DIAGENESIS OF UPPER TRIASSIC RESERVOIR DETRITAL ROCKS IN WEST AND NORTHWEST OF SICHUAN BASIN

The upper Triassic includes the Xujiahe Formation and the Xiangxi Group in the west and north-west of Sichuan Basin.The main types of sedimentary facies are fluvial-dominated deltas,embayment,lake and alluvial fans.The upper Triassic reservoir rocks consist of feldspathic litharenite,lithic arkose,litharenite,subarkose and sublitharenite.Clastic grains,which contain abundant feldspar and rock fragments,make up 81% to 95% of rocks by volume.Sandstones are characterized by low composition maturity and high texture maturity.Primary porosity lost 45.0% to 77.5% due to compaction effect.Cementation,including precipitation of authigenic quartz,feldspar,clay minerals and carbonate minerals,is the second process contributing to porosity reduction.Influenced by organic materials,the δ¹³C values for carbonate cements,which range from-4.85‰ to 2.91‰(PDB),are low.Because of leaching with meteoric water and deep burial depth,the δ¹⁸O values for carbonate cements,which range from-16.97‰ to-7.86‰(PDB),are also low.Dissolution effect,which is mainly the leaching of feldspar,is the most important process for reservoir quality improvement.Clay minerals are mainly illite,and smectite is absent.Vitrinite reflectance ranges from 0.81% to 2.48%,indicating 85-160 ℃ for most ancient geotemperatures.Diagenesis proceeds to middle-late stage.Fractures have improved reservoir characteristics to a great extent.Because the influence of compaction and cementation is strong,and the influence of dissolution is weak,reservoir rocks are characterized with low porosity and permeability.