Petrographic and stable isotopic evidences of CO2-induced alterations in sandstones in the Lishui sag, East China Sea Basin, China

Abstract

Dawsonite-bearing sandstones in the Lishui sag, East China Sea Basin, China, have been investigated as a natural occurrence for CO2-induced diagenesis using petrography, X-ray diffraction and stable isotopes. The sandstones are predominantly litharenite, with the order of cements growth comprising clay coatings and pyriteⅠ, siderite, pyriteⅡ, quartz overgrowths, ankerite, kaolinite and microcrystalline quartz, dawsonite and late calcite. Dawsonite, which has recently been considered as trapping mineral after massive CO2 charging, constituted up to 7% of the whole rock volume, occurring as pore-filling clusters or replacement of detrital grains. Stable isotopic data suggest that the CO2 in equilibrium with dawsonite and calcite display a mantle-magmatic origin and is in close proximity to those of the CO2 gas now present in the reservoir, indicating the same origin for both. The elevated CO2 concentrations induced by a mass influx of magmatic CO2 into host rock was favourable for dawsonite cementation. Plagioclase was then considered as the precursor of dawsonite. K-feldspar could also serve as the aluminum source of dawsonite with an external supply of Na+ in a geochemically open system.