Effective Permeability in Tidal Heterolithic Cross-Bedded Sandstones

Massart, Benoit Y.; Jackson, Matthew D.; Hampson, Gary J.; Legler, Berit; Johnson, Howard D.; Jackson, Christopher A.; Ravnas, Rodmar; Sarginson, Marcus

Tidal heterolithic sandstone reservoirs are heterogeneous at the sub-metre-scale, often due to the presence of mud drapes of varying frequency and continuity along the foresets and toesets of cross-beds. It is well known that permeability is highly scale-dependent in such heterogeneous rocks, and conventional permeability measurements fail to sample a representative volume. Previous studies have used ‘mini-models' to calculate permeability values at appropriate length-scales in flaser-wavy-lenticular bedded intervals, but no studies have modelled tidal heterolithic cross-bedded sandstones. We investigate the impact of mud drape distribution on effective permeability using 3-D mini-models that accurately capture the geometry of cross-beds.

The surface-based modelling methodology uses template surfaces to represent the geometries of cross-bed set and foreset-toeset lamina boundaries. Geometrical input parameters are extracted directly from subsurface and outcrop analog data. The impact on effective permeability of seven parameters that characterize heterogeneity has been determined: (1) total sandstone fraction (‘net-to-gross'), (2) thickness of sand laminae, (3) mud drape continuity, (4) toeset dip, (5) angle of climb between successive foreset-toeset surfaces, (6) proportion of foresets to toesets, and (7) the trough or tabular geometry of the cross-beds.

Calculated permeability decreases as the sandstone fraction decreases and is highly anisotropic: the vertical permeability (averaged over the other 6 parameter values) falls to c. 0.5% of its value in a sandstone-dominated plug, while the horizontal permeability falls to c. 5% and c. 50% of the sandstone value in the dip and strike directions respectively. A relationship between sandstone fraction and calculated permeability in each direction can be identified, although there is considerable spread, because the other 6 parameters investigated can each have a significant impact, depending upon the flow direction and sandstone fraction. The calculated permeability also depends strongly on model volume, but converges to consistent (effective) values at the largest volumes investigated. The results yield improved estimates of effective permeability in cross-bedded, heterolithic intervals, which can be used to populate reservoir-scale model grid blocks, using estimates of sandstone fraction obtained from wireline or core data, and geometrical parameters obtained from core data or outcrop analogs.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013