--> Abstract: The Role of Diagenesis on the Formation of Fluid Flow Barriers Along Parasequence Boundaries, by J. Marcelo Ketzer, S. Morad, and Luiz Fernando De Ros; #90039 (2005)

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The Role of Diagenesis on the Formation of Fluid Flow Barriers Along Parasequence Boundaries

J. Marcelo Ketzer1, S. Morad2, and Luiz Fernando De Ros3
1 Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
2 Uppsala University, 752 36 Uppsala, Sweden
3 Universidade Federal do Rio Grande do Sul, Porto Alegre/RS, Brazil

Sequence stratigraphy is a powerful tool to predict reservoir facies and to estimate their depositional porosity and permeability. Important changes in depositional reservoir properties, such as porosity reduction owing to cementation, however, may take place during diagenesis, particularly along key sequence stratigraphic surfaces. Systematic studies on three different siliciclastic units (the Ferron Sandstone Member, Upper Cretaceous of Utah; the Mullaghmore Formation, Lower Carboniferous of Ireland; and the Rio Bonito Formation, Permian of Southern Brazil) revealed that pervasive carbonate cementation and formation of pseudomatrix occurs preferentially along parasequence boundaries. Such carbonate cementation is related to one or more of the following reasons: (1) abrupt changes in pore water chemistry due to abrupt increase in water depth, (2) prolonged residence time of the sediments under suitable geochemical conditions, close to the sediment-water interface, (3) concentration of carbonate bioclasts in lags, (4) occurrence of organic-rich deposits such as coal, and (5) intense bioturbation. Formation of pseudomatrix occurs by concentration of mud intraclasts within transgressive lags associated to parasequence boundaries, and further mechanical compaction and squeezing of these ductile grains into intergranular pore space, during burial. Both carbonate-cemented and pseudomatrix-rich sandstones have typically low porosity and permeability. They form laterally-extensive horizons that act as baffles and barriers for vertical fluid flow at parasequence boundaries, compromising hydraulic connectivity between amalgamated sand bodies of adjacent parasequences, and introducing strong reservoir heterogeneity.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005