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Permeability Anisotropy and Heterogeneity of a Sandstone Reservoir Analogue: An Estuarine to Shoreface Depositional System in the Virgelle Member, Milk River Formation, Writing-on-Stone Provincial Park, Southern Alberta
In this study we investigate the patterns of permeability anisotropy and multi-directional heterogeneity of permeability structure based on a large number of lithofacies-specific horizontal and vertical cylindrical plugs from Upper Santonian Virgelle Member sandstones at Writing-on-Stone Provincial Park, southern Alberta. Diagnostic lithofacies of sandstone-rich, upward-coarsening siliciclastic marine shorefaces and overlying estuarine coastal plain are very common in the geological record. Hence, corresponding permeability distributions are thought to be a good analogue for broadly similar lithofacies and marginal marine successions known to make-up important petroleum reservoirs in Alberta and worldwide (e.g. Belly River Formation, Misoa Formation, Brent Group).
The use of permeability distribution types and vertical-to-horizontal permeability ratios (kv/kh) derived in this study can be readily transferred and tested on reservoir models in the subsurface. Highlighted among the results is the relative homogeneity of vertical permeability (kv) distributions that may arise from the baffling effect of laminar, current-induced fabrics acting as filter. Cross-laminar fluid flow in such facies is expected to form a more uniform and efficient displacement front, bypassing fewer pores relative to a heterogeneous, 'fingering' layer-parallel flow profile.
Overall, the permeability anisotropy in major lithofacies is low, with mean ratios kv/kh of 0.7–1, consistent with the absence of thick, continuous clay laminae, and only minor baffling by thin, discontinuous carbonaceous laminae or bands of mud chips. The ratio kv/kh of upper shoreface and channel lithofacies varies from 0.25 to > 2, but these lithofacies are all, to some degree, laminated at a scale of 1–30 mm that is smaller than the size of plugs. Hence, the magnitude of kv/kh is obscured due to preferred orientation of laminae and the interlaminar contrast in horizontal permeability (kh). In the Virgelle Member and laminated reservoir sandstones in general, the plug-scale kv/kh ratio can only rarely be considered truly representative of permeability anisotropy, and improved definition of subvertical, cross-layer flow requires distinction of anisotropy at a smaller scale.
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