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Insights on the Opening of the South Atlantic from the Evolution of the Pre-Salt Lacustrine Carbonates

Author(s): Teresa Sabato-Ceraldi BP Exploration, United Kingdom
D. Green Fixme
Alex Bump BP Exploration, United Kingdom

The discovery of the Lula field in the Santos Basin in Brazil has uncovered a pre-salt play in lacustrine carbonate reservoirs, and since then many significant discoveries in both Brazil and Angola have been made in the same play. Little was known of the lacustrine systems and how the systems were aligned to the evolution of the opening of the South Atlantic. Moreover, the understanding of the tectono-stratigraphic systems that lie beneath a thick layer of salt could provide insights into the formation and history of the early South Atlantic opening.

The observations coming from a conjugate pair of regional seismic lines between Kwanza and South Campos basins, help define the key megasequences: pre-rift, rift, sag, salt basin, post-rift carbonates, post-rift clastic. The subsidence and stratigraphy of the pre-salt deposits can be further described by 3 critical events:

1) Syn-rift lakes (Valanginian- Hauterivian to Early Barremian): Onset of continental break and syn-rift grabens with formation of several' deep fresh-water basins (overfilled) lakes above the sea level. with clastic-rich fluvial-lacustrine deposits. 2) Sag 1 (Barremian to Early Aptian): Syn-kinematic stretching of the continental crust and/or continuous rifting with continuous subsidence. In this phase the lacustrine sediments still maintain a high biodiversity, stable isotope analysis suggests the system is evolving into wide, potentially interconnected lakes with fresh-brackish water in balanced filled conditions 3) Sag 2 (Early- Late Aptian): Continuous subsidence the basin: the lake becomes hydrologically isolated (underfilled lake) thus provoking a dramatic change in the lake chemistry. The lake becomes very alkaline as demonstrated by the limited diversity of fauna, widespread occurrence of microbialite in most well penetrations, and prevalence of authigenic Stevensite. Stable isotope analysis supports the concept of a large closed hyper-saline alkaline lake connected from Angola to Brazil.

Carbonate reservoir intervals developed in both steps 2 and 3 above. Coarse-coquina grainstones, deposited as shoreface deposits along the lake margin and paleo-highs constitute the best reservoir in Step 2. Microbial deposits predominate in Step 3, and occur along the basin margin but are most prolific near basement-rooted paleo-highs away from the basin margin (outboard highs). Authigenic Stevensite with calcite spherulites, are commonly associated with the microbia lite textures, likely representing the deeper/lower energy areas of the lake.

The transition from Sag 1 to Sag 2 stage is the most critical stage in the evolution of the pre-salt salt tectono-stratigraphy. This could possibly represent the time when the lake level falls below the sea level forcing the lake to become hydrologically isolated and underfilled. In this talk we describe an holistic approach based on the integration of seismic data, well data, stable isotopes analysis with lake hydrology and subsidence modelling that results in a regional

Insights on the Opening of the South Atlantic from the Evolution of the Pre-Salt Lacustrine Carbonates
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BP Exploration
United Kingdom
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