Changes

===Sandstone classification===

===Sandstone classification===

[[File:Sandstone-Fig-3.png|thumb|300px|{{Figure number|3}}Dott's (1964) sandstone classification scheme (after North<ref name=North>North, F. K., 1985, Reservoir rocks, ''in'' F. K. North, Petroleum Geology: Boston, Allen & Unwin, p. 117-218.</ref>).]]

[[File:Sandstone-Fig-3.png|thumb|300px|{{Figure number|3}}Dott's (1964) sandstone classification scheme (after North<ref name=North>North, F. K., 1985, Reservoir rocks, ''in'' F. K. North, Petroleum Geology: Boston, Allen & Unwin, p. 117-218.</ref>).]]

==Sandstone depositional environments==

==Sandstone depositional environments==

The depositional environments are very important and determine the reservoir quality. They sandstone beds range from terrestrial to deep marine, including: Fluvial ([[alluvial]] fans, river sediments); Deltaic (levees, distributary deposits ,mouth bars and other sediments formed where river meets a lake or sea); Aeolian(wind-blown dune sands formed in coastal and desert environments); Shoreline ( beaches, barrier bars, tidal deltas and similar deposits formed in coastal areas);Glacial(sandy materials in tillites and other glacier deposits); and Deep-sea sediments, including contourite sands formed by ocean-bottom currents, turbidites and submarine fan deposits, formed by gravity-driven mass movements.(figure 4). However, Depositional conditions at any instant vary from one location to another which results in lateral as well as vertical changes within the reservoir and within individual rock units. These changes result in variations in porosity, fluid distribution, and permeability.

The depositional environments are very important and determine the reservoir quality. They sandstone beds range from terrestrial to deep marine, including: Fluvial ([[alluvial]] fans, river sediments); Deltaic (levees, distributary deposits ,mouth bars and other sediments formed where river meets a lake or sea); Aeolian(wind-blown dune sands formed in coastal and desert environments); Shoreline ( beaches, barrier bars, tidal deltas and similar deposits formed in coastal areas);Glacial(sandy materials in tillites and other glacier deposits); and Deep-sea sediments, including contourite sands formed by ocean-bottom currents, turbidites and submarine fan deposits, formed by gravity-driven mass movements.([[:File:Sandstone-Fig-4.png|Figure 4]]). However, Depositional conditions at any instant vary from one location to another which results in lateral as well as vertical changes within the reservoir and within individual rock units. These changes result in variations in porosity, fluid distribution, and permeability.

 

===Fluvial sandstones===

===Fluvial sandstones===

Fluvial or [[alluvial]] sandstones are not so well sorted sand deposits containing carbonaceous debris and clay trapped in the spaces within the framework grains. They are formed in river channel deposits and terrestrial environments, and are commonly developed on unconformity surfaces. Individual sands in meandering and braided stream deposits are as thick as the depth of the river. They often rest on eroded base with a fining upward particle size sequence especially with meandering streams; where sands grade into shales and possibly even coals deposited in associated flood plain areas. Trough and planar cross beds occur in main part of sandstone beds (figure 5) dipping essentially downstream, with ripples occurring towards the top. In plain view, fluvial sandstones have abrupt terminations of sand bars. The individual channel bar and point bars deposits are interleaved with shales and other muddy sediments of flood plain origin; they tend to elongated parallel to flow direction especially in braided streams compared to direction parallel to the depositional strike. The beds then developed on unconformity surfaces may thicken in the valley (down cut) areas and be rich in sand, and become shaly and thin in the between-valley (intervening) sections.

