The geological setting of Xia 9 wellblock is a nose structure which uplifted and disappeared toward northeast direction with no structural trap and up dip wedge out trap. Several different scale reservoirs were discovered in the lower part of this nose structure in the Triassic and Jurassic strata. To a same sand body, the lower part of the structure is oil layer; however, the higher part of the structure is water layer. This abnormal phenomenon has been an enigma for a long time. Recently, based on the actual data and research, it is considered that diagenetic shading is the key factor in controlling the distribution of the reservoirs in Xia 9 wellblock, and that the structural break slope is an essential condition for the forming of diagenetic shading. The intensity of diagenetic shading is closely related to physical properties of reservoir. The accumulation scale of this diagenetic trapped reservoir is inversely correlated with the physical properties. The discovery and the understanding of the diagenetic trapped reservoirs in Xia 9 wellblock provide a newtarget for oil and gas exploration.