Abstract: Well Zhong-2 is the first exploration well drilled into the upper Ordovician Lianglitage formation on the southern slope of Tazhong area. The establishment of reef-flat combination sequence and the analysis on geological structure of platform margin on the southern slope of Tazhong area based on the data of core, thin section and 3D seismic are of great significance to understand geological features in the area and to guide next exploration. There are 7 medium-high-energy sedimentary cycles of grain shoal, reef knoll, micrite mound and thin interbedded interbanks in Lianglitage formation of Well Zhong-2 vertically with high overall sedimentary energy. 4 typical reef-flat facies combinations are developed in the formation including combination A (grain shoal→micrite mound→reef knoll→grain shoal), combination B (interbank→grain shoal→micrite mound→grain shoal), combination C (interbank→grain shoal→reef knoll→grain shoal) and combination D (interbank→grain shoal), and combinations of B and D are dominant. The geological structure of platform margin on the southern slope of Tazhong area is a typical platform margin of sedimentary steep-slope type which is similar to that of the eastern section of the northern slope and has a broader marginal belt with the width of 4~6 km. The reef flat facies is widely distributed in the inner side of the platform marginal belt. Well Zhong-2 is located at the transitional position from platform margin facies to foreslope facies. The size of a single reef/mount is relatively small and serious crush occurs to reef-building organism in the reef front or the area adjacent to the slope of the platform margin. The main part of reef flat facies is located in the area next to the inner side of the platform marginal belt in the well. It is suggested that the later exploration should revolve around these zones