琼东南盆地深水区L18气田上新统地层圈闭气田形成条件及成藏模式

Formation condition and accumulation of Pliocene strata-trapped gas field L18 in the deepwater area of the Qiongdongnan Basin

近期在琼东南盆地超深水区发现了L18气田上新统地层圈闭气田,但在聚气背景、烃源岩、储层沉积成因及天然气输导体系等气田形成条件和成藏模式认识存在争议。通过对该气田形成条件的综合分析,认为上新世轴向古洼槽内地层圈闭、陵水凹陷东洼下渐新统崖城组浅海相烃源岩、上新统限制型重力流砂岩储层和渐新统-中新统断裂垂向沟源通道是形成上新统地层圈闭气田的4个基本条件。中中新世以来盆地中央继承性发育轴向古洼槽和限制型重力流沉积,随着后期地层沉积迁移、差异压实作用,上新统莺歌海组砂岩顶面在轴向洼槽内起伏,并被周边泥岩封盖、封堵,形成了地层圈闭;约3.4 Ma BP,陵水凹陷东洼下渐新统崖城组浅海相烃源岩生成了成熟天然气,沿渐新统-中新统断裂向上运移到上新统莺歌海组重力流沉积砂岩中,再侧向运移至地层圈闭中聚集成藏,具有"烃源岩、圈闭、断裂+砂岩输导层"三要素控藏的上新统地层圈闭成藏模式。

The Pliocene strata-trapped gas field L18 has been discovered recently in the ultra-deepwater area of the Qiongdongnan Basin.There are considerable debates regarding what the accumulation background is and how the source rock developed and the reservoir deposited and what the gas migration pathway and the gas accumulation model are in the gas field L18.Our integrate study reveals that besides developing strata-trap within axial paleo-groove where the natural gas accumulated Pliocene,the formation conditions of the gas field L18 are required to develop the mature marine source rock in the Oligocene Yacheng Formation and the axial confined gravity flow sandstone reservoirs Pliocene,as well as the fault migration pathway in the Oligocene and Miocene in the eastern Lingshui Sag.Our comprehensive research on accumulation model show that the sandstone reservoirs that deposited in the axial confined paleo-groove,and their tops fluctuated due to the later depositional migration and the differential compaction,and are sealed by the adjacent abyssal mudstone deposits,formed the stratigraphic traps of the Pliocene Yinggehai Formation.The mature natural gas has been generated from the marine source rock in the Oligocene Yacheng Formation in the eastern Linshui Sag since 3.4 Ma BP,migrated upward to the sandstone reservoirs in the Yinggehai Formation,then moved laterally into the strata-trapped area and accumulated.The source rock,the valid trap and the migration pathway composing of faults and sandstones together contributed to the natural gas accumulation in the gas field L18.