Hydrothermal Dolomites in Brazilian Precambrian Brecciated Carbonate Sequences: From Analogue for Deep Basin Oil Reservoirs?

Nilo S. Matsuda¹, Almerio B. Franca¹, Ricardo J. Jahnert², Jose M. Neto³, Antonio M. Rebelo³, and Anelize M. Bahniuk^3
1E&P/EXP/GEO, PETROBRAS S. A., Rio de Janeiro, Brazil
²E&P-EXP/IABCS, PETROBRAS S. A., Rio de Janeiro, Brazil
³Geologia, UFPR, Curitiba, Brazil

Dolomite formed by hydrothermal processes is an important oil and gas reservoir around the world. Important examples come from the Paleozoic of North America and Western Canada Basins.

Along the Brazilian Atlantic Passive Margin there are thick carbonate plataforms ranging in age from Early Cretaceous to Cenozoic with several dolomitized horizons. Petrography and 18Oxygen isotopes signatures show evidence of dolomites formed by hydrothermal processes, but no conclusive proofs have been found yet.

There are good examples of hydrothermally-formed dolomites, confirmed by petrography and isotopic analysis, from Precambrian carbonates in southern and central regions of Brazil. One is from a cement industry quarry close to the town of Curitiba showing a vertical brecciated zone about 70 m in thick, composed of pure dolomites. Such brecciated zone has been interpreted as related to deep transcurrent faults that were also the pathway for the hot fluids responsible for the dolomitization.

Petrographically, it is composed of an intergrowth of saddle dolomites and goethite. Another example are the Precambrian brecciated dolomites associated with Pb/Zn ore bodies (willemite) located in the town of Vazante, in the central part of Brazil. As Pb/Zn ore bodies occur commonly as Mississippi Valey-Type (MVT) deposits in low-temperature hydrothermal dolomitic associations, most likely, the huge zinc deposit in Vazante also is associated with brecciated dolomites, apparently formed as a collapse structure along a fault plane associated to a 300km-long transcurrent fault system. The Vazante ore deposit is an amazing example of fractured reservoir created by an association of faulting, brecciation, and hydrothermal dolomitization

These two examples of hydrothermally formed dolomites from Precambrian carbonates might be good analogues for Atlantic Basin passive margin oil reservoirs.

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