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AAPG Bulletin

Abstract

AAPG Bulletin, V. 87, No. 7 (July 2003),

P. 1073-1082.

Copyright copy2003. The American Association of Petroleum Geologists. All rights reserved.

Secondary porosity formed by deep meteoric leaching: Botucatu eolianite, southern South America

A. B. Franccedila,1 L. M. Arauacutejo,2 J. B. Maynard,3 P. E. Potter4

1Petrobraacutes-EampP-BC, Av. Chile 65, Rio de Janeiro RJ, Brazil
2Petrobraacutes-EampP-Corp., Av. Chile 65, Rio de Janeiro, RJ, Brazil
3Department of Geology, University of Cincinnati, P.O. Box 210013, Cincinnati, Ohio, 45221; [email protected]
4Department of Geology, University of Cincinnati, P.O. Box 210013, Cincinnati, Ohio, 45221

AUTHORS

Almeacuterio B. Franccedila received his B.S. degree in geology from the Universidade de Brasiacutelia, Brazil in 1974 and his Ph.D. from the University of Cincinnati in 1987, studying the stratigraphy and reservoirs of glacially influenced deposits in the Gondwana of the Paranaacute basin, southern Brazil. He joined Petrobraacutes in 1975 working on exploratory projects in basins of the Amazonas and offshore areas. In 1980, he joined Paulipetro on the exploratory project of the Paranaacute basin, finished in 1984. Almeacuterio returned to Petrobraacutes in 1987, and currently, he is with the exploration department of Petrobraacutes, working on the Campos basin.

Laury M. Arauacutejo received his B.S. degree in geology from the Universidade do Vale do Rio do Sinos, Brazil in 1978 and his Ph.D. from the Universidade Federal do Rio Grande do Sul in 2001, studying the stratigraphy and geochemistry of the Irati source rock from the Gondwana of the Paranaacute basin, southern Brazil. He joined Petrobraacutes in 1979, working on exploratory projects in the marginal Campos basin and the intracratonic Paranaacute basin. Since 2001, he has been working with geochemistry applied to basin modeling in the exploration department of Petrobraacutes.

J. Barry Maynard received his Ph.D. from Harvard in 1972. He has worked in resource geology of both oil and minerals, in South Africa, Brazil, Mexico, Cuba, and China in addition to the United States, with focus on the inorganic geochemistry of shales and sandstones. His current work involves the application of S isotopes to understanding ore deposit genesis and improving mine reclamation.

Paul E. Potter received his Ph.D. in geology from the University of Chicago in 1952 and his M.S. degree in statistics from the University of Illinois in 1959. His professional interests include shales, sandstones, basins, big river systems, and geomorphology. His work experience includes Illinois State Geological Survey, Johns Hopkins University, Indiana University, University of Cincinnati, Universidade Estadual de Sao Paulo, and Universidade Federal de Rio Grande do Sul, plus consulting in Algeria, Brazil, and the United States.

ACKNOWLEDGMENTS

We thank Petrobraacutes for their key support and also for support from the Universidade Estadual de Satildeo Paulo (Rio Claro, SP Brazil), the Universidade Federal de Rio Grande do Sul (Porto Alegre, RS, Brazil), and the University of Cincinnati (Cincinnati, Ohio). Samples were provided by many geologic organizations, both public and private and especially by the geological and hydrological surveys of Argentina and Uruguay. Professor A. C. Rebouccedilas of the Universidade de Satildeo Paulo provided early insightful discussions and we have benefited greatly from comments by L. F. De Ros of the Universidade Federal de Rio Grande do Sul. Para todos acima, nosso mais sincero obrigados.

ABSTRACT

The Botucatu Sandstone of the Paranaacute basin of Brazil, a Jurassic eolianite, exhibits unusually deep, basinwide penetration of meteoric water. This flow of dilute groundwater has removed calcite cement for an average distance of about 140 km from the outcrop and down to a burial depth of about 250 m. Feldspar has been leached from the framework for about 30 km downdip, transforming what was deposited as a feldspathic sandstone into a diagenetic quartz arenite on outcrop. These two dissolution processes have generated appreciable secondary porosity by weathering reactions at sites far removed from contact with the atmosphere. Between 100 and 500 pore volumes of water have passed through the aquifer system to produce this leaching, making this an end-member case of extensive meteoric diagenesis of sandstone. Consideration of other basins with meteoric incursion suggests that basin-scale leaching is favored when uplift of one margin provides a steep gradient and there is a downdip escape route via faults or outcrop of the aquifer. Cratonic basins close to rifted-continental margins seem to provide the best chance of meeting these conditions.

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