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

Abstract Turbidite deposits, overpressure, hydrothermal fluid activity, and CO 2 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 CO 2 accumulation on reservoir quality. A comparative study between area DFX-1 (CO 2 -rich) and area DFX-2 (CO 2 -poor) has illustrated the differences in diagenetic alteration and the effects of CO 2 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 CO 2 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 CO 2 -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 δ 13 C (−3.3‰ to −1.6‰, PDB), which is consistent with formation from inorganic carbon. The timing of dissolution by inorganic CO 2 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. CO 2 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.