深水区——南海北部大陆边缘盆地油气勘探新领域

南海北部大陆边缘经历了前裂谷期、裂谷期、热沉降期和新构造期4大构造演化阶段,形成了多个新生代盆地。在深水区(水深大于300m)裂谷期(古近纪)伸展断裂活动强烈,形成凸凹相间的构造格局,凹陷中沉积了厚度为3 000-6 000m的古近系;热沉降期(早—中中新世)发生了区域性拗陷作用,形成了隆坳相间格局,沉积厚度小于2 000m;新构造期(晚中新世—第四纪)沉降幅度加大,并沉积了巨厚的地层,断裂活动东强西弱。上述3期构造演化均形成了沉积厚度很大的叠加伸展盆地。深水区生烃凹陷是裂谷期的断陷,已经证实的主力烃源岩是渐新统下部中深湖相泥岩、海陆过渡相泥岩、海相泥岩与煤系地层,可能的烃源岩是始新统中深湖相泥岩,潜在的烃源岩是渐新统上部和中新统海相泥岩,生、排烃产物主要是天然气。深水区至少有3套储层,即渐新统海陆过渡相砂岩、新近系海相砂岩及生物礁、始新统陆相砂岩。盖层发育广泛,区域性盖层是新近系海相泥岩和渐新统泥岩。圈闭数量较多,其类型主要有披覆背斜、断层圈闭和深水扇体等。油气运移方式随时间和空间的不同变化很大,受新构造期断裂活动控制,琼东南盆地深水区以侧向运移为主;白云凹陷早期以侧向运移为主,晚期以垂向运移或断坡-断坪运移为主。深水区成藏单元有凸起、凹中次凸、断裂带和深水扇等。深水区油气勘探的策略是优先勘探凹陷内部及其周边正向构造上的构造圈闭,然后逐渐向凹陷内的非构造圈闭拓展。

Deepwater area-the new prospecting targets of northern continental margin of South China Sea

The deepwater area with over 300 meters in water depth in the northern continental margin of South China Sea experienced pre-rift phase,rift phase,post-rift phase and Neotectonic phase.In this area,a bump-sag pattern was formed.The Paleogene system with 3~6 kilometers in thickness was deposited in the sags during the rift phase.The thickness of Lower-Middle Miocene deposited during postrift phase was less than 2000 meters.The late Miocene-Quaternary system with a tremendous thickness was formed and also reformed in east part during Neotectonic phase.The potential hydrocarbon sags in deepwater area are the faulted depressions formed in the rift phase.The proven source rocks are the early Oligocene mid-deep lacustrine mudstone,transitional mudstone,marine mudstone and coal system.The probable source rock is the Eocene mid-deep lacustrine mudstone,and the possible one is the upper Oligocene and Miocene marine mudstone.There are at least three reservoir cases in deepwater,including Oligocene transitional sandstones,Neogene-Quaternary marine deepwater sandstone and reef,and Eocene terrestrial sandstone.The seal rocks are developed widely,and the Neocene marine mudstone is the regional seal.There are a lot of traps with the main styles of draping anticline,fault trap and deepwater fan.The oil-gas migration style varies with time and space and is controlled by the Neotectonic fault activities.The parallel migration dominates in the deepwater of Qiongdongnan Basin and in the early phase of Baiyun Depression,and the vertical migration dominates in the late phase of Baiyun Depression.The deepwater play and traps contain bulge,secondary bulge in sag,fault zone and deepwater fan.Exploration activity should first aim at the structural traps in the sag and in the forward direction of structure around the sag and then gradually develop toward the non-structural trap.