Shallow hydrocarbon accumulation model and migration-accumulation chains in Bonan area,Bohai Sea
Migration is a key factor controlling shallow hydrocarbon accumulation.Based on drilling,core,seismic,laboratory data,and exploration practices,shallow hydrocarbon accumulation characteristics and distribution laws in the Bonan area,Bohai Sea were investigated,and the hydrocarbon accumulation model of Neogene large-scale reservoirs was established.The results show that:①Four large-scale shallow hydrocarbon accumulation models exist in the Neogene strata,including uplift accumulation model with deep far-source migration-accumulation and vertical fault transport,gentle slope accumulation model with deep early-convergence and late adjustment of strongly active faults,steep slope accumulation model with deep multi-source strong drive and strong transport of composite faults,and sag zone accumulation model with strongly drive of"secondary transition"and lateral transport in continuous sand body.②Two types of hydrocarbon migration-accumulation chains are found from source to reservoirs:a."Z"-shaped cascade hydrocarbon migration-accumulation chain in the gentle slope-uplift area,or in other words,for the hydrocarbon generated in the sag zone,some hydrocarbon accumulates in the deep strata of the gentle slope zone through lateral transport in deep delta sand bodies,while other hydrocarbon accumulates in shallow uplift on a large scale through vertical and lateral cascade migration through faults and sand bodies,respectively.b."Inverted C"-shaped gas migration-accumulation chain forms in the steep slope-sag zone,or in other words,the hydrocarbon from the sag zone accumulates in the deep strata of the steep slope zone firstly and forms"transfer station"-type reservoirs,and then it migrates to shallow strata by fault activity adjustment and forms shallow reservoirs in steep slope zone.As migration and accumulation intensify,hydrocarbon continues to migrate laterally along the large area of shallow sand bodies to the high part of the sag zone to form a large-scale reservoir.By summarizing the shallow hydrocarbon migration-accumulation chain in the Bonan area,the hydrocarbon exploration insights along the shallow chains were established,providing effective guidance for shallow hydrocarbon exploration in the Bonan area and other exploration areas in the Bohai Sea.
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