Reformation effect of atmospheric water on volcanic clastic rocks：a case study in Tamtsag Basin, Mongolia

The volcanic clastic rocks from the Tamtsag Basin were studied. The experiments of atmospheric water and volcanic clastic rock reaction were conducted under different temperatures. It has been concluded that the soluble minerals in volcanic clastic rocks, such as feldspar and calcite, can be dissolved by atmospheric water, and the dissolution intensity will be enhanced with the increasing of temperature. Tuff in volcanic clastic rocks could be dissolved easily and is the main dissolution mineral. The researches of reservoir properties in the Southern Tamtsag Sag showed that within 30 m beneath the unconformity surface of the Tongbomiao Formation, the physical properties of reservoir were improved obviously by the eluviations of atmospheric water. Porosities increased by 5%-14% and the secondary porosities accounted for 88.48%-95.00%. There was no effect away from the scope of 80 m. The eluviations of atmospheric water play the main contribution to the secondary porosity formed in the Tongbomiao Formation of the Southern Tamtsag Sag, and the lowest limit is 30 to 80 m below unconformity surface.