Ichnological Controls on Hydrocarbon Shale Properties in the Light of Three-Dimensional Volumetric Reconstructions of Shale Ichnofabric
Bednarz, Malgorzata; McIlroy, Duncan
The present study is to evaluate the impact of ichnofabric on the shale-hydrocarbon reservoir quality in terms of porosity, permeability and fracturability. Phycosiphon-like burrows and Chondrites are common ichnofabric forming trace fossils for shale hydrocarbon reservoir facies, contributing to redistribution and concentration of quartz grains within mudstones and siltstones. Silty and sandy conduits resulting from burrowing activities of infaunal organisms comprise complex geometries in ultra-low permeability organic-enriched rocks. These ichnofabrics are here reconstructed in three dimensions in order to understand spatial geometries, density and distribution of burrows in shale-hydrocarbon facies, and allow consideration of connectivity and volumetrics.
Samples containing Chondrites and Phycosiphon-like burrows were accurately ground with precise computer-controlled machinery and photographed in order to examine mineralogy and grain distribution. The photographs of known spacing are the basis of three-dimensional reconstructions modeled with volume-visualizing computer software.
Examination of reconstructed 3-D ichnofabrics and individual burrows indicate that, even with low density of Phycosiphon and Chondrites bioturbation, the quartzose strips are connected throughout the sample in vertical and horizontal planes. Highly tortuous burrow geometries create extensive surface area that can be more than twice the surface of the prism-shaped bioturbated sample.
Ichnofabric may greatly increase porosity and permeability of reservoir facies. Frameworks created by biogenically concentrated quartzose material within clay-rich shale provides or/and improves zones that are susceptible for natural or induced fractures. Enhanced porosity and inherent natural fracture responsiveness of quartzose material within burrow framework provide additional space for hydrocarbon molecules increasing reservoir capacity and storativity. Significant surface area of the interface between organic-rich host sediment and permeable ichnofabric material enhance fluid migration into the fracturable framework increasing deliverability of the reservoir. Thus comprehensive understanding of density, structure and distribution of ichnofabric in three dimensions is a prerequisite condition for accurate reservoir assessment in bioturbated shale-hydrocarbon facies.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013