巴西桑托斯盆地盐下层序油气地质特征与有利区预测

巴西桑托斯盆地经历了裂谷期、过渡期和被动大陆边缘期3个构造演化阶段,发育了盐下、盐岩和盐上3套层序。盐下下裂谷期烃源岩是盆地内的主力烃源岩,具有良好的生烃能力;盐下储层主要为上裂谷—坳陷期生物灰岩、上裂谷期介壳灰岩;盐岩层在深水区厚度大,呈连续分布,封盖能力强;基底断垒带有利于形成各类构造圈闭,有利的地质条件为盆地盐下巨型油气藏的形成提供了良好基础。盆地北部深水区的基底断垒带是盐下油气聚集的有利场所,这一区域盐下烃源岩演化程度适中,巨厚连续分布的盐岩层提供了良好的保存条件;基底断垒形成的古浅水区有利于碳酸盐岩储层的发育,断垒、翘倾断块、披覆背斜圈闭十分丰富,周边地堑区生成大量的油气就近运移到地垒区聚集成藏。其他地区盐下层序存在各种风险,成藏条件略差。

Petroleum geology characteristics and exploration targets of pre-salt formations in Santos Basin,Brazil

The Santos Basin in Brazil has experienced three tectonic evolution stages (rifting, transitional and passive continental) with three sequences deposited (pre-salt, salt and post-salt). Source rocks in the pre-salt formations in the lower rift were the most effective source rocks in the basin. Microbial limestones in the upper rift and depression, and coquina limestones in upper rift served as the main reservoirs for the pre-salt source rocks. The salt formations had strong capping capacity thanks to great thickness and continuous distribution in the deep water area. The basement horst belt helped the formation of various structural traps. All these advantageous geologic conditions provided a good basis for the generation of giant oil and gas pools in the pre-salt formations in the basin. The basement horst belt in the deep water area in the north of the basin was a favorable location for the pre-salt hydrocarbon accumulation, where the source rocks were moderately mature, and the thick, continuously distributed salt formations provided a good preservation environment. In the shallow water area formed by a basement horst, carbonate reservoirs were well-developed, and traps such as horsts, tilted fault blocks and drape anticlines were very common. Hydrocarbons generated in the surrounding grabens migrated to and accumulated in the horsts. The other pre-salt formations had poor accumulation conditions.