Abstract: Lunnan area is one of the most significant targets in Tarim in that of oil and gas were abundantly accumulated. The mudstone of Carboniferous,serving as the regional caprock of both carboniferous sandstone reservoir and ordovician limestone fractured reservoir,Owns strong capillarily sealing ability. Morever, the formation period of this capillarily sealing bed well matched for that in which oil and gas were signifccantly generated in the sowrce rock of mid-upper Ordovician in Manjiaer depression. therefore, the mudstonecaprock of carboniferous is favorable to accumulating the oil and gas in Carboniferous and Ordovician to formpools.