Libra: A Newborn Giant in the Brazilian Pre-Salt Province

Abstract

As the operator of several exploratory blocks in ultradeep waters, Petrobras was responsible for many presalt oil discoveries in Santos basin such as Lula, Lapa, and Sapinhoá fields. In 2010, in partnership with the National Petroleum, Natural Gas and Biofuels Agency (ANP), Petrobras drilled the 2-ANP-2A-RJS well, which resulted in a world-class presalt hydrocarbon discovery – named as Libra.

In 2013, Libra was publicly offered by the Brazilian Government as the first production sharing contract in Brazil. The winning consortium is comprised of Petrobras (operator), Shell, Total, CNOOC (China National Offshore Oil Corporation), CNPC (China National Petroleum Corporation), and Pré-Sal Petróleo S.A. (PPSA).

Libras’ light oil accumulation is found in carbonate reservoirs occurring in a structural trap covering approximately 417 km2 (161 mi2). Libra lies at the northeastern flank of a regional structural feature known as Santos External High, where the majority of the most significant Brazilian presalt discoveries occur.

The predominantly shallow water and probable high-energy paleo environment over the Santos External High decisively influenced the deposition of lacustrine carbonate reservoirs with excellent porosities and permeabilities. From Neobarremian through Eoaptian, during the rift stage, high-energy bivalve grainstones and rudstones (coquinas) of the Itapema Formation were deposited. These rocks are the best reservoir facies found at the 2-ANP-2A-RJS well. The Barra Velha Formation carbonates were deposited in the Aptian, during the post rift stage, characterized by the transition from continental to a shallow marine setting (SAG interval). Grainstones and packstones composed of stromatolite fragments predominate at the base of the Barra Velha Formation. To the top, in situ carbonates such as stromatolites and laminites are the prevalent facies.

The source rocks are interpreted as organic-rich shales deposited in the distal lower-energy areas of the Itapema Formation. The top seal is a thick salt sequence (Ariri Formation) deposited during the late Aptian.

The 2-ANP-2A-RJS well found a 329 m (1079 ft) oil column (29o API gravity) with a gas-to-oil ratio from 416 to 448 m3/ m3 and CO2 content around 44%. Since 2014 eleven appraisal wells were drilled, confirming the preliminary estimates that indicated volumes of oil in place ranging between 7 and 15 billion BOE for the Northwest area (Central and Southeast areas are still under evaluation).

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018