| According to the characteristics of rock particle composition, the sandstone reservoir of the Xujiahe Member 4
of Upper Triassic in western Sichuan Depression can be divided into three categories: feldspar lithic sandstone, lithic
quartz sandstone and lithic sandstone, of which the quality of feldspathic lithic sandstone reservoir is the best. By means of
cast thin section observation, cathodoluminescence and electron microprobe analysis, it is considered that carbonate
cement is an important factor affecting reservoir quality and continuous calcite and calcite cement with grain structure
play a very important role in affecting reservoir quality. The enrichment law and trace element content change of
carbonate cements are analyzed. It is found that different types of sandstone develop crystal stock calcite, crystalline
calcite and dolomite cements, and the characteristics of cement content, cathodoluminescence and trace element
composition reflect the differences of their sources. The cathodoluminescence of crystal stock calcite is bright yellow,
with high Fe2+ , Mn2+ contents, and low Sr2+ content. It has the characteristics of trace element composition caused by
atmospheric fresh water, which confirms that it is affected by the leaching of atmospheric fresh water. Microcrystalline,
very fine crystalline, fine crystalline and medium crystalline calcite has the cathodoluminescence of orange red to orange
yellow, and the calcite filling in feldspar dissolved pore has the cathodoluminescence of dark and bright for multiple
generations, reflecting the variation of Fe/Mn value from high to low. The middle crystalline calcite and calcite filling in
feldspar dissolved pore are characterized by high Fe2+, Mn2+ contents and low Mg2+ content. It is considered that because
feldspar and clay minerals are affected by organic acid dissolution during burial diagenesis, the resulting high Fe2+ and
Mn2+ concentrations in diagenetic system, and the high distribution coefficient of Mn2+ and Fe2+ in the diagenetic process
of carbonate cement determine the distribution characteristics of trace element in the cements. Therefore, ions forming
crystalline carbonate cement mainly comes from the dissolution of feldspar and clay minerals during burial. |
