| Based on the task of"The Post-Sedimentational Water-Rock Interaction of Xujiahe Formation gas reservoir of Xiao-Xing-He Gas Field, West Sichuan Depression"and by analyzing the whole rock chemical composition of the deep Xujiahe Formation gas reservoir of Xingchang Gas Field, West Sichuan Depression , combining the Optical microscopy, XRD, CLM, SEM analyses on core samples from three wells, This thesis presents how the Chemical composition indicates the Chemical diagenesis of Xujiahe sandstone. this thesis gives some conclusions in the following:1) Based on the description of well cores, the identification of the thin section and the grain size analysis, we got the lithologies of the research area. As a result, we found that abundance of lithic sandstone occupies the absolutely superiority. Carbonate is the most major authigenic minerals, kaolinite only to be found in Xu4. Xu4 contains higher calcite cements and authigenic illite content than that of in Xu2. However, authigenic quartz content, chlorite, dolomite cement content in Xu2 was significantly higher than Xu4. Dissolution of feldspar is the most important factor to improve the Reservoir quality of Xujiahe sandstone, local stratigraphic interval of high dissolved is the main objective of our study.2) According to theδ13C values of calcite cement and the homogenization temperature of fluid inclusion in quartz cements, the diagenetic process of Xu4 sandstone involves higher degrees of organic carbon participation.3) 3)According to whole-rock chemistry, Xu2 contains higher SiO2 content and Xu4 contains higher Al2O3 content, suggestes Xu2 sandstone has a higher compositional maturity, and SiO2 present in Xu4 may exists mainly in the Al-rich minerals; the fact that MgO and CaO enriched in Xu4 is relative to the higher content of carbonate minerals, which shows a significant positive correlation between them, the related coefficient (R2) between calcium and magnesium in Xu2 up to 0.89, revealed the MgO mainly in dolomite; With the exception of kaolin and albite, the major aluminium-bearing minerals in sandstones (K-feldspar, mica and illite) also contain significant potassium, causing a strong positive correlation between potassium and aluminium. However, the K/Al molar ratio in K-feldspar is 1/1, but varies between 1/3 and 1/4 in illite.So,Variations in the abundance of these minerals cause the differences in the total potassium and aluminium content of the sandstones. This is the key theoretical point used in this thesis to investigate the chemical composition relative to the feldspar dissolution and secondary porosity formation mechanism. However, alumina content is relatively high in samples with higher plagioclase contents. Therefore, aluminium in plagioclase(albite), corresponding to the molar Na2O, is subtracted from the total Al2O3 content in the calculation of K/Al molar ratio, and this modied ratio has been used instead. The bulk K/Al molar ratios in the Xu4 sandstones basically below 1/4 shows a lack of potassium-rich feldspar mineral. However, the K/Al molar ratios are relatively high in the Xu2 sandstones, where K/Al molar ratios vary between 1/1 and 1/3. might be related to higher amounts of K-feldspar and authigenic illite.; the top of Xu4, close to X5 mudstone, with high secondary porosity , shows sandstone with low K2O/Al2O3 index, In low K zone, The K2O/Al2O3 index shows a good positive correlation with porosity, indicating the sandstone subjected to strong feldspar dissolution.
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