Influence of Bioturbation on the Reservoir Quality of Miocene Slope Carbonates

Abstract

Bioturbation alters sediment properties in complex and highly variable ways, often leading to either enhanced or diminished reservoir quality and heterogeneity. In carbonate studies, the impact of post-depositional, biogenic modification is often overlooked. However, if carbonate reservoir characterizations, estimates of performance, and models are to be accurate, the biogenic control on reservoir quality must be well understood and accounted for. This study focuses on highly cyclic, heavily bioturbated slope carbonates from the Great Bahama Bank (ODP Leg 166). The objectives are to (1) conduct an ichnological analysis of middle-upper Miocene core intervals from the leeward slope of the Great Bahama Bank to document trace fossil assemblages, bioturbation indices, ichnofabrics, and ichnofacies, and (2) collect samples with differing ichnofabrics for further examination using a variety of techniques. A detailed sedimentological and chronostratigraphic framework is provided by previous work. The middle-upper Miocene lithologies record high frequency cycles of sea-level rise and fall, represented by alternating, decimeter-meter scale beds of light, moderately bioturbated, lithified neritic material, and darker, more heavily bioturbated and less-lithified sediment. Preliminary results show the main ichnogenera define a range of ichnofacies, including Cruziana, Zoophycos, Nereites, and Glossifungites, and that certain ichnogenera correspond to dark and light lithologies. Point counting, image analysis, and other techniques allow detailed visualization of bioturbation and assessment of relationships between porosity, permeability, and bioturbation. Initial results indicate that sample porosities range from 9.5% to 17%, and that the principal form of porosity in these rocks is intra-particle porosity contained within the shells of planktonic foraminifera, while inter-particle micro-porosity within the micrite matrix is also common. Samples also indicate that burrow fill can be more or less porous than the surrounding matrix, showing that bioturbation can impart complex heterogeneities in reservoir quality in fine-grained carbonates.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018