Fluvial or [[alluvial]] sandstones are not so well sorted sand deposits containing carbonaceous debris and clay trapped in the spaces within the framework grains. They are formed in river channel deposits and terrestrial environments, and are commonly developed on unconformity surfaces. Individual sands in meandering and braided stream deposits are as thick as the depth of the river. They often rest on eroded base with a fining upward particle size sequence especially with meandering streams; where sands grade into shales and possibly even coals deposited in associated flood plain areas. Trough and planar cross beds occur in main part of sandstone beds ([[:File:Sandstone-Fig-5.png|Figure 5]]) dipping essentially downstream, with ripples occurring towards the top. In plain view, fluvial sandstones have abrupt terminations of sand bars. The individual channel bar and point bars deposits are interleaved with shales and other muddy sediments of flood plain origin; they tend to elongated parallel to flow direction especially in braided streams compared to direction parallel to the depositional strike. The beds then developed on unconformity surfaces may thicken in the valley (down cut) areas and be rich in sand, and become shaly and thin in the between-valley (intervening) sections.

===Deltaic sandstones===

===Deltaic sandstones===

Principal reservoirs are in delta-front sands.<ref name=North /> These deltaic sandstones form as deltas build out over the deeper-water deposits, and are common in post-organic sedimentary basins from Devonian to Recent for example Clinton sandstone in Ohio (Silurian), Jurassic Brent deltaic sandstones in North Sea. Vertical stacking in deltaic sandstones is common, giving rise to multi- pay zones. This is mainly due to individual delta lobes typically building out over and beside earlier lobes as subsidence proceeds and basin fills. Deltaic sandstones typically have a coarsening upward sequence, with individual sand bodies often elongate, lobate,or parallel to the paleo- shoreline but discontinuous along strike. During [[diagenesis]]; deltaic sandstones, although coarser, may often be cemented by early-formed silica and are commonly less productive. The cementation may block migration of hydrocarbons from the deeper sources into beds such as onshore fluvial deposits higher up in the prograding delta-fill sequence.

Principal reservoirs are in delta-front sands.<ref name=North /> These deltaic sandstones form as deltas build out over the deeper-water deposits, and are common in post-organic sedimentary basins from Devonian to Recent for example Clinton sandstone in Ohio (Silurian), Jurassic Brent deltaic sandstones in North Sea. Vertical stacking in deltaic sandstones is common, giving rise to multi- pay zones. This is mainly due to individual delta lobes typically building out over and beside earlier lobes as subsidence proceeds and basin fills. Deltaic sandstones typically have a coarsening upward sequence, with individual sand bodies often elongate, lobate,or parallel to the paleo- shoreline but discontinuous along strike. During [[diagenesis]]; deltaic sandstones, although coarser, may often be cemented by early-formed silica and are commonly less productive. The cementation may block migration of hydrocarbons from the deeper sources into beds such as onshore fluvial deposits higher up in the prograding delta-fill sequence.

===Shoreline sandstones===

===Shoreline sandstones===

Shoreline sandstones are convex upwards sand deposits with associated marine and lagoonal shales serving as both source rock and as well as seals; these sands are best developed along gently sloping shoreline areas, subject to transgression and/or regression with fluctuations in sea level, basin subsidence or sediment supply, and are mainly deposited in coastal and shallow marine environments, but most significantly are those formed by beaches and offshore (figure7);including coastal barriers and those associated with tidal channels. With slow transgression or regression, the individual shoreline sands may progressively overlap to form sand sheets such as transgressive sand sheets which occur on unconformity surfaces as the sea level rises and encroaches on over the land. Shoreline sandstones deposited on beaches are narrow, linear, elongate and oriented parallel to the shoreline while barrier shoreline sandstones are finer grained than the beach deposits, and are more discontinuous laterally. However, both beach and barrier shoreline sands are typically clean, well sorted; with little muddy matrix and good porosity and permeability.

[[File:Sandstone-Fig-7.png|thumb|300px|{{Figure number|7}}Shoreline sandstone; Lake Superior, Michigan's Upper Peninsula, USA. (Modified from Pettijohn et al.<ref name=Pettijohn />).]]

[[File:Sandstone-Fig-7.png|thumb|300px|Figure 7: Shoreline sandstone; Lake Superior, Michigan's Upper Peninsula, USA. (Modified from Pettijohn et al.<ref name=Pettijohn />).]]

