Changes

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===

[[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|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).]]

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

([[: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====

* '''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.

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.  

* '''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.

 

====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.

===Burial-diagenetic modifications on reservoir quality===

===Burial-diagenetic modifications on reservoir quality===