By means of the datums, such as ordinary thin slice and cast thin slice of the core hole and the data of scanning electronic microscope, the main diagenesis affecting the clastic rock reservoir in Xiaermen oilfield are compaction, cementation and denudation. According to the micro-pore structure characteristics, combined with the distribution of sedimentary facies, the authors believe that, the favorable reservoir of Xiaermen oilfield develops in the underwater distributary channel and fore delta sand bar of braided delta front sub-facies. These new understandings provide a geological guide for adjustment and tapping potential of the oilfield.