Abstract: Reservoir Facies in the Proterozoic São Francisco Basin - Mid-Continent of Brazil

Toledo, Marco A.S.; Pedro V. Zalán - Petrobras/E&P; Maria Dolores de Carvalho - Petrobras/Cenpes

The São Francisco sag basin is situated in the mid-continent of Brazil encompassing an area of about 300,000 km². The sedimentary filling is composed predominantly of Late Proterozoic sedimentary rocks that belong to several lithostratigraphic units: Paranoá (mostly quartzites, transitional), Macaúbas (conglomerates and diamictites, marine/glacial) and Traíras (carbonates and shales, marine restricted) Groups, all three being approximately laterally equivalent and covered by the Bambuí Supergroup (open marine). This unit is composed from base to top of five formations: Sete Lagoas (carbonates), Serra de Santa Helena (siltstones/carbonates), Lagoa do Jacaré (intercalation of carbonates and shales), Serra da Saudade (shales) and Três Marias (deltaic sandstones). Shallow crystalline metamorphic rocks and deep Middle Proterozoic aulacogens constitute economic basement.

Only four wells had already been drilled for hydrocarbon prospecting in the basin. The last one had its set of well logs completed by an ultrasonic borehole image. The integration of the processed logs, including mineral discrimination, image enhancement, 3D visualization, besides core and flow test analysis, allowed a better evaluation of the Proterozoic reservoirs. They are dominated by secondary porosity and very poor preservation of matrix porosity, deteriorated by advanced chemical and mechanical compaction, as determined by petrographic analysis. Three types of secondary porosity facies were defined: fractured Bambuí carbonates, dolomitized Bambuí carbonates and fractured Paranoá sandstones.

The fractured Bambuí facies, present in the Serra de Santa Helena and Lagoa do Jacaré formations, can be split into five sub-types on basis of micro-acoustic, image log: scarcely fractured, fracture concentrated, fenestral-like, vugular-like and oblique fracture. In the four first sub-types the fractures are dominantly sub- horizontal and their associated occurrences suggest a progressive sequence related to events of carbonate dissolution from scarcely or concentrated fractures to vugular-like facies. The best gas shows are associated to shallow intervals presenting high concentration of sub-horizontal fractures, some of which with associated vugular dissolution (Fig. 1).

The dolomitized Bambuí reservoir, regionally present at the top of Sete Lagoas Formation, is represented by medium- to coarse-grained crystalline dolomite. This dolomite pervasively replaced packstone and oolitic grainstone facies producing secondary intercrystalline and vugular porosity estimated by log analysis to be up to 8% (Fig. 2). Under cathodoluminescence microscopy dolomite shows dull red luminescent cores. Permeability in Paranoá quartzites is indicated due to water flows on DST in one of the wells and by thermal springs that flow 1,000,000 bpd of hot water from outcropping fracture networks in the western basin edge.

In conclusion, there is storage capacity in the basin.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil