Changes

===Stratigraphy===

===Stratigraphy===

[[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=Jngl/>.]]

[[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]]).

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

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

[[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).]]

[[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<ref name=Jngln/>.]]

* 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.

* 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.