Bioturbation Influence on Permeability Distribution within the Lower Triassic Montney Formation of the Western Canadian Sedimentary Basin

Abstract

The Montney Formation is one of western Canada's most lucrative unconventional hydrocarbon plays. Within the very fine-grained formation, many areas possess alternating layers of bioturbated and non-bioturbated facies. Bioturbation greatly influences the distribution of permeability and porosity by disrupting primary sedimentary structures, altering pore-throat distribution and concentrating cementation and dissolution processes during diagenesis. These small-scale features can significantly impact reservoir properties. Cored wells throughout the study area were investigated to understand how macroscopic bioturbation impacts vertical and horizontal permeability within the Upper Montney Formation. Multiple cores were logged and permeability was measured every 10 cm along the cores using a Pressure Decay Permeameter. Trace fossil assemblages were identified and are dominated by a dense Phycosiphon fabric with subordinate Teichichnus and Cylindrichnus also occurring. Cryptic (microscopic) bioturbation also occurs. Within each bioturbated zone, individual traces are isolated within specific beds or extend to multiple bedsets, with bioturbation intensity reaching up to 100%. Biogenically modified siltstone units posses permeabilities ranging from 0.2 to 1.0 md, whereas non-bioturbated, planar laminated siltstone possessed permeability ranging from 0.01 to 0.08 md. Although the measured permeability is low, the order of magnitude difference between bioturbated and non-bioturbated zones suggests a dual-porosity flow media. Increased permeability within the bioturbated zones may be due to (1) concentrating higher permeability within silty halos of Phycosiphon, (2) vertical traces breaching sedimentary structures and acting as flow conduits, and (3) the ichnofacies being relatively more homogeneous as a result of bioturbators disrupting original grain alignment and sedimentary structures, which would act as flow baffles if left unaltered. Although geomechanical analyses were not included as part of this study, it is recognized herein that the presence or absence of bioturbation also plays a significant role in the response of Montney facies to hydraulic fracturing. Understanding the influence of bioturbation on reservoir characteristics is essential in developing accurate reservoir models and resource assessments for the Montney Formation and will help provide ichnological models that can be applied to other unconventional, fine-grained reservoirs around the world.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016