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Depositional, Infiltrated and Authigenic Clays in Fluvial Sandstones of the Jurassic Sergi Formation, Recôncavo Basin, Northeastern Brazil

By
Marco A. S. Moraes
Marco A. S. Moraes
Petrobras/Cenpes/Diger, llha do Fundão Cid. Universitária Qd. 7, Rio de Janeiro RJ 21910, Brazil
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Luiz F. de Ros
Luiz F. de Ros
Petrobras/Cenpes/Diger, llha do Fundão Cid. Universitária Qd. 7, Rio de Janeiro RJ 21910, Brazil
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Published:
January 01, 1992

Abstract:

The Sergi Formation, a Jurassic pre-rift sequence composed mostly of fluvial sandstones, is one of the major hydrocarbon reservoirs of the Recôncavo Basin in northeastern Brazil. Interstitial clays are important components of sandstones and exert significant control on reservoir properties, including permeability, irreducible water saturation and residual-oil saturation. These clays can be grouped into three types: (1) depositional clays; (2) mechanically infiltrated (MI) clays; and (3) authigenic (neoformed) clays. Each type shows a characteristic petrographic aspect that permits recognition and quantification using thin sections.

Depositional clays were incorporated into the rocks as mud intraclasts resulting from reworking of overbank fines by fluvial processes. Early mechanical compaction crushed the mud clasts among more rigid grains, forming a compaction matrix. Mechanically infiltrated (MI) clays occur chiefly as coatings of tangentially accreted particles (cutans) or, locally, as complete pore fills. MI clays appear to be concentrated within the upper part of the formation. These clays can modify the pore geometry of sandstones. Shrinkage porosity, developed by diagenetic transformation of clays, is the dominant porosity type in the upper part of the Sergi Formation.

Authigenic clays are kaolinite and chlorite. Kaolinite occurs as pore fills in large secondary pores and, where present in large amounts, may generate high microporosity in the reservoirs. Chlorite occurs as pore linings and, locally, as pore fills. In the reservoirs, chlorite causes permeability reduction and is related to the presence of low resistivity in water-free, oil-producing zones. These authigenic clays show a distinct distribution within the basin. Kaolinite dominates in the western portion, where Sergi reservoirs are found at shallow depths (above 1,000 m), whereas chlorite is dominant in the eastern portion, where Sergi reservoirs are found at greater depths. The distribution of these clay minerals is the result of differences in burial/temperature histories, which are still reflected by present depths.

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SEPM Special Publication

Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones

David W. Houseknecht
David W. Houseknecht
Department of Geological Sciences University of Missouri Columbia Missouri 65211
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Edward D. Pittman
Edward D. Pittman
Department of Geosciences University of Tulsa Tulsa Oklahoma 74104
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SEPM Society for Sedimentary Geology
Volume
47
ISBN electronic:
9781565761728
Publication date:
January 01, 1992

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