Abstract: The sedimentary facies of the middle member of Permian Jiamuhe formation in Xiaoguai oilfield in northwestern margin of Junggar basin was always considered as alluvial fan or fan delta facies deposit. However, from point of view of petrology, it is definitely different from the standard alluvial fan facies. As early as 1996 it was considered as volcanic eruptive facies, but at that time,“fan control”theory was prevalent, this understanding was not paid more attention to. On the fifteenth anniversary of the reservoir exploration, this paper presents the understandings about it, so as to deepen the recognition of petroleum geology in this area