Supergene karstification in the Sinian Dengying Formation, Sichuan Basin

The Sinian Dengying Formation strata in the Sichuan Basin are interpreted to be the oldest marine carbonate strata and gas reservoirs in China. In the last geologic periods, the Dengying Formation underwent multiple phases of karstification, of which the supergene karstification have played an important part in the formation of the excellent reservoir rocks in the Formation. Therefore, the emphasis in the present paper is placed on the supergene karstification in the Sinian Dengying Formation on the basis of macroscopic and microscopic features, and multi-parameter experimental analysis of samples from the field outcrops and drilled cores in the Sichuan Basin. The supergene karstification occurs dominantly in three horizons, including the topmost parts of the 4th member and the 2nd member, and lower and middle parts of the 2nd member of the Dengying Formation. The tectonic movements during the Tongwan I and II episodes are believed to be the main controlling factors for the supergene karstification in the topmost parts of the 4th member and 2nd member of the Dengying Formation. The supergene karstification in the lower and middle parts of the 2nd member of the Dengying Formation is mostly constrained by the large-scale sea level changes caused by climatic variations..