Abstract: The evolution of Caohu sag in northern Tarim Basin is discussed in this paper based on geological interpretation of two-dimensional seismic profiles and analysis of drilling data.We present new ideas about the evolution of Caohu sag since Paleozoic.The time and mechanism of formation of the eastern and western slopes of Caohu sag are apparently different.In the western side of the sag,the Middle-Upper Ordovician show distinct overlapping characteristics from east to west,indicating that the western slope of Caohu sag was developed mainly in the Middle Caledonian as the result of the subsiding of Caohu area and the uplifting of Akekule Uplift under the continuous regional compression.The western slope has the characteristics of depositional sag slope.While in the eastern side of the sag,the Silurian-Ordovician were strongly uplifted and eroded and underlie unconformably the Upper Devonian-Carboniferous,which in turn underlie unconformably the Triassic.This demonstrates that the eastern slope was developed in the Late Caledonian as well as the Early and Late Hercynian as the result of the intense tectonic deformation,with the Late Hercynian being the dominant.The eastern slope shows the characteristics of structural sag slope.The western side of Caohu sag is favorable for the development of Late Ordovician karst stratigraphic traps,while the eastern side is favorable for the development of overlapped lithologic traps in the Silurian,Carboniferous and Triassic,unconformity stratigraphic traps in the Upper-Middle Ordovician,as well as the karst stratigraphic traps in the Lower Ordovician.They are the major potential exploration targets in the future.

Key words: depositional sag slope, structural sag slope, petroleum trap, Paleozoic, Caohu sag, Tarim Basin