A large number of in-situ volcanic reservoirs have been discovered from the Meso-Cenozoic rift basin group in eastern China.Based on drilling results in combination with geological and geophysical analysis,a case study from the Early Cretaceous Xujiaweizi fault-depression shows that the formation mechanism of in-situ volcanic reservoirs is characterized by“fault-controlled body,body-controlled facies,facies-controlled reservoir,and reservoir-controlled accumulation”.In other words,deep faults control the volcanic eruption type,volcanic body,and gas reservoir distribution;the volcanic body determines the spatial distribution of volcanic facies and volcanic gas reservoir size;the volcanic facies control reservoir physical properties and effective thickness of gas formation;the volcanic reservoir properties control gas reservoir type and gas productivity.The result is useful to guiding the discovery of in-situ volcanic gas reservoirs in faulted basins in both theory and practice. A large number of in-situ volcanic reservoirs have been discovered from the Meso-Cenozoic rift basin group in eastern China. Based on drilling results in combination with geological and geophysical analysis, a case study from the Early Cretaceous Xujiaweizi fault-depression shows that the formation mechanism of in-situ volcanic reservoirs is characterized by “fault-controlled body, body-controlled facies, facies-controlled reservoir, and reservoir-controlled accumulation ”. In other words, deep faults control the volcanic eruption type, volcanic body the volcanic body determines the spatial distribution of volcanic facies and volcanic gas reservoir size; the volcanic facies control reservoir physical properties and effective thickness of gas formation; the volcanic body determines the spatial distribution of volcanic facies and volcanic gas reservoir size; is useful to guiding the discovery of in-situ volcanic gas reservoirs in faulted basins in both theory and practice.