Sedimentary characteristics of the third member of Xujiahe Formation in Yuanba area

High oil yield was obtained from the 3th member of Xujiahe Formation of YL7 and YL10 wells in Yuanba area,which subverts the traditional understanding that the 3th member of Xujiahe Formation( T3X3) is hydrocarbon generation rock and the calcarenaceous sandstone is non-reservoir rock. The sedimentary facies of T3X3is studied deeply according to geologic,logging and seismic data. The results show that,①there is sandstone and mudstone interbeds of unequal thickness in the western Yuanba area,while in the eastern Yuanba area is overlaying layer of sandstone and mudstone,the sandstone at the bottom and the mudstone on the top. In vertical direction,five sand groups overlap each other. ②In the sedimentary period of T3X3,the northern segment of Longmen mountain and Michang- Daba mountain provide sedimentary source at the same time. Due to the different sedimentary source,there are two kinds of sandstone in Yuanba area,calcarenaceous sandstone deposits mainly in the west,while lithic quartz sandstone deposits mainly in the east. ③ in the west of T3X3develops a braided river delta sedimentary system,which is characterized by the acceleration sedimentary sequence from delta front-delta plain to basin in vertical direction. In the east of T3X3,develops a meaning river delta sedimentary system,which is characterized by the retrogradation sedimentary sequence from delta front to basin edge.