The Upper Triassic reservoirs in Sichuan Basin are composed of sandstones.The porosity and permeability of them are very poor

i.e.2-10%(averaging 6.8%)<1md

pore-throat radius is less than 1μm.The voids

observed in rock slice

casting slice and SEM

are mainly secondary solution pores and the intercrys-tal pores of authigenic clays.The development of these secondary pores are related to the diagenesis which is controlled by the component

fabric and sedimentary environment of the sandstones.With poor compressive resistance

strong mechanical compaction

the lithic sadstone and graywacke are considered to be sediments of alluvial fan.The porosity here is less than 2%.Littoral quartzose sandstone is cemented by silica with well-developed quartzovergrowth.The secondary porosity is not developed so well for lack of solu-ble minerals

and the average porosity is only 2%.The original intergranular pores of the feldspathic and lithic quartz osesandstones are vanished owing to mechanical compaction

secondary changes of debris and the infilling of authigenic clay mineral during diagenesis

but the in-tercrystal pores of authigenic illite and kaolinite

and dissolution pores in the feldspars and calcites are well developed.These pores become the main reservoir pore spaces of the sandstones in the Xujiahe Formation.Being deposited as deltaic or fluvial sediments of meandering stream

the porosity and permeability of these sandstones are rather low.