In order to clarify the developmental characteristics and formation mechanism of authigenic illite in the Long 7 dense sandstone reservoir of the Yanchang Formation in the Shimoushiwan area and its influence on the physical properties of the reservoir, a study was carried out using thin section analysis, scanning electron microscopy, and X-diffraction. Thin-section analysis, scanning electron microscopy, X-diffraction and other testing methods were used to study the content, production and development mechanism of authigenic illite in the Long 7 reservoir. The results show that the authigenic clay minerals in the study area are mainly illite, with an average content of 4.3%, which is mainly endowed with three types of production, such as thin film, intergranular pore cementation and filling, etc. The illitization of montmorillonite and the formation of illite by the dissolution of potassium feldspar are the two main mechanisms of authigenic illite genesis in the study area. In the early orogenic B stage, the stratigraphic temperature reaches 70~100℃, and the authigenic illite formed by the dehydration of montmorillonite is mainly in the form of flakes and honeycomb. In the middle diagenetic stage A, the dissolution of potassium feldspar in the acidic diagenetic environment directly formed fibrous or filamentary authigenic illite. The authigenic illite developed in the form of granular envelope in the study area prevents the secondary increase of quartz to a certain extent and thus protects the preservation of primary pores, but it has great limitations. In general, the development of authigenic illite in the study area can block the pore space, which leads to the decrease of the quality of sandstone reservoirs and negatively affects the physical properties of the reservoirs.

Wang Ying, Huang Xingyu. Mechanism of genesis of long 7 authigenic Illite colluvium in the Xiasiwan area and its effect on physical properties. Academic Journal of Environment & Earth Science (2024), Vol. 6, Issue 1: 67-74. https://doi.org/10.25236/AJEE.2024.060110.

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