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Clay Minerals; June 2008; v. 43; no. 2; p. 213-233; DOI: 10.1180/claymin.2008.043.2.06
© 2008 Mineralogical Society of Great Britain and Ireland
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Research Paper

Dickite in shallow oil reservoirs from Recôncavo Basin, Brazil: diagenetic implications for basin evolution

J. DE BONA1, N. DANI1, J. M. KETZER2 and L. F. DE ROS1,*

1 Institute of Geosciences, Federal University of Rio Grande do Sul, UFRGS, Porto Alegre, RS, Brazil, and 2 Pontifical Catholic University of Rio Grande do Sul, PUC-RS, Porto Alegre, RS, Brazil

* E-mail: lfderos{at}inf.ufrgs.br

(Received 11 February 2007; revised 6 November 2007)

Fluvial and aeolian sandstones of the Sergi Formation are the most important reservoirs of the Recôncavo Basin, Brazil. Optical and scanning electron microscopy, X-ray diffraction and infrared spectroscopy revealed the occurrence of dickite, a clay mineral indicative of deep burial conditions (T >100°C), in the shallow Buracica (630–870 m) and Água Grande (1300–1530 m) oilfields. Vermicular dickite replaces K-feldspar and plagioclase grains, and fills intra- and intergranular pores. Its vermicular habit is a product of pseudomorphic kaolinite transformation during burial. The presence of dickite is in accordance with the intensity of compaction, post-compactional quartz cementation and {delta}18O values of calcite cements (T up to 109°C). These petrological features of deep burial, as well as apatite fission-track analyses, indicate that uplift and erosion of at least 1 km, and probably >1500 m, affected the central part of the Recôncavo Basin and possibly the entire region. This uplift has not been detected previously by conventional structural and stratigraphic models.

KEYWORDS: diagenesis, sandstones, reservoirs, dickite, uplift, Recôncavo Basin, Brazil






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