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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 910

Last Page: 910

Title: Reservoir Quality of Sandstones Influenced by Mineralogy and Texture: Example of Brazilian Offshore Basins: ABSTRACT

Author(s): Hung Kiang Chang, Severino Xavier De Menezes, Earle F. McBride

Article Type: Meeting abstract


Lower Cretaceous sandstones in four basins off Brazil illustrate that reservoir quality is controlled by diagenetic events that were pre-programmed by (1) detrital mineralogy, (2) grain size, and (3) sorting.

The northeasternmost basin (Cassipore) received volcanic-rock detritus. At depths less than 3,000 m all pores in these lacustrine turbidite sands were cemented by pervasive corrensite and local laumontite. Secondary porosity is trivial.

Five hundred km to the southeast, the Ilha de Santana basin received granitic and reworked red-bed detritus. Samples from fluvial and lacustrine turbidite sandstones between 1,500 and 2,800 m show that sands lost porosity by cementation by patchy calcite, minor quartz, and pervasive mixed-layer clays. Modest secondary porosity developed by dissolution of calcite and clay cements, and detrital plagioclase.

Five hundred km farther to the southeast, the Ceara and Potiguar basins received granitic detritus and minor metamorphic-rock debris. Lacustrine deltaic deposits of the Ceara were sampled between 1,500 and 2,700 m and fluvial deposits of the Potiguar between 1,600 and 2,500 m. Except for the presence of kaolinite beneath an unconformity in the Ceara basin, the basins had a similar history. Porosity was lost successively by precipitation of clay coatings, quartz and calcite cement, and by compaction. Good secondary porosity developed by dissolution of calcite and plagioclase, but much porosity was lost subsequently by precipitation of mixed-layer clays derived from reaction of pore fluids with feldspars.

The best secondary porosity developed in the coarser and better sorted sandstones. Coarser sandstones (1) had more calcite cement that yielded clean secondary-pores and (2) have larger pore throats that were affected less by clay cement. Fine sandstones (1) have more ductile micas and rock fragments that compacted and plugged pores and (2) have smaller pore throats that were strongly affected by clay cement.

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