南海北部超深水区双峰盆地构造演化与油气地质条件

Tectonic evolution and hydrocarbon geological conditions of Shuangfeng Basin in the ultra-deep water area of the northern South China Sea

近年来南海超深水区的油气勘探随着陵水18气田的发现而日益受到关注。双峰盆地位于南海北部超深水区、珠江口盆地南部隆起以南。盆地基底主体为洋壳,分3个次级坳陷。双峰盆地内沉积地层以新近系为主;古近系发育局限,其下渐新统地层不均匀分布于双峰盆地周缘。南海边缘海构造旋回中的新南海扩张作用是双峰盆地结构及沉积充填的主要成因。双峰盆地是在新南海洋壳上形成的沉积盆地,作为新南海扩张的西北端,双峰盆地的演化分为前洋壳期、洋壳形成期、洋壳拗陷期3个阶段。早渐新世前洋壳期,双峰盆地尚未形成,古南海北部大陆边缘整体受陆缘断拗作用控制;从32 Ma开始受新南海扩张作用双峰盆地进入洋壳形成期,盆地由东向西呈"剪刀式"扩张并初步形成;23.8 Ma时期,西北次海盆受到西南次海盆扩张和中西沙稳定地块的限制停止扩张,双峰盆地进入洋壳拗陷期,稳定沉积大套下中新统—第四系海相地层,期间在16.5 Ma新南海扩张整体停止。双峰盆地具有良好的油气地质条件,一方面洋壳扩张期在渐新统发育的陆源海相烃源岩指标较好;另一方面在洋壳拗陷期盆地西缘发育的双峰深水扇带来乐东扇—中央峡谷水道优良的砂岩储集体,加之盆地良好的构造与岩性-地层圈闭条件、厚层海相泥岩盖层以及高地温梯度,有利于油气的成藏,双峰盆地以双西南坳陷为代表是南海超深水领域有利的油气勘探远景区。

Recently,oil and gas exploration in the ultra-deep water area of the South China Sea（SCS）has been increasingly concerned since the discovery of Lingshui 18 gas field.Shuangfeng Basin is located in the ultra-deep water area of the northern SCS,south to the southern uplift of the Pearl River Mouth Basin.The basin basement is mainly constituted by oceanic crust,which is divided into 3 secondary sags.Shuangfeng Basin is dominated by Neogene sedimentary strata,where Paleogene strata are limitedly developed,and the lower Oligocene strata are nonuniformly distributed on the periphery of Shuangfeng Basin.New SCS expansion in the tectonic cycle of SCS marginal sea is the main cause of structure and sedimentary filling in Shuangfeng Basin.This basin is a sedimentary basin formed on the new SCS oceanic crust.As the northwestern end of New SCS expansion,the evolution of Shuangfeng Basin is divided into 3 stages,i.e.,pre-formation stage of crust stage,formation stage of oceanic crust and downwarping stage of oceanic crust.During the early Oligocene pre-formation stage of crust stage,Shuangfeng Basin was not yet formed;the northern continental margin of the paleo-SCS was wholly controlled by the marginal fault-depression effect.Due to the New SCS expansion since 32 Ma,Shuangfeng Basin entered into the formation stage of oceanic crust,and it expanded from east to west in a scissor-like shape and was initially formed.In23.8 Ma,due to the expansion of the southwestern sub basin and restriction of stable blocks in Zhongsha and Xisha islands,the northwestern sub basin ceased expansion.Then Shuangfeng Basin entered into the downwarping stage of oceanic crust,and large sets of lower Miocene-Quaternary marine strata were steadily deposited.New SCS expansion had entirely ceased in 16.5 Ma.Shuangfeng Basin has good hydrocarbon geological condition.On the one hand,there are good indicators for the terrigenous marine source rocks developed in Oligocene during the period of ocean-ic crust expansion;on the other hand,excellent sandstone reservoirs are formed in Ledong fan and central canyon channel due to the development of Shuangfeng deep-water fan developed on the west margin of the basin during the depression stage of oceanic crust,which favorable for oil and gas accumulation in combination with favorable structural and lithologic-stratigraphic trap conditions,thick marine mudstone caprock and high geothermal gradient.Represented by Shuangxinan sag,Shungfeng Basin is a favorable petroleum exploration prospect area in the ultra-deep water area of the SCS.