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CO2 Mineralization in Natural Analogues of the Yinggehai Basin, Northern South China Sea

Abstract

To better understand the interaction between hydrogeological and mineral entrapment of CO2, we investigated two natural CO2 accumulations (C-1 and C-2) in the Yinggehai Basin, northwestern South China Sea, using fluid inclusion, XRD, and carbon and oxygen isotopes analyses. The hydrogeological CO2 trapping in the C-1 and C-2 fields was dynamic and static, respectively. Fluid inclusion salinity decreases with rising homogenization temperature up to 170 °C in the C-2 reservoirs, suggesting the dilution of primary formation water by low-salinity hot fluids. While a monotonously positive correlation between fluid salinity and temperature in the C-1 reservoirs shows the absence of salinity dilution after leaking of primary formation water. The C-1 caprock has a smaller amount of chlorite and a larger amount of kaolinite relative to the C-2 caprock, confirming that the dynamic CO2 leakage induced the mineral alteration in the C-1 caprock. CO2 was mineralized into ankerite and calcite in the C-1 and C-2 reservoirs, respectively. A positive correlation between oxygen and carbon isotopes indicates that CO2 was locked into calcite in the C-2 reservoirs at various temperatures before present-day. In contrast, carbon and oxygen isotopes of ankerite in the C-1 reservoirs are almost constant at -2‰ and -10‰ respectively, suggesting that ankerite was primarily precipitated at a reservoir temperature of recent burial depth. The thermodynamic equilibrium caused the recent sluggish precipitation of calcite and residue of chlorite in the C-2 reservoirs. While the far-from-equilibrium during the dynamical hydrogeological trapping induced the recent activity precipitation of ankerite and exhaustion of chlorite and early carbonate cement in the C-1 reservoirs.