Mechanisms of quartz and dolomite vein formation in the Ediacaran carbonate reservoirs, Sichuan Basin, SW China: Implications for petroleum origin and accumulation

Sichuan Basin experienced repeated burial and tectonic uplift events, as well as the influence of the Emeishan Large Igneous Province (ELIP), driving multiple fluid flow, hydrocarbon charging, and phase transformations in its ancient carbonate reservoirs. This study combines petrological observations, elemental and silicon-oxygen isotope geochemistry, dolomite U-Pb dating, and fluid inclusion analysis to investigate the formation of dolomite and quartz veins in the Ediacaran Dengying carbonate reservoirs. The results reveal fluid activities closely associated with petroleum migration and accumulation. Dolomite veins formed earlier than quartz veins and solid bitumen in the fractures, with U-Pb dating indicating an age of 328 ± 20 Ma. The rare earth element and yttrium (REY) patterns of the dolomite veins match those of typical marine carbonates. The dolomite- forming fluids originated from diagenetic fluids in the surrounding rocks during the Caledonian tectonic uplift. The quartz veins show relatively low and variable trace element contents. Together with the silicon-oxygen isotope data, the homogenization temperatures (141-178 °C) of primary aqueous inclusions, and the positive Eu anomaly in the quartz REY patterns, these features indicate that the quartz precipitated from low-temperature hydrothermal fluids. Bitumen-bearing inclusions, mainly trapped in quartz veins, confirm that oil charging and quartz precipitation occurred simultaneously in the fractures. Calculated oxygen isotope values (5.1 to 12.9‰) suggest quartz- forming fluids were unaffected by meteoric water, indicating the oil originated from the Lower Cambrian shales. The quartz-forming fluid originated from siliceous diagenetic fluids released from the Lower Cambrian shales and was subsequently mixed with deep silica-rich fluids. The minimum homogenization temperature of primary inclusions constrains that oil charge and quartz vein formation occurred at ca. 263 Ma, which coincides with the onset of ELIP eruption. Under the impact of ELIP, the Lower Cambrian shales matured rapidly, with the generated oil and silicon-rich fluid migrating to the Ediacaran Dengying reservoirs. This study demonstrates the spatial and temporal coupling of fluid activity, hydrocarbon generation and accumulation, and ELIP eruption, providing crucial geological evidence for understanding the petroleum system within petroliferous basins.