Genesis and Hydrocarbon Significance of Vesicular Ignimbrite: A Case Study from Fengcheng Formation, Wu-Xia Area, Junggar Basin, NW China
摘要
On the basis of a large number of cores and thin-sections observations, the petrologic and pore evolution characteristics of ignimbrite in the Fengcheng Formation in the Wu-Xia area, Junggar Basin, are studied, and the genesis of vesicular ignimbrite and formation mechanism of vesicles are analyzed. Volcanic action in the Fengcheng Formation is characterized by welded pyroclastic rocks. There are predominant plastic shards with rounded edges and obvious devitrification in the rocks. Plastic debris is deformed and wrinkled, and crystal fragments are usually corroded. A lot of lithophysae occur in the ignimbrite, and welded texture is obscured. Vesicles are the cavities of lithophysae, which are formed by solidification contraction. They have little or no late filling cement because the volatile constituent and water vapour in the bubbles have escaped with the help of devitrification. Lithophysae distribution within a cooling unit obviously has the characteristic of zonation in vertical: increasing upwards and then decreasing gradually. The cavities of lithophysae greatly improve the reservoir capability of ignimbrite. Influenced by intensive welded process, lithophysae are not subject to collapse or damage. In the study area, deeply buried vesicular ignimbrite is an important hydrocarbon reservoir rock, and the vesicular ignimbrite has good petroliferous property.