Abstract:

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.