Types, genesis, and formation of CO₂-rich reservoirs in the Yangjiangdong Sag, Pearl River Mouth Basin

By studying the composition of the fluid wells, P-T phase diagram, crude oil biomarkers, natural gas components, isotopes and fluid inclusions of high CO₂ oil and gas reservoirs in the Yangjiangdong Sag, the types, genesis, and formation of CO₂-rich reservoirs were analyzed. Results show that the CO₂-rich reservoirs could be divided into three types: i.e., conventional reservoir with CO₂ -dissolved gas, volatile reservoir with high CO₂ dissolved gas, and CO₂ gas reservoirs containing dissolved hydrocarbons. The crude oil in the reservoir is mainly originated from shallow–semi-deep lake facies and semi-deep–deep lake facies in the mature stage. The hydrocarbon gas is crude oil associated, and CO₂ belongs to mantle-derived inorganic gas. The charging period in EP20-A well is 12～10.7 Ma, and that of CO₂ is 5.6～1.7 Ma. Two stages of crude oil charging and two stages of CO₂ charging in EP20-C well were recognized. The first stage of crude oil charging is 14～6 Ma, which is the main charging period; and the second stage is 4～0 Ma. The first stage of CO₂ charging period is 11～10 Ma, and the second is 5 ～1.5 Ma. The relative magnitude of CO₂ charging intensity show that the CO₂ gas reservoir containing dissolved hydrocarbon was formed in well EP20-C, volatile reservoir containing CO₂ dissolved gas was formed in well EP20-A, and conventional reservoir containing CO₂ dissolved gas was formed in well EP20-B.