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Source-Rock Contribution and Petroleum System Efficiency - Pre-Salt Cluster Area, Santos Basin, Offshore Brasil

Matthieu Pontet and Simone Sciamanna
Repsol Sinopec Brasil, Rio de Janeiro, Brazil

The equivalent of Kazakhstan reserves (ab. 30 Bbbl probable reserves) have been discovered in the pre-salt Cluster Area of the Santos basin, Brazil, since 2007. With only few public wells available, the petroleum system approach helped in deciphering the occurrence and contribution of the lacustrine pre-salt source-rocks, proven or hypothetical. Migration pathways have been identified and migration efficiency has been calculated based on a mass balance approach. Geochemistry analyses corroborated the interpretation of our source-rock and migration schemes. By de-risking the exploration of future prospects or farm-in opportunities the understanding of the petroleum system directly impacted management decisions and strategy.

A regional model was build with lithology distribution based on 3D seismic. Lithofacies where defined from logs and subsequently reservoir and non-reservoir facies were populated in the model using impedance data.

In-house kerogen kinetic from immature source-rock sample allowed a detailed description of the maturity evolution together with a rigorous salt restoration to control the pre-salt section burial. Model was calibrated with pressure, temperature and maturity indicators.

Considering that the source-rock potential is the less constrained parameter of the study, a sensitivity analysis on the source-rock potential and the number of source-rock was performed using invasion-percolation simulations. Original generated masses have to be multiplied by a factor 2 to fit with the observed accumulations.

In parallel, an attempt to determine the petroleum system yield (The petroleum system: A fundamental tool, Biteau et al, 2003) of the Santos Cluster was done and gave an estimate of 6 % (optimistic case) to 3% (pessimistic case) efficiency. By mass balance exercise, the initial source-rock potential would require an higher efficiency 11%. This leads to the same conclusion as previously obtained by invasion-percolation simulations and the same requirement of applying at least a factor 2 to the generated masses.

Inversion-percolation and Darcy migration methods were applied and present similar results in term on saturation, validating the migration scheme. Distinct timings for prospect infill are related to the physical differences between the two methods and define a range for prospect charging scenarios.

The last part of the study includes biomarker comparison from 3 wells validating the migration scheme established by modeling. It emphasized the occurrence of two migration schemes depending of the maturation of the involved source-rocks and the migration distance.

The study successfully provides a framework to fully explain and calibrate the latest exploration results in the southeast Brazil Off-shore basin.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012