Abstract: Calcarenaceous sandstone is a type of sandstone in terrigenous clastic rocks in terms of genesis.The diagenesis in the burial process,reservoir space and hydrocarbon production capacity of calcarenaceous sandstones are determined to be quite special due to its uncommon compositions.Given the observation of the core,common thin sections,casting thin sections,cathode luminescent thin sections,scanning electron photomicrograph,as well as X-ray diffraction data,mercury intrusion data,and physical properties,we made a systematic study on the pore types,as well as quantitative evolution and validity of pores in the calcarenaceous sandstones of the 3rd member of Xujiahe Formation (T₃x₃) in western Yuanba area,Northeastern Sichuan Basin.The results show that there are fewer primary pores in these sandstones,and the secon-dary pores are mainly categorized into three types:intercrystalline micropores,dissolved pores and fractured pores.They are different in terms of genetic mechanism-the formation of intercrystalline micropores are mainly related to the recrystallization of calcareous debris and cements,and some intercrystalline micropores of calcareous debris are a result of the dolomitization of host rocks; while there are two causes for fractures:the intergranular fractures were developed out of unsynchronized recrystallization of calcareous debris and cements,and the structural fractures were formed due to the intense tectonic movements during the deposition of the T₃x₃.In addition,the dissolved pores were less developed and just formed partial "sweet spots",unable to improve the reservoir quality as a whole; while the well-developed intercrystalline pores,most of which are effective,serve as major contributors to the reservoir space of gases in calcarenaceous sandstones,combined with the communication provided by microfractures.Areas with high contents of calcareous debris and calcareous cements (such as high-energy distributary channels) are the high-quality reservoir zones in the T₃x₃,and could be the key areas for exploration and development,since strong recrystallization functions to produce a large number of intercrystalline micropores and the connection provided by fractures is also effective.

Key words: type of pores, quantitative evolution of pore, validity of pore, intercrystalline micropore, calcarenaceous sandstone, the 3rd member of Xujiahe Formation, Yuanba area, Northeastern Sichuan Basin