The Alteration of Reservoir-Cap’s System During CO2 Charging in Huangqiao Region, China

Revealing the alteration mechanism of reservoir-cap rock system during CO₂-rich fluid charging is meaningful to the study of CO₂ geological storage, as well as when CO₂ enhance oil recovery. The study is taking the Permian Longtan reservoir formation and Dalong cap layer of Huangqiao and Jurong region in Lower Yangtze area in China as comparative study objects, in order to understand the differences between presence and absence of CO₂ in the similar geological background. The samples of reservoirs and cap rock in both regions are analysized through petrological and geochemistry method. The authigenic minerals in the reservoirs of Huangqiao region are mainly overgrowth quartz and kaolinite. A small amount of dawsonite is developed in Huangqiao, while undeveloped in Jurong region due to the absent of CO₂. The content of secondary quartz is lower in Jurong than in Huangqiao. The reservoir’s average porosity in Huangqiao is obviously higher than in Jurong, because of the feldspar’s dissolution during CO₂ charging. The cap rocks in the two areas are both black block mudstones. There were micro-cracks developed in the cap rocks of Huangqiao region, in which have been refilled with calcite veins. Carbon isotope data shows that calcite was formed from CO₂-water-rock interaction. The result indicates that CO₂ charging could cause a major dissolution of feldspar in reservoir, and precipitate a typical authigenic mineral assemblage of dawsonite, secondary quartz and kaolinite. The continuous activity of the CO₂-rich fluid leads to re-precipitation of carbonate minerals in cap rock, which is improving its sealing ability.