Abstract:

The leaching and corrosion of feldspar as framework grain of sandstone is an important role of the formation of secondary porosity in diagenesis. Because the mobility of Al is the lowest in the corrosion of feldspar, Al can be migrated under strong hydrodynamic conditions of fluid. The kaolinite distribution and feldspar corrosion are controlled by migration and enrichment ability of Al. So it has important significance to study the migration of Al. In this paper, the diagenetic characteristics of reservoir in Dongying depression are analyzed through quantity thin section analysis, core observation, scanning electron microscope and X-ray powder diffraction. The feldspar corrosion of sandstone reservoir is general and the characteristics of migration and enrichment of Al are obvious in Dongying depression. We calculate the kaolinite contents through chemical reaction equation of the formation of kaolinite by plagioclase dissolution on well He 140(2,921.3 to 2,925.8 m). The difference between the theoretical calculation and the actual formation of kaolinite content shows that the migration and accumulation characteristics of Al in the relatively open system. Through the study of kaolinite distribution of the sandstone in the Niuzhuang sag, we find the enrichment of kaolinite occurs near the sand body at the interface of sandstone and mudstone. It shows that the Al as feldspar dissolution product has migrated and enriched under certain hydrodynamic conditions. The distribution of kaolinite from feldspar corrosion is controlled by the sandstone-mudstone boundary where the fluid activity is active and reservoir physical property in research area. Taking the formation of kaolinite and quartz from albite and potash feldspar as examples, we have calculated the volume changes before and after diagenesis. The results show that if the Al of the feldspar dissolution does not have a large scale migration, the nearly equal volume of secondary minerals after feldspar dissolution will precipitate and fill the pores. Petrographic criteria of secondary porosity alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement. It only means redistributional secondary porosity, dissolution of framework grains and reservoir heterogeneity. The kaolinite content is not the mark of effective porosity on the sandstone reservoir. The reservoir physical property can be increased if the Al is moved out.

Key words: clastic reservoir, feldspar corrosion, migration and enrichment of Al, secondary pore