Characteristics Of Salt Structure In The Lower Congo-Congo Fan Basin And Its Control On Hydrocarbon Accumulation

The Lower Congo-Congo fan basin,which is a typical passive continental margin basin,is located in the central part of the West African coast.It is a combination of the Lower Congo Basin and the huge Congo fan in basin's upper part.The basin is rich in oil and gas reserves,and the salt structures are highly developed,which play an important role in controlling hydrocarbon accumulation above the salt.Based on the data of literature,IHS,research project and C&C database,according to plate tectonics and analysis theory of petroleum-bearing Basin,the 2Dmove software and physical simulation experiment of salt structures were used to identify salt structures,discuss the characteristics of structural evolution and salt activities in the Lower Congo-Congo fan basin,study the controlling effect of salt structural activities on hydrocarbon accumulation above the salt,and finally establish the hydrocarbon accumulation model of the Lower Congo-Congo fan basin above the salt.The results are as follows.(1)The Lower Congo-Congo fan basin experienced three stages of evolution: the rifting period,the transition period and the passive continental margin period.The salt structure has zoning characteristics.From land to sea,it is the salt raft structural belt in the extensional zone,the salt diapir structural belt in the transitional zone,and the thick-layer salt structural zone in the compressional zone,respectively.(2)The salt activities of the basin are divided into three stages.From the Albian to late Cretaceous,the salt rock in the extensional area slid down and formed a series of faults,and the salt rock were partially thickening and thinning in the central transitional zone and the front marginal compression zone.From Paleogene to the late Pliocene,salt rock in the extensional zone underwent large-scale slippage toward the sea,forming a small salt-rolling and salt-welding structure.The graben began to develop on top of the salt diaper,and the puncture diapir was formed in the compressional zone.After the Pliocene,the early salt diapir structure continued to develop,and penetrated the Tertiary strata with a tendency of extending toward the sea.(3)The salt affects the static accumulation factors,which can accelerate the maturity of the source rocks above the salt,but delays the oil-producing window at the thick salt rock.It can change the shape of the reservoir,and form a good combination of reservoir and cap with other caprocks.Salt activity controls the forming and shaping of traps associated with salt structures.The fault system and salt window structures associated with salt activities are important passways for hydrocarbon migration.(4)There are three types of hydrocarbon accumulation above salt in the Lower Congo-Congo fan basin.First,in Cretaceous,the oil and gas generated below the salt in salt raft structural belt migrated to the Upper Cretaceous carbonate reservoir through the salt window and faults in the extensional zone.Second,in Neogene salt raft belt,the oil generated from source rocks above salt migrates vertically and/or laterally to the trap and reservoir nearby through the salt margin fault.Third,in the transition zone and the compression zone,the oil and gas generated from the source rocks above salt migrate to the Tertiary turbid water reservoir through the deep faults.