Genesis of ultra-deep dolostone and controlling factors of large-scale reservoir: A case study of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin

This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs, focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin. The study involves petrology, microscale X-ray diffraction, trace element analysis, and C-O-Sr-Mg isotope experiments to provide a detailed analysis. The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement. The Dengying Formation, which developed under a sedimentary background of a restricted platform, contains special microbial and microcrystalline dolostones. The dolomite grains are small (30 µm), high order degree (Min=0.7), small unit cell parameters, high Na content (Max=432 ppm), and low Fe and Mn content. The δ 26 Mg and δ 13 C values are consistent with the contemporaneous seawater, while the δ 18 O and 87 Sr/ 86 Sr values are higher than those of the contemporaneous seawater. There is mutual coupling between multiple-period varying δ 26 Mg values and sedimentary cycles. The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater. The thickness and scale of the dolost