Hydrocarbon Charging and Accumulation Process of the Large Bozhong19-6 Condensate Gas Reservoirs in the Southwestern Bozhong Sub-Basin, Bohai Bay Basin, China

The Bozhong19-6 (BZ19-6) condensate gas reservoirs, located in the southwestern Bozhong sub-basin, Bohai Bay Basin, China, were paleo-oil reservoirs in the geological past and have undergone at least three successive hydrocarbon charging events. The hydrocarbon migration and accumulation process of “early oil and late gas” has occurred in the current reservoirs. At the end of the sedimentation of the Guantao Formation (N 1 g , ∼12 Ma), the reservoirs began to fill with first stage low-moderate mature crude oil. At the late stage of the Lower Minghuazhen Formation (N 1 m l ) (∼6.7 Ma), the reservoirs were largely charged with second stage high mature crude oil. Since the deposition of the upper Minghuazhen Formation (N 2 m u , ∼5.1 Ma), the paleo-oil reservoirs were transformed into shallow Neogene reservoirs due to the reactivation of basement faults. From the late stage of the N 2 m u to the present day (∼2.8–0 Ma), the reservoirs were rapidly filled by natural gas within a short period. In addition, analysis of the formation of the reservoir bitumen and the conspicuous loss of the lower molecular weight n -alkanes in the crude oil reveal that the injection of a large amount of gas in the late stage caused gas flushing of the early charged oil.