Sylvia Maria Couto Anjos¹, Carlos Manuel de Assis Silva¹, Cristiano Leite Sombra¹

(1) PETROBRAS/CENPES, Rio de Janeiro RJ, Brazil

ABSTRACT: Diagenetic Clay Minerals in Brazilian Turbidite Reservoirs

Most of the Brazilian oil reserves are in turbidite sandstones. The detrital composition is usually formed of quartz, feldspars and, to a minor extent, of rock fragments. In some of them, diagenetic clays strongly influence porosity and permeability. This work aims to present how the main diagenetic clay minerals occur in Brazilian turbidite sandstones. In proximal settings, close to basin margins, clastic wedges are efficient pathways to meteoric water infiltration. As a result of meteoric water action, kaolinite, smectite and mixed layers illite-emectite are the main clay mineral diagenetic products. Kaolinite occurs mainly filling pore space as booklets, in vermicular form and replacing framework grains, mainly feldspars and micas. Examples occur in Campos, Espírito Santo, Sergipe and Potiguar basins. Smectite and mixed-layers illite-smectite occur as authigenic pore-lining fringes and pore-filling aggregates as observed in Espírito Santo Basin turbidites. In deep water settings, where diagenetic phases are usually found in very early stages of evolution, diagenetic clays may be important porosity/permeability modifiers when related to either bioturbation or to alteration of unstable volcanic rock fragments. Smectite, chlorite and mixed layers illite-emectite are the main diagenetic clays. In deeply buried settings, chlorite is the most common diagenetic clay. Chlorite occurs as pore-lining and pore-filling cement, and is typically found in deep reservoirs and in sandstones enriched in basic volcanic rock fragments as observed in some Santos Basin sandstones. Illite is not a very common cement in Brazilian turbidite reservoirs.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado