AAPG Annual Convention and Exhibition

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Pre-Salt Carbonates of the South Atlantic


The “Pre-salt play” in South Atlantic has emerged as being one of the most exciting and prolific petroleum systems in the world. Since 2005, many billions of barrels of oil reserves have been discovered in Campos and Santos basin sweet spots in Brazil. In Angola, recent large discoveries within this play suggest that perhaps a similar steep climb up the pre-salt creaming curve has begun. The vast majority of these high-impact, giant and supergiant discoveries lie in carbonate reservoirs generated during the late rift to sag phases of the South Atlantic break up. Reservoirs are juxtaposed alongside thick, rich, and widespread source rocks and are ultimately trapped by evaporite strata in base ‘salt’ closures. Exploration success is mostly determined by the presence and quality of effective reservoir, which are notoriously difficult to predict. These carbonates rocks are bizarre. Remaining mostly unknown to both the industry and academic communities alike, they have a surprisingly limited variety of sedimentary textures but a large range of complicated pore types developed under complex and poorly-understood diagenetic processes in a linked carbonate-silicate-pH-temperature system. Depositional analogues are unclear although laminated to thrombolitic carbonates generated in alkaline, volcanic continental rift-lake systems share many similarities. The abundance of purely crystalline components comprising these rocks remain without analogy at scale. Well-developed sequences of skeletal biogenic sediments can also form prolific reservoirs separate from these mostly crystalline fabrics. Finally, direct reservoir comparisons between these reservoirs on a basin scale can be difficult due to differential diagenetic processes that can significantly alter original depositional fabrics. But not all is lost – it is within these challenges where scientific advances can be made. As more subsurface data become available to the general community, this session has sought to assemble experts from industry and academia working on the subsurface aspects of this complex and intriguing system. We would like to use this time to capture what we have seen in this session and to have an interactive and exciting discussion on the state-of-the-art understanding of what may very well be a new paradigm in carbonate geoscience.