--> Abstract: Chlorite Occurrence and Distribution Pattern: The Importance of the Sedimentary Processes and Depositional Environment Conditions, Santos Basin, Brazil, by Rogério S. Souza and Dorval C. Dias Filho; #90082 (2008)

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Chlorite Occurrence and Distribution Pattern: The Importance of the Sedimentary Processes and Depositional Environment Conditions, Santos Basin, Brazil

Rogério S. Souza and Dorval C. Dias Filho
Research & Development Center, Petrobras, Rio de Janeiro, Brazil

Numerous works have reported the possible sources of Fe-chlorite, including depositional clay minerals, replacement of early precursors, burial diagenesis and alteration of Fe-rich rock fragments. However, because of the poor understanding of the controls on the chlorite occurrence and distribution patterns, presently the pre-drilling prediction of the best permeability trends to help guide exploration and appraisal strategies is a difficult task. Fe-rich chlorite is often found as cement, and replacing framework grains and intergranular fine-grained matrix in sandstones in the Santos Basin, southeastern Brazil. The present study suggests that the depositional processes and the depositional environment conditions have primary importance in the amount, occurrence and distribution of Fe-rich chlorite in this basin. Thick beds of sandstones deposited as short-lived hyperpicnal flows during catastrophic river flooding have high amounts of chlorite, while associated thinner turbiditic beds deposited by long-lived turbulent flows with lamination and cross stratification have no or lower content of interstitial chlorite. Petrographic, SEM/XRD/WDS, and REE analysis indicate that Fe and Al were primarily delivered by flooding rivers from the granitic-gnaissic basement rich in mafic Fe-rich minerals. As the fresh water flow entered the sea, a Fe-rich clay mineral precursor (likely as a smectite clay) precipitated as a thin coating around the framework grains and as fine-grained floccules in the thick beds deposited as hyperpicnal flows. After addition of Mg from sea water and further deep burial most of the precursor was converted to Fe-rich chlorite. Sandstones of the thinner beds with clear evidences of traction as major depositional process present neither precursor nor fine matrix, and diagenesis evolved differently with strong quartz, feldspar and calcite cementation.

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