The Impact of Biogenic Porosity Alteration: a Thin Section Petrographic Study of the Ben Nevis and Avalon Formations, Whiterose Field, Offshore Newfoundland, Canada
Awareness of the importance of ichnological activity as an agent of porosity and permeability alteration is becoming increasingly widespread. The expansion of this field of research and consequent greater recognition of localized heterogeneities has considerable potential for the refinement of reservoir models that previously relied on the assumption of uniformly homogenous permeability properties.
The results presented in this study focus on Cretaceous strata from the Avalon (Barremian-Aptian) and Ben Nevis (Aptian-Albian) formations encountered within the Whiterose Field, Jeanne D’Arc Basin, offshore Newfoundland, Canada, which has previously been the subject of limited petrographic and ichnological research.
Thin section petrographic analysis reveals that porosity varies both dramatically and abruptly from the millimetre scale upwards. Porosity heterogeneities are observed as a consequence of factors that include bioturbation, with both primary (selective grain emplacement and alignment) and secondary (diagenetic and differential cementation effects) being noted. These properties are particularly apparent amongst burrows that exhibit a zoned fill, where the core and mantle display distinct porosity properties, as well as in burrows with a simpler burrow lining. Differential cementation is also present independently of any association with biogenic activity, the presence of sharp cementation fronts being locally observed. Thin section analysis of both bioturbated and ichnologically depauperate intervals reveals porosity variation locally exceeding 20% and this variation frequently occurs across abruptly bounded margins.
Evidence for pressure solution is extensive and includes stylolites, as well as concavo-convex and sutured grain contacts that are present in both silicate grains and carbonate allochems. Selective dissolution of aragonite fossil material is observed, including complete dissolution of some fully aragonitic bivalves and gastropods, as well as partial dissolution of serpulids. Deformation and dissolution of carbonate bioclasts indicates localized compaction, ranging up to as much as 40% for some serpulid material.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009