--> Burial Diagenesis of Clay Minerals in Jurassic Sandstones, Reconcavo Basin, Brazil, by C. R. O. Rodrigues and S. M. C. Anjos; #90986 (1994).

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Abstract: Burial Diagenesis of Clay Minerals in Jurassic Sandstones, Reconcavo Basin, Brazil

Carlos R. O. Rodrigues, Sylvia M. C. Anjos

Sandstones and conglomerates of the Sergi Formation are the main petroleum reservoirs of the Reconcavo basin. These rocks were deposited by braided rivers under arid to semi-arid conditions at the end of the Jurassic. The break up of Pangea and consequent rifting of the Reconcavo basin led the Sergi Formation to occur in a wide range of depths. Samples from surface to over 3000 meters were analyzed by conventional petrography, scanning electron microscopy and X-ray diffraction allowing the identification of diagenetic processes and trends with increasing depth.

The clay content of these rocks includes mechanically infiltrated smectites, neoformed kaolinites and chlorites and transformed mixed-layered illite-smectite and chlorite-smectite. The contents of discrete illite, chlorite and the percentage of illite in the illite-smectite increased with increasing depth whereas kaolinite and mixed layer illite-smectite decreased.

The gradual clay contents variation with depth together with the packing proximity index allowed the separation of the Sergi Formation in shallow (up to 1000 meters), medium (between 1000 and 2000 meters) and deep (more than 2000 meters) reservoirs. These differences are related to the maximum burial these rocks have been subjected to during the tectonic evolution of the basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994