Shoreline sandstones are convex upwards sand deposits with associated marine and lagoonal shales serving as both source rock and as well as seals; these sands are best developed along gently sloping shoreline areas, subject to transgression and/or regression with fluctuations in sea level, basin subsidence or sediment supply, and are mainly deposited in coastal and shallow marine environments, but most significantly are those formed by beaches and offshore ([[:File:Sandstone-Fig-7.png|Figure 7]]); including coastal barriers and those associated with tidal channels. With slow transgression or regression, the individual shoreline sands may progressively overlap to form sand sheets such as transgressive sand sheets which occur on unconformity surfaces as the sea level rises and encroaches on over the land. Shoreline sandstones deposited on beaches are narrow, linear, elongate and oriented parallel to the shoreline while barrier shoreline sandstones are finer grained than the beach deposits, and are more discontinuous laterally. However, both beach and barrier shoreline sands are typically clean, well sorted; with little muddy matrix and good porosity and permeability.

===Deep sea sandstones===

===Deep sea sandstones===

===Glacial sandstones===

===Glacial sandstones===

Glacial sandstones are poorly sorted matrix-bouldery sand deposits consisting of sands, lenses left by melt water streams with reservoir potential and other glacial deposits left by retreating glaciers as moraine deposits. Examples of these glacial sandstones include tillites, with little value as a reservoir due to low porosity. Glacial sandstones may also be deposited in marine areas, giving rise to muddy sediments with dispersed boulders. These may however be locally reworked by wave action in shallow area resulting into potential reservoir sand bed for example Ordovician oil- bearing glacial sands found in Algeria and Permo-[[carboniferous]] hydrocarbon glacial sands in southern Oman.

Glacial sandstones are poorly sorted matrix-bouldery sand deposits consisting of sands, lenses left by melt water streams with reservoir potential and other glacial deposits left by retreating glaciers as moraine deposits. Examples of these glacial sandstones include tillites, with little value as a reservoir due to low porosity. Glacial sandstones may also be deposited in marine areas, giving rise to muddy sediments with dispersed boulders. These may however be locally reworked by wave action in shallow area resulting into potential reservoir sand bed for example Ordovician oil- bearing glacial sands found in Algeria and Permo-[[carboniferous]] hydrocarbon glacial sands in southern Oman.

===Aeolian sandstones===

===Aeolian sandstones===

Aeolian sandstones are wind-blown sand dunes of coastal and desert environments. Dunes are heaped-up shaped ridges of sand with crests either parallel to wind direction (longitudinal dunes) and perpendicular to the prevailing wind (transverse dunes) although they may also be crescent- shaped. Dunes advance down- wind direction by erosion of sand from up-wind side and depositing sand on the down-wind (lee side).the up-wind face clearly has a gentle inclination while the down- wind face has a steeper slope. sand added to this steeper slopes runs down this slope adding to it and building cross beds which when buried within the dune forms large scale high-angle cross beds (Figure 9); whose [[dip]]s are in the same direction as wind direction, thus providing an indication of paleocurrent direction in ancient dune sands. Aeolian sandstones are extremely well sorted because of their wind-blown nature and hence have good porosity and permeability. However, with desert deposits, they are less likely than other types of sand to be associated with organic accumulations that would act as hydrocarbon source rocks and provide a petroleum charge. Aeolian sandstones are commonly associated with evaporites which are salt deposits. They can thus have a complex diagenetic history, which may in some cases block the pores of the sand with chemically precipitating cementing materials.

[[File:Sandstone-Fig-9.png|thumb|300px|{{Figure number|9}}Navajo Sandstone, Utah, USA (Modified from Pettijohn et al.<ref name=Pettijohn />).]]

[[File:Sandstone-Fig-9.png|thumb|300px|Figure 9: Navajo Sandstone, Utah, USA (Modified from Pettijohn et al.<ref name=Pettijohn />).]]

