E. A. Prochnow*1, M. V. D. Remus*, J. M. Ketzer*2, J. C. R. Gouvea Jr.*, R. Schiffer de Souza3 and L. F. De Ros*+
* Inst. de Geociências, Universidade Federal do Rio Grande do Sul – UFRGS, Av. Bento Gonçalves, 9500, Porto Alegre , Brazil.
+ corresponding author, email: Lfderos@inf.ufrgs.br
1 permanent address: Universidade Luterana do Brasil – ULBRA, Rua Miguel Tostes, 101, Canoas, Brazil.
2 Present address: Pontifícia Universidade Católica do Rio Grande do Sul – PUCRS, Porto Alegre, RS, Brazil.
3 PETROBRAS Research Center – CENPES, Ilha do Fundão, Rio de Janeiro, Brazil.
Turbidite sandstone reservoir rocks in the Campos Basin, offshore eastern Brazil, show evidence of a number of interactions among organic and mineral phases during diagenesis. These include extensive dissolution of plagioclase, precipitation of kaolinite into intergranular and intragranular pores, dissolution of carbonate cements and grains, as well as selective deposition of organic films on kaolinite aggregates, dissolved plagioclase and carbonate grains. These processes are connected with the biodegradation of hydrocarbons within the reservoir, related to the influx of meteoric fluids possibly during sea-level lowstands, through major stratigraphic sequence boundaries and large-scale faults. Oil biodegradation generated organic solvents responsible for mineral alteration, as well as heavy oils rich in asphaltenes. The action of organic solvents in the reservoirs is indicated by the extensive dissolution of feldspars, quartz, garnets and kaolinite, and the negative mass balance between dissolved feldspar and precipitated kaolinite, indicating aluminum mobilization. Asphaltene films and coatings may affect the wettability of the reservoir rocks and the evaluation of hydrocarbon saturation in the reservoirs by resistivity logs.
These various inorganic and organic processes are apparently interrelated in the diagenesis of the Campos Basin turbidite reservoir sandstones. These interactions should be considered in future models of the basin and its petroleum system, as well as for the creation of geochemical models capable of predicting the quality of the reservoirs and the occurrence of degraded heavy oils.
JPG Home (opens in this window)