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The Middle Montney Altares Member: Lithology, Depositional Setting and Significance for Horizontal Drilling and Completion in the Altares Field, British Columbia
The Montney Formation is dominated by fine-grained lithologies, primarily siltstone and very fine-grained sandstone. Regionally pervasive hydrocarbon saturation and a lithological framework that is both relatively easy to drill horizontally, as well as sufficiently brittle to respond satisfactorily to hydraulic fracture programs have rendered this the primary unconventional reservoir target in the Western Canada Sedimentary Basin.
Recent drilling programs in the Altares Field of northeastern British Columbia have encountered heterogeneities in the Altares Member of the Montney Formation that present challenges to fracture stimulation. The Altares Member, consist of centimetre to decimetre-scale interbeds of bituminous siltstone, very fine-grained sandstone and bioclastic packstone to grainstone beds. These beds accumulated in a proximal offshore to offshore transition setting, above maximum storm wave base but below fairweather wave base. Bioclast-rich beds were subjected to syndepositional/early post-depositional calcite cementation, whereas bituminous siltstone horizons are largely devoid of calcite cements. These beds are characterized by geomechanical properties (changes in Poisson’s Ratio and Young’s Modulus determined via triaxial testing and analyses of dipole sonic logs) that are distinct from that of the host-rock.
Frequent vertical contrasts in geomechanical properties in the interbedded siltstone/bioclastic packstone/grainstone beds provide numerous planes of weakness. Common bedding plane slickensides attest to lateral movement at lithofacies contacts, likely during Laramide tectonism. In the Altares Member in the Altares Field area these bedding-parallel contrasts in geomechanical properties act as vertical barriers to fracture stimulations, causing the water to follow the contrast instead of breaking through it, thus preventing effective stimulation. In addition to facilitating ineffective fracture propagation, bedding plane slippage can have a deleterious effect on the vertical well bore including, in several cases, crimping of well casings.
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