Changes

===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.