A comparison of hydrothermal events and petroleum migration between Ediacaran and lower Cambrian carbonates, Central Sichuan Basin

Hydrothermal minerals and associated organic matter have been frequently found in the Ediacaran and lower Cambrian carbonate reservoirs, separated by internal shale source rocks, in the Central Sichuan Basin. This work provides a thorough comparison of petrography, geochemical signatures, fluid inclusions and ages between the hydrothermal minerals in the Ediacaran Dengying Formation (Fm) and the lower Cambrian Longwangmiao Formation. This comparison leads to a better understanding of the behaviour and migration history of hydrothermal fluids and petroleum. During the Ediacaran–Cambrian transition, the flux of silicon-rich hydrothermal fluid triggered by seafloor-related volcanic events, resulted in the exclusive occurrence of silicification in the Dengying Fm. During P3–T3, two different hydrothermal systems were found in the Dengying and Longwangmiao formations. The hydrothermal fluids in the Dengying reservoirs were dominated by a mixture of basement-derived Mississippi Valley Type [MVT] ore fluids and shale-derived oil-bearing brines, while those in the Longwangmiao Fm were mainly the oil-bearing brines. The upwards migration of the MVT ore fluids, characterized by very hot, saline, radioactive and Eu-rich conditions, was promoted by the activity of the Emeishan mantle plume 259.4–248 Ma and then blocked by the lower Cambrian shales and trapped only within the underlying Dengying reservoirs. The expulsion of the warm, less saline and moderately radioactive brines and oil was induced by thermal maturation of the organic-rich shales, which might have occurred over a longer duration (261–218.7 Ma). During 170.7–125.8 Ma (J2–K2), the combined effects of the overmature source rocks and Yanshanian tectonic compression facilitated the migration of much hotter shale-derived brines, as well as gas hydrocarbons, across the Ediacaran to lower Cambrian carbonate reservoirs. This comparative study provides some insights into the behaviour of hydrothermal fluids and petroleum in carbonates with internal organic-rich shales.