Abstract

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 (k_v/k_h) 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 (k_v) 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 k_v/k_h 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 k_v/k_h 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 k_v/k_h is obscured due to preferred orientation of laminae and the interlaminar contrast in horizontal permeability (k_h). In the Virgelle Member and laminated reservoir sandstones in general, the plug-scale k_v/k_h 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.