Aeolian sandstones are wind-blown sand dunes of coastal and desert environments. Dunes are heaped-up shaped ridges of sand with crests either parallel to wind direction (longitudinal dunes) and perpendicular to the prevailing wind (transverse dunes) although they may also be crescent- shaped. Dunes advance down- wind direction by erosion of sand from up-wind side and depositing sand on the down-wind (lee side).the up-wind face clearly has a gentle inclination while the down- wind face has a steeper slope. sand added to this steeper slopes runs down this slope adding to it and building cross beds which when buried within the dune forms large scale high-angle cross beds ([[:File:Sandstone-Fig-9.png|Figure 9]]); whose [[dip]]s are in the same direction as wind direction, thus providing an indication of paleocurrent direction in ancient dune sands. Aeolian sandstones are extremely well sorted because of their wind-blown nature and hence have good porosity and permeability. However, with desert deposits, they are less likely than other types of sand to be associated with organic accumulations that would act as hydrocarbon source rocks and provide a petroleum charge. Aeolian sandstones are commonly associated with evaporites which are salt deposits. They can thus have a complex diagenetic history, which may in some cases block the pores of the sand with chemically precipitating cementing materials.

==Sandstone reservoir quality==

==Sandstone reservoir quality==

Sandstone is the best known petroleum reservoir rock with an average porosity of about 15% and permeability of 25-100 Darcies which are mainly depend on the depositional environment, the character of the minerals forming the matrix and diagenesis. Diagenesis significantly reduces the porosity and permeability of sandstone reservoirs as illustrated in the case study below:

Sandstone is the best known petroleum reservoir rock with an average porosity of about 15% and permeability of 25-100 Darcies which are mainly depend on the depositional environment, the character of the minerals forming the matrix and diagenesis. Diagenesis significantly reduces the porosity and permeability of sandstone reservoirs as illustrated in the case study below:

==A case study of reconstruction of the diagenesis of the fluvial- lacustrine deltaic sandstones and its influence on the reservoir quality evolution<ref>Luo Jinglan, Morad, S., Zhang Xiaoli, Yan Shike, Wufuli, Li Yuhong and Xue Junmin. (2002). Reconstruction of the Diagenesis of the Fluvial-Lacustrine deltaic Sandstones and its Influence on the Reservoir Quality Evolution. (7th edition), Vol 45. Science in China, pp. 615-634.</ref>==

A case study of reconstruction of the diagenesis of the fluvial-lacustrine deltaic sandstones and its influence on the reservoir quality evolution<ref>Luo Jinglan, Morad, S., Zhang Xiaoli, Yan Shike, Wufuli, Li Yuhong and Xue Junmin. (2002). Reconstruction of the Diagenesis of the Fluvial-Lacustrine deltaic Sandstones and its Influence on the Reservoir Quality Evolution. (7th edition), Vol 45. Science in China, pp. 615-634.</ref>

===Location of study area===

===Location of study area===

===Stratigraphy===

===Stratigraphy===

The Yanchang Formation of the Upper Triassic is a suit of terrestrial fluivolacustrine-deltaic sequence, consisting one of the main hydrocarbon-producing intervals in the area. The Upper Triassic Yanchang Formation can be divided into three facies from the bottom to the top.

The Yanchang Formation of the Upper Triassic is a suit of terrestrial fluivolacustrine-deltaic sequence, consisting one of the main hydrocarbon-producing intervals in the area. The Upper Triassic Yanchang Formation can be divided into three facies from the bottom to the top.

(Figure 10).

([[:File:Sandstone-Fig-10.png|Figure 10]]).

 

[[File:Sandstone-Fig-10.png|thumb|300px|Figure 10: Typical stratigraphic section of the Lower-Middle Jurassic and Upper Triassic fluvio- deltaic facies showing general lithostratigraphy and sedimentary structures 1). 1, [[Oil shale]]; 2, trough [[cross-bedding]]; 3, tubular cross-bedding; 4, parallel bedding; 5, ripple cross-bedding; 6,contorted bedding; 7, [[conglomerate]]; 8, gravel sandstone;; 9, sandstone; 10, silt; 11, muddy sandstone; 12, muddy silt; 13, silty mudstone; 14, mudstone. (Modified after Luo Jinglan et al, 2002).]]

===Evolutional diagenesis.===  

===Evolutional diagenesis.===