源下油气成藏主控因素与成藏模式——以北部湾盆地乌石凹陷乌石17X下始新统流沙港组三段油藏为例

Main Controlling Factor of Hydrocarbon Accumulation under Source Rock and Pool-forming Pattern——a Case Study of the Field of Early Eocene Third Member of Liushagang Formation in Wushi Sag,Beibu Gulf Basin

北部湾盆地乌石凹陷下始新统流沙港组三段发育源下断块油藏,其原油来自上覆流沙港组二段超压成熟烃源岩,其成藏受"源储压差"、沉积相和有效断块圈闭三因素控制。"源储压差"控制了油气运移方向,油气由流沙港组二段源内超压区向下部、侧向流沙港组三段常压区运移,在断裂圈闭带中聚集。流沙港组二段超压还起到了封盖油气作用。流沙港组三段近岸水下扇扇中辫状水道微相储层物性较好,是"甜点"储层。下始新统流沙港组三段具有"高压源、源封盖、源储接触"的源下油气成藏模式。乌石凹陷东部流沙港组三段始新世裂陷幕断裂圈闭带是"源下"油气富集的区带。

Fault block pools distributed under source rock were discovered in the early Eocene third member of Liushagang formation in Wushi sag,Beibu Gulf basin,and oil derived from overlay overpressured and matured second member of Liushagang formation.Main controlling factor of hydrocarbon accumulation were pore fluid pressure difference between source rock and reservoir,sedimentary micro-facies and valid trap.Pore fluid pressure difference between source rock and reservoir controlled hydrocarbon migration direction,and made hydrocarbon migrated from second member of Liushagang formation downward to third member of Liushagang formation,then accumulated in the fault block trap belt.Otherwise,overpressured second member of Liushagang formation played sealing role for pool of third member of Liushagang formation.Braided channel conglomerate sandstone reservoir of middle part of subacqueous fan had good reservoir property,which was sweet spot reservoir.Pooling model of third member of Liushagang formation was characterized by overpressured source rock made hydrocarbon migrated downward,meanwhile acted as sealing rock,and source rock contacted with reservoir.Fault-related trap belts during Eocene rift episode were favorable hydrocarbon accumulation trend in the eastern part of Wushi sag.