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Abstract: Chlorite Prediction in Deep Reservoirs of Santos Basin, Brazil

Anjos, Sylvia M. C.; Carlos M. A. Silva* and Marcelo S. Almeida - Petrobras/ Cenpes

Cretaceous sandstones deposited as turbidite and shallow marine systems are major targets for petroleum exploration in Santos Basin, an offshore passive margin basin in southeastern Brazil (Fig.1). They occur up to 5000 meters deep and reach up to 60 meters in thickness.

Detailed petrographic analyses, along with X-ray diffraction and SEM/EDS studies were performed in several samples from more than 20 wells.

Lithic arkoses are composed of quartz, feldspars and volcanic rock fragments. The volcanic rock fragments (VRF) are of basic and acidic composition occurring in both turbidite and shallow marine facies. The VRFs were supplied by basalts from the adjacent Paran√° Basin as indicated by provenance studies.

Chlorite, quartz overgrowth and calcite are the main diagenetic phases in these sandstones. Calcite occurs mainly as a poikilotopic cement and was precipitated after chlorite.

Chlorite occurs replacing grains, as pore-filling rosettes and mainly as thick rims around framework grains. Chlorite coatings prevented later quartz cementation preserving 20% porosity in reservoirs as deep as 5000 meters. Chlorite is considered to be formed by the late transformation of a precursor smectite thin coating, generated from early diagenetic alteration of basic VRF. This hypothesis is supported by the presence of mixed layers chlorite-smectite coatings, preserved under calcite cement and detected by X-ray diffraction analysis.

The clay mineralogy of the sandstones and associated shales shows that below 3500m, chlorite is the main clay mineral whereas mixed-layers illite/smectite and chlorite/smectite predominate at shallow depths.

These data suggest that chlorite occurrence is depth-controlled and presents a basin-wide distribution and therefore quartz cementation is probably inhibited basin-wide.

Intense quartz cementation occurs mostly in shallow marine facies reservoirs probably due to the lack of the precursor smectite coating or because it was not in sufficient amount to promote pervasive chlorite growth. Although these reservoirs present lower porosity values due to its high content of quartz overgrowth, the permeability is higher due to the absence of thick chlorite coatings which favor the occlusion of pore throats.

Turbiditic reservoirs with thin and discontinuous chlorite coatings also prevented quartz overgrowth and developed even higher permeability reservoirs than those with thick chlorite coatings.

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