--> Bioclastic Reservoirs of the Distal Montney “Shale” Play

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Bioclastic Reservoirs of the Distal Montney “Shale” Play

Abstract

Sedimentologic and stratigraphic analysis of 1500m of full-diameter core, integrated with petrographic description of cuttings from wells in the most basinward subsurface extent of the Montney formation, northeastern British Columbia have identified two of the three most productive reservoir units as bioclastic in origin. Their excellent hydrocarbon deliverability makes the origin, lateral variability and heterogeneity of these reservoir units an issue of economic significance within a leading North American resource play. Referred to for years as “the turbidite zone”, the lower of the two bioclastic units is a monospecific, life assemblage of Claraia sp. “flat clams”, interpreted as a biostrome, interbedded with highly bituminous, parallel laminated siltstones deposited out of suspension near storm- weather wave base on a low gradient, predominantly siliciclastic, ramp. The biostrome reservoir unit is bounded by a marine flooding surface below, and maximum flooding surface above, and grades basinward into a dolosiltstone facies of hemipelagic origin and paleolandward into lower shoreface coarse siltstones to sandstones. The younger of the two bioclastic units is interpreted as a mixed carbonate/clastic ramp facies association. Individual bioclastic beds are sharp based, normally graded and comprised of an admixture of bivalves, brachiopods and echinoderms, showing evidence of wave and/or storm transport, hence interpreted as tempestites. There are three parasequences within this reservoir interval each of which grades basinward into bituminous siltstones and hemipelagic dolosiltstone. Paleolandward (east) they become thinner through overlying erosion by a regional sequence boundary. The lateral distribution of bioclastic intervals form reservoir “sweet spots” within the Montney. Bioclastic beds are densely calcite cemented with minimal measurable porosity (1-2%) and only rarely naturally fractured. However, the interbeds of siltstone in both successions are highly bituminous (TOC range 2-4%) and of relatively high total porosity averaging 5-7%. It is concluded that the hydrocarbon deliverability of the bioclastic reservoirs has less to do with primary or secondary reservoir quality and is more a function of geomechanical rock properties attributable to high frequency interbedding of brittle-ductile facies resulting in significant permeability and geomechanical anisotropy leading to more effective reservoir stimulation through hydraulic fracturing.