Abstract: Great much of gas and liquid may be created in the process of incoalation,which can change high valence oxides in the contacted rocks into low valence oxides in the process of migration and produce a lot of authigenic minerals,such as siderite oolite and nodular. At the same time,the red rocks can be changed into grey rocks or purple porphyry.Trace made by above changes may be used for tracing the migration of coal gas and finding out the shallow gas reservoir.