Forming mechanisms of chlorite films in Yanchang Formation, Ordos Basin

Key words:  Ordos Basin  Chang 8 sandstone  chlorite film  double layer structure  destructive diagenesis

Abstract:

Using inclusion analysis, casting slice, scanning electron microscope analysis with energy disperse spectroscopy, the chlorite films in Chang 8 sandstones of the Yanchang Formation in the Ordos Basin were observed and probed, in order to analyze the forming mechanisms and processes of the films and the relationship between the films and the physical property. The results show that the chlorite films have double layers, referred to as the inside layer film and the outside layer film from the surface of the grain to the pore, respectively. The outside layer film with the isopachous feature was developed after the compaction,whose chlorite crystals precipitated from the pore fluid and densely cover the grain. The primary porosity provides enough space for the outside layer film, which leads to large chlorite crystals with uniform shape and regular accumulation. Owing to bi-directional blocking of the outside layer film, the inside layer film was developed in the small cavity formed by the dissolution of the framework grain. The cavity is narrow which leads to small chlorite crystals of the inside layer with irregular shape and chaotic mélange. With the change of the diagenetic environment, the inside and outside films around the quartz fragment simultaneously grow and cease repeatedly after the first formation of the outer layer, but the growth of the inside layer alternates with the growth of the outside layer around the felspar fragment. The cementation of the chlorite films is a destructive diagenesis process, such that the formation of the chlorite films does not lead to good physical properties in sandstones; on the contrary the good physical properties of sandstones facilitate formation of chlorite films.