The Role of Bioturbation in Permeability Distribution in the Upper Montney Formation, Northeastern British Columbia

The Lower Triassic Montney Formation in the Western Canada Sedimentary Basin comprises a westward thickening wedge of dominantly clastic sediment. Throughout most of the basin the Montney is grain size restricted, consisting primarily of siltstone and very fine-grained sandstone. In northeastern British Columbia the Montney is pervasively gas saturated rendering this unit Canada’s premier unconventional gas play. Despite the limited grain-size variability, lithological heterogeneities result in highly variable distributions of porosity and permeability. Detailed analysis of physical and biogenic sedimentary structures in core, completed in association with selective spot-permeameter analyses, has revealed that bioturbation plays an integral role in natural reservoir quality in the Upper Montney Formation in the west Groundbirch area of northeastern British Columbia. The bioturbation index (BI) of the Upper Montney Formation was assessed in Upper Montney core from Shell Groundbirch 16-2-78-22W6. The study interval consisted of zones with pervasive bioturbation by a Phycosiphon-dominated assemblage (BI 4-5) interbedded with zones characterized by planar lamination and devoid of trace fossils (BI 0). Although petrological analyses have not revealed any mineralogical or grain-size differences between laminated and bioturbated siltstone samples, permeability characteristically 1 to 2 orders of magnitude higher in the burrowed media than in laminated strata. Planar laminated dolomitic siltstone flow media possess permeability ranges of 0.04 to 0.08 md. Biogenically mediated dolomitic siltstone flow media possess permeability ranges of 0.2 to 1.0 md. Thus the bioturbated levels in the Upper Montney Formation constitute a dual-porosity flow media. Owing to the broad nature of biogenic flow media, the trace fossils are analogous to natural fracs, as their substantial surface areas promote flow interactions with lower permeability matrix. As such, these results suggest that detailed ichnological analyses are an important component of assessments of unconventional Montney reservoir intervals.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California