Carbobates in Brazil: New Challenges in Reservoir Studies

Spadini, Adali R.¹; Denicol, Paulo S.²; Madrucci, Vanessa ³
(1)Exploration, Petrobras, Rio de Janeiro, Brazil. (2) Exploration - Petrophysics and Formation Evaluation, Petrobras, Rio de Janeiro, Brazil. (3) Exploration - Geology Applied to Exploration: Stratigraphy and Sedimentology, Petrobras, Rio de Janeiro, Brazil.

More than 60% of the oil reserves in the world are in carbonate rocks, but due to their susceptibility to diagenesis, carbonates form heterogeneous reservoir and exhibit complexes porosity versus permeability relationships that impose immense challenges when predicting reservoir quality. In Brazil, Petrobras has produced from carbonates since 1970's but until one year ago this type of reservoir has contributed with only 4% of the total reserves, mostly from the Albian carbonates. Outstanding discoveries from the Pre-Salt carbonates, SE Brazil, in the last few years are progressively changing this scenario. Last year, Petrobras incorporated 1.25 billion of barrels to its reserves because of Pre-Salt carbonate discoveries in Santos basin. Now, carbonates reservoirs comprises approximately 10% of the proven reserves.

The Albian carbonates show a wide spectrum of porosity and permeability, but in general, there is linear correlation between depositional facies and permeability. The oolitic/oncolitic grainstones show good permeabilities within a narrow range of porosity, whereas reservoirs containing microporosity show a linear porosity versus permeability relationship in a larger spectrum of porosity.

Since the early 1970's and 1980's that predict reservoir quality in such type of reservoirs is not an easy task. Permeability estimates were done using Gamma-Ray, Density and Resistivity logs. Anomalies in the Photoelectric curve (Pe) and processing of the Stoneley wave applied in the late 1990's helped to extract petrophysical attributes from logs.

The Pre-salt reservoirs include both in-situ and redeposited carbonates. The former comprises stromatolites and laminites, including a wide spectrum of fabrics. SEM studies show that microbial mats and associated EPS are important in the formation of rocks and their porosity evolution. Porosity versus permeability correlations for the Pre-Salt carbonates are more complex than for the Albian. The spectral gamma-ray and NMR logs integrated with image logs, are used to estimate permeability. These data, together with capillary-pressure curve, clarified the distribution of micro, meso and macroporosity.

In summary, carbonate reservoirs studies throughout the last decades benefited from the rock studies integrated with log technology advances, and led Petrobras to a better understanding of permeability and porosity prediction from carbonate rocks of the SE Brazilian marginal basins.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.