Genesis of differential cementation by authigenic kaolinite in deep-buried tight sandstone reservoirs for the upper Triassic Xujiahe Formation, western Sichuan foreland basin, China

Abstract

The tight sandstone reservoirs of the Xu3 Member in the Depression Zone of the Western Sichuan Foreland Basin (WSFB-DZ) are characterized by the widespread development of kaolinite cement, with notable cementation variations observed between the northern and southern regions of the depression zone. Utilizing an integrated methodology that encompasses cast thin section petrography, SEM, XRD, CL, fluid inclusion homogenization temperatures, in-situ micro-area carbon and oxygen isotopes, and trace element analysis, this study compared the differential cementation traits and genetic mechanisms of authigenic kaolinite within the Xu3 Member tight sandstone reservoirs, distinguishing between the northern and southern regions of WSFB-DZ. The study indicates that the northern Xu3 Member tight sandstone reservoirs are predominantly litharenite. The authigenic kaolinite crystals exhibit well-defined structures, predominantly, manifesting as pseudo-hexagonal platy single crystals and vermicular-to-booklet-like aggregates. These crystals fill predominantly intragranular dissolution pores and intergranular pores, and coexist mainly with quartz cement and carbonate cement. In contrast, the southern region is primarily litharenite and sublitharenite. The crystal morphology of authigenic kaolinite in this area is less well-developed compared to that in the northern region, with rare vermicular aggregates. Additionally, kaolinite cement in this region underwent substantial transformation into illite and chlorite through late diagenetic alteration. The Xu3 Member tight sandstone reservoirs in the WSFB-DZ exhibit two stages of kaolinite cementation, early and late. These cements originated from the dissolution of easily soluble grains such as feldspar and rock fragments by humic acids derived from early coal-bearing strata and organic acids generated during organic matter maturation and decarboxylation in the late stage. The southern Xu3 Member sandstone reservoirs exhibit a relatively high content of K-feldspar grains, and the dissolution of K-feldspar by organic acids released a significant amount of K⁺ ions. Additionally, the late-stage carbonate cementation consumed only a small amount of Fe²⁺ and Mg²⁺ ions, resulting in late diagenetic fluids that were rich in K⁺, Fe²⁺, and Mg²⁺ ions. These geochemical conditions favor the widespread transformation of authigenic kaolinite into illite and chlorite in the southern Xu3 Member sandstone reservoirs. Consequently, the southern sandstones are characterized by a low content of kaolinite cement and a relatively high content of illite and chlorite cement. In contrast, the northern Xu3 Member sandstone reservoirs have a lower content of K-feldspar grains, resulting in reduced K⁺ ions release from organic acid dissolution. Simultaneously, the late-stage carbonate cementation sequestered substantial Fe²⁺ and Mg²⁺ ions, leading to late diagenetic fluids depleted in K⁺, Fe²⁺, and Mg²⁺ ions. These conditions resulted in only partial illitization and chloritization of the kaolinite cement, primarily characterized by a relatively high content of kaolinite cement and a low content of illite and chlorite. The differences in the K-feldspar grains content and the carbonate cementation in the Xu3 Member sandstones, which lead to variations in trace element ion concentrations in the late diagenetic fluids, are the key factors responsible for the differential cementation of authigenic kaolinite in the Xu3 Member sandstones between the northern and southern regions of the WSFB-DZ.