosion features, Wind direction, Eolian facies, Grain size distribution, Sedimentary structures, Stratigraphy, Paleoclimate, Eolian reservoirs, Reservoir quality, Permeability, Porosity, Diagenesis, Compaction, Cementation, Well log analysis, Reservoir characterization — **FIGURE 1**

In terms of known traps, structural, stratigraphic, and combination situations are well-studied. The Nugget sandstone, for example, is now widely accepted as being mainly eolian, and forms one of the major reservoirs along the Western Overthrust Belt of the Rocky Mountain region. Figure 2 (Northwest-southeast cross section through Painter Reservoir field showing complex structure typical along the western Overthrust.

Northwest-southeast cross section, Desert environment, Eolian Environments, dunes, sand, sandstone, erosion, abrasion, deflation, desertification, aeolian processes, windward, leeward, cross-bedding, slip face, ripple marks, yardangs, desert pavement, sand sheets, saltation, suspension, wind velocity, dune fields, ergs, transverse dunes, longitudinal dunes, barchan dunes, star dunes, blowouts, loess, dust storms, sand storms, haboobs, desert varnish, rock pedestals, hoodoos, arches, mesas, buttes, canyons, playas, pan dunes, sand shadows, wind tails, zibars, salt flats, salt pans, quicksand, wind ripples, deflation hollows, sand seas, desert ecology, aridity, erosion control, sand mining, wind turbines, dust devils, sand spouts, desert flora, desert fauna, geologic time, eolian sediments, fossil dunes, desert pavement, ergodic hypothesis, paleoclimate, desert hydrology, aeolian transport, ventifacts, desert geology, desertification control, wind erosion, eolian landforms, sand seas, dune migration, geomorphology, desertification, atmospheric sciences, aeolian dust, eolian climate archives, eolian soil, eolian deposits, aeolian landforms, eolian geomorphology, Eolian environment, Petroleum geology, Sediment transport, Aeolian deposition, Wind-blown sediments, Dune morphology, Cross-bedding, Sand accumulation, Dust deposition, Erosion features, Wind direction, Eolian facies, Grain size distribution, Sedimentary structures, Stratigraphy, Paleoclimate, Eolian reservoirs, Reservoir quality, Permeability, Porosity, Diagenesis, Compaction, Cementation, Well log analysis, Reservoir characterization — **FIGURE 2**

Note that the trap is still a relatively coherent anticline.) and (Figure 3,

coherent anticline, Desert environment, Eolian Environments, dunes, sand, sandstone, erosion, abrasion, deflation, desertification, aeolian processes, windward, leeward, cross-bedding, slip face, ripple marks, yardangs, desert pavement, sand sheets, saltation, suspension, wind velocity, dune fields, ergs, transverse dunes, longitudinal dunes, barchan dunes, star dunes, blowouts, loess, dust storms, sand storms, haboobs, desert varnish, rock pedestals, hoodoos, arches, mesas, buttes, canyons, playas, pan dunes, sand shadows, wind tails, zibars, salt flats, salt pans, quicksand, wind ripples, deflation hollows, sand seas, desert ecology, aridity, erosion control, sand mining, wind turbines, dust devils, sand spouts, desert flora, desert fauna, geologic time, eolian sediments, fossil dunes, desert pavement, ergodic hypothesis, paleoclimate, desert hydrology, aeolian transport, ventifacts, desert geology, desertification control, wind erosion, eolian landforms, sand seas, dune migration, geomorphology, desertification, atmospheric sciences, aeolian dust, eolian climate archives, eolian soil, eolian deposits, aeolian landforms, eolian geomorphology, Eolian environment, Petroleum geology, Sediment transport, Aeolian deposition, Wind-blown sediments, Dune morphology, Cross-bedding, Sand accumulation, Dust deposition, Erosion features, Wind direction, Eolian facies, Grain size distribution, Sedimentary structures, Stratigraphy, Paleoclimate, Eolian reservoirs, Reservoir quality, Permeability, Porosity, Diagenesis, Compaction, Cementation, Well log analysis, Reservoir characterization — **FIGURE 3**

Isopach map and discovery well log (gamma ray/neutron/density) of Painter Reservoir field.for isopaching the net effective reservoir, 10% minimum porosity was used) shows an instance of the complex structure that characterizes Nugget fields along the producing fairway in southwesternmost Wyoming.

More generally, eolianites share with alluvial fan and braided stream facies the potential drawback of being deposited at considerable distances from marine source rocks. Certainly, the “telescoping” together of entire stratigraphic sections which takes place during regional thrust faulting (Figure 2) is one way nature has used to juxtapose widely disparate facies. At the same time, dune deposition does, in fact, occur quite frequently in proximity to coastlines. Furthermore, the nature of this deposition reduces the opportunity for subfacies development and, therefore, large-scale stratigraphic traps are possible, for example, at the updip margins of marine basins. Regional pinch-out into, or capping by, evaporitic facies is relatively common in the sedimentary record, as shown by the Rotliegendes reservoirs of the North Sea.