Abstract:

The three oil units of the Carboniferous in Tazhong-4 oilfield feature in a hydrocarbon distribution of gas-bearing formation being clipped between oil-bearing formations.The Carboniferous reservoirs in Tazhong-4 oilfield were formed mainly in two stages,the earlier oil-charging stage and the later gas-charging stage.By combining the oil and gas charging times with the analysis of mudstone compressive strength,fluid residual pressure and normal pressure on fault surface in different periods,and the relationship among these three parameters,it is suggested that the fault cutting through the three units were open in the late Hercynian,leading to accumulation of oil and gas in the Unit C_Ⅲ and C_Ⅰ;while in the Himalayan period,fluid residual pressure of Unit C_Ⅲ was greater than the normal pressure on fault surface,resulting in late accumulation of gas in the Unit C_Ⅱ rather than Unit C_Ⅰ.Moreover,the gas charged in C_Ⅱ unit could not migrate upward through the fault into Unit C_Ⅰ due to the large thickness of the upper shale interval.The analysis of the whole process of the pooling in the Carbonife-rous reservoirs in Tazhong-4 oilfield indicates that the western section of the central fault-horst zone is favorable for finding secondary oil pools in Unit C_Ⅰ and C_Ⅱ.

Key words: fault transport system, fault sealing property, genetic mechanism, Tazhong-4 oilfield, Tarim Basin