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.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019