Abstract: Compositional Genetic Potential, Diagenetic Evolution and Reservoir Quality of Eastern Brazilian Margin Turbidites

De Ros, L. F. - Universidade Federal do Rio Grande do Sul

Upper Cretaceous and Tertiary turbidite sandstones are the main oil reservoirs of Brazil. Detrital composition plays an essential role on their patterns of diagenetic evolution and on their reservoir quality. Five compositional petrofacies (Fig. 1) are defined based on this compositional genetic potential, and ranked according to decreasing reservoir quality:

Petrofacies 1 comprises ?clean? arkoses and subarkoses of proximal, massive, channelled turbidite deposits. This petrofacies shows the highest quality potential, due to the preservation of primary porosity related to minor cementation, early oil emplacement, and/or late subsidence. Carbonate cementation is commonly scattered and concretionary, and laterally continuous cementation occurs only along contacts with interbedded shales. Meteoric influx occurred in some reservoirs during major sea-level falls, promoting dissolution of feldspar and early carbonate cement and kaolinization of feldspar and mica.

Petrofacies 2 comprises sandstones rich in volcanic rock fragments which were eodiagenetically altered to smectitic clays. During mesodiagenesis, chlorite rims decreased permeability, but preserved porosity, by inhibiting late quartz and carbonate cementation. The better quality potential occurs where thin chlorite rims preserve high porosities even in deep reservoirs.

Petrofacies 3 comprises hybrid arenites rich in mud intraclasts and/or glaucony peloids eroded from platform edge and upper slope, with decreased reservoir quality potential due to the generation of abundant compaction pseudomatrix. Silicification of pseudomatrix to opal and microquartz associated to radiolaria-rich shales promotes high microporosity and low-resistivity anomalies. Local kaolinization of pseudomatrix occurred due to meteoric influx.

Petrofacies 4 comprises hybrid arenites rich in carbonate bioclasts with low reservoir quality due to pervasive, eodiagenetic, microcrystalline calcite cementation. Carbonate precipitation is commonly accretional, with oxic/suboxic cements covered by sulfate-reduction and fermentation cements. The lateral continuity of the cemented areas is essentially controlled by the original depositional extension of the bioclastic beds and by dissolution due to meteoric influx which affected some turbidites. Cemented Petrofacies 4 areas usually show a lenticular/concretional geometry with little influence on fluid flow and development of the porous Petrofacies 1 reservoirs in which they occur.

Petrofacies 5 comprises cycle-top and levee sandstones with the lowest reservoir quality potential, due to fine grain-size and mud matrix introduced by bioturbation and fluidization. The permeability anisotropy due to micaceous or clay laminae is usually accentuated by the selective precipitation of eogenetic, microcrystalline siderite, dolomite or ankerite. The mud fraction may be either affected by silicification associated to radiolaria-rich shales, by kaolinization due to meteoric influx, or by choloritization associated to volcanic fragments.

Petrofacies 5 comprises fine-grained, cycle-top and levell sandstones. Besides for their grain size, these deposits show the lowest reservoir quality potential also due to: a) the common presence of mud matrix introduced by bioturbation and fluidization, and b) the permeability anisotropy promoted by micaceous or clay laminae which is usually accentuated by the selective precipitation of eogenetic, microcrystalline carbonate (particularly siderite, dolomite or ankerite). The primary, bioturbation or fluidization mud fraction may be either affected by silicification, in successions associated with radiolaria-rich shales, by kaolinization, in sequences affected by meteoric influx, or by chloritization in volcanic fragments-rich sequences associated to Petrofacies 2.

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