The origin of authigenic illite in tight sandstones and its effect on the formation of relatively high-quality reservoirs:A case study on sandstones in the 4th member of Xujiahe Formation,western Sichuan Basin

Abstract

The 4th member of Xujiahe Formation in the western Sichuan Basin is composed of typical tight sandstones that contain abundant debris but very scarce feldspar in skeleton grains.Authigenic clay minerals are dominated by illite and kaolinite,wherein authigenic illite can be divided into three principal types: grain-coatings,netted aggregates filled in secondary intragranular dissolved pores and non-netted aggregates filled in intergranular space.The first type transforms from the early-formed smectite through I/S mixed layer,the second type is derived from illitization by K-feldspar during burial metamorphism,and the third type is a product of illitization by K-feldspar and the early-formed kaolinite during deep burial.The formation of authigenic illite has a dual effect on the development of relatively high-quality reservoirs in tight sandstones.On the one hand,it can reduce sandstone permeability and lower the quality of reservoirs,and on the other hand,the grain-coaling illite can preserve the primary porosity by preventing quartz from cementation,while the transformation of smectite to illite and the illitization of K-feldspar and kaolinite during burial diagenesis can accelerate the dissolution of K-feldspar to form secondary pores.Therefore,the formation of authigenic illite generally plays a positive role in the primary porosity preservation and the development of secondary pores of sandstones,and consequently favors the formation of relatively high-quality reservoirs.