| On the basis of the analysis on growth-development law of lacustrine reef, applying all available data, this paper gives a comprehensive research on reservoir characterization and forming mechanism of reservoir in Es4 formation of SiKou sag, including oil-source correlation, migration passways, the dynamic procedure of reservoir forming, hydrocarbon accumulation model and enrichment law.Based on sub-phase classification, strata correlation and physical property calculation of reef reservoir, the reef internal structure and reservoir distribution law are illustrated. The conclusions are as follows: the reef has a little size, and is retail distributed; reef and reef flat are chaotic, superimposed and cross connected. The physical property of reef reservoir is well, but it is heterogeneous. Reef distribution is controlled by palaeo -geomorphology, alternation of lake level and contemporaneous fault, the formation of available reservoir is affected by sedimentary faces, dissolution and tectonization.The oil-source correlation demonstrated that reef reservoir oil is mainly from source rocks of Es3x and Es4s, the oil can be divided into 3 types. From the analysis of hydrocarbon migration system, we can know that the main migration pathways are faults and crevices, which combined with reef reservoir form there types of migration systems. From the study on acoustic pressure, migration directions and reservoir injection patterns, the conclusions are as follows: the reservoirs have normal pressure system now and the formation Es3x has a higher abnormal pressure system than the formation Es4s in reservoir forming time; the oil from source rock of Es3x has an upward migration, and the oil from source rock of Es4s has an downward migration, the migration direction is from the center of sag to structural high part in the plane. Based on the accumulation mechanism study, three types of reservoir-forming pattern are established, and the main influencing factors are trap type and structure position.
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