The Vertical Heterogeneity Of Oolitic Schoal Of Triassic Feixianguan Formation In Yudongzi Outcrops Of Erlangmiao, Jiangyou

Four phases of oolitic shoal are respectively developed in the inner of parasequence set 1, parasequence set 2, parasequence set 3 and parasequence set 4 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 developing 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 between 0.3 and 0.8mm, well-sorted and commonly-rounded 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 which are all the main body of oolitic shoal develop at 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 beween 0.2mm and 1.8mm, poorly-sorted and well-rounded. Parasequence set 3 is composed of parasequence 1 and parasequence 2. Both of them are the main body of oolitic shoal and develop 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 beween 0.1mm and 0.8mm, poorly-sorted and poorly- rounded. Parasequence set 4 is composed of parasequence 1, parasequence 2, parasequence 3, parasequence 4 and parasequence 5. The four laters which develop on the mobile fringe are the main body of oolitic shoal. Ooids at the bottom of each parasequence which is dominated by dolomitic-oolitic limestone are small with the diameter beween 0.2mm and 0.8mm, poorly-sorted and well-rounded with the contents of 50-60%, while on the top which is dominated by residual oolitic dolostones they are much smaller with the diameter beween 0.2mm and 0.5mm, well-sorted and well-rounded with the contents of over 80%. Viewing the sedimentary evolution of the four phases of oolitic shoal, with the cyclical fluctuations of sea level and the overall decline, the palaeoenvironment of Yudongzi district altered from the edge of the stabilized sand flat around area with deep water to the stabilized sand flat ,then to the edge of the stabilized sand flat around area with the mobile fringe ,and finally to the mobile fringe. That indicated that the oolitic shoal continuously prograded direction to the broad ocean and the distrbition amplitude gradually expanded. By the analogs of oolitic shoal and testing of physical properties, it is found that high-quality reservoir are developed in the 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^-3μm² in 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.