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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 name=Jngln>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===

[[File:Sandstone-Fig-10.png|thumb|300px|{{Figure number|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).]]

[[File:Sandstone-Fig-10.png|thumb|300px|{{Figure number|10}}Typical stratigraphic section of the Lower-Middle Jurassic and Upper Triassic fluvio-deltaic facies showing general lithostratigraphy and sedimentary structures. 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<ref name=Jngln/>.]]

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

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

===Evolutional diagenesis.===  

===Evolutional diagenesis===  

Diagenesis is mainly classified into stages:

[[Diagenesis]] is mainly classified into stages:

* '''Early diagenesis:''' Early [[diagenesis]] was controlled by the depositional facies and detrital composition. Early diagenesis mainly includes mechanical compaction, early diagenetic carbonate cementation, dissolution of the detrital fragments, and the mechanical infiltration of grain coating smectitic clay, and precipitation of kaolinite. Mechanical compaction reduced porosity and permeability through increased grain packing and the bending and rupturing of mica and plastic [[deformation]] of ductile rock fragments and mud intraclasts. Interaction of meteoric water with sandstones resulted in the dissolution of detrital fragments (mainly feldspars) and the precipitation of kaolinite and smectite.  

* '''Early diagenesis:''' Early diagenesis was controlled by the depositional facies and detrital composition. Early diagenesis mainly includes mechanical compaction, early diagenetic carbonate cementation, dissolution of the detrital fragments, and the mechanical infiltration of grain coating smectitic clay, and precipitation of kaolinite. Mechanical compaction reduced porosity and permeability through increased grain packing and the bending and rupturing of mica and plastic [[deformation]] of ductile rock fragments and mud intraclasts. Interaction of meteoric water with sandstones resulted in the dissolution of detrital fragments (mainly feldspars) and the precipitation of kaolinite and smectite.  

* '''Late diagenesis:''' Late diagenetic alterations include chemical compaction, the albitization of plagioclase, dickitization and illitization of kaolinite, illitization and chloritization of smectite and the precipitation of [[quartz]], ankerite and type II calcite cements. In addition to the increase in temperature and pressure, the spatial and temporal distributions of the late diagenetic alterations, and hence of porosity-permeability evolution of the sandstones, were influenced by the early diagenetic modifications and oil emplacement.

* '''Late diagenesis:''' Late diagenetic alterations include chemical compaction, the albitization of plagioclase, dickitization and illitization of kaolinite, illitization and chloritization of smectite and the precipitation of [[quartz]], ankerite and type II calcite cements. In addition to the increase in temperature and pressure, the spatial and temporal distributions of the late diagenetic alterations, and hence of porosity-permeability evolution of the sandstones, were influenced by the early diagenetic modifications and oil emplacement.

* '''Epidiagenesis:''' The uplifting of sediments to the depths shallower than 2km promoted the meteoric water invasion (evidenced by the presence of formation-waters with a brackish composition), leading to the dissolution of silicate grains such as feldspars (mainly plagioclase) and precipitation of kaolinite. The extent of plagioclase kaolinitization varies widely even within the same well, being most extensive in the medium- to coarse-grained fluvial sandstones. Compared with the early diagenetic kaolinite, which is most common in the Jurassic sandstones, late-epidiagenetic kaolinite reveals no evidence of dickitization or illitization.

* '''Epidiagenesis:''' The uplifting of sediments to the depths shallower than 2km promoted the meteoric water invasion (evidenced by the presence of formation-waters with a brackish composition), leading to the dissolution of silicate grains such as feldspars (mainly plagioclase) and precipitation of kaolinite. The extent of plagioclase kaolinitization varies widely even within the same well, being most extensive in the medium- to coarse-grained fluvial sandstones. Compared with the early diagenetic kaolinite, which is most common in the Jurassic sandstones, late-epidiagenetic kaolinite reveals no evidence of dickitization or illitization.

====Effect of compaction on reservoir quality====

====Effect of compaction on reservoir quality====

Mechanical compaction during the early diagenetic phase reduced porosity and permeability through increased grain packing, displacement and rearrangement of the detrital fragments, plastic [[deformation]] of the ductile mud intraclasts to pseudo matrix as well as bending and squeezing of Biotite between rigid grains. The overall small amounts of early diagenetic cements rendered compaction to be more important than cementation in reducing porosity and permeability (figure 11).

 

Mechanical compaction during the early diagenetic phase reduced porosity and permeability through increased grain packing, displacement and rearrangement of the detrital fragments, plastic [[deformation]] of the ductile mud intraclasts to pseudo matrix as well as bending and squeezing of Biotite between rigid grains. The overall small amounts of early diagenetic cements rendered compaction to be more important than cementation in reducing porosity and permeability ([[:File:Sandstone-Fig-11.png|Figure 11}}).

====Effect of cementation on reservoir quality====

====Effect of cementation on reservoir quality====

* Carbonate cements are more abundant, and hence have a greater control on porosity and, particularly, permeability of the deltaic sandstones than of the fluvial sandstones.(figure 12).A higher overall content of carbonate cement of the delta-front and pro-delta sandstones (av. 4.0% and 4.2%, respectively) accounts partly for the considerably lower reservoir quality. However, the formation of a small amount of early diagenetic [[quartz]] cement and homogeneously distributed early diagenetic carbonates in sandstones weakened part of the mechanical compactional effect, and hence the preservation of part of the primary porosity in sandstones.

 

* Carbonate cements are more abundant, and hence have a greater control on porosity and, particularly, permeability of the deltaic sandstones than of the fluvial sandstones.([[File:Sandstone-Fig-12.png|Figure 12}}). A higher overall content of carbonate cement of the delta-front and pro-delta sandstones (av. 4.0% and 4.2%, respectively) accounts partly for the considerably lower reservoir quality. However, the formation of a small amount of early diagenetic [[quartz]] cement and homogeneously distributed early diagenetic carbonates in sandstones weakened part of the mechanical compactional effect, and hence the preservation of part of the primary porosity in sandstones.

* The clay minerals have variable influence on reservoir quality evolution. The grain- rimming chlorite and, particularly, illite caused a substantial deterioration of reservoir quality by decreasing the permeability through blocking the pore throats and by increasing the micro porosity.

* The clay minerals have variable influence on reservoir quality evolution. The grain- rimming chlorite and, particularly, illite caused a substantial deterioration of reservoir quality by decreasing the permeability through blocking the pore throats and by increasing the micro porosity.

* The grain-coating chlorite rims and illite around [[quartz]]. Nevertheless, chlorite fringes induced the preservation of a considerable proportion of the primary inter/granular macroporosity through inhibiting the precipitation of quartz overgrowths. This resulted in a considerable decrease in permeability, yet often low permeability, in the deltaic sandstones and less commonly in the fluvial sandstones.

* The grain-coating chlorite rims and illite around [[quartz]]. Nevertheless, chlorite fringes induced the preservation of a considerable proportion of the primary inter/granular macroporosity through inhibiting the precipitation of quartz overgrowths. This resulted in a considerable decrease in permeability, yet often low permeability, in the deltaic sandstones and less commonly in the fluvial sandstones.

* The illitization of intergranular kaolinite(due to the only small degree of albitization of K- feldspar grains), and hence further deterioration of permeability, was inhibited due to the earlier transformation of kaolinite into dickite, which is more resistant to illitization due to its better ordered crystal structure.

* The illitization of intergranular kaolinite(due to the only small degree of albitization of K- feldspar grains), and hence further deterioration of permeability, was inhibited due to the earlier transformation of kaolinite into dickite, which is more resistant to illitization due to its better ordered crystal structure.

====Effect of dissolution on reservoir quality====

====Effect of dissolution on reservoir quality====