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Lessons from Hamelin Pool and the Maldives for the Coquina Reservoirs in Libra, Santos Basin

Abstract

Libra is one of the giants in the Brazilian Presalt Province. It is situated on the Santos External High about 200 km from the coastline. The discovery well was placed in the flank of a structure elongated in a northwest-southeast direction that consists mostly of bivalve coquina accumulations with excellent reservoir quality. It is draped by the Barra Velha Formation whose facies consists of stromatolites, spherulitic limestones, grainstones, and laminites. This facies association is reminiscent of the modern deposits in the Hamelin Pool, Australia, where stromatolites and coquina also coexist. The stromatolites in Hamelin Pool are distinctly different from the microbialites (spherulites and shrubs) in Libra while the coquina accumulations are more comparable. As in Libra, the loose coquinas and the cemented coquinas (pavement) are volumetrically much larger than the stromatolites. The coquinas are easily transported to form subaqueous bars and beach ridges. In addition, they are cemented early with microbial cements that dramatically increases the stiffness of the coquina aggregates. This early diagenesis, makes the coquina beds in the Hamelin Pool very resistant to compaction. In Libra, the coquina deposits form a prograding unit with foresets of 500 - 700m height. A cross-line at the position of the discovery well shows a mounded feature with bidirectional downlaps. The entire flank is interpreted as coquina wedges that prograde towards north-northwest (Carlotto et al., 2017). The observed bi-directional downlap together with petrophysical results of coquina hardgrounds in the Hamelin Pool show that these geometries are not caused by post-depositional compaction. The geometries of the triangular shaped prograding flank in Libra is reminiscent to the coarse-grained drift deltas discovered in the Inner Sea of the Maldives. The drift delta is deposited by a semi-continuous, sediment laden surface current that flows through the Kardiva Channel and releases its sediment load when reaching the deeper water of the Inner Sea. A case can be made that the coquinas at Libra are also deposited by currents that transport coquinas from the highs into deeper areas. This transport mechanism requires a wind-driven current system within “Lake Tupi” that with its size and geographical position most likely experienced easterly winds. Carlotto M.A., et al., 2017, Libra: A Newborn Giant in the Brazilian Presalt Province. AAPG Memoir 113, 165-176.