Abstract: In this paper,the Ordovician weathering crust reservoir in west Lungu region of Tarim Basin is taken as an example.Through the systematic analysis of geological,seismic,logging,and geochemical data derived from more than 20 exploration wells,the carbonate reservoir characteristics and accumulation features of west Lunnan region are studied,and then,the reasons leading to exploration failure are discussed by detailed analysis on the wells.The study shows that in west Lungu region,the Ordovician Yingshan Group is demonstrated as the main oil-bearing formation,in which hydrocarbons are distributed within 180m beneath the weathering crust surface.The reservoir space here mainly includes intergranular pore,intragranular pore,structural fractures and corroded fissures.Hydrocarbons are initially charged and accumulated here as a reservoir in late Hercynian period,then undergone an intense biodegradation due to tectonic uplift,which lead to the characteristics of high density,high viscosity,high-quality asphalt,high wax,high sulfur and low saturated /aromatic of the petroleum.The failure of those wells can be generally summarized as: strong carbonate reservoir heterogeneity,ascertainment of traps and complex distribution of oil-water in the karst fracture-cave system,accuracy of seismic interpretation,etc.The palaeogeomorphology,karst reservoir and fault are critical factors for oil and gas accumulations,high petroleum production usually occurs near discordogenic fault where the fracture and cave are well developed.Therefore,deeper drillings are suggested to be performed in this zone in future.

Key words: carbonate rock, hydrocarbon accumulation, reservoir heterogeneity, failure well, west Lungu region, Tarim Basin