Elements Controlling Hydrocarbon Accumulation In The Passive Marginal Basins In West Africa

Salt-bearing basins are one of the basins that are most enriched oil and gas resources in the world.Currently,salt-bearing basins control 89% of the world's proven oil reserves and 80% of natural gas reserves.Most important passive continental margin basins in West Africa have a thick layer of evaporite deposited in the Late Cretaceous Aptian period.They are typical salt-bearing basin on the east coast of the South Atlantic Ocean and the hot spot for hydrocarbon exploration.Guided by the relevant theories of sedimentology,petroleum geology and oil and gas accumulation.Seismic profile and well logging data are used to analyze typical oil and gas reservoirs,by seismic interpretation,seismic facies,sedimentary facies and analysis.Mainly obtain the following knowledge:The analysis results of oil-fields indicate that the study area can be divided into“pre-salt generation and pre-salt reservoir” ? “pre-salt generation and post-salt reservoir”?“post-salt generation and post-salt reservoir” based on the relationship between the source-reservoir cover configuration and evaporite.There are three types of reservoir-forming combinations Elements that control reservoir-forming are: the structural pattern of the rift period,Albian reservoir and post-salt source rock.The uplift zone inside the subsalt rift controls the distribution of “pre-salt generation and pre-salt reservoir” fields by affecting the development of lacustrine carbonate reservoirs.Through the interpretation of seismic data combined with the analysis of gravity and magnetic data,the structural pattern of the subsalt rift period as "three depressions and two uplifts" was clarified.The Albian period shallow sea carbonate reservoirs are divided into five tertiary sequences.Among them,the oolitic beach,sandy shallow beach and mixed beach in SQ1 and SQ2 have good porosity and permeability.“pre-salt generation and post-salt reservoir” fields are mainly oil and gas enriched facies belts,and high-quality reservoirs are distributed in “stripes” along the shallow sea area“post-salt generation and post-salt reservoir” fields are distributed around mature source rocks on the salt.The interpretation of seismic data and thermal simulation methods are used to divide the source rocks on the salt into three types of seismic facies.The central part has the largest thickness and the highest maturity,and gradually decreases toward the north and south sides.Comprehensive analysis of the development rules of the main reservoir control elements and prediction of favorable exploration areas.The results show that the distribution area around the uplift zone and the low uplift zone in the subsalt rift basin is a favorable exploration area,while the shallow sea carbonate development zone and source hydrocarbon generation zone distributed along the 200 m isobath on the salt is a favorable exploration area.