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AAPG Bulletin

Abstract


Volume: 73 (1989)

Issue: 5. (May)

First Page: 598

Last Page: 612

Title: Diagenetic Evolution of Cretaceous-Tertiary Turbidite Reservoirs, Campos Basin, Brazil

Author(s): Marco A. S. Moraes (2)

Abstract:

Three sandstone turbidite sequences of the Campos basin (offshore, state of Rio de Janeiro, Brazil) were petrologically studied: (1) the Albian-Cenomanian Namorado Sandstone on the Macae Formation, (2) the Upper Cretaceous Carapebus Member of the Campos Formation, and (3) the Eocene Carapebus Member of the Campos Formation. The sequences represent deep marine deposits consisting mostly of massive sandstones rather than classical turbidites, indicating sand-rich submarine fans were the main depositional system of these sequences. The framework composition of the sandstones averages for quartz, feldspar, and lithics are Q60F40Ltr for the Cretaceous rocks and Q71F29Ltr for the Eocene rocks, plotting granitic r ck fragments at the feldspar pole.

The main diagenetic phases that affected the sandstones studied were (1) development of a clay matrix due to compaction of rip-up mud clasts, (2) partial replacement of the matrix by opal, (3) precipitation of small pyrite framboids, (4) widespread direct precipitation or replacement of different materials by calcite, (5) intense generation of secondary porosity, (6) localized kaolinite development, (7) minor precipitation of quartz and feldspar overgrowths, (8) development of dolomite, ferroan dolomite, and ankerite, and (9) replacement of different materials with minor direct precipitation of late pyrite.

Geologic and geochemical evidence lets us infer the main processes that controlled diagenetic transformations and mass transfer within the sequence studied. The principal source of carbonate cements was pressure solution of the underlying Macae Formation. Most of the diagenetic evolution of the sandstones was apparently controlled by the relative balance between the activity of CO2 and carboxylic acid species in the formation waters, both related to organic matter transformations within adjacent marine shales. The timing of hydrocarbon (generated in deeper lacustrine shales) migration to the reservoirs was an important factor in preserving diagenetically enhanced porosity.

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