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

.