Abstract: An unconformity exists in Yingshan formation of the Lower Ordovician in Tazhong area, in which significant breakthrough for petroleum exploration has been made. This paper restudies the unconformity by means of unconformity method, geophysics, logging response feature, lithology and colour of cavern sand and shale as well as microfossil information, combining with the results of high-accuracy seismic reservoir prediction and evaluation. It suggests that this large karst unconformity is widely distributed in Tazhong slope area, with good stratified and connected features, being as a favorable reservoir. This study elabrates the key event for hydrocarbon accumulation and petroleum resource extent of the Ordovician carbonate reservoir, hence re-understanding such an unconformity, forming its new strategic idea for carbonate reservoir exploration and extending new domain for petroleum prospecting.

Key words: Tarim basin, Lower Ordovician, unconformity, determination