Keywords: 
• Xujiahe Formation /
• mechanism /
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•  /

Abstract: Xujiahe Formation in southern Sichuan Basin deposited in distributary channel of braided river delta plain and is dominated by lithic arenite, lithic quartz arenite. The Upper Triassic tight reservoirs with average porosity of  4.04%  and average permeability of  0.147×103μm2 are in a current depth of 02 673 m (mostly 800 ~ 2 000 m). The causes of low porosity and permeability include four aspects. (1) The relative short transportation distance from provenance to deposition area resulted in large quantities of shale, phyllite and other plastic debris as well as poorly sorted grain pattern. (2) The early mudstone was easily eroded and mixed with sand grains because of the hydropower instability and quickly lateral diversion of braided river during the deposition process. (3) During the burial process, strong deformation of plastic particles caused by mechanical compaction combined with quartz cementation during the afterwards chemical compaction resulted in significant reduction of primary porosity. (4) The deficiency of acid fluid from the source rocks because the long distance from deposition center of basin resulting in weak dissolution after densification of sandstone. Hence, limited amount of secondary pores generated.