Oolitic shoal composition and its implication of Feixianguan Formation in Yudongzi Section of Erlangmiao, Jiangyou, Sichuan, China

Four phases of oolitic shoal developed in the inner parasequence set 1, parasequence set 2, parasequence set 3 and parasequence set 4 respectively in the forepart of Feixianguan Formation in Yudongzi section of Erlangmiao, Jiangyou. Parasequence set 1 is composed of parasequence 1 and parasequence 2. The later development at the edge of the stabilized sand flat around area with deep water is the main body of oolitic shoal. It is dominated by ooid-silty-fine limestone and oolitic limestone in which ooids are smaller with the diameter ranging from 0.3 mm to 0.8 mm, moderate-sorted and rounded-subrounded with the increasing of contents ascending from about 20% to 50%-60%. Parasequence set 2 is composed of parasequence 1, parasequence 2 and parasequence 3. The upside of parasequence 1, parasequence 2 and parasequence 3 are all the main body of oolitic shoal developed at the edge of the stabilized sand flat. They are dominated by pisolitic-oolitic limestone and oolitic-pisolitic limestone in which the contents of grains are 40%-65%. Ooids are bigger with the diameter varying from 0.2 mm to 1.8 mm, poorly-sorted and rounded-subrounded. Parasequence set 3 is composed of parasequence 1 and parasequence 2. Both of them are the main body of oolitic shoal and developed at the edge of the stabilized sand flat around area with the mobile fringe. They are dominated by oolitic limestone in which the contents of grains are 40%-70%. Ooids are a little smaller with the diameter ranging from 0.1 mm to 0.8 mm, moderate or poorly-sorted and subrounded. Parasequence set 4 is composed of parasequence 1, parasequence 2, parasequence 3, parasequence 4 and parasequence 5. The last four developed on the mobile fringe are the main body of oolitic shoal. Ooids at the bottom of each parasequence which are dominated by dolomitic-oolitic limestone are small with the diameter ranging from 0.2 mm to 0.8 mm, moderate or poorly-sorted and well-rounded with the contents of 50%-60%, while those on the top dominated by residual oolitic dolostones are much smaller with the diameter ranging from 0.2 mm to 0.5 mm, well-sorted and well-rounded with the contents of over 80%. By the analogs of oolitic shoal and testing of physical properties, it is found that high-quality reservoir is developed in the mobile fringe where the hydrodynamic conditions are stronger. The reservoir in which the contents of grains in the oolitic shoal are more than 55% and the contents of dolomite are more than 60% have better physical properties with porosities of 10.94% and permeabilities of 0.076×10-3 μm2 on average. Thus it can be seen the mobile fringe is the position where potential high-quality reservoirs develop and the forming of them are highly related with their original sediment conditions.