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Line 84: − + + + − [[File:Sandstone-Fig-11.png|thumb|300px|Figure 11: Plot of intergranular volume (%) versus cement (%) for the Jurassic-Triassic fluvio- deltaic sandstones with a depositional intergranular volume of 40% (after Li Renwei et al<ref>Li Renwei, Lu Jiacan, Zhang Shukun et al., 1999, Organic carbon isotopic composition in black shales of Sinian and early Cambrian. (6th edition), vol.42, Science in China, pp. 595-603.</ref>).]] −  − + + + − − [[File:Sandstone-Fig-12.png|thumb|300px|Figure 12: Plots of porosity versus total carbonates (a), and permeability versus total carbonates (b) for the sandstones in the study area. (After Luo Jinglan et al, 2002).]]
Sandstone sedimentology and stratigraphy (view source)
Revision as of 19:31, 9 January 2024
, 9 January→Sandstone reservoir quality
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.
====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).
[[File:Sandstone-Fig-11.png|thumb|300px|{{Figure number|11}}Plot of intergranular volume (%) versus cement (%) for the Jurassic-Triassic fluvio- deltaic sandstones with a depositional intergranular volume of 40% (after Li Renwei et al<ref>Li Renwei, Lu Jiacan, Zhang Shukun et al., 1999, Organic carbon isotopic composition in black shales of Sinian and early Cambrian. (6th edition), vol.42, Science in China, pp. 595-603.</ref>).]]
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.
[[File:Sandstone-Fig-12.png|thumb|300px|{{Figure number|12}}Plots of porosity versus total carbonates (a), and permeability versus total carbonates (b) for the sandstones in the study area. (After Luo Jinglan et al, 2002).]]
* 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====