南大西洋深水盐下碳酸盐岩沉积特征、模式及油气成藏

Sedimentary characteristics,model and hydrocarbon accumulation of deep water pre-salt carbonate rocks in South Atlantic

近年来,南大西洋深水盐下碳酸盐岩陆续取得重大发现。根据已发现的深水油气田特征,结合该地区地质、地球物理资料,研究认为:南大西洋深水盐下碳酸盐岩形成背景是南美与非洲板块白垩纪差异裂解,沉积环境为障壁条件局限湖泊沉积,岩性主要为灰岩、藻灰岩,多位于构造高部位和斜坡地带。其沉积过程主要是板间差异裂解,受南部瓦尔维斯火山脊阻挡作用,水体逐渐变浅,局部高地及斜坡地区形成碳酸盐岩,当周期性灌入的海水不足以抵消古盐盆蒸发量时,盐岩开始析出,持续欠补偿发育的古盐盆形成巨厚的盐岩沉积。该套碳酸盐岩储层受盐岩控制作用明显,其油源来自下部裂谷期湖相泥岩,油气主要是经断裂、不整合短距离运移,在构造高部位且连续性盐岩发育区形成有效储层,盖层为上覆盐岩,以构造和地层圈闭为主。

Recently, there have been great discoveries in the deep water pre-salt carbonate rocks in South Atlantic. According to the characteristics of discovered deep water oil and gas fields, regional geological and geophysical data, the South Atlantic deep water pre-salt carbonate formation, which mainly locates in the high structural areas and slope zones, belongs to limited lake sedimentation under the barrier conditions during the period of the South American and African plate cracking, and is characterized by main limestone and algal limestone. The carbonate develops in the local highlands and slope areas due to the shallow water which is the result of the cracking of different plates and the resistance of volcanic ridge in Walvis. When the evaporation of the ancient salt basin is larger than the amount of water provided by the ocean, the salt rocks begin to precipitate and the development of the ancient salt basin continues. The carbonate reservoirs are mainly controlled by the salt rocks and sourced by the bottom lacustrine mudstone formed （luring the rift stage. The oil and gas migrate through the faults and the short distance unconformities to accumulate as effective reservoirs in the high structural areas and the developmental zones of salt rocks. These reservoirs covered by the overlying formation are characterized by structural and stratigraphic traps.