Research paperDiagenesis and reservoir quality of overpressured deep-water sandstone following inorganic carbon dioxide accumulation: Upper Miocene Huangliu Formation, Yinggehai Basin, South China Sea

- The late overpressure made a little contribution to porosity preservation in sandstone.
- Ankerite and spherosiderite in CO2-rich area result from reactions between CO2-water-rock.
- CO2-rich hydrothermal fluid flows along faults lead to a significant amount of secondary pore formation.

Turbidite deposits, overpressure, hydrothermal fluid activity, and CO2 accumulations are the basic geologic characteristics of the Upper Miocene Huangliu Formation sandstone reservoir in the Dongfang gas field, Yinggehai Basin. This study aims to unravel the timing and origin of dissolution in sandstone and evaluate the effects of inorganic CO2 accumulation on reservoir quality. A comparative study between area DFX-1 (CO2-rich) and area DFX-2 (CO2-poor) has illustrated the differences in diagenetic alteration and the effects of CO2 accumulation on the sandstone reservoir. Diagenetic alteration of sandstones in area DFX-1 was characterised by intense dissolution and medium cementation. Dissolution of silicates following inorganic CO2 charging in the DFX-1 area created a higher percentage of dissolution porosity (41.0%-53.9% of total visible porosity), compared with minor dissolution porosity in the DFX-2 area (16.7%-29.6% of total visible porosity). Small amounts of kaolinite, authigenic quartz, ankerite, and siderite were precipitated by CO2-water-rock reactions in area DFX-1; these minerals are almost absent in area DFX-2. The carbonate cement of the Huangliu Formation sandstone in area DFX-1 is characterised by light δ13C (−3.3‰ to −1.6‰, PDB), which is consistent with formation from inorganic carbon. The timing of dissolution by inorganic CO2 is suggested to be at about 0.4 Ma based on the previous simulation experiments. In the two areas studied, overpressure was caused by pressure transmission along deep open faults and was not related to mudstone undercompaction. Overpressure did a little to contribute to porosity preservation. CO2 accumulating at low partial pressures related to hydrothermal fluid flow cannot make a net increase in porosity in the Huangliu Formation sandstone of the Dongfang gas field.