The Study On Petroleum Migration Driven By The Free Fluid Dynamic Field In Nanpu Sag

Nanpu Sag is an important oil-producing area in Bohai Bay Basin and has a high degree of exploration and development.However,there are some controversies on the partial oil source,petroleum migration and quantitative analysis in the petroleum migration systes.Based on the comprehensive methods of petroleum geology,petroleum geochemistry and geophysics,this dissertation analyzes the single elements of migration system,including faults and sand bodies of No.1 and No.2 structural belt in the shallow sea area of Nanpu sag,clarifying the controlling mechanism of migration system elements on petroleum accumulation,and finally establishes the petroleum migration and accumulation mode of No.1 and No.2 structural belt in Nanpu sag.The biomarkers of Ed₃ source rocks are characterized by"two low and three high"indexes-low gammacerane index（<0.1）,low sterane content,high ratio of Pristane to Phytane,high tricyclic terpene content,and high C₂₄/C₂₃ tricyclic terpene content.The Es₁ source rock are characterized by"five medium"indexes-medium gammacerane index,medium sterane content,medium ratio of Pristane to Phytane,medium tricyclic terpene content,medium C₂₄ tetracyclic terpene/C₂₃ tricyclic terpene content.The Es₃ source rocks are characterized by"two high and three low"indexes-high gammacerane index,high sterane content,low ratio of Pristane to Phytane,low tricyclic terpene content,and low C₂₄/C₂₃ tricyclic terpene content.The combination of gammacerane index,C₂₄tetracyclic terpene/C₂₃ tricyclic terpene index and C₁₉/C₂₃ tricyclic terpene index can completely distinguish the three sets of source rocks.Through the research on the fault and sand body under the free fluid dynamic filed in the study area,the basic development characteristics of the single element of migration system can be clearly concluded.According to the spatio-temporal matching relationship between faults and source rocks,the oil-source faults can be divided into 3 categories,including the oil-source faults extending to Es₃,the oil-source faults extending to Ed₃to Es₁,and the adjusting faults extending above Ed₃.And then we could recalculate the vertical sealing capacity of the major faults,thus to establish the vertical sealing capacity evaluation standard of faults under different lithologic caprocks,on the basis which we could put forward 3 kinds of fault-caprock sealing modes:the connected type,the limited connected type and the sealed type.Based on the principal component analysis of oil-bearing properties of fault traps,the main controlling factors of oil-bearing properties of fault traps,such as fault displacement,buried depth,dip angle,normal stress,sand-to-strata ratio and SGR,were determined.The oil-bearing properties of fault traps were quantitatively characterized by statistical methods,and the petroleum enrichment index FI in fault block traps could be established.Based on the quantitative characterization of the oil-bearing property of sand body and the relationship between fault-sand configuration,the evaluation index of lateral migration of sand body can be established.It is considered that when the ratio of sand to strata is more than 20%and the thickness of sand body is greater than the"threshold"of the critical oil column height,it is favorable for the lateral migration of sand body.Based on the comprehensive matching analysis of the migration and accumulation elements,a set of typical hydrocarbon migration and accumulation mode which is suitable for the No.1 and No.2 structural belt in the shallow sea area of Nanpu sag is established,which could provide a theoretical basis for thoroughly understanding the controlling of migration system on the petroleum accumulation and better guiding the production and development for Jidong oil field.