Print this pageUnravelling Biogenic Pore Networks in Shale Gas Reservoir Facies
Most Proterozoic Earth surface environments experience animal-sediment interactions. In this regard, organic-rich mudstones are no different. Such reductant-rich, fine-grained deposits are ideal habitats for sediment-dwelling deposit feeders. The action of such deposit feeders on sediments include, chemical effects e.g. oxidation of organic matter which facilitate early diagenesis in the near burrow environment and—most importantly for shale gas reservoirs—physically reorganising sedimentary grains into silt and clay- grain sized wavy-discontinuous, sub-horizontal lamina-sets.
Many shale gas reservoir models assume the presence of primary, continuous silt-rich laminae, very thin siltstone beds, and fractures for storage and migration of gas. Our work demonstrates that some deposit feeding organisms, particularly those making ‘phycosiphoniform’ burrows have the ability to create good quality permeability in their burrow walls. Furthermore, this oblique-to-bedding bioturbational activity has the potential to connect horizontal sedimentary laminae separated by impermeable mudstone layers. This potential for net increase in both Kv and Kh suggests that burrowing is an underestimated component to effective permeability in shale gas reservoirs.
Our study focuses on ‘phycosiphoniform’ burrows from the Cretaceous Rosario Formation of Mexico. In this study, we have used serial grinding, petrography and tomographic methods in order to reconstruct the 3D geometry of phycosiphoniform burrows in a clay-rich siltstone, similar to many shale gas reservoirs. Burrow geometry is linked to probe-permeametry measurements that demonstrate that, in the material studied, the effects burrowing may have a significant positive impact on porosity and permeability